Solid-State Fermentation: An Overview

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Abstract
Solid-state fermentation (ssf) is defined as the growth of microbes without free flowing aqueous phase. The ssf is alternative to submerged fermentation for production of value added products like antibiotics, single cell protein, PUFA’s, enzymes, organic acids, biopesticides, biofuel and aroma production. However, the advantages of ssf in various processes are found to be greater than in submerged fermentation. This paper reviews the advantages of solid-state fermentation over submerged in production of different value added products, important features of various bioreactor designs, recent developments in utilization of various agro-industrial residues as substrates and the importance of mathematical modeling. With advances through modeling and optimization techniques, production-using ssf is advantageous and appropriate for production of many value added products like enzymes, antibiotics, and organic acids. This technique not only decreases the cost of the process but also makes product cheaper for consumers.
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Solid-state Fermentation: An Overview
S. Bhargav,aB. P. Panda,a,* M. Ali,aand S. Javedb
aPharmaceutical Biotechnology Laboratory, Faculty of Pharmacy,
Jamia Hamdard (Hamdard University), Hamdard Nagar,
New Delhi, India – 110062
bMolecular Biology and Biotechnology Laboratory, Faculty of Science,
Jamia Hamdard (Hamdard University), Hamdard Nagar,
New Delhi, India – 110062
Solid-state fermentation (SSF) is defined as the growth of microbes without free
flowing aqueous phase. The SSF is alternative to submerged fermentation for production
of value added products like antibiotics, single cell protein, Poly unsaturated fatty acids,
enzymes, organic acids, biopesticides, biofuel and aroma production. However, the advan-
tages of SSF in various processes are found to be greater than in submerged fermentation.
This paper reviews the advantages of solid-state fermentation over submerged in produc-
tion of different value added products, important features of various bioreactor designs, re-
cent developments in utilization of various agro-industrial residues as substrates and the
importance of mathematical modeling. With advances through modeling and optimization
techniques, production-using SSF is advantageous and appropriate for production of many
value added products like enzymes, antibiotics, and organic acids. This technique not only
decreases the cost of the process but also makes product cheaper for consumers.
Key words:
Solid-state fermentation, submerged, aroma, biocontrol, biofuel, antibiotics, aroma, bio-
reactors, substrates, enzymes, acids, biochemical engineering, modeling, PUFA, exo-
polysaccharides
Introduction
Solid-state fermentation (SSF) is defined as the
fermentation process in which microorganisms
grow on solid materials without the presence of free
liquid.1The concept of using solid substrates is
probably the oldest method used by man to make
microorganisms work for him. In recent years, SSF
has shown much promise in development of several
bioprocesses and products. However, SSF has also
some disadvantages. There are some processes in
which solid-state fermentation cannot be used as in
bacterial fermentation.
Solid-state offers greatest possibilities when
fungi are used. Unlike other microorganisms, fungi
typically grow in nature on solid substrates such as
pieces of wood, seeds, stems, roots and dried parts of
animals such as skin, bones and fecal matter i.e. low
in moisture.2In SSF, the moisture necessary for mi-
crobial growth exists in an absorbed state or in com-
plex with solid matrix. However, SSF differs from
solid substrate fermentation. In solid substrate fer-
mentation, the substrate itself acts as a carbon source
and occurs in absence or near absence of free water.
However, in solid-state fermentation, the process oc-
curs in absence or near absence of free water by em-
ploying a natural substrate or inert substrate as solid
support.3The aim of SSF is to bring cultivated fungi
or bacteria in tight contact with the insoluble sub-
strate and to achieve the highest nutrient concentra-
tion from the substrate for fermentation. This tech-
nology so far is run only on a small scale, but has an
advantage over submerged fermentation.
Two types of SSF systems have been distin-
guished depending on the type of solid phase used.
The most commonly used system involves cultivation
on a natural material and less frequently on an inert
support impregnated with liquid medium.4Solid-state
fermentation processes can also be classified based on
whether the seed culture for fermentation is pure or
mixed. In pure culture SSF, individual strains are used
for substrate utilization and with mixed culture, differ-
ent microorganisms are utilized for the bioconversion
of agro-industrial residues simultaneously.
Biochemical engineering aspects
of solid-state fermentation
A considerable amount of work has been done
in recent years to understand the engineering as-
pects of solid-state fermentation.
S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008) 49
*Corresponding author: Bibhu Prasad Panda,
E-mail: bibhu_panda31@rediffmail.com;
Telephone no.: +91-11-26059688 Ext. 5627;
+91-11-65631138
Review
Received: November 14, 2006
Accepted: September 15, 2007
Moisture and water activity in SSF
The concept of water availability in substrate
becomes important. Water activity, Awis defined as
the ratio of vapor pressure of an aqueous solution to
that of pure water at the same temperature.5Scott in
1953 introduced to microbiology the concept of
water activity (Aw), which is a different way of de-
scribing the water relations in medium from the
term “water content”. In solid-state, the low mois-
ture within the substrate limits the growth and me-
tabolism of microorganisms when compared to sub-
merged fermentation. Awis a very useful parameter
for measuring water potential, characterizing the
energetic state of water.6
Water relations in SSF have been studied under
quantitative aspects.7,8,9 Water activity of substrates
has a strong influence on microbial activity. Awde-
termines the type of organisms, that can grow in
SSF. Awof the medium has been attributed as a fun-
damental parameter for mass transfer of water
across the microbial cells. The control of this pa-
rameter could be used to modify microbial meta-
bolic production and its excretion.10
Grevais and Molin studied the effect of water
in solid culture medium on fungal physiology such
as cellular mechanisms, radial growth rate and ori-
entation of fungus. They observed that the radial
extension rate of mycelia related to the water activ-
ity value. The optimum water activity was found to
be 0.99 for T. viride, and below 0.90 no fungal de-
velopment occurred, for Penicillum roquefortii the
optimal water activity was 0.97.11 Molin et al. de-
scribed the effect of water activity on hyphal and
radial growth.12 Moreover, hyphal growth rate was
always greater than radial growth rate, showing that
hyphae do not simply extend in radial directions.
Fungal spore production is also influenced by
water activity. Maximum sporulation value of
Penicillum roquefortii was obtained at water activ-
ity of 0.96 i.e. at lower water activity maximum
sporulation occurs. The influence of low water ac-
tivity value on fungal fermentation is not well un-
derstood; at low water activity there is loss of es-
sential nutrients necessary for fungal growth.13,14
Production of secondary metabolites also de-
pends on water activity. It was found that there is a
direct relation between the amounts of enzyme pro-
duced vs. Aw. In a solid culture medium of T. viride
biosynthesis of polygalactouronase, D-xylanase and
b- galactosidase is affected by water activity of sub-
strate. It was found that maximum polygalactouro-
nase and D-xylanase production occur at Awof 0.99.
However, b-galactosidase formation was optimum
between Awof 0.96 and 0.98.15 Lipase production
by Candida rugosa was enhanced by mixed solid
substrate with initial Awvalue of 0.92.16 Awalso ef-
fects the fungal aroma production, bioconversion of
2-heptanone from octanoic acid by T. viride was
optimum at water activity value of 0.96.11
Moreover, higher Awalso enhanced substrate
conversion to fungal biomass as reduced water ac-
tivity causes lower mass transfer and little water
availability for microorganisms. This results in
spore production. Increase in Awof substrate in-
creases specific growth rate and spore germination
time of the fungus.5
Grajek and Grevais reported that decrease of
Awby 0.01 (equivalent to 1 % of relative air humid-
ity) causes reduction in biomass production and
protein content of culture medium.17 Narahara
found that optimum Awfor Aspergillus spp. was be-
tween 0.970 and 0.990.18 Fungus was unable to
grow below Awof 0.97. These data prove that fun-
gal growth and their secondary metabolite produc-
tion during SSF are strongly affected by Awof sub-
strate.11,17,18
Temperature and heat transfer
Fungal growth and secondary metabolite pro-
duction in SSF are greatly influenced by tempera-
ture and heat transfer processes in the substrate bed.
During SSF a large amount of heat is generated,
which is proportional to the metabolic activities of
the microorganism. However, fungus can grow over
a wide range of temperatures from range of J=2C
to 55 °C. Nevertheless, optimum temperature for
fungal growth could be different from that required
for product formation.19 The substrates used for
SSF have low thermal conductivities that decrease
heat removal and increase its accumulation. There-
fore, the key issues in SSF are heat removal, and
hence most studies are focused on maximizing heat
removal.
The heat transfer in or out of an SSF system is
closely associated with microbial metabolic activity
and aeration of the fermentation system. High tem-
perature effects fungal germination, metabolites
formation and sporulation.20 The effect of tempera-
ture on kinetic parameters of T. ressei QM9414
have been estimated by measuring radial growth of
fungus, its glucosamine content and specific respi-
ration activity using wheat bran as substrate. The
fungal activity declined exponentially when opti-
mum temperature for growth reached above maxi-
mum.21
Mitchell and Meien used modeling experi-
ments to study the potential of Zymotis packed bed
bioreactor. The rate of cooling was higher at the top
of the bed as compared to the bottom. Vertical tem-
perature gradients were decreased. Bioreactor per-
formance was estimated by determining the time re-
quired for volume average biomass concentration to
50 S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008)
reach 90 % of maximum biomass concentration.
The highest biomass concentration was above the
bottom of the bioreactor where inlet air temperature
is lower than optimum temperature. The biomass
near the top was less than maximum biomass indi-
cating unfavorable temperature.22
Coupled control of temperature and moisture is
also an important issue for consideration. The low
moisture and poor thermal conductivity of the sub-
strate makes heat transfer and temperature control
difficult in SSF. Minimizing the substrate bed
height can solve heat transfer problems, but it is
possible only in small-scale solid-state fermenta-
tion. Good mixing of substrate with sparged oxygen
can also solve heat transfer problems. Moreover
mixing not only aids in the homogeneity of the bed
but also ensures an effective heat and mass transfer.
Continuous mixing along with addition of water is
advantageous for simultaneous control of tempera-
ture and moisture in large-scale SSF.20 Adjusting
the aeration rate controls temperature during evapo-
rative cooling in SSF. If the temperature is too low,
the aeration rate decreases, which increases temper-
ature in the bed due to respiration of the microor-
ganism. But if temperature is too high, increasing
the aeration rate promotes cooling of the sub-
strate.23,24,25 However, with larger size particles or
by forming aggregates of available agro-industrial
residues, such as in the case of wheat bran, heat re-
moval can be achieved with forced cold aeration, as
larger particles have greater space between them.
Biomass and growth kinetics
Biomass is a fundamental parameter for char-
acterizing the fungal growth and is essential for
measuring growth kinetics in SSF. However, cultur-
ing fungus over membrane filter can perform direct
estimation of biomass. A disadvantage of this
method is that the membrane filter prevents pene-
tration of fungal hyphae within the substrate.26 Fun-
gal biomass in an SSF system can be estimated by
different techniques, like scanning electron micro-
scope, reacting fungal biomass with specific flouro-
chrome probes, respiration rate of organism, and
reflectance infrared (IR) spectroscopy.27,28,29
Determination of fungal biomass in SSF can
also be done through some biochemical methods.
Changes in three fungal constituents such as Glu-
cosamine, Ergosterol and total sugar represents
the effect on fungal biomass. Among these, Gluco-
samine is considered a good biomass indicator if
the fermentation media has the same constituents
but not the same carbon-to-nitrogen ratio. The
amount of Ergosterol varies throughout develop-
ment of fungus with different medium composition
and appears only when the fungi sporulate. Hence,
it does not allow estimation of mycelial growth but
it is a good indicator for sporulation. In conclusion,
Glucosamine is the most reliable measure for
mycelial biomass determination but it can only be
used for optimization studies of nutrient concentra-
tion. Ergosterol is, however, an unreliable measure
of biomass but when determined along with Gluco-
samine gives useful information regarding spore
amount and fungal sporulation ability. Estimation
of total sugar is not a good biomass indicator be-
cause it depends on fungus age and medium com-
position. The disadvantage of these manual bio-
chemical methods are that they are time consuming,
and require lengthy analytical procedures.29
Direct methods such as measurement of CO2or
O2consumed are most powerful when coupled with
the use of a correlation model, which correlates bio-
mass with a measurable parameter.30 Desgranges et
al. estimated the biomass in solid-state fermentation
by Infrared measurement of cell components such
as Glucosamine, Ergosterol, medium residues (su-
crose, nitrogen) and CO2evolution rate. Gluco-
samine estimation is not used when medium con-
tains insoluble nitrogen. When manual methods for
biochemical estimations are not possible, the CO2
production rate and IR measurement are more bene-
ficial. In SSF system CO2production correlates
well with other parameters. Moreover, it can indi-
cate even low physiological activity since the
method is very sensitive.31
Mass transfer
In SSF the fungal hyphae forms a mat on the
substrate surface and penetrates by secreting second-
ary metabolites and enzymes.32,33 Interparticle con-
centration gradients due to nutrient consumption in
combination with mass transfer limitations can have
a strong effect on the rate and efficiency of the pro-
cess.34,35,36 Mass transfer in SSF involves micro-scale
and macro-scale phenomenon. Micro-scale mass
transfer depends on the growth of microorganisms
which depends on inter and intra particle O2and CO2
diffusion, enzyme, nutrient absorption and metabo-
lites formation. Macro-scale mass transfer includes
airflow into and out of the SSF system, types of sub-
strate, mixing of substrate, bioreactor design, space
between particles, variation in particle size and mi-
croorganisms within the SSF system.20,37,38
Micro-scale mass transfer
Limitation of O2affects the aerobic SSF per-
formance. In SSF fungal mycelia develops on a
solid surface and in a void area within the substrate.
Fungal mycelia are filled with water and oxygen in
the space between substrate.39 Oxygen consumption
takes place at interface between the substrate parti-
S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008) 51
cles and fungal hyphae. Several researchers have
modeled O2transport between the water and fungal
hyphae by considering fungal hyphae as a biofilm
of unicellular organisms. Mitchell et al. studied
intra-particle oxygen transfer of Rhizopus oligo-
sporus in SSF conditions on a nutritionally defined
medium and reported that O2transfer depends on
the interfacial gas-liquid surface area and thickness
of the wet fungal layer. These two factors play an
important role for O2transfer in the SSF system.
Similar to O2transfer, CO2diffusion takes place
from fungal hyphae through the void area of the
solid substrate.32
Apart from O2,CO
2diffusion micro-scale mass
transfer includes transfer of nutrients, enzymes, sec-
ondary metabolites and different metabolic prod-
ucts. Enzymes secreted from the fungal hyphae act
upon the complex organic substrate converting it
into simplified carbon sources, which are again uti-
lized by fungus. Therefore, in SSF there is mass
transfer of enzymes from fungus to substrate and
transfer of nutrients from substrate to fungus. When
Rhizopus oryzae is cultured on a complex tannin
substrate, the secreted tannase breaks the substrate
into glucose and gallic acid, and the glucose is uti-
lized by the fungus as a carbon source.37
Macro-scale mass transfer
Macro-scale mass transfer is directly influ-
enced by type and design of the bioreactors used in
SSF: transfer rate of O2,CO
2into and out of the
bioreactor, transfer of water in the bioreactor, mix-
ing speed of solid substrate, water and microorgan-
isms, thickness of bioreactor wall, and temperature
of the cooling agent in the bioreactor. The optimum
design and suitable selection of a bioreactor is re-
quired for maximum mass transfer process.20 How-
ever, a particle simulation model for optimizing
better design of rotating drum bioreactor was devel-
oped by Schutyser et al. and particle simulation
models provide better details about the transport
process and optimizes high mass transfer.40
Modeling
Mathematical modeling in the SSF system is an
important tool for optimum design and operation of
bioreactors. The cost procured in predicting fermen-
tation conditions at larger scale eases with model-
ing. Models in bioreactor can be of two types: Ki-
netic models and Transport models. A kinetic
model describes how the microorganisms are influ-
enced by different process parameters, while the
transport model describes the mass and heat trans-
fer within the bioreactor SSF systems. Kinetic mod-
eling depends on particle size, packing density, res-
piration rate, pore size of substrate particles, depth
of fungal mycelium penetration within the substrate,
and water content of the substrate bed. Transport
modeling depends on the growth rate of fungus,
rate of airflow through the bed, width of bioreactor
walls, height of the bed, and rate of the heat re-
moval process. Moreover, mathematical models
help in finding the relationship between O2and the
substrate consumption rate, substrate consumption
rate and biomass synthesis rate, oxygen uptake rate
and biomass synthesis rate, heat evolved and its re-
lation with the biomass synthesis rate.
Modeling and simulation of biological pro-
cesses provide a basis for economic and ecological
evaluation, which enables integrated optimization
of the processes. This helps in prediction of the pro-
cess at larger scale. Ecological assessments are
based on material and energy balance of the process
to identify the most relevant materials and process
steps.41
Microbial growth kinetics along with operating
variables such as moisture, temperature, product ac-
cumulation, heat and wastes accumulation play an
important role in continuous SSF processes. Lage-
maat and Pyle, attempted to develop an unstruc-
tured growth model, to form a basis of continuous
tannase production. The model described the uptake
and growth kinetics of Penicillum glabrum on inert
impregnated polyurethane foam. The time delay be-
tween biomass production and tannase and spore
formation was described using logistic kinetics,
since tannase is not produced after stationary
phases.42
Determination of biomass production through
fungal indicators such as Glucosamine, Ergosterol
is time-consuming but using sensitive methods like
determining respiration saves time and improves ac-
curacy of the process. A kinetic model for monitor-
ing bed conditions in SSF was developed using re-
spiratory gases such as O2,CO
2. The model was
calibrated using Gibberella fujikoroi on wheat bran
starch in an agitated aseptic SSF bioreactor. The
various assumptions were divided into 4 groups: ki-
netic, evaporation, energy balance, and water bal-
ance modules. The CO2production rate is a direct
indicator for biomass growth. First CO2production
rate, water content and bed temperature were ob-
tained through water and energy balance models,
but due to the complexity of the process these esti-
mations were considered to result in significant
errors. Determination of the CO2production rate
through kinetic models reduced the chances of
errors and noise production. Simulations in single
step were enabled with help of kinetic models.43
A mathematical growth model for converting
the absolute mass ratio to relative mass ratio was
52 S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008)
developed using logistic equation kinetics. How-
ever, by growing Rhizopus oligosporus on cassava
and gel solid based substrates validated the model.
The model was used for determining indirect mea-
surements of growth such as protein estimations.44
The curvature in graphs between biomass and mi-
crobial growth kinetics represents the dependence
of biomass on the length of the growth phase. It is
better to study biomass produced in lag phase
through logistic equations. Logistic equations ex-
plain final limitations of biomass production in the
stationary phase.45 Since, data interval particularly
representing growth kinetics is required for fitting
an equation; hence, interpretation of growth kinet-
ics is important for developing an appropriate
bioreactor vessel. Logistic equations are simple and
give an approximation of the whole growth curve
including the lag phase and stoppage of growth in
other stages of fermentation.46 These logistic equa-
tions fail to provide complete change in biomass
and production of secondary metabolites during the
death phase.47
Mitchell et al. have reviewed the use of trans-
port modeling optimum operation of bioreactors.
Mathematical modeling provides easier way of op-
timization among different operating variables for
maximum productivity. Heat and mass transfer in
tray bioreactors is limited due to intraparticle oxy-
gen diffusion, growth rate of fungus and lack of
heat exchange processes. Many differential and al-
gebraic equations are used for correlating variables
like bed height with oxygen, diffusion in particles
and increase in temperature. These studies influ-
ence the removal of heat and enhance production of
important metabolic compounds. Scale-up in tray
bioreactors can be achieved by increasing the num-
ber of trays. Oxygen limitation is absent in packed
bed reactors due to forced aeration. Packed bed bio-
reactors are affected by superficial velocity of air
and heat removal through convective, evaporative
cooling. Decrease in water content of substrate bed
affects mass transfer within the bed, causing shrink-
age of the bed. The shrinkage directly affects
growth kinetics of the inoculated organism. Axial
temperature gradients are unavoidable in packed
bed due to unidirectional flow of air. The Zymotis
packed bed reactor minimizes axial temperature
gradients due to vertical arrangement of cooling
plates. Heat removal through rotating drum bio-
reactors can be controlled by regulating rotational
speed. Mass transfer in such bioreactors depends on
particle size of substrates, growth rate of fungal
mycelia, rate of inlet air, and bed height. Rate of in-
let airflow and particle size have a significant effect
on heat and mass transfer in bioreactors with forced
aeration such as fluidized bed reactors. These fac-
tors directly influence the conductive cooling
through the bioreactor walls and exchange of heat
between the solid and gaseous phase.48 Recently,
various other reviews stressed the use of mi-
cro-scale mathematical modeling in SSF for im-
proving bioreactor performance and providing a
better yield of product with improved characteris-
tics.46,49
One of the major problems in SSF bioreactors
is heat accumulation. Changing design of
bioreactors through mathematical modeling can
solve the problem, which affects cooling rate by
bringing thermodynamic changes in the reactor.
Oostra et al. evaluated the applicability of mixed
and non-mixed systems for producing spores of
bio-control agent Coniothyrium minitans. Conduc-
tive cooling through the walls of the non-mixed re-
actor was evaluated by simple mathematical models
based on parameters such as thermal conductivity
of bed and metabolic heat production from fungus
(calculated from O2consumption rate). Increase in
bed diameter with decrease in bed height enhanced
the temperature of the core, which decreased the ra-
dial growth rate and relative activity of fungus.
Hence, this showed requirement of additional evap-
orative cooling. Additional simulations also showed
that large volumes could be cooled via conductive
cooling through the walls at low mixing intensities
and small temperature driving forces. The effect of
mixing depends on the fungal hyphae used in the
experiment. Experimental studies showed no detri-
mental effect of mixing on spore production by C.
minitans. The spore production yield in a conti-
nuously mixed scraped-drum reactor (ns=0.2min
–1)
was 5 · 1012 spores per kg of dry oats after t=
450 h.50
Heat removal is an important aspect of packed
bed reactor. Packed bed reactors are suitable for
static SSF processes. Modeling in packed bed sys-
tems has been done to study the factors for heat re-
moval. Packed bed systems have a static bed with-
out forced aeration. Ashley et al. have evaluated the
phenomenon of air reversal and mixing by heat
transfer dynamics in packed bed for preventing
overheating. In air reversal, air moves from right to
left and vice versa. The time in which air flows
from one side to the other, the temperature of the
cooled area increases due to accumulation of meta-
bolic heat. As a result, significant temperature fluc-
tuations were reported. However, discontinuous
mixing at controlled rotational speed and bed height
can be highly useful for controlling overheating in
packed beds.51 The design of the Zymotis bioreactor
with internal cooling plates has solved the problem
of heat removal in a packed bed reactor. The cool-
ing plates are arranged at a distance of l= 5 cm ver-
tically. The distance between the plates is increased
for fast growing organisms, and for proper loading
S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008) 53
and unloading of the reactor. The aim of Zymotis is
to minimize vertical temperature gradients and
maximize the heat transfer through cooling water.22
In conclusion, most researchers for bioreactor
development have used mathematical modeling at
microscopic scale using simple logistic kinetics.
Logistic equations are simpler than complex partial
differential equations and represent whole data
without complexity. Modeling at microscopic level
directly affects heat and mass transfer in bio-
reactors, which affects scale-up of the bioprocess.
Continuous operating bioreactors based on agro-in-
dustrial residues are to be modeled. These bio-
reactors should have arrangements for substrate ad-
dition, water replacement and air inlet. However,
validation of artificial results at smaller scale is nec-
essary in reality at a larger scale for success of the
mathematical modeling.
Bioreactors
In fermentation processes, bioreactor systems
provide the environment for growth, cultivation of
microbes. However, some of the factors affecting
the growth of the product in SSF bioreactors are
temperature, humidity of substrate bed, type of sub-
strate used, size of the bioreactor, aeration, cooling
rate, height of bed and fungal morphology. When
compared to submerged fermentation, SSF is car-
ried out in simple bioreactor systems; SSF bio-
reactors are fitted with a humidifier and with or
without an agitator unit. Poor thermal conductivity
of the substrate bed presents a great challenge to
bioreactor design, but composition, particle size,
porosity and water-holding capacity of the substrate
used also affects the bioreactor.
Various researchers have classified the SSF
bioreactors broadly20,50,52,53 but most bioreactors can
be distinguished by a factor whether they are used
at small scale and large scale.
Small-scale bioreactors
Solid-state fermentation at laboratory scale is
carried out in Petri dishes, jars, wide mouthed Erlen-
meyer flasks, roux bottles, and roller bottles. These
systems are simple and experiments are carried
out easily.53 There are several forms of small-scale
bioreactors such as column bioreactors. Mauris
Raimbault and J. C. Germon in ORSTOM (Institut
Français de Recherche Scientifique pour le De-
veloppment en Cooperation, France) laboratory of
soil microbiology designed it in 1980, for growing
Aspergillus niger on cassava meal in solid-state.
Column bioreactors consist of small columns (di-
ameter: D= 22 mm, height: H= 210 mm) which
can hold 20 g of pre-inoculated solid material. Ap-
proximately 24 such columns were put together in
thermo-regulated water baths and water-saturated
air was passed through each column at a flow rate
of Q= 4–6 L h
–1. Column bioreactors are useful
for optimization of the medium, and constitute an
important part of research. However, difficulty lies
in obtaining the product and poor heat removal.
Pandey et al.54 determined the performance of the
column bioreactor for glucoamylase production.
Columns of different diameters were used and ar-
ranged with different substrate bed heights of h=
4.5, 9, 18, 22.5 cm. Enhancement in substrate bed
height increased the enzyme synthesis. The column
with 18 cm of bed height produced maximum en-
zyme in 48 h with an aeration rate of Qa= 1 to 1.5
LL
–1 min–1. However, with further increase in bed
height, enzyme synthesis decreased. The decrease
may be due to the reduced aeration rate with in-
creased height of the substrate bed.
A few years later the INRA (Institut National
de la Recherche Agronomique, Dijon, France) team
developed a reactor with one liter working volume
fitted with a relative humidity probe, a cooling coil
in heating circuit, and a cover for the reactor vessel.
These reactors were filled with pre-inoculated solid
material, and a computer controlled all parameters
of the reactor. Sampling for analysis was easier in
INRA bioreactors. This was the advantage over
ORSTOM bioreactors. The automatic control of
relative humidity and temperature makes these re-
actors useful in scale-up studies.
Apart from non-agitated column reactors, there
are bioreactor systems with agitator systems such as
perforated-drum reactor and horizontal paddle
mixer (with or without water jacket). Drum bio-
reactors are designed to mix the solid substrate by
rotating horizontal rotation vessels (which are with
or without baffles). The rotation or agitation creates
tumbling in the matrix of the solid medium by min-
imizing the heat produced. However, mixing of
inoculum with substrate is uniform in drum bio-
reactors but fungi morphology is readily damaged
by high shear, stress is not used in such bioreactors.
However, an intermittent mixing strategy is in-
volved in these bioreactors, which gives balance
between positive effects of increasing temperature
on deleterious effects on fungal hyphae.52,55 A pad-
dle mixer was developed by Wageningen Univer-
sity of Agriculture. It consists of a number of
blades making mixing more efficient than rotating
drum.52
Various advancements have been made through
rotating drum bioreactors. Kalogeris et al. have de-
veloped a laboratory scale intermittent agitation ro-
tating drum type bioreactor for SSF of thermo-
phillic organisms. The main parts of the apparatus
are: perforated (pore size, S=1mm
2) cylindrical
54 S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008)
drum (diameter, D= 0.15 m; length, L= 0.59 m; ca-
pacity, V= 10 L), vessel with water jackets, motor
connected to controller, heat exchanger, condenser
at air outlet, and humidifier.56 Kalogeris et al.de
-
termined the efficiency of the bioreactor by produc-
ing hemicellulases and cellulases with a thermo-
phillic fungus, Thermoascus aurantiacus, using
wheat straw as substrate. He examined the effects
of temperature, moisture and aeration. Among the
various kinetic models tested, Le Duy model de-
scribed the relationship between T. auranticus
growth and enzyme production. However, increase
in aeration rate and high moisture levels favored the
enzyme and enzyme production. The bioreactor can
be used for production of hydrolases from thermo-
phillic organisms due to their superior thermo-sta-
bility, optimum activity at elevated temperatures,
and high rates of substrate hydrolysis.57
AZymotis packed bed reactor was developed at
laboratory scale by the ORSTOM team. Zymotis
bioreactors are a modification of packed bed bio-
reactors with internal plates in which cold water cir-
culates at an optimum temperature. Variation in op-
timum temperature retards the initial growth of the
culture used. In packed bed bioreactor, large axial
temperature gradients arise leading to poor micro-
bial growth at the end of the base near the outlet,
with an increase in vertical temperature gradients.22
Another bioreactor (Growtek) was developed
by a team of scientists at IIT (Indian Institute of
Technology) Kharagpur, India, for plant tissue cul-
ture but later it was used for solid-state fermenta-
tion. This bioreactor has a cylindrical vessel (width
D= 11.3 cm, height H= 16 cm) with a spout at the
base, inclined at an angle of a= 15° having a diam-
eter of d= 2.6 cm and length of l= 8.5 cm. Both
vessel and spout have lids. A float of area A=72
cm2is provided inside the vessel, which consists of
a perforated base. Fermentation of solid substrate is
carried out on a float to which the seed culture is
added, while liquid salt solution is kept below the
float. An advantage of the Growtek bioreactor is
that the inoculated solid comes into direct contact
with the liquid medium. The products formed dur-
ing the fermentation process leach in liquid me-
dium. This type of bioreactor was used for produc-
tion of gallic acid from tannin-rich solid material.37
Ahamad et al. used a Growtek bioreactor for pro-
duction of mevastatin by Penicillium citrinum from
wheat bran.58
Large scale bioreactors
The successful operation of large-scale bio-
reactors depends on design features obtained after
mathematical modeling, susceptibility of the sub-
strate and fungal morphology to increase in temper-
ature, effect of particle size and voids between
them, quantity of the substrate used, and height of
the substrate bed. There are several forms of
large-scale bioreactors used in SSF.
Koji types of bioreactors are the simplest and
without forced aeration. These types of bioreactors
are also known as tray bioreactors. The trays are
made of wood, plastic, metal. It is not necessary
that trays should be perforated. Trays are arranged
one above the other with suitable gaps between
them and placed in climatic controlled chamber un-
der circulating air, which maintain uniform temper-
ature. An advantage of this technology is that by in-
creasing the number of trays, the scale-up is easier.
However, the requirement of sterility, large space
and labor makes the process difficult. Rodriguez et
al. reported the tray bioreactor to be appropriate for
producing laccase by T. versicolor in solid-state
conditions operating with lignocellulosic supports
besides their several disadvantages.59 Rodriguez et
al. also proved superiority of grape seeds over ny-
lon sponge supports using tray configuration during
laccase production (with Trametes hirsuta under
SSF conditions).60 A Shallow tray fermentor was
used at laboratory scale for cellulase production
with Trichoderma reesei ZU02 strain on corncob
residues.61
However, BIOCON India has used this tech-
nology for large-scale production of immuno-sup-
pressants. They have simplified problems of steril-
ity by keeping the trays in HEPA (High Efficiency
Particulate Air) filtered air, using automated ma-
chines for layering the substrate in trays.
Packed bed bioreactors are modifications of
laboratory-scale column reactors. A sieve present at
the bottom is used for holding the substrate. Passing
forced air through the static bed provides aeration.
Heat produced during fermentation within the sub-
strate bed is a major limitation of these reactors.
However, reduction in water content within the sub-
strate bed increases its hardening. This decreases
fungal penetration within the bed, causing reduction
in product formation. Hence, it is advantageous to
use thermo-stable fungus for producing a thermo-
-stable product (especially enzymes) in packed bed
reactors. Heat produced in the packed column can
be reduced by increasing the moisture of air inside
it, which is achieved by passing cold saturated air
or using heat exchangers as in Zymotis. Pressure
drop is an important feature of the packed reactor.
High-pressure drop reduces airflow from the sub-
strate favoring channeling within the bed.53 The
channeling decreases optimum temperature re-
quired for growth of organisms in the substrate bed.
Some large-scale bioreactors like PLAFRACTOR62
use heat exchangers for controlling temperature
during fermentation. However, enhancing agitation
S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008) 55
in the unmixed static bed solves the problem of heat
removal and scale-up. Oostra et al.50 using the
model of a scraped drum reactor have proved this.
Another bioreactor used at industrial scale is
the rotating drum bioreactor (RDB). The RDB
mixes the substrate with the cultivating organism,
while rotation can be continuous or intermittent de-
pending on the bed height and speed of rotation.
Some critical factors affecting RDB are type of sub-
strate, height of bed, fungal physiology, and size of
substrate particles. Incorporation of baffles may im-
prove the operation of RDB. Various researchers
have studied the impact of baffles within RDB.
Schutyser et al. have used discrete particle simula-
tors to predict that curved baffles have better radial
mixing characters than other designs. Curved baf-
fles are independent of particle rotation, and contin-
uous rotation of drum is possible with them.40 RDB
are characterized as mixed bioreactors. However,
with higher shear rates and rotational speed, the
fungal physiology may be destroyed.20,50,63
In fluidized bed reactors, the solid substrate is
fluidized by upward airflow. The bioreactor oper-
ates on the flow of air and velocity with which the
substrate moves upwards. Sufficient velocity of air
is supplied to fluidize the solid substrate. The col-
umn of the bioreactor is high enough for bed expan-
sion. Widening of the column near the top allows
disengagement of solids from the gas stream. Main-
tenance of uniform conditions throughout the sub-
strate and increase in surface area is an advantage
of this bioreactor.
After many advances in SSF reactors, heat con-
trol and scale-up still require consideration through
modeling.
Substrates used in solid-state
fermentation
For SSF processes, different agro-industrial
wastes are used as solid substrates. Selection of
agro-industrial residues for utilization in SSF de-
pends on some physical parameters such as particle
size, moisture level, intra-particle spacing and nutri-
ent composition within the substrate. In recent
years, some important agro-industrial residues such
as cassava bagasse, sugarcane bagasse, sugar beat
pulp/husk, orange bagasse, oil cakes, apple pomace,
grape juice, grape seed, coffee husk, wheat bran,
coir pith etc. have been used as substrates for
solid-state fermentation.
Cassava bagasse and sugarcane bagasse offer
an advantage over other substrates such as rice
straw, wheat straw, because of their slow ash con-
tent.64 One of the factors making cassava an opti-
mum substrate for SSF is its high water retention
capacity.65 Cassava bagasse is highly used for pro-
duction of citric acid, flavors, mushrooms, and dif-
ferent biotransformation processes. However, in
comparison to sugarcane bagasse, cassava offers an
advantage as it does not require pretreatment, and
can probably be decomposed by most organisms for
various purposes.64 Some experiments have proved
that the protein mass fraction of cassava may be im-
proved from w= 1.67 % to 12 % by carrying out its
biotransformation with tray bioreactors.54 However,
sugarcane bagasse and wheat bran are used for
commercial production of most compounds using
SSF, but their potential is still to be explored com-
pletely. Sugarcane bagasse has many efficient and
economical applications, such as production of pro-
tein-enriched cattle feed and protein.64 Pretreatment
of sugarcane bagasse is an important aspect since
assimilation and accessibility of microorganisms to
substrate becomes easy, thus resulting in decompo-
sition of hemi cellulose and lignin.66
Another upcoming agro-industrial substrate is
coffee pulp/husk due to its rich organic nature and
high nutritive value. Obtaining coffee pulp or cof-
fee husk usually depends on the method employed
for refining the coffee cherries, i.e. wet or dry
method. However, fungi Basidiomycetes are found
mostly in husk rather than in pulp.67
Okara (soybean curd residue), rich in water-in-
soluble ingredients, is a useful substrate for micro-
bial fermentation. The main disadvantage of okara
is natural spoilage when it is not refrigerated. Dehy-
dration of the soybean curd residue has been re-
ported to improve its utilization. Ohno et al. suc-
cessfully produced an antibiotic iturin from okara.68
Substrates such as agro-industrial residues are
proved by many researchers to be better for fila-
mentous fungi. The morphology of filamentous
fungi supports them to penetrate the hardest surface
due to the presence of turgid pressure at the tip of
their mycelium. Hence, the raw materials consid-
ered as waste are used for production of value
added fine products and reducing pollution prob-
lems.69
Applications of solid-state fermentation
Production of enzymes by SSF
Enzyme production is one of the most impor-
tant applications of SSF. SSF has advantages over
submerged fermentation such as high volumetric
productivity, low cost of equipment involved, better
yield of product, lesser waste generation and lesser
time consuming processes etc.
The type of strain, culture conditions, nature of
the substrate and availability of nutrients are the
56 S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008)
other important factors affecting yield of enzyme
production.70 It is crucial to provide optimized wa-
ter content and control the water activity for good
enzyme production. Agro-industrial substrates are
considered best for enzyme production in SSF. The
cost of enzyme production by submerged fermenta-
tion is higher compared to SSF. Tangerdy et al.
have also proved this by comparing cellulase pro-
duction costs in SSF and SMF.71
Protease
Various attempts for producing acid, neutral
and alkaline protease have been done using agro-in-
dustrial residues with SSF, depending on their cata-
lytic activity and optimum pH. Proteases are pro-
duced by fermentation of agro-industrial substrates
such as rice bran, wheat bran, soy meal, green gram
husk, rice husk, spent brewing grain, coconut oil
cake, palm kernel cake, sesame oil cake, jackfruit
seed powder, olive oil cake, lentil husk etc.
Ikasari and Mitchell72 used rice bran for pro-
ducing acid proteases with Rhizopus oligosporus, as
no toxin production occurred during SSF. Acid pro-
tease production has also been obtained using bac-
teria Mucor bacilliformis in SSF.73 Tunga et al.
produced extra-cellular alkaline serine protease
using Aspergillus parasiticus and Rhizopus oryzae
NRRL-21498 with wheat bran as substrate.74
Prakasham et al. obtained maximum alkaline prote-
ase production from green gram husk.75 The alka-
line proteases obtained using SSF of Aspergillus
parasiticus was thermo-stable at J= 50 to 60 °C,
high pH, in the presence of surfactants, metals and
oxidizing agents.74 However, most researchers have
used wheat bran for production of alkaline protease.
Protease production in SSF is higher than with
submerged fermentation. Down streaming and ex-
traction of the product is cheaper in SSF due to
lesser water content of the substrate. Bacillus
subtillis obtained from excreta of larvae Bruckdaole
pachymerusnucleorum was also used for protease
production using soy cake as substrate. Maximum
protease productivity was P= 15.4 U g–1 h–1 for
SSF but it was P= 1.3 U g–1 h–1. The enzyme pro-
ductivity was 45 % higher in SSF.76 Some critical
fermentation parameters for obtaining maximum
protease production are accumulation density of
microbe within the substrate and height of the sub-
strate bed. Sandhya et al. studied the effect of pa-
rameters such as incubation temperature, pH,
inoculum size and incubation time on neutral prote-
ase production through submerged fermentation
and SSF. In solid-state, the total enzyme yield was
31.2 U enzyme per gram of fermented substrate, but
in submerged fermentation the yield was 8.7 U en-
zyme per gram of fermented substrate. The yield in
SSF was 3.5 times higher than in SMF.77 Similarly,
George et al. compared the production of proteases
by Bacillus amyloliquefaciens in solid-state and
submerged fermentation conditions. In submerged
state, 8 · 106units of enzyme were obtained in
aV= 20 L fermentor under optimal conditions.
However, in solid-state the enzyme production was
25 · 104units g–1. They reported SSF to be simpler
in operation than submerged fermentation.78
Lipase
In recent years, considerable increase in lipase
production from microbes and agro-industrial
wastes using SSF has gained importance. However,
fungi are best lipase producers.79 Most researchers
have used wheat bran for maximum lipase produc-
tion. Kamini et al. used gingelly oil cake for the
production of lipase by Aspergillus niger. However,
supplementation of various nitrogen sources, carbo-
hydrates and inducers to substrates was ineffec-
tive.80 In contrast, SSF of babassu oil cake with
Penicillum restrictum proved that lipase activity de-
pends on the type of supplementation. Enrichment
of high carbon containing substances such as
peptone, olive oil, resulted in high lipase activity.81
Enhancement in lipase activity by olive oil
supplementation has also been proved by Cordova
et al.82 The fungal spp. Rhizopus oligosporus has
been found to be the best mould among 10 different
fungal cultures for production of lipases without the
addition of supplements.83
Castilho performed the economic analysis of
solid-state and submerged fermentation for produc-
tion of lipase using Penicillum restrictum. He re-
ported that, for the production scale of 100 m3
lipase concentrate per year, a total capital invest-
ment needed for submerged process was 78 %
higher than that for SSF. The unitary product cost
in submerged processes was 68 % higher than the
product selling price.84
Mahadik et al. compared lipase production in
submerged and solid-state fermentation. In SMF a
synthetic oil-based medium was used, but SSF used
wheat bran supplemented with olive oil, which re-
sulted in increased lipase production.79 Mateos Diaz
et al. reported that lipase from solid-state to be
more thermo-tolerant than from submerged fermen-
tation (thermal stability, half life at J= 50 °C was
t= 0.44 h with SMF, and 0.72 h with SSF). The
temperature at which maximum activity of lipases
occurs was J= 30 °C with SMF, and J=40 °C
with SSF. The lipase produced was from fungus
Rhizopus homothallicus. Process development re-
quires proper utilization of enzymes. Hence, they
should show high functional stability. The enzymes
obtained from thermophillic fungus have high ther-
S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008) 57
mal stability, and activity at mesophillic tempera-
tures are generally utilized in chemical processes.85
Some of the agro-industrial wastes used in SSF
for lipase production are babassu oil cake, wheat
bran, rice bran, gingelly oil cake, almond meal,
mustard meal, coconut meal, rice husk, sugarcane
bagasse, cassava bagasse, coconut oil cake, olive oil
cake, etc.
Cellulase
Cellulase is an enzyme complex used for the
conversion of lignocellulosic residues and used for
production of ethanol, single-cell protein, bleaching
of pulp, for treatment of waste papers and for fruit
juice extraction. In SSF, using lignocellulosic
wastes as substrates can reduce the cost of cellulase
production.61 Lignocellulosic materials are cheaper
and pretreatment is required to improve their utili-
zation. Pretreatment of lignocellulosic matrix in-
creases the potential of cellulases to act on
cellulose, hemicelluloses.
The concerted action of enzymes like endo-
glucanases, exoglucanases and b- glucosidase is
used for hydrolyzing cellulose. The rate-limiting
step is the ability of endoglucanases to reach amor-
phous regions within the crystalline matrix and cre-
ate new ends with which endoglucanases can act.86
Ethanol production from lignocellulose biomass re-
quires hydrolysis by cellulase and hemicellulase for
converting lignocellulosic biomass to biofuel.87
Some factors like moisture content, particle
size, pH, incubation temperature, inoculum size, in-
cubation period and enrichment of medium with
carbon and nitrogen were considered optimum for
cellulase production by bacterial strain Bacillus
subtilis on banana fruit stalk wastes. However, total
enzyme production was 12 times higher in SSF
than in submerged fermentation under similar ex-
perimental condition.88 It is necessary to consider
these factors for cellulase production from ligno-
cellulosic wastes as their nutrients are already de-
pleted.
Water content of substrate and aeration rate are
critical factors in cellulase production using SSF.
Corncob residue was used for cellulase production
with Trichoderma reesei ZU02 in shallow tray fer-
mentors. Xia and Cen used a deep trough fermentor
with forced aeration for cellulase production.
Forced aeration enhanced the mass transfer to a
greater extent, which increased cellulase activity to
305 IU per g of cellulose.61 It has been reported by
Fujian et al. that substrates in solid-state with con-
tinuous circulation of air and convective diffusion
with pressure are better for fungal propagation than
static cultures. This periodic air circulation in-
creases the looseness of substrates and enhances
cellulase activity. The filter paper activity of cellu-
lase enzyme increased to 20.4 IU g–1 at a bed height
of h= 9 cm in t= 60 h, while maximum filter paper
enzyme activity was 10.8 IU g–1 in 84 h within
static cultures. The work was performed using
steam-exploded wheat straw as carrier with Peni-
cillum decumbens in SSF.89 However, changes in
the amplitude of air pressure increased the oxygen
availability to the cultures used and heat removal.
The variations enhanced the cellulase production by
Trichoderma viride in SSF.90
Co culturing of two fungi in SSF enhances the
enzyme production. Co culturing of Trichoderma
reesei mutants with Aspergillus spp. increased the
cellulase production by 50 % and improved the
cellulase glucosidase ratio, by partially removing
product inhibition and its hydrolysis.91 Co culturing
Aspergillus ellipticus and Aspergillus fumigatus re-
sulted in improved hydrolytic and b-glucosidaes ac-
tivity.61,71 However, some newly developed agro-in-
dustrial wastes used for cellulase production are
banana wastes, rice straw, corn cob residue, rice
husk, wheat straw, banana fruit stalk, and coconut
coir pith.
Pectinase
Pectinases are constitutive or inducible en-
zymes produced by microbes for breaking pectin.
Different substrates used for production of pecti-
nase are wheat bran, soy bran, apple pomace, cran-
berry pomace, strawberry pomace, beet pulp, coffee
pulp & husk, cocoa, lemon & orange peel, combi-
nation of sugarcane bagasse and orange bagasse,
wheat bran etc.
Production of polygalactouronase (PG) and
pectinesterase (PE) was 6.4 times higher in SSF
compared to submerged fermentation. PE and PG
activity was measured to be 500 U L–1 and 350
UL
–1 at 24 h of incubation with pectin as the sole
carbon source with SSF, but in submerged fermen-
tation enzyme production was 127 U L–1 and 55
UL
–1 at t= 48 h of incubation. Supplementation of
glucose decreased the production of enzymes due to
catabolite repression in submerged fermentation.
However, PE and PG enzymes increased by 30 %
and 33 % respectively with addition of glucose.92
Similarly, exopectinase activity increased from 623
to 7150 (IU L–1) in SSF, but decreased from 1714 to
355 (IU L–1) in submerged fermentation in the pres-
ence of sucrose at Awof 0.995. Similar results were
at Awof 0.96. Increase in water activity increased
pectinase activity in SSF.93 Production using
deseeded sunflower head as substrate resulted in
step-up of exopectinase and endopectinase enzyme
using SSF. In SMF the endopectinase was 18.9
UmL
–1, which increased to 19.8 U mL–1 in SSF.
58 S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008)
Similarly, exopectinase increased from 30.3 U mL–1
in SMF to 45.9 U mL–1 in SSF, at optimum
moisture of s= 65 % and particle size of d= 500
mm.94,95 Polyurethane, when used as inert support
for exopectinase production using Aspergillus niger
in Erlenmeyer flasks, resulted in direct measure-
ment of biomass production, substrate uptake and
enzyme activity in culture medium.93 Botella et al.
used grape pomace for exopectinase production
with Aspergillus awamori in SSF. Initial substrate
moisture content of 65 % and supplementation of
6 % glucose as carbon source enhanced enzyme ac-
tivity. However, particle size did not influence the
increased enzyme production, which is contradic-
tory to earlier reports.96,97
Besides moisture, the production of pectinase
enzymes also depends on physical parameters of
the bioreactor used, amount of substrate used, type
and pH of substrate and supplements added during
fermentation. A mixture of orange bagasse, wheat
bran (1:1) was used for production of endo-poly-
galactouronase, exo-polygalactouronase and pectin-
lyase by Penicillum viridicatum in Erlenmeyer
flasks and polypropylene packs at 2 different mois-
ture s= 70 %, 80 %. However higher production of
all three enzymes was obtained in polypropylene
packs at s= 70 % moisture, but lower moisture con-
tent was highly significant for Pectinlyase produc-
tion. Increase in pectinase enzymes using poly-
propylene packs favored it for scale-up.98
Martins et al. used the Thermoascus auranticus
for pectinlyase and polygalactouronase production.
The quantity of pectinlyase and polygalactouronase
was found to be higher when compared with other
pectinolytic strains such as Aspergillus niger, Peni-
cillum italicum, Aspergillus foetidus. However, ad-
dition of fibrous material such as sugarcane bagasse
to the mixture of wheat bran, orange bagasse, re-
sulted in intra-particle spacing, causing increase in
aeration, nutrient and enzyme diffusion, which sup-
ported pectinlyase production. These results proved
that media composition did not affect polygalac-
touronase production.99 Pectinase and polygalac-
touronase using Moniellia, Penicillum spp. were
produced using orange bagasse, sugarcane bagasse
and wheat bran as substrate. Addition of fibrous
material such as sugarcane bagasse caused inter-
particle spacing resulting in increase of aeration and
nutrients supply. However, sugarcane bagasse as
the sole carbon source, did not allow the growth of
fungi, indicating that microorganisms are unable to
hydrolyze cellulose, hemicelluloses fibers but they
can support mycelia formation. The sugarcane ba-
gasse was used as the inert support for growth &
pectinase production by Aspergillus niger, Peni-
cillum viridicatum, Thermoascus auranticus.100
Solid substrates resulted in higher pectinase
production as they supply nutrients to microbial
cultures along with anchorage. Nutrients, which are
unavailable or are in sub-optimal concentration, are
usually supplemented externally. A Bacillus sp.
DT 7 was used for production of pectinase using
wheat bran, rice bran, apple pomace as substrates in
Erlenmeyer flasks. Maximal enzyme production
was obtained from wheat bran supplemented with
polygalactouronic acid and neurobion. Supplemen-
tation of 27 mL of neurobion enhanced pectinase
production by 65.8 %.101
Phytase
Several strains of bacteria, yeasts, fungi such
as Bacillus subtillis, E. coli, Bacillus amyloliquefa-
ciens, Schwanniomyces castellii, Schwanniomyces
occidentalis, Hansenula polymorpha, Aspergillus
flavipes, Aspergillus fumigatus, Aspergillus oryzae
and Aspergillus ficcum are employed for phytase
production in SSF systems. Some of the substrates
generally used for phytase production are canola
meal, coconut oil cake, wheat bran, black bean
flour, cowpea meal, mustard cake, cotton cake,
palm kernel cake, sesame oil cake, rapeseed meal,
olive oil cake, groundnut oil cake etc.
Phytase production increases with fermentation
of mixed substrates. The increase may be due to the
availability of different nutrients from different sub-
strates in the same reactor simultaneously. This may
step-up phytase production. Roopesh et al. used
SSF for phytase production from various substrate
combinations of wheat bran and various oil cakes
using Mucor racemosus under SSF. Combination of
wheat bran and oil cake yielded greater Phytase
when compared to their individual production. Op-
timization of conditions resulted in 44.5 U g–1 of
phytase with the combination of wheat bran and oil
cake, which were 1.5 times and 4 times higher, re-
spectively, when the oil cake and wheat bran were
used as substrates. The type of carbon and nitrogen
source used is an important factor for consideration
in any fermentation process.102 However, along
with imbalance in the carbon-to-nitrogen ratio,
many parameters affect phytase production such as
incubation time, initial moisture content and incu-
bation temperature. Similarly, Ramachandran et al.
used Rhizopus spp. with coconut oil cake, sesame
oil cake, palm kernel cake, groundnut oil cake, cot-
tonseed oil cake and olive oil cake for phytase pro-
duction. Cottonseed oil cake and olive oil cake
poorly supported the phytase production indivi-
dually, but mixed substrate fermentation of both
increased enzyme production to 35 U/gds.
Supplementation of glucose further enhanced
phytase activity to 52 U/gds. Enhancement was re-
S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008) 59
ported due to the combined effects of nutrients in
them.103
Various other substrates like wheat bran, oil
cakes such as groundnut oil cake, canola meal,
rapeseed meal and soybean meal when supple-
mented with surfactants resulted in higher produc-
tion of phytase. Aspergillus ficcum was used for
phytase production on canola meal using SSF.104
Extra cellular phytase produced using thermo-toler-
ant Aspergillus niger was maximum with cowpea
meal. Increase in membrane permeability of fungus
has been reported to be the optimum cause for
phytase enhancement in SSF.105
L-glutaminase
L-glutaminase is used as antileukaemic and fla-
vor enhancing agent. Solid-state fermentation was
found to be more suitable than submerged fermen-
tation for biosynthesis of L-glutaminase using Pseu-
domonas flourescens, as 25-fold enhancement was
obtained.106 The moisture of substrate highly effects
enzyme production. L-glutaminase was produced by
yeast Zygosacharomyces rouxii using wheat bran
and sesame oil as substrate with s= 64.2 % mois-
ture, t= 48 h old inoculum and J= 30 °C incuba-
tion temperature. Addition of 10 % NaCl and sea-
water to wheat bran and sesame oil enhanced en-
zyme production. The enzyme produced from
wheat bran and sesame oil cake in dry state was 2.2
and 2.17 U/gds but with supplementation, the con-
centration increased to 7.5 and 11.61 U/gds respec-
tively.107 Beauveria spp. an alkalophilic and salt-to-
lerant fungus isolated from marine sediment, was
used for L-glutaminase production using seawa-
ter-based medium supplemented with L-glutamine
(w= 0.25 %) as substrate.108 Seawater being a natu-
ral reserve for marine organisms can provide them
sufficient nutrients when used as supplement in SSF
for production of industrially important enzymes.
Amylase
Amylase has potential application in a number
of industrial processes such as food, fermentation,
textiles and paper industries. The two major classes
of starch degrading amylases are glucoamylases
and a-amylase. Solid-state fermentation holds tre-
mendous potential for production of these enzymes
industrially on a large scale.
Production of a-amylase is not limited to fun-
gal cultures but it is also done by bacterial cultures
of genus Bacillus. Bacterial cultures reduce the time
required for fermentation, reducing a lot of expen-
diture involved. Wheat bran has been reported to be
the best substrate even for bacterial cultures. Babu
and Satyanarayana produced a-amylase with Bacil-
lus coagulans in SSF using wheat bran as substrate
and supplemented with tap water as moistening
agent. They compared the production of enzyme in
different vessels.109 Ramesh and Lonsane produced
a-amylase with Bacillus licheniformis M27 under
SSF conditions, using wheat bran as substrate.38
An advantage of SSF over submerged fermen-
tation is the inhibition of catabolic repression by
regulation of end product synthesis from the prod-
uct formed. a-amylase production by Bacillus
licheniformis decreased from 420 to 30 units mL–1
with increase in fraction of starch from 0.2 to 1 %
in SMF. However, in SSF the enzyme activity en-
hanced 29-fold with a 42 times increase in concen-
tration of starch. Hence, inhibition of enzyme by
product is overruled in solid-state fermentation.110
Ramachandran et al. used coconut oil cake, a dry
product obtained from copra for production of
a-amylase with Aspergillus niger in SSF.
Supplementation with starch, peptone and glucose
increased the enzyme synthesis.111
Glucoamylase is still produced by large-scale
submerged fermentation, but the production cost re-
mains high in all respects. There are less reports of
glucoamylase production in SSF. However, cost of
production is lower in SSF. Cost reduction can be
achieved by using cheaper substrates such as agro-
-industrial residues. Selection of the proper substrate
is an important parameter for maximum enzyme ac-
tivity. Ellaiah et al. optimized different substrates
like wheat bran, green gram husk, black gram bean,
barley flour, jowar flour, maize bran, rice bran, and
wheat rawa for maximum glucoamylase production
with Aspergillus spp. under SSF. However, wheat
bran showed maximum enzyme activity to be 247
Ug
–1. Supplementation of w= 1 % fructose and
w= 1 % urea enhanced enzyme activity. A very
high moisture content of 80 % favored maximum
enzyme production.112 Tea waste was used as sub-
strate for maximum glucoamylase production using
Aspergillus niger. Supplementation of tea waste
with mineral solution containing salts such as K,
Mn, Mg, Zn and Ca resulted in 198.4 IU/gds of
enzyme. The results were at t= 96 h with 4 %
inoculum, 60 % moisture, and pH of 4.5. Supple-
mentation with w= 1 % sucrose enhanced enzyme
activity to 212.6 IU/gds at 96 h. Further addition of
w= 1 % malt extract increased enzyme activity to
226 IU/gds at 72 h after inoculation.113
Anto et al. produced glucoamylase using rice
flakes (categorized as coarse, fine and medium),
wheat bran and rice powder as substrates with
Aspergillus spp. HA-2. Supplementation of sucrose
as carbon source to wheat bran and glucose to
coarse, medium waste enhanced enzyme produc-
tion. Addition of yeast extract and peptone to the
substrates also enhanced enzyme production. Maxi-
mum enzyme production was obtained in wheat
60 S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008)
bran (264 ±0.64 U/gds) but glucoamylase produc-
tion in different categories of rice flakes varied.
Maximum enzyme was produced in coarse (211.5
±1.44 U/gds) and medium wastes (192.1 ±1.15
U/gds). The factors affecting enzyme activity with
combination of wastes were attributed to the in-
crease in agglomeration of particles, which resulted
in reduced aeration and penetration by fungal
mycelia.114
Nutrient supplementation from organic sources
increases enzyme production to a greater extent
than inorganic sources. It has also been found that
most researchers used wheat bran as substrate for
a-amylase production because it contains sufficient
nutrients, it remains loose even in moist conditions,
and has a larger surface area. Due to these factors,
aeration and mycelial penetration are easier in
wheat bran.109
Ligninase
Lignin is a biopolymer with complex phenyl-
propanoid structure and contributes to environmen-
tal pollution. The most common organisms for
lignin degradation are white rot fungi. However, ac-
tivity of Phanerochaete chrysosporium is found to
be most suitable for efficient lignin degradation.
Enzymes secreted by white rot fungi are ligninases.
They are Lignin Peroxidase (LiP), Manganese
Peroxidase (MnP) and H2O2generating enzymes.
Secondary metabolism of these enzymes is trig-
gered by C, N, S depletion.115,116
The respiration rate of an organism is indicator
of the organism’s activity. Cordova et al. deter-
mined the relation between the CO2evolution rate
and enzymatic activity of fungus when grown on
sugarcane bagasse pith. There was an increase in
CO2evolution rate with increase in metabolism. Ac-
tivity of manganese peroxidase (MnP) was ex-
pressed in the idiophase where the residual glucose
level was the least in the medium. The MnP activity
enhanced and then decreased rapidly due to simul-
taneous protease activity. Lignin peroxidase activity
was reported during the exponential phase of the
organism’s growth. However, it decreased after fun-
gal secondary metabolism. This indicates that LiP is
less sensitive to MnP for proteolytic action.117 CO2
generation took place after uptake of O2by fungus.
However, LiP and MnP activities cannot be studied
in an anaerobic media.
Fujian et al. compared lignolytic enzyme activ-
ities in submerged and solid-state fermentation us-
ing steam-exploded wheat straw as substrate. LiP,
MnP activities in optimum conditions were 61.67
UL
–1 and 27.12 U L–1 in submerged fermentation,
but in solid-state the activities were 365.12 U L–1
and 265 U L–1, respectively. However, in SSF the
maximum enzymatic activities reached 2600 U L–1
(LiP) and 1375 U L–1 (MnP).118 This proves the fea-
sibility of solid-state over submerged fermentation.
Developing semi solid-state conditions within
the bioreactor also enhances lignin enzymes.
Dominguez et al. developed a bioreactor based on
rotating drums for producing lignolytic enzymes.
The enzyme activities at 1 L L–1 min–1 of air were
found to be 1350 U L–1 (LiP) and 364 U L–1 (MnP)
respectively. Semisolid-state conditions were main-
tained by using nylon sponges for providing inert
support, which is rotated by regularly passing
through the nutrient medium. Nylon sponges were
used due to its hydrophobicity, porous nature, affin-
ity for fungus and moisture retaining capacity.
Higher aeration rate avoided the support clogging
and made nutrient availability to organism easier.116
Similarly, Couto et al. used polypropylene sponges
as inert support for lignolytic enzyme production.115
Lignin-degrading enzymes were obtained from
mushroom Bjerkandera adusta by immobilizing it
on polyurethane foams.119
Degradation by lignin enzymes is a non-spe-
cific reaction on the basis of free radicals resulting
in destabilization of bonds and finally breaking of
macromolecule. This characteristic increases the
potential for chemical industries; environmental in-
dustries; coal industries and extracting important
metabolites from natural sources.
Xylanase
Different food processing with products such
as straws and brans of different cereals, corn, hull
and cobs, sugarcane and cassava bagasse, various
saw dusts and different fruit processing and oil pro-
cessing residues have been used for producing
xylanase. Xylanase production requires substrates
in very high concentration, with prominent water
absorbance capacity. Xylanase are produced mainly
by Aspergillus and Trichoderma spp.120,121 Xylanase
production was achieved successfully by Asper-
gillus fischeri, Aspergillus niger using wheat bran
and wheat straw as main substrates.122,123
Addition of nitrogen source as supplement is
an important step for xylanase production.123 So-
dium nitrite played a significant role in production
of alkali stable cellulose free xylanase by Asper-
gillus fischeri. Xylanase in conjunction with
cellulolytic enzymes is used for bioconversion of
lignocellulosic material to produce fuels and other
chemicals.122
The Koji process was used for production
of xylanase by Aspergillus sulphureus using dry
koji.124 Ghanem et al. produced xylanase using
Aspergillus terreus on wheat straw medium.55
Topakas et al. used the Sporotrichum thermophile
S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008) 61
for xylanase production in a horizontal bioreactor
with wheat straw and destarched wheat bran as sub-
strate.125
Pretreatment of lignocellulosic substrates was
found to be advantageous in many cases, however
in some it proved disadvantageous.26 Pretreatment
for xylanase production using wheat straw by
Aspergillus terreus resulted in reduction of enzyme
activity.123 Some other molds that can be used for
production of xylanase using SSF are Melano-
carpus albomyces,126 Thermomyces lanuginosus,127
Humicola insolens128 and Aureobusidium pullu-
lans.129
Production of organic acids under SSF
Fermentation plays a key role in the production
of organic acids. The production of organic acids
progressed with development of SSF. However,
biotechnological processes for large-scale produc-
tion of organic acids are still in early phases of de-
velopment. Organic acids are the most common in-
gredients of food and beverages because of their
three main properties: solubility, hygroscopic qual-
ity, and their ability to chelate.130 Some of the acids
produced using SSF are citric acid, lactic acid, gal-
lic acid, fumaric acid, g-linoleic acid, and kojic
acid.
Citric acid
Various agro-industrial residues such as sugar-
cane press mud, coffee husk, wheat bran, cassava
fibrous residue, de-oiled rice bran, carob pods, ap-
ple pomace, grape pomace, kiwi fruit peels, okara,
pineapple wastes, mausami wastes, kumara etc. are
most potential substrates for production of citric
acid in SSF.
Solid-state fermentation has been proved ad-
vantageous over SMF in many respects. Metal ions
such as Fe+2,Mn
+2 and Zn+2 repress biosynthesis of
citric acid by Aspergillus niger in submerged fer-
mentation, but this does not happen in SSF. It is re-
ported by Kumar et al. that addition of these metal
ions enhances production of citric acid by 1.4 – 1.9
times in SSF.3Kumar et al. compared wheat bran
and sugarcane bagasse for maximum citric acid
production with Aspergillus niger DS-1 strain. Sug-
arcane bagasse proved to be a better carrier when
compared with wheat bran. Agglomeration in wheat
bran caused non-uniform mixing of the substrate,
affected heat and mass transfer along with growth
and product formation. Bagasse did not show ag-
glomeration even when moisture level was in-
creased from 65 % to 85 %.3Supplementation of
methanol to agro-industrial residues enhanced citric
acid production. Kumar et al. utilized pineapple,
mixed fruit and mausami residues for producing cit-
ric acid using Aspergillus niger DS-1 in SSF.
Supplementation of methanol to substrates was
done at different moisture levels. Methanol being a
dehydrating agent enhanced the moisture require-
ment of substrates. In the presence of methanol
sugar consumption decreased but citric acid pro-
duction increased. The maximum citric acid yield
was Y= 51.4, 46.5 and 50 % (based on sugar con-
sumed) from pineapple, mixed fruit and mausami
residues, respectively.131 Similar reports with carob
pods and corncobs have been published by Roukas
and Hang et al., respectively. Addition of methanol
to the substrate increased citric acid produc-
tion.132,133 Figs were used for production of citric
acid due to their high carbohydrate content and
yielded 8 % citric acid. However, the addition of
methanol increased citric acid production from
64 – 490 g kg–1 dry fig.134 Enhancement in citric
acid also depends on other additives such as carbon
and nitrogen sources and metal ions.
Researchers have used different bioreactors for
production of citric acid. Flasks proved their superi-
ority over tray and rotating drum bioreactors when
compared among different bioreactors using pine-
apple waste as substrate for citric acid production,
with strains of Aspergillus.135 Packed bed reactors
proved their superiority when compared to flask
cultures in production of citric acid.Kumara,
starch-containing substrate was used for citric acid
production using Aspergillus niger with a packed
bed bioreactor. Kinetic analysis of the packed bed
bioreactor showed an overall reactor productivity of
P= 0.82 g h–1 citrate per kg wet mass kumara on 5th
day, which in the flask was 0.42 g h–1 citrate per kg
wet mass kumara on 8th day. Some adverse effects
of packed bed such as reactor blockage due to fun-
gal biomass, forced aeration and limited space,
were dominated by optimizing particle size of sub-
strate (dp= 4 – 8 mm) and airflow rate (Q=0.5–1.5
Lmin
–1). Higher bed height and larger particle size
retard the citric acid production. Larger particle size
promotes aeration at lower bed height in the reactor,
which can destroy filamentous fungi. The high pro-
ductivity in packed bed was due to initial utilization
of starch in the substrates for citric acid production
and occurrence of CO2throughout the process.136
However, some of the results above were contrain-
dicated when apple pomace was used as substrate
for production of citric acid using Aspergillus niger
in packed bed bioreactor. Adjustment of the aera-
tion rate provided sufficient O2to filamentous
fungi, increase in bed height promoted physical sta-
bility and better air distribution in the reactor, opti-
mum moisture content provided proper diffusion to
fungal cells, and increase in particle size prevented
compaction of the substrate promoting aeration
in the reactor. Hence, 124 g citric acid was pro-
62 S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008)
duced from 1 kg dry apple pomace with a yield of
Y= 80 % based on total sugar.137
Prado et al. related the metabolic activity of
fungus Aspergillus niger to citric acid production
on cassava bagasse. The cassava bagasse containing
different percentages of gelatinized starch was used
during the experiment. Gelatinization of starch in
cassava bagasse made it more susceptible to fungal
action. In horizontal drum bioreactor and tray
bioreactor, it was observed that the lower respira-
tion rate i.e. high O2uptake and low CO2produc-
tion, favored the citric acid production in horizontal
drum, but the presence of high partial pressure of
CO2within the cassava bed also favored citric acid
due to entrapment of fungal spores. Maximum citric
acid production in tray bioreactor was with 80 %
gelatinized starch. But in horizontal drum bio-
reactors, the maximum results were obtained with
100 % gelatinized starch. Increasing the bed height
in tray bioreactors enhanced citric acid produc-
tion.138 When efficiency of different substrates such
as cassava bagasse, sugarcane bagasse and coffee
husk were compared for citric acid production with
Aspergillus niger, cassava bagasse proved to be
better since it increased the protein content of fer-
mented matter. The presence of calcium, phospho-
rus, vitamin B2, thiamine and niacin in cassava may
have enhanced the yield of citric acid.139
Aspergillus niger has been found to be the
most suitable strain for citric acid production. Most
strains are unable to produce citric acid in accept-
able yields since it is a metabolite of energy metab-
olism. Its accumulation rises in appreciable
amounts in drastic conditions. The main advantages
of using Aspergillus niger are its easy handling and
its ability to ferment a wide variety of cheap raw
materials. Enhancement in citric acid depends on
the selection of proper nutrient supplements, organ-
ism and substrates to prevent drastic changes in
pH.140 However, significant optimization may make
production cheaper in SSF.
Lactic acid
Some of the substrates used for production of
lactic acid are wheat bran, wheat straw corncob,
cassava, sweet sorghum, sugarcane bagasse, sugar-
cane press mud and carrot processing wastes. Aera-
tion of moistened medium is an important factor for
the SSF. It provides humidity to solid support and
oxygen for growth. Soccol et al. used Rhizopus
oryzae NRRL 395 on sugarcane bagasse impreg-
nated with glucose and CaCO3to produce lactic
acid. Lactic acid production in SSF and SMF was
g= 137.0 and 93.8 g L–1 respectively. Thus, produc-
tivity was g= 1.38 g L–1 per hour in liquid medium
and g= 1.43 g L–1 per hour in solid medium, which
makes SSF suitable for higher production of lactic
acid.141 Miura et al. utilized corncobs for L-lactic
acid production using Acremonium thermophilus
and Rhizopus in an airlift bioreactor.142
Inert support when used should provide good
conditions for fermentation along with the purity of
the product. Sugarcane bagasse impregnated with
the sugar solution from gelatinized cassava bagasse
was used as an inert support for production of lactic
acid using Lactobacillus delbruecki.143
Naveena et al. used statistical analysis to opti-
mize medium for lactic acid production from wheat
bran using Lactobacillus amylophilus GV6 in SSF.
Wheat bran not only makes the process economical
but also brings the organism closer to its natural
habitat. Lactobacillus amylophilus has been found
to be efficient in direct fermentation of starch to
lactic acid, avoiding the multi-step processes of si-
multaneous saccharification and fermentation.144
Optimization of nutrients by response surface meth-
odology resulted in production of 36 g of lactic acid
per 100 g of wheat bran having 54 g of starch, with
the Lactobacillus amylophilus strain. The increase
in lactic acid production was 100 % (from 0.18 to
0.36). The conversion from rough surface with ex-
tensive complex mesh to smooth surface particles
after inoculation confirmed alteration of raw starch
to glucose, which then converted to lactic acid.145
Some parameters to be considered for lactic acid
production are aeration rate, substrate selection and
nutritional supplementation.
However, lactic acid production from cheaper
substrates is still a challenge in SSF, which has to
be overcome by development of low cost mediums.
Gallic acid
Gallic acid is a phenolic compound. Tannase
enzyme is used for converting tannin to gallic acid
using Rhizopus oryzae on tannin-rich substrate in
a Growtek bioreactor.37 In the Growtek bioreactor,
the solid substrate comes into direct contact with
the liquid medium, and thus heat removal is easy.
This reactor was used for producing gallic acid
with mixed cultures of filamentous fungi such as
Rhizopus oryzae and Aspergillus foetidus. Co-cul-
turing of two organisms has advantage of providing
internal regulation and product formation.146
Secondary metabolites production
under SSF condition
Solid-state fermentation can be used for pro-
duction of secondary metabolites. Most of these are
accumulated in later stages of fermentation (idio-
phase). However, product formation has been found
superior in solid-state processes. Problems associ-
ated with secondary metabolite production in liquid
S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008) 63
fermentation are shear forces, increase in viscosity
due to metabolite secretion, fungal morphology,
and reduction in metabolite stability.
Gibberellic acids
Gibberellic acid is a fungal secondary metabo-
lite produced in its stationary phase. The SSF sys-
tem not only minimizes production and extraction
costs, but also increases the yield of gibberellic ac-
ids. Accumulation of gibberellic acid was 1.626
times higher in SSF than SMF using wheat bran as
substrate with Gibberella fujikuroi P3.147 Limitation
in nitrogen sources stops the exponential growth of
fungus triggering production of secondary metabo-
lites such as gibberellins.148
Various bioreactors using different substrates
have been used for production of gibberellic acid in
SSF. Bendelier et al. developed an aseptic pilot
scale reactor for production of gibberellic acid us-
ing Gibberella fujikoroi in fed batch SSF.149 Gelmi
et al. used amberlite as inert solid support in glass
column reactors under different conditions of tem-
perature and water activity for gibberellic acid pro-
duction.148 A model using maize cob particles
soaked in an amylaceous effluent was developed
for studying factors affecting gibberellic acid pro-
duction in SSF. The factors were particle diameter,
volume of liquid phase and substrate concentra-
tion.150 Tomasini et al. produced gibberellic acid on
different SSF systems such as cassava flour, sugar-
cane bagasse, and polyurethane foam. With SSF
250 mg of gibberellin per kg of dry solid medium
was produced in 36 h on cassava, but 23 mg of
gibberellin per L of the medium was produced in
submerged fermentation.151 Scaling up and optimiz-
ing the operation of solid-state cultivation bio-
reactors will be simplified if there is availability of
accurate process models, but problems may still ex-
ist due to absence of online measuring devices.47
Aroma production
Microorganisms play an important role in gen-
eration of natural compounds, such as fruity aroma.
Although bacteria, yeast and fungi produce aroma
compounds only a few spp. of yeast and fungi have
been preferred due to their GRSA (Generally Re-
garded as Safe) status. Solid-state fermentation has
been used for production of aromas by cultivating
Neurospora spp, Zygosaccharomyces rouxii,Asper-
gillus spp. and Trichoderma viride using pre-gelati-
nized rice, miso, cellulose fibers, and agar.
Medeiros et al.usedKluyveromyces marxianus
on cassava bagasse, as substrate in packed bed reac-
tor with forced aeration at two different flow rates of
Q= 0.06 and 0.12 L h–1 g–1. However, with lower
aeration, the total volatile content increased at t=24 h
but with higher aeration the total volatile content was
less, and the rate of production decreased. Acetal-
dehyde, ethyl acetate and ethanol were the three ma-
jor volatile compounds. Production of acetaldehyde
and ethyl acetate decreased with increase in aeration
rate. This has been attributed to the decrease in vapor
pressure within the fermented medium with the in-
crease of the aeration rate.152 Strain of Kluyvero-
myces marxiamus was cultivated on five different
agro-industrial residues such as cassava bagasse, gi-
ant palm bran, apple pomace, sugarcane bagasse, and
sunflower seeds. Palm bran and cassava bagasse
proved feasible for aroma production. Ethanol and
ethyl acetate were in highest concentration with
palm bran and cassava bagasse respectively in t=72
h. Difference in composition of fermentation me-
dium highly affected the aroma production.153
Ceratocystis fimbriata was used for producing
volatile compounds in column and horizontal drum
bioreactors over coffee husk as substrate. However,
production was higher in the horizontal drum
bioreactor. The dominant compounds in the process
were ethyl acetate, acetaldehyde and ethanol. These
were extracted by using porous absorbents. Resins
like (Tenax and Amberlite) showed the best results
for volatile compounds. In the case of Tenax,
acetaldehyde was recovered in significant amounts
(n= 649.7 mmol). The type of reactor used and ab-
sorbent influences the production.154 Ceratocystis
fimbriata when grown on different agro-industrial
residues medium containing cassava bagasse, apple
pomace, amaranth and soybean resulted in aroma
production. The medium containing amaranth pro-
duced pineapple aroma. However, the medium con-
taining cassava bagasse, apple pomace and soybean
resulted in fruity aroma. The aroma produced was
found to depend on the growth rate of fungus and
its respiratory rate. The greater the respiratory ac-
tivity of fungus the greater is its growth.155 Hot wa-
ter treated coffee husk was used for production of
aroma production by Ceratocystis fimbriata in SSF.
Supplementation of glucose, leucine increased the
aroma production but an opposite effect was seen
by addition of saline solution.156
Ferron et al. reviewed the microbial prospects
of aroma production using SSF.157 Longo and Sanro-
man reviewed the production of aroma compounds
for the food processing industry by microbial cul-
tures. Production form microorganisms has great ad-
vantages over traditional methods, such as decrease
in production costs, ease of downstreaming processes,
and use of cheaper agro-industrial substrates.158
A cheap alternative to agro-industrial residues
for aroma production is cereal grain. Various spp.
of Aspergillus, Penicillum, Rhizopus etc, can be
cultivated on it for production of various aroma
compounds such as esters, aldehydes, alcohols etc.
64 S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008)
The development of novel, cheap production pro-
cesses such as solid-state fermentation may solve
some current limitations of microbial flavor produc-
tion, as well as widen the spectrum of some bio-
technologically accessible compounds.
Antibiotics
Many antibiotics such as penicillin,cephamycin
C, neomycin, iturin, cyclosporin A, cephalosporins
are produced by SSF.
Penicillin was produced by using Penicillum
chrysogenum with substrates such as wheat bran of
high moisture content (s= 70 %) and sugarcane
bagasse.159,160,161 Cephamycin C is produced by a
variety of microorganisms including Streptomyces
cattleya, Streptomyces clavuligerus and Nocardia
lactamdurans. Wheat raw supplemented with cot-
tonseed-de-oiled cake and sunflower cake was used
for production of cephamycin C using SSF.162
Wheat raw supplemented with raspberry proved to
be optimum for production of neomycin by SSF.
Some critical parameters considered to be optimum
for production of neomycin are particle size of sub-
strate, initial moisture content, inoculum volume,
and incubation temperature.163
Iturin, an anti-fungal antibiotic, produced by
SSF had stronger antibiotic activity due to its
longer side-chain.164 Dehydrated okara (containing
s= 82 % moisture) mixed with wheat bran, when
treated with Bacillus subtilis produced Iturin.165,68
The amount of iturin produced per unit mass of wet
substrate in SSF was found to be 5–6 times higher
than in submerged fermentation when wheat bran
was treated with Bacillus subtilis NB22.164 Several
parameters such as perforation in SSF trays, solid
substrate thickness, type and size of inoculum and
effect of relative humidity have been optimized for
cyclosporin A production with wheat bran by
Tolypocladium inflatum in SSF.166,167
Adinarayana et al. optimized the additives and
parameters such as incubation temperature, moisture
content, and inoculum level etc. for maximum
cephalosporin C production using various substrates
with Acremonium chrysogenum.However, wheat
raw was found to be the best substrate.168 Strains of
Streptomyces were assessed for tetracycline produc-
tion using various agro-industrial residues as sub-
strates. All the strains produced maximum tetracy-
cline with peanut as carbohydrate source.169
Production of poly unsaturated fatty acids
(PUFA) under SSF
Poly unsaturated fatty acids (PUFA) have to be
supplied in diet, as they are not produced in the
body. Submerged and solid-state fermentation can
be used for PUFA production. Fungus involved in
PUFA production decreases anti-nutrient substances,
such as phytic acid, within the substrate, and par-
tially hydrolyzes the biopolymers in it making them
suitable for food and feed supplement. An impor-
tant parameter to be considered in SSF is mass
transfer during fermentation. Problems involved in
scale-up also require solving.170
Gamma linoleic acid (GLA) is the most exten-
sively studied PUFA by SSF. In addition, SSF
processes have also been used for production of
arachidonic acid, eicosapentanoic acid-rich byprod-
ucts.170 Cunninghamella japonica when grown on
various cereal substrates such as unhurled barley,
pearl barley, peeled barley, hulled wheat, hulled
millet, and polished rice in SSF produced GLA and
lipids. The highest amount of lipid, GLA was ob-
tained during SSF of rice and millet.171 Similarly,
optimization and screening of cereals was done in
roller bottles and cultivation bags for maximum
GLA production. Pearled barley resulted in highest
GLA production. Supplementation of peanut oil
further enhanced the production.172 Among various
fungal strains, the highest GLA was obtained from
Thamnidium elegans in a roller bottle with apple
pomace and spent malt grains as substrate.173 Com-
mercially, GLA is being produced in Japan using
the genus Mortierella and Mucorales.171,173
Production of poly gamma glutamate (PGG)
under SSF
Poly gamma glutamate (PGG) is an anionic, wa-
ter-soluble, and highly viscous polypeptide. PGG is
used as thickener, humectant, drug carrier, heavy
metal absorber and feed additive. Uncontrollable
foaming, limitation of O2and mass transfer decreases
PGG production in submerged fermentation, but
control over foaming and cost of substrates is
achieved in SSF. A high protein-containing material
is a good substrate for PGG production using Bacil-
lus subtilis. Hence, wheat bran supplemented with
soybean cake powder and additives such as gluta-
mate, citric acid as substrate resulted in maximum
PGG production (83.61 g per kg of dry substrate) at
s= 60–65 % moisture level. However, with optimi-
zation a four-fold enhancement was observed.174
Production of poly hydroxy alkanoates (PHA)
under SSF
Poly hydroxy alkanoates are microbial polyes-
ters accumulated by microbes as energy reserve.
Production using SSF decreases environmental
problems and costs involved in the process. Sub-
strates such as olive oil, babassu oil cake treated
with Ralstonia eutropha can be used for PHA pro-
duction by SSF. Addition of sugarcane molasses to
soy cake increases PHA production.175
S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008) 65
Exo-polysaccharides production under SSF
Exo-polysaccharides such as succinoglycan,
xanthan are the future products of SSF.65 Agrobac-
terium spp. has been used for succinoglycan pro-
duction on an industrial scale as they are non-patho-
genic and produce good yield of polysaccharide.
Succinoglycan was produced using Agrobacterium
tumaefaeciens on agar medium, spent malt grains,
ivory nut and grated carrot in a horizontal bio-
reactor.176
Similarly, Xanthomonas compestris was grown
on a variety of solid substrates such as spent malt
grains, apple pomace and citrus peels for xanthan
gum production. Fermentation was done in rotating
culture bottles.177
Production of biocontrol agents under SSF
Development of sustainable agriculture through
eco-friendly pesticides is needed due to their stabil-
ity and reliability. They are used as an alternative to
toxic residues. Among the various microbial agents
as biocontrol agents, fungal agents are found to
have greater potential because of their different
modes of action. Liagenidium giganteum, a fungal
agent used for control of mosquitoes, act by encyst-
ing on their larvae. It uses the larvae as a substrate
for growing by entering inside it. This fungus acts
by producing motile zoospores.30
The recognition of the fungal strain with pesti-
cide activity is significant for development of infec-
tive propagules such as conidiospores, blastospores,
chlamydospores, oospores and zygospores,65 along
with assessment of its biocontrol activity. Similarly,
the mechanism of infection is also an important pa-
rameter to be considered for large-scale production
strategy.3Retention of moisture is an important as-
pect for development of biopesticide formulation.
Vrije et al.179 illustrated the various mode of action
and requirements of down stream processing for the
biocontrol agents.
Spores produced in SSF are more heat resistant
and are more stable through SSF.70 Heat evolution
and retention are the big problem in SSF system,
but using lignocellulosic substrates, which increase
the water retention capacity and forced aeration,178
can prevent it.
E. nigrum produced on peat, vermiculite and
lentil meal is an antagonist of Monilinia laxa,re
-
sponsible for loss in stone fruits. Production using
SSF is done in plastic bags because increasing the
number of bags can do scale-up.180 A glass column
reactor was used with forced aeration with different
substrate bed height and diameter for the produc-
tion of spores of Metarhizium anisophilae. Forced
aeration reduces heat transfer.128 Similarly, a packed
column bed reactor has been used to study interac-
tions between Fusarium culmorum and its potential
biocontrol agent, Trichoderma harzianum.
Solid-state fermentation of broiler litter has been
done for the production of spores of B. thuringe-
nesis and P. flourescens.181 Oostra et al. used mixed
bioreactors for large-scale production of spores for
biocontrol agent Coniothyrium minitans and evalu-
ated the heat produced during the process. C.
minitans has the ability to grow and sporulate in a
wide temperature range and is responsible for con-
trolling Sclerotinia sclerotinium infections.50
Production of biofuel by SSF
Ethanol is the most widely used biofuel today.
Although it is easier to produce ethanol using sub-
merged fermentation, SSF is preferred due to its
lower water requirement, smaller volumes of fer-
mentation mash, prevention of end product inhibi-
tion, and disposal of less liquid water, which de-
creases pollution problems.
Cellulosic materials are receiving major atten-
tion for ethanol production because of their abundant
availability.182 Solid-state fermentation of apple
pomace supplemented with ammonium sulfate and
controlled fermentation with Saccharomyces cerevi-
siae has been reported to produce ethanol.183 Kargi et
al. used the rotating drum fermentor to estimate the
influence of rotational speed on the rate of ethanol
formation. The rate of ethanol formation decreased
with the increase in rotational speed.184 Various spp.
of yeasts that can be used for ethanol production by
SSF are Saccharomyces cerevisiae, Kloeckera api-
culate, Candida stellata, Candida pulcherrina and
Hensenula anomala. Bacteria like Zymomonas mobi-
lis and fungus like Fusarium oxysporum also have
the ability to produce ethanol. Ethanol production
using thermotolerant yeasts has been supported be-
cause of lower cooling costs required and faster de-
termination rates.149,185 Various substrates that can be
used for alcohol production are sweet sorghum,
sweet potato, wheat flour, rice starch, soluble starch
andpotatostarchusingSaccharomyces cerevisiae.
However, maximum ethanol production is achieved
by using mixed substrates.186,187
Conclusions
There is a continuous development in SSF
technology over the last two decades. The advan-
tages of SSF processes overweigh the obstacles due
to engineering problems involved in fermentation
processes. Presently, in most SSF systems fungi are
more suitable than bacterial strains and yeasts, but
genetically improved or genetically modified bacte-
rial and yeast strains may be made to suite SSF pro-
cesses. Bacterial cultures decrease the time required
66 S. BHARGAV et al., Solid-state Fermentation: An Overview, Chem. Biochem. Eng. Q. 22 (1) 49–70 (2008)
for fermentation and hence reduce the capital in-
volved. Many difficulties are involved in SSF, that
require extensive attention, such as: difficulty in
scale-up, requirement for controlling process vari-
ables like heat generation, unavailability of direct
analytical procedures to determine the biomass di-
rectly in the substrate bed, and heterogeneous fer-
mentation conditions. It has been noted that the use
of inert support conditions provides good condi-
tions for fermentation along with the purity of the
product.93,143 Improvement in bioreactors, process
control for continuous SSF is required in the bio-
technology industry for producing most value
added products. Analysis of existing literature has
proved that most value added products could be
produced in higher amounts by SSF than by sub-
merged fermentation. Optimization of the proper
substrate and additives are an important part of the
process. Recent developments made by various re-
searchers, show that control of heat transfer,
scale-up in SSF should be solved through prior lab-
oratory-scale mathematical modeling.
List of symbols
Afloat of area, cm2
Awwater activity
Ddiameter of column, mm, m
ddiameter, mm
dpparticle diameter, mm
Hheight of column, mm, m
hheight of substrate bed, cm
llength, m
namount of substance, mmol
nSstirring speed, min–1
Pproductivity, U g–1 h–1
Qvolume flow rate, L h–1
smoisture, %
Shole surface, mm2
ttime, h
Vvolume, L
wmass fraction, %
Yyield, %
ainclination angle, °
gmass concentration, g L–1
Jtemperature, °C
List of abbreviations
SSF solid-state fermentation
SMF submerged fermentation
GDS gram of dried substrate
IU/gds International Unit per gram of dried substrate
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  • ... Une méthode plus directe peut également être utilisée, comme la mesure de la production de dioxyde de carbone. Celle-ci peut notamment être corrélée à plusieurs paramètres mesurables comme la perte de masse sèche par exemple129,130.Chapitre I32Tableau 6. Bilan des méthodes les plus utilisées pour l'estimation de la biomasse en FMS127,128 131,107,132,133 CatégorieCible Commentaire Composants cellulaires spécifiques Ergostérol Propre à la biomasse fongique, instable, varie en fonction du stade de développement et des conditions de culture, difficulté de la méthode Glucosamine et chitine Propre à la biomasse fongique, varie en fonction des conditions de culture et de l'état physiologique, difficulté de la méthode Spores Fortes variations suivant les conditions de culture, peu précis, pas de distinction spore/mycélium pour la viabilité Acides nucléiques et ADN Fortes variations, interférences suivant le milieu de culture, difficulté de la méthode Protéines Non applicable si le substrat contient également des protéines Activités métaboliques Métabolites secondaires Dépendants du stade de développement, difficulté des méthodes Respiration (consommation d'O 2 et production de CO 2 ) Nécessite des installations étanches ou des mesures en ligne, rapidité de la méthode Activité enzymatique Dépendante du substrat, de l'état physiologique et des conditions de culture Consommation de nutriments (ex sucres) Dépendante du substrat, de l'activité métabolique et de l'état physiologique du microorganisme Production d'ATP Estime l'activité microbienne, non corrélée à la croissance, résultats peu fiables Perte de masse sèche Mesure corrélée à la respiration, nécessite une précision de mesure Propriétés physiques Analyse d'image par des techniques de microscopie Equipements onéreux, difficile à appliquer, nécessite de développer des logiciels Production de chaleur métabolique, réflectance (réflexion de la lumière) et capacitance (charge électrique) Mesures en ligne, interférences liés à la nature et à l'hétérogénéité du substrat3. Les bioréacteurs3.1. ...
    ... Propriétés physicochimiquesLes propriétés du substrat solide affectent considérablement le procédé de fermentation en milieu solide et peuvent compromettre le changement d'échelle. En plus de la composition chimique, la taille des particules, la porosité, la résistance mécanique ainsi que la capacité de rétention en eau de la matrice solide influencent le choix de la technologie ainsi que la conduite du procédé en bioréacteur.A l'échelle du microorganisme ces propriétés agissent sur la surface disponible à la colonisation ainsi que sur les transferts d'oxygène, de chaleur, de nutriments et d'eau qui sont indispensables à leur Chapitre II 66 développement118,129. Parmi les résidus d'agroindustries les plus utilisés en FMS, le son de blé est un des plus intéressants, il a donc été choisi pour le comparer au tourteau de colza (Tableau 12).Tableau 12. Caractéristiques physicochimiques du tourteau de colza et du son de blé Substrat Tourteau de colza Son de blé Condition Produit brut Produit à 40% de MS Produit brut Produit à 40% de MS Matière sèche (%) 85,71 ± 0,06 40,89 ± 0,08 88,27 ± 0,19 38,83 ± 0,12 Densité volumique (g/L) 571,11 ± 9,44 648,14 ± 8,79 158,77 ± 2,10 277,12 ± 2,13 Capacité maximale de rétention en eau (%) 71,54 ± 0,49 83,63 ± 0,27 pH spontané humide 5,46 ± 0,21 6,53 ± 0,10 Le son de blé est un substrat bon marché couramment utilisé en FMS pour la production d'enzymes et de métabolites secondaires. ...
    Thesis
    La fermentation en milieu solide (FMS) est un procédé biotechnologique particulièrement bien adapté au traitement de la biomasse végétale. Cette technologie, extrapolable à l’échelle industrielle, peut répondre aux besoins actuels du marché de l’alimentation animale en développant des produits fermentés visant à améliorer les qualités des matières premières d’un point de vue nutritionnel et fonctionnel. Les travaux de recherche présentés dans cette thèse développent ces aspects à partir du tourteau de colza issu du procédé de trituration des graines.Un premier criblage a mis en évidence le potentiel de certaines souches fongiques en prenant en compte les performances de croissance et leur capacité à enrichir le tourteau en protéines totales et digestibles. Des teneurs comparables à celles du tourteau de soja, principale source de protéine en nutrition animale, ont d’ailleurs été atteintes. Une activité biologique d’intérêt a ensuite été démontrée in vitro sur cellules immunitaires. Cette voie, inexplorée jusqu’à présent à partir de culture en milieu solide, confère une valeur ajoutée au produit fermenté via un apport en composés immunomodulateurs naturels d’origine fongique comme les β-glucanes. Ces derniers suscitent un intérêt croissant car ils représentent une alternative à l’utilisation des antibiotiques comme facteur de croissance. L’étude consacrée à la phase d’optimisation du procédé FMS a permis d’accroître les performances de croissance de la souche A. sojae et de définir une stratégie visant à maintenir les conditions optimales de croissance en réacteur pré-pilote à couche profonde. Des complications liées à la granulométrie très fine du tourteau de colza et à la qualité microbiologique qui évolue en cours de fermentation dans des conditions non stériles ont pu être identifiées mais le procédé mis en place a alors permis de les maîtriser en partie.
  • ... This has been established due to the fact that these substrates can support fungal growth along with their natural metabolism (secreted enzymes). Importantly, fungi can grow on the solid substrate in the same way they typically grow in nature (Bhargav et al., 2008;Kumar and Kanwar, 2012). Furthermore, SSF involving fungi offers high volumetric productivity and high yields of enzyme production. ...
    ... Furthermore, SSF involving fungi offers high volumetric productivity and high yields of enzyme production. In this method, enzymes can be easily extracted with water and the process is recognized as being less expensive, easier to use and less time-consuming (Bhargav et al., 2008). The process has been widely used in the fermentation industry, particularly for enzyme production (Pandey et al., 1999;Soccol et al., 2017). ...
    Article
    Full-text available
    Phytases are a group of enzymes that hydrolyze the phospho-monoester bonds of phytates. Phytates are one of the major forms of phosphorus found in plant tissues. Fungi are mainly used for phytase production. The production of fungal phytases has been achieved under three different fermentation methods including solid-state, semi-solid-state, and submerged fermentation. Agricultural residues and other waste materials have been used as substrates for the evaluation of enzyme production in the fermentation process. Nutrients, physical conditions such as pH and temperature, and protease resistance are important factors for increasing phytase production. Fungal phytases are considered monomeric proteins and generally possess a molecular weight of between 14 and 353 kDa. Fungal phytases display a broad substrate specificity with optimal pH and temperature ranges between 1.3 and 8.0 and 37–67°C, respectively. The crystal structure of phytase has been studied in Aspergillus. Notably, thermostability engineering has been used to improve relevant enzyme properties. Furthermore, fungal phytases are widely used in food and animal feed additives to improve the efficiency of phosphorus intake and reduce the amount of phosphorus in the environment.
  • ... The experimental results show that, at higher temperatures, the growth of the microorganism was delayed, decreasing its metabolic activity and its action on the compounds of interest. The filamentous fungi grow in an optimal temperature range, however during the process, due to the metabolic activity, heat is generated, increasing the internal temperature and retarding the growth of the microorganism (Bhargav et al., 2008). In tray bioreactors keeping the temperature in the optimum range is complex, because the bed is kept fixed during the processes and the heat generated is not removed so that the microorganism can suffer alterations (Dalsenter et al., 2005). ...
    ... It can also affect sporulation, enzyme production, and secondary metabolites. Low water activity can cause low mass transfer and poor water availability for the microorganism (Bhargav et al., 2008). Bhoite and Murthy (2015) found that the moisture suitable for the production of enzyme tannase from the coffee pulp was 50% because high moisture produces a reduction in the porosity of the solid matrix decreasing the oxygen transfer. ...
    Article
    Biotechnological advances offer potential opportunities for economic utilization of agro-industrial residues such as coffee pulp. Coffee pulp is a fibrous and mucilaginous residual material obtained during the processing of coffee cherries by the wet or dry process. The coffee pulp contains some amount of caffeine and tannins, which makes it toxic in nature, resulting in the disposal problem, however, the coffee pulp is characterized by its nutritional high-value. To increase the use of the coffee pulp (Arabica or Robusta) in animal feed, these compounds were removed by biodetoxification through solid-state fermentation using Rhizopus oryzae (MUCL 28,168). Temperature, moisture, and inoculum size were those studied parameters that had a significant influence on the decrement of the antiphysiological compounds. It was found that the highest decrease of caffeine (52%) and condensed tannins (52%) was achieved at pH 6.0, temperature 30 °C, initial moisture 70%, and inoculum 1 × 10⁶ spores/g. The results show that with this process a reduction of these compounds can be achieved, allowing efficient use of the coffee pulp in different industries.
  • ... With the development of second-generation feedstocks (advanced biofuels), it becomes more obvious that biomass can be sufficiently produced for industrial chemical production and renewable fuels "with little negative impact on the supply chain of food products for human consumption" for the ever-growing world population (Biermann et al., 2011). Application of solid-sate fermentation technology have opened another frontier of valuable chemicals, enzymes, antibiotics, surfactants and industrial aromas (Bhargav et al., 2008). Exploring valuable chemicals from biomass is in tandem with the concept of green chemistry which focuses on transforming conventional chemical reactions with the more environmentally benign industrial process. ...
    Article
    Full-text available
    Reduction in the amount of food loss and waste requires appropriate quantification method of the amount of food loss annually on the one hand and devising alternative use for foods that would otherwise end up as loss or waste. In this review, food loss and waste (FLW) were classified as avoidable, non-avoidable and possibly avoidable wastes based on inherent composition of several food categories. The current disposal methods of FLW were evaluated for its effectiveness and overall environmental impacts presented by landfills, composting and incineration. Alternative and sustainable alternative for management of food loss and waste include feedstocks for biofuel production, valuable chemicals and coproducts. This approach is renewable, environmentally friendly, improved social status through job creation for local communities and overall improved quality of life.
  • ... With the development of second-generation feedstocks (advanced biofuels), it becomes more obvious that biomass can be sufficiently produced for industrial chemical production and renewable fuels "with little negative impact on the supply chain of food products for human consumption" for the ever-growing world population (Biermann et al., 2011). Application of solid-sate fermentation technology have opened another frontier of valuable chemicals, enzymes, antibiotics, surfactants and industrial aromas (Bhargav et al., 2008). Exploring valuable chemicals from biomass is in tandem with the concept of green chemistry which focuses on transforming conventional chemical reactions with the more environmentally benign industrial process. ...
    Article
    Full-text available
    Reduction in the amount of food loss and waste requires appropriate quantification method of the amount of food loss annually on the one hand and devising alternative use for foods that would otherwise end up as loss or waste. In this review, food loss and waste (FLW) were classified as avoidable, non-avoidable and possibly avoidable wastes based on inherent composition of several food categories. The current disposal methods of FLW were evaluated for its effectiveness and overall environmental impacts presented by landfills, composting and incineration. Alternative and sustainable alternative for management of food loss and waste include feedstocks for biofuel production, valuable chemicals and coproducts. This approach is renewable, environmentally friendly, improved social status through job creation for local communities and overall improved quality of life.
  • ... The bioreactor is one of the most important equipment in the development of bioprocess, and numerous types of bioreactors have been used for fermentation technologies in the production of different secondary metabolites (i.e., antibiotics, enzymes, bioactive compounds, pigments) or biofertilizers, biopesticides, and biofuels from solid agro-industrial residues (Barrios-Gonzalez et al. 2005) (please see Fig. 4.1). The use and design of bioreactors for SSF have been reviewed previously (Pandey 1991b(Pandey , 2003Mitchell et al. 2000;Raghavarao et al. 2003;Robinson and Nigam 2003;Durand 2003;Prabhakar et al. 2005;Krishna 2005;Mitchell et al. 2006a;Couto and Sanromán 2006;Ruiz-Leza et al. 2007;Bhargav et al. 2008;Singhania et al. 2009;Ali and Zulkali 2011;Chen 2013;Ashok et al. 2017;Arora et al. 2018;Krishania et al. 2018), taking into account the advantages and disadvantages in terms of operational conditions, scale-up, substrate, microbial strain, etc. Pino et al. (2018) reported that the bioreactor provides optimal conditions for enzymes and microorganisms, improving the yields in the bioprocess. The bioreactor for SSF can be operated in different modes, for example: ...
    Chapter
    Full-text available
    Industrial enzymes are biocatalysts that are commercially used in a variety of commercial sectors such as pharmaceuticals, chemical production, biofuels, food and beverages, and consumer products. Due to advancements in recent years, biocatalysts are considered more economical than use of whole cells and can be used as a unit operation within a process to generate the desired product of interest. Industrial biological catalysis through enzymes has experienced rapid growth in recent years due to their ability to operate at mild conditions, high specificity, and high productivity. Industrial enzymes can be produced by both submerged fermentation (SmF) and solid-state fermentation (SSF). In contrast to the first process, the second bioprocess (SSF) is the cultivation of microorganisms under controlled conditions in the absence of free water. Examples of bioproducts of SSF include industrial enzymes, fuels, and nutrient-enriched animal feeds. Most industrial enzymes are manufactured using the traditional bioprocess of SmF, where microbial cells are suspended in a large volume of water that is stirred and aerated using mechanical devices; such culture conditions dictate the overall physiological behavior of microorganisms provoking biochemical and structural changes affecting the quantity and activity of biocatalysts produced. Among the main advantages of SSF over SmF is a higher volumetric productivity, secretion facilities to get extracellular bioproducts with higher stability, being usually simpler with lower energy requirements, resembling of the natural habitat of some microorganisms, and easier downstream processing. In this chapter we summarize, compare, analyze, and discuss the technological, biochemical, and microbiological advantages of SSF to produce industrial enzymes. Furthermore, culture conditions, aggregation and diffusional phenomena, bioreactors, genetic expression, and protein regulation are covered.
  • ... Some J o u r n a l P r e -p r o o f agricultural waste can be used as solid substrates, which leads to a reduction of capital costs and potentially provides a superior productivity [12]. Another advantage of solid fermentation is that it is a well-known technology, because it is used in different fields such as the production of enzymes, antibiotics, organic acids, unicellular proteins, biopesticides, biofuels [13,14]. In Ecuador, the cultivation of broccoli is widespread because the demand for this vegetable from the European continent has grown in recent years [15]. ...
    Article
    Full-text available
    Alternaria japonica causes annual losses of up to 25% of the world broccoli crops, for this reason this research focused on the development of biopreparations containing Bacillus megaterium to prevent the outbreak of this disease caused by Alternaria japonica in the crop of Brassica oleracea var. italica. During the laboratory phase two types of biopreparations were evaluated, the first biopreparation was obtained by liquid fermentation composed of 40 g.L⁻¹ of fava bean flour and 5 g.L⁻¹ of ground brown sugar. This showed a maximum cell growth of 3.8 × 10⁸ CFU. mL⁻¹; while the second biopreparation was obtained by solid fermentation composed of wheat bran and it achieved a maximum cell growth of 4.7 × 10⁹ CFU. g⁻¹. In the fieldwork phase the aforementioned biopreparations were applied in an open-field crop. At the end of the cultivation period, the degree of the disease in leaves and in the inflorescences was measured and through the statistical analysis, a significant difference was evidenced (α = 0.05). On the broccoli leaves the disease index values do not exceed 15.56% and the disease index for postharvest florets was around 38%. The evaluated variables showed a statistical similarity with the chemical treatment, thus determining the effective effect of the biopreparations.
  • ... It is characterized as the maturation prepare in which microbes develop on strong substantial devoid of the absence of clear fluid. Idea of utilizing strong base material is most likely most seasoned strategy utilized for the production of enzymes to work for welfare of mankind (Bhargav et al., 2008). ...
  • ... 9 Solid-state fermentation is a fermentation process that occurs on solid substrates without free water. 10 The use of SSF in biorefinery processes is an alternative strategy to conventional physical and chemical treatments to produce value-added products from agro-industrial wastes. 11 Filamentous fungi are good degraders of lignocellulosic materials because they release lignocellulolytic enzymes and exhibit hyphal penetration into the substrate were they grow. ...
    Article
    Olive pomace is characterized by its low nutritional value and high phenolic content, which hinders its direct use as animal feed, fertilizer, or as a substrate in bioprocesses such as solid‐state fermentation (SSF). A possible strategy for bioprocessing olive pomace by SSF is the mixture of olive mill wastes with other wastes produced in the same region, such as winery wastes. This may improve the production of bioactive compounds like enzymes and antioxidant phenolics. A simplex‐centroid design was used to evaluate the use of olive mill and winery wastes alone or in combination as a substrate for SSF with Aspergillus niger and Aspergillus ibericus. Synergistic effects of combinations of crude olive pomace (COP), exhausted olive pomace (EOP), vine trimming shoots (VTS), and exhausted grape marc (EGM) were observed in the production of xylanases, cellulases, β‐glucosidases, and in the variation in total phenolics and antioxidant activity of SFF extracts. A multiple response optimization was carried out, leading to the following optimal mixture of substrates: for A. niger, 23% (w/w) COP, 30% EGM, 33% VTS, 14% EOP; for A. ibericus, 30% EGM, 36% VTS, 34% EOP. The scale‐up to tray bioreactor with optimal substrate made it possible to achieve the maximum xylanase, cellulase, and β‐glucosidase production of 189.1 ± 26.7, 56.3 ± 2.1 and 10.9 ± 0.8 U/g, respectively. The antioxidant activity of fermented wastes was also improved 2.2‐fold as compared with unfermented wastes. Thus, a combination of olive mill and winery wastes in SSF is a potential strategy to increase their value and to develop a circular strategy in these industries. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.
  • Chapter
    Full-text available
    The management practice of the fresh market wastes (wastewater and solid waste) plays an important role in the presence or absence of the adverse effect on the environment which received the disposed of wastes. The management process included the collection, segregation, transportation, treatment, and final disposal of the wastes into the environment. These processes aim to minimize the health risk associated with the fresh market wastes in terms of infectious agents. The health 6 risk of the fresh market wastes is coming with the presence of pathogenic bacteria transferred from the fruits, vegetables as well as meats and fishes. The present chapter aimed to highlight the management role for reducing the infectious agents in fresh market wastes before the final disposal. It appeared that most of the infectious agents in fresh market wastes are pathogenic for plants. However, many of the infectious agents coming from meat and fishes might also cause some diseases for human. The main concern lies in the potential of fresh market wastewater to induce the eutrophication in terms of harmful algae blooms (HABs). However, the technologies used for the inactivation of HABs in the freshwater have been reviewed in this chapter.
  • Article
    The growth ofRhizopus oligosporus on a model solid substrate consisting of cassava starch and other nutrients embedded in a kappa-carrageenan matrix was followed microscoplcally. There were two distinct growth phases: (1) germination of spores and development of mycellum to form a loose network over the whole substrate surface; (2) increase in the density of the mycellum. Although some penetrative hyphae were produced, the biomass was malnly restricted to the surface. Consequently, starch utilization was greatest near the surface. Based on these observations a descriptive model for the growth ofRhizopus ollgosporus on the model substrate was proposed. The five steps are: (1) release of glucoamylase by the mycelium; (2) diffusion of the glucoamylase to the starch; (3) hydrolysis of the starch by the glucoamylase, releasing glucose; (4) diffusion of glucose; (5) absorption of glucose by the mycellum at the surface to produce new blomass.
  • Article
    Substrate (wheat bran) particle degradation studies were conducted in solid state fermentation employing Aspergillus niger CFTRI 1105. Changes in the substrate weight and variations in the microscopic appearance of the particle were made during the course of fermentation. The sequential release of extracellular enzymes by the fungus recorded at different fermentation times. The maximum reduction in size and total weight of the substrate particle and the release of all the major hydrolytic enzymes were observed after 36 h of fermentation. A mathematical model was proposed for the degradation of the substrate particles during fermentation. Model predictions coincide with the experimental observations of the extent of reduction in particle size.
  • Article
    The relationship between CO2 evolution and the extent of ligninolytic and proteolytic activities of Phanerochaete chrysosporium grown on sugarcane bagasse pith was investigated. A crude extract from the solid state fermentation (SSF) system was obtained and manganese peroxidase (MnP), lignin peroxidase (LiP) and proteolytic activities were determined without further concentration. CO2 evolution profile showed that MnP activity was expressed during the idiophase when glucose was limited, then decreased due to the protease activity. The respiratory quotient (QR) practically remained around one. This value was an indicator that the relation between CO2 production and O2 consumption was equimolar. This indicates that CO2 generation was basically biotic.
  • Article
    The production of GA3 in solid-state culture was studied using systems composed of maize cob particles soaked in an amylaceous effluent, as a function of three variables-particle diameter, volume of liquid phase, and substrate concentration—with a determinant influence on the response. The empirical model obtained provides an explanation of the behavior of the system in terms of a series of interactions with underlying problems of water activity and mass transfer, that can, however, be manipulated in a simple and reproducible manner. This model also allows the suitable orientation of production improvement, the interpretation of the mechanism of substrate inhibition detected, as well as means of correcting it without reducing the initial concentration of the substrate, and leads to an 11-fold increase in the production obtained in the preliminary assays.
  • Article
    Influence of water activity (aw) on biosynthesis of polygalacturonase, d-xylanase and β-glucosidase in solid culture system of Trichoderma viride TS was studied. It was found that the production of enzymes was strongly affected by water activity of substrate and nature of aw depressor used. The polygalacturonase and d-xylanase production were maximized at aw = 0.995 whereas β-glucosidase formation was favored at aw = 0.96–0.98. The influence of water activity on catalytic effect of enzymes using sodium chloride, glycerol and sorbitol as aw depressor was also investigated. It was observed that sorbitol improved the thermal stability of polygalacturonase and d-xylanase.
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    Direct monitoring of bed conditions in large scale solid-state fermentation (SSF) bioreactors is difficult due to lack of reliable and affordable instrumentation. Although, relevant variables such as average bed temperature and water content can be inferred from energy and water balances, these estimations are prone to significant error since the process is complex and measured variables are extremely noisy. Hence, to obtain better estimation of bed conditions, simple and robust models, and effective noise suppression methods should be developed. A kinetic model was applied to obtain noise free CO2 production rate (CPR) values, which in turn was used in water and energy balances. The evolution of other relevant variables including total and active biomass, O2 consumption rate, secondary metabolite production and dry mass degradation were also calculated. The proposed model was calibrated with data obtained in an agitated aseptic SSF pilot-scale bioreactor of 200kg, growing Gibberella fujikuroi on wheat bran and starch. The calibration was achieved by means of a non-linear optimization routine. With this model better average bed temperature estimates, compared with a model that uses noisy (CPR) measurements, were attained. Hence, the proposed model can be used on-line to estimate more reliable average bed conditions and to develop model based control strategies. In addition, a predictive model for bioreactor design can be derived from the proposed model, after good outlet gas temperature estimation has been achieved.