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Gluconic acid is a mild organic acid derived from glucose by a simple oxidation reac-tion. The reaction is facilitated by the enzyme glucose oxidase (fungi) and glucose de-hydrogenase (bacteria such as Gluconobacter). Microbial production of gluconic acid is the preferred method and it dates back to several decades. The most studied and widely used fermentation process involves the fungus Aspergillus niger. Gluconic acid and its deriva-tives, the principal being sodium gluconate, have wide applications in food and pharma-ceutical industry. This article gives a review of microbial gluconic acid production, its properties and applications.
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ISSN 1330-9862 review
Gluconic Acid: Properties, Applications and
Microbial Production
Sumitra Ramachandran
, Pierre Fontanille
, Ashok Pandey
and Christian Larroche
Laboratoire de Génie Chimique et Biochimique (LGCB), CUST Université Blaise Pascal,
24, avenue des Landais, B.P. 206, F-63 174 Aubière Cedex, France
Biotechnology Division, Regional Research Laboratory, CSIR, Trivandrum 695 019, India
Received: December 12, 2005
Accepted: March 12, 2006
Gluconic acid is a mild organic acid derived from glucose by a simple oxidation reac
tion. The reaction is facilitated by the enzyme glucose oxidase (fungi) and glucose de
hydrogenase (bacteria such as Gluconobacter). Microbial production of gluconic acid is the
preferred method and it dates back to several decades. The most studied and widely used
fermentation process involves the fungus Aspergillus niger. Gluconic acid and its deriva-
tives, the principal being sodium gluconate, have wide applications in food and pharma-
ceutical industry. This article gives a review of microbial gluconic acid production, its
properties and applications.
Key words: gluconic acid, glucose oxidase, microbial production, Aspergillus niger
Gluconic acid (pentahydroxycaproic acid, Fig. 1) is
produced from glucose through a simple dehydrogena
tion reaction catalysed by glucose oxidase. Oxidation of
the aldehyde group on the C-1 of b-
D-glucose to a car
boxyl group results in the production of glucono-d-lac
tone (C
, Fig.1) and hydrogen peroxide. Glucono
-d-lactone is further hydrolysed to gluconic acid either
spontaneously or by lactone hydrolysing enzyme, while
hydrogen peroxide is decomposed to water and oxygen
by peroxidase. The gluconate pathway is detailed in Fig.
2. The conversion process could be purely chemical too,
but the most commonly involved method is the fermen
tation process. The enzymatic process could also be con
ducted, where the conversion takes place in the absence
of cells with glucose oxidase and catalase derived from
A. niger. Nearly 100 % of the glucose is converted to glu
conic acid under the appropriate conditions. This me
thod is an FDA approved process. Production of gluco
nic acid using the enzyme has the potential advantage
that no product purification steps are required (1)ifthe
enzyme is immobilised, e.g. the use of a polymer mem
brane adjacent to anion-exchange membrane of low-
-density polyethylene grafted with 4-vinylpyridine (1).
However, this approach is not yet common in the indus
try, and it will not be considered in the review.
S. RAMACHANDRAN et al.: Gluconic Acid: A Review, Food Technol. Biotechnol. 44 (2) 185–195 (2006)
*Corresponding author; Fax: ++33 473 407 829; E-mail:
Fig. 1. Formula of gluconic acid (A) and glucono-d-lactone (B)
Gluconic acid production dates back to 1870 when
Hlasiwetz and Habermann discovered gluconic acid (2).
In 1880 Boutroux (3) found for the first time that acetic
acid bacteria are capable of producing sugar acid. In
1922 Molliard (4) detected gluconic acid in the Sterig
matocystis nigra, now known as Aspergillus niger. Later,
production of gluconic acid was demonstrated in bacte
rial species such as Pseudomonas, Gluconobacter, Acetobac
ter, and various fungal species. Studies of Bernhauer (5–
7) showed that A. niger produced high yields of gluconic
acid when it was neutralised by calcium carbonate and
the production was found to be highly pH dependent.
However, it was found that with Penicillium sp., the pH
dependence is not as critical when compared to A. niger,
indicating that there was some correlation between the
amount and time-dependent appearance of organic ac
ids, such as gluconic acid, citric acid, oxalic acid, which
are formed under different conditions. Gluconic acid pro
duction has been extensively studied by May et al. (8),
Moyer (9), Wells et al. (10), and Stubbs et al. (11) using
A. niger. Using Penicillium luteum and A. niger Currie et
al. (12) filed a patent employing submerged culture, giv
ing yields of gluconic acid up to 90 % in 48–60 h. Later
Moyer et al. (13) used A. niger in pilot plant studies and
produced as high as 95 % of theoretical yields in glucose
solution of 150 to 200 g/L in 24 h. Porges et al. ( 14)
found that the process could be run semicontinuously,
by the reuse of the mycelium for nine times repeatedly
where the inoculum was recovered either by filtration
or centrifugation. Findings of Moyer et al. (13) showed
that efficiency of more than 95 % could be achieved by
the addition of glucose at 250 g/L and boron com
pounds (1 % in solution of 250 g/L glucose) at later
stages of the fungal growth with the reuse of mycelium
in cycles of 24 h each.
Current commercial production of sodium gluco
nate uses submerged fermentation with A. niger and is
based on the modified process developed by Blom et al.
(15). It involves fed-batch cultivation with intermittent
glucose feedings and the use of sodium hydroxide as
neutralising agent. pH is held at 6.0–6.5 and the temper
ature at about 34 °C. The productivity of this process is
very high, since glucose is converted at a rate of 15 g/
Physicochemical behaviour
Gluconic acid is a noncorrosive, nonvolatile, nonto
xic, mild organic acid. It imparts a refreshing sour taste
in many food items such as wine, fruit juices, etc.So
dium gluconate has a high sequestering power. It is a
good chelator at alkaline pH; its action is comparatively
S. RAMACHANDRAN et al.: Gluconic Acid: A Review, Food Technol. Biotechnol. 44 (2) 185–195 (2006)
Fig. 2. General gluconate pathways
better than EDTA, NTA and other chelators. Aqueous
solutions of sodium gluconate are resistant to oxidation
and reduction at high temperatures. It is an efficient
plasticizer and a highly efficient set retarder. It is easily
biodegradable (98 % at 48 h). It has an interesting prop
erty of inhibiting bitterness in foodstuffs. Concentrated
gluconic acid solution contains certain lactone structures
(neutral cyclic ester) showing antiseptic property. The
characterisitics are described in Table 1.
In the European Parliament and Council Directive
No. 95/2/EC, gluconic acid is listed as a generally per
mitted food additive (E 574). The US FDA (Food and
Drug Administration) has assigned sodium gluconate a
GRAS (generally recognized as safe) status and its use
in foodstuff is permitted without limitation (16).
There are several methods for the determination of
D-gluconic acid and D-glucono-d-lactone. Among them,
isotachophoretic method (17) and hydroxamate method
(18) are the most commonly used ones for the determi
nation of gluconic acid. The concentration of gluconic
acid is also determined by gas chromatography of their
trimethylsilyl (TMS) derivatives prepared according to
Laker and Mount (19) with inositol as internal standard.
A widely used enzymatic method (20) is based on
the following principle:
D-gluconic acid is phosphoryla
ted to
D-gluconate-6-phosphate by ATP in the presence
of the enzyme gluconate kinase with the simultaneous
formation of ADP. In the presence of NADP,
nate-6-phosphate is oxidatively decarboxylated by 6-phos
phogluconate dehydrogenase to ribulose-5-phosphate with
the formation of reduced NADPH. The NADPH is stoi
chiometrically formed and its measurement allows di
rect determination of the amount of
D-gluconic acid.
Gluconic acid is abundantly available in plants, fruits
and other foodstuffs such as rice, meat, dairy products,
wine (up to 0.25 %), honey (up to 1 %), and vinegar. It
is produced by different microorganisms as well, which
include bacteria such as Pseudomonas ovalis (21), Aceto
bacter methanolicus (22), Zymomonas mobilis (23), Acetobac
ter diazotrophicus (24), Gluconobacter oxydans (25–27), Glu
conobacter suboxydans (28,29), Azospirillum brasiliense (30),
fungi such as Aspergillus niger (8–10), Penicillium funiculo
sum (31), P. variabile (32), P. amagasakiense (33), and vari
ous other species such as Gliocladium, Scopulariopsis, Gona
tobotrys, Endomycopsis (34) and yeasts such as Aureobasidium
pullulans (formerly known as Dematium or Pullularia pul
lulans)(35,36). Ectomycorrhizal fungus Tricholoma robus
tum, which is associated with the roots of Pinus densi
flora, was found to synthesise gluconic acid (37).
Gluconic acid is a mild organic acid, which finds ap
plications in the food industry. As stated above, it is a
natural constituent in fruit juices and honey and is used
in the pickling of foods. Its inner ester, glucono-d-lac
tone imparts an initially sweet taste which later becomes
slightly acidic. It is used in meat and dairy products,
particularly in baked goods as a component of leaven-
ing agent for preleavened products. It is used as a fla-
vouring agent (for example, in sherbets) and it also finds
application in reducing fat absorption in doughnuts and
cones. Foodstuffs containing
D-glucono-d-lactone include
bean curd, yoghurt, cottage cheese, bread, confectionery
and meat.
Generally speaking, gluconic acid and its salts are
used in the formulation of food, pharmaceutical and hy-
gienic products (Table 2). They are also used as mineral
S. RAMACHANDRAN et al.: Gluconic Acid: A Review, Food Technol. Biotechnol. 44 (2) 185–195 (2006)
Table 1. General characteristics of gluconic acid
Gluconic acid
Nature Noncorrosive, mildly acidic,
less irritating, nonodorous,
nontoxic, easily biodegradable,
nonvolatile organic acid
Relative molecular mass 196.16
Chemical formula C
Synonym 2,3,4,5,6-pentahydroxyhexanoic
pKa 3.7
Melting point (50 % solution) Lower than 12 °C
Boiling point (50 % solution) Higher than 100 °C
Density 1.24 g/mL
Appearance Clear to brown
Solubility Soluble in water
Degree of sourness (sourness
of citric acid is regarded as
Mild, soft, refreshing taste
Table 2. Applications of gluconic acid and its derivatives
Components Applications
Gluconic acid Prevention of milkstone in dairy industry
Cleaning of aluminium cans
Latent acid in baking powders for use in dry
cakes and instantly leavened bread mixes
Slow acting acidulant in meat processing
such as sausages
Coagulation of soybean protein in the manu
facture of tofu
In dairy industry for cheese curd formation
and for improvement of heat stability of
Sodium salt of
gluconic acid
Detergent in bottle washing
Metallurgy (alkaline derusting)
Additive in cement
Derusting agent
Textile (iron deposits prevention)
Paper industry
Calcium salt of
gluconic acid
Calcium therapy
Animal nutrition
Iron salt of
gluconic acid
Treatment of anaemia
Foliar feed formulations in horticulture
supplements to prevent the deficiency of calcium, iron,
etc. and as buffer salts. Different salts of gluconic acid
find various applications based on their properties. So
dium salt of gluconic acid has the outstanding property
to chelate calcium and other di- and trivalent metal ions.
It is used in the bottle washing preparations, where it
helps in the prevention of scale formation and its re
moval from glass. It is well suited for removing calcare
ous deposits from metals and other surfaces, including
milk or beer scale on galvanised iron or stainless steel.
Its property of sequestering iron over a wide range of
pH is exploited in the textile industry, where it prevents
the deposition of iron and for desizing polyester and
polyamide fabrics. It is also used in metallurgy for alka
line derusting, as well as in the washing of painted
walls and removal of metal carbonate precipitates with
out causing corrosion. It also finds application as an ad
ditive to cement, controlling the setting time and in
creasing the strength and water resistance of the cement.
It helps in the manufacture of frost and crack resistant
concretes. It is also used in the household cleaning com
pounds such as mouthwashes.
Calcium gluconate is used in pharmaceutical indus
try as a source of calcium for treating calcium deficiency
by oral or intravenous administration. It also finds a
place in animal nutrition. Iron gluconate and iron phos
phogluconate are used in iron therapy. Zinc gluconate is
used as an ingredient for treating common cold, wound
healing and various diseases caused by zinc deficiencies
such as delayed sexual maturation, mental lethargy, skin
changes, and susceptibility to infections.
Organic acids represent the third largest category
after antibiotics and amino acids in the global market of
fermentation. The total market value of organic acid will
rise to $3 million in 2009 (38). Citric acid dominates the
market of organic acids due to its application in various
fields. The market of gluconic acid is comparatively
smaller. However, 60 000 tonnes are produced world
wide annually and it is available in the market as 50 %
technical grade aqueous solution (by mass).
The main product among the gluconic acid deriva
tives is the sodium gluconate due to its properties and
applications. Manufacturers of gluconic acid and its salt
in the United States are Pfizer Inc., New York, Bristol
–Meyers Co., New York, Premier Malt Products Inc., Wis
consin. European gluconate producers include Roquette
Frères in France, Pfizer in Ireland, Benckiser in Germa
ny. Fujisawa and Kyowa Hakko are the manufacturers
of gluconate in Japan. Calcium gluconate is also an im
portant product among the derivatives of gluconic acid
and it is available as tablets, powder, and liquid for di
etary supplements.
Production of Gluconic Acid
There are different approaches available for the pro
duction of gluconic acid, namely, chemical, electrochem
ical, biochemical and bioelectrochemical (39–41). There
are several different oxidising agents available, but still
the process appears to be costlier and less efficient com
pared to the fermentation processes. Although the con
version is a simple one-step process, the chemical me
thod is not favoured. Thus, fermentation has been one
of the efficient and dominant techniques for manufac
turing gluconic acid. Among various microbial fermen
tation processes, the method utilising the fungus A. ni
ger is one of the most widely used ones. However, the
process using G. oxydans has also gained significant im
portance. Irrespective of the use of fungi or bacteria, the
importance lies on the product which is produced, for
example, sodium gluconate or calcium gluconate, etc.As
the reaction leads to an acidic product, it is required that
it is neutralised by the addition of neutralising agents,
otherwise the acidity inactivates the glucose oxidase, re
sulting in the arrest of gluconic acid production. The con
ditions for the fermentation processes in the production
of calcium gluconate and sodium gluconate differ in ma
ny aspects such as glucose concentration (initial and fi
nal) and pH control. In the process involving calcium
gluconate production, the control of pH results from the
addition of calcium carbonate slurry. Another important
point to be noted is about the solubility of calcium glu
conate in water (4 % at 30 °C). At high glucose concen
tration, above 15 %, supersaturation occurs, and if it ex
ceeds the limit, the calcium salt precipitates on the
mycelia and inhibits the oxygen transfer. The neutralis-
ing agent should also be sterilized separately from the
glucose solution to avoid Lobry de Bruyn-van Eken-
stein reaction, which alters the conformation of glucose,
which results in the reduction of yield for about 30 %.
On the contrary, the process for sodium gluconate is
highly preferable as the glucose concentration of up to
350 g/L can be used without any such problems. pH is
controlled by the automatic addition of NaOH solution.
Sodium gluconate is readily soluble in water (39.6 % at
30 °C).
Gluconic acid production by filamentous fungi
Glucose oxidase
The reaction involving the conversion of glucose to
gluconic acid by filamentous fungi is catalysed by the
enzyme glucose oxidase (b-
D-glucose: oxygen 1-oxidore
ductase, E.C. The enzyme was first isolated
from a press juice obtained from Penicillium glaucum by
Müller (42). The enzyme was crystallised by Kusai et al.
(33) from P. amagasakiense. The enzyme was previously
known as notatin. Glucose oxidase is a flavoprotein which
contains one very tightly but noncovalently bound FAD
cofactor per monomer and is a homodimer with a mo
lecular mass of 130–320 kDa depending on the extent of
glycosylation. It catalyses the reaction where glucose is
dehydrated to glucono-d-lactone, while hydrogen is trans
ferred to FAD. The resulting FADH
is regenerated to
FAD by transmission of the hydrogen to oxygen to form
hydrogen peroxide (Fig. 3). Glucose oxidase is a glyco
protein. The native enzyme is glycosylated, with a car
bohydrate mass percentage of 16–25 % (43,44). The en
zyme from A. niger contains 10.5 % carbohydrate, which
is believed to contribute to the stability without affect
ing the overall mechanism (45).
S. RAMACHANDRAN et al.: Gluconic Acid: A Review, Food Technol. Biotechnol. 44 (2) 185–195 (2006)
The enzyme is induced in the presence of high lev
els of glucose in the medium, pH around 5.5 and ele
vated oxygen levels. The enzyme is stable between pH=
4.0 and 6.0 at 40 °C for 2 h but is unstable above 50 °C.
Liu et al. (46) conducted a study on the effects of metal
ions on simultaneous production of glucose oxidase and
catalase and found that calcium carbonate induced the
synthesis of both enzymes. The induction of calcium car
bonate was accompanied by a metabolic shift from the
glycolytic pathway (EMP) to direct oxidation of glucose
by the enzyme. The enzyme is found to be inhibited by
hydrogen peroxide, the by-product of gluconic acid pro-
duction (47). A study on glucose oxidase inactivation
showed that only the reduced form of glucose oxidase is
highly sensitive to hydrogen peroxide (48).
The enzyme is used in various fields such as food,
clinical analysis, mainly as glucose sensor, in the quanti-
tative determination of glucose in body fluids and urine.
It is used in food processing in the removal of glucose
prior to the preparation of products such as dried eggs
to reduce the nonenzymatic browning. It is also used in
removing residual oxygen from fruit juices, beer, and
wine and also from dehydrated packaged foods.
Reports on glucose oxidase localization are ambigu
ous. Van Dijken and Veenhuis (49), and Witteveen et al.
(50) reported that the enzyme of A. niger is intracellular
and found in peroxisomes, whereas Mischak et al. (51)
reported it as extracellular. There are also reports which
have stated that it is intracellular prior to fungal auto
lysis (52). These varying reports on its location in the
cell could be attributed to the differences of parameters
and conditions adopted for the growth or due to the age
of the fungal cultures. Very little is known about the
mechanisms of glucose oxidase export. Zetelaki (53)as
sociated export with autolysis of the fungus, whereas
Mischak et al. (51) reported that the glucose oxidase of
A. niger was excreted after synthesis.
Aspergillus niger
A. niger produces all the enzymes required for the
conversion of glucose into gluconic acid, which include
glucose oxidase, catalase, lactonase and mutarotase. Al
though crystalline glucose monohydrate, which is in the
alpha form, is converted spontaneously into beta form
in the solution, A. niger produces the enzyme mutaro
tase, which serves to accelerate the reaction. During the
process of glucose conversion, glucose oxidase present
in A. niger undergoes self-reduction by the removal of
two hydrogens. The reduced form of the enzyme is fur
ther oxidised by the molecular oxygen, which results in
the formation of hydrogen peroxide, a by-product in the
reaction. A. niger produces catalase which acts on hy
drogen peroxide releasing water and oxygen. Hydroly
sis of glucono-d-lactone to gluconic acid is facilitated by
lactonase. The reaction can be carried out spontaneously
as the cleavage of lactone occurs rapidly at pH near
neutral, which are brought about by the addition of cal
cium carbonate, or sodium hydroxide. Removal of lac
tone from the medium is recommended as its accumula
tion in the media has a negative effect on the rate of
glucose oxidation and the production of gluconic acid
and its salt. There are reports stating that the enzyme
gluconolactonase is also present in A. niger (54), which
increases the rate of conversion of glucono-d-lactone to
gluconic acid.
Production of gluconic acid is directly linked with
the glucose oxidase activity. Depending on the applica
tion, the fermentation broths containing sodium gluco
nate or calcium gluconate are produced by the addition
of solutions of sodium hydroxide or calcium carbonate
respectively, for neutralisation. The general optimal con
dition for gluconic acid production is as follows (2):
¿ Glucose at concentrations between 110–250 g/L
¿ Nitrogen and phosphorus sources at a very low
concentration (20 mM)
¿ pH value of medium around 4.5 to 6.5
¿ Very high aeration rate by the application of ele-
vated air pressure (4 bar).
There are two key parameters which influence the
gluconic acid production. These are oxygen availability
and pH of the culture medium. Oxygen is one of the
key substrates in the oxidation of glucose as glucose oxi-
dase uses molecular oxygen in the bioconversion of glu
cose. The concentration of oxygen gradient and the volu
metric oxygen transfer coefficient are the critical factors,
which monitor the availability of oxygen in the medium.
These two factors highly influence the rate of the trans
fer of oxygen from gaseous to aqueous phase. Several
reports are available on this particular aspect. The aera
tion rate and the speed of agitation are the two parame
ters which affect the availability of the oxygen in the
medium. Gluconic acid production is an extremely oxy
gen-consuming process with a high oxygen demand for
the bioconversion reaction, which is strongly influenced
by the dissolved oxygen concentration. Oxygen is gener
ally supplied in the form of atmospheric air; however,
in some studies high-pressure pure oxygen has also
been provided. For example, Sakurai et al. (55) supplied
high-pressure oxygen at approx. 6 bar and maintained
dissolved oxygen at 150 ppm. They found that immobi
lised mycelium of A. niger grown using pure oxygen pro
duced high titres of gluconic acid in comparison with
mycelium grown in air. Kapat et al. (56) found that at an
agitation speed of 420 rpm and aeration of 0.25 vvm, the
dissolved oxygen concentration was optimal for glucose
oxidase production. The K
value of glucose oxidase for
oxygen lies in the range of air saturation in water (57).
Lee et al. (58) obtained high volumetric productivity of
gluconic acid using relatively high pressure (2–6 bar), re
S. RAMACHANDRAN et al.: Gluconic Acid: A Review, Food Technol. Biotechnol. 44 (2) 185–195 (2006)
Fig. 3. Oxidation of glucose by Aspergillus niger
sulting in an increase in dissolved oxygen up to 150
mg/L. Generally, during the course of fungal growth,
the distribution of oxygen becomes uneven, as the size
of gas bubbles increases, resulting in insufficient oxygen
supply (59). The oxygen absorption rate is also influ
enced by the viscosity of the culture. A rapid decrease is
observed in the absorption rate of oxygen with an in
crease in mycelial concentration (60).
pH is another important parameter that influences
the gluconic acid production. A. niger produces weak or
ganic acids such as citric acid, gluconic acid and oxalic
acid, and their accumulation depends on the pH of the
nutritive medium (61). pH below 3.5 triggers the TCA
cycle and facilitates the citric acid formation. The pH
range of the fungi for the production of gluconic acid is
around 4.5 to 7.0. pH=5.5 is generally considered as op
timum for Aspergillus niger (62). Franke (63) collected
some data concerning the relative activity of glucose oxi
dase at different pH levels and reported 5 and 35 % ac
tivity at pH=2.0 and 3.0, respectively, based on 100 %
activity at pH=5.6. Report by Heinrich and Rehm (64)
states that gluconic acid production occurs even at
pH=2.5 in the presence of manganese in fixed bed and
stirred bed reactors, possibly because of the difference
in intracellular and extracellular pH.
Cheaper raw materials as substrates
Glucose is generally used as carbon source for mi-
crobial production of gluconic acid. However, hydroly-
sates of various raw materials such as agro-industrial
waste have also been used as substrate. Kundu and Das
(65) obtained a high yield of gluconic acid in media con-
taining glucose or starch hydrolysate as the sole carbon
source. Vassilev et al. (66) used hydrol (corn starch hy-
drolysate) as the fermentable sugar to produce gluconic
acid by immobilized A. niger. Rao and Panda (67) used
Indian cane molasses as a source of glucose. The cane
molasses was subjected to different pre-treatments such
as acid treatment, potassium ferrocyanide treatment, salt
treatment, etc. Potassium ferrocyanide treatment gave a
promising result. Gluconic acid synthesis was influenced
by various metal ions such as copper, zinc, magnesium,
calcium, iron, etc. Mukhopadhyay et al. (68) used depro
teinised whey as a nutritive medium for gluconic acid
production. Lactose was used as a substrate and 92 g of
gluconic acid was produced from 1 L of whey contain
ing 0.5 % glucose and 9.5 % lactose by A. niger immobi
lized on polyurethane foam. Ikeda et al. (69) used sac
charified solution of waste paper with glucose concentra
tion adjusted to 50–100 g/L for bioconversion with A.
niger. The yields were 92 % in Erlenmeyer flasks and 60
% in repeated batch cultures in the turbine blade reactor
with 800 mL of working volume. Another striking fea
ture in the study was when xylose and cellobiose were
used as the sole carbon sources, yields of gluconic acid
obtained were 83 and 56 %, respectively.
Singh et al. (70) observed that grape must and ba
nana must resulted in significant levels of gluconic acid
production, i.e. 63 and 55 g/L respectively. The purifica
tion of grape and banana must leads to a 20–21 % in
crease in gluconic a cid yield. They also used molasses,
where the gluconate production was 12 g/L, but a sig
nificant increase in production of 60 g/L with a yield of
61 % was observed following treatment of the molasses
with hexacyanoferrate. Rectified grape must appeared to
be the best suited substrate, which after 144 h resulted
in 73 g/L of gluconic acid with 81 % yield when com
pared to the value of 72 % obtained from the rectified
banana must. Buzzini et al. (71) also used grape must
and rectified grape must and they found that the latter
substrate was better, with a production of 67 g/L and a
yield of 96 % in 72 h. Citric acid was also observed as a
Use of solid-state fermentation (SSF)
SSF has been widely described for the production of
industrial enzymes and organic acids (72–76). However,
for the production of gluconic acid, there are only a few
reports using SSF. Roukas (77) reported the production
of gluconic acid by solid-state fermentation on figs. The
maximal gluconic acid concentration was 490 g/kg of dry
fig with 63 % yield. The addition of 6 % methanol into
the substrate helped to increase the production of glu
conic acid from 490 to 685 g/kg. Singh et al. (78) per
formed SSF by using HCl pretreated sugarcane bagasse
and the highest level of gluconic acid (107 g/L) with 95 %
yield was obtained. In comparison with the submerged
culture, the degree of conversion was higher in SSF. The
increased rate of product formation might be due to the
variations of osmotic pressure, water content and dis-
solved oxygen. A study by Moksia et al. (79) used a two-
-step process, the first being the production of spores of
A. niger by SSF on buckwheat seeds, and the second
step, the bioconversion of glucose to gluconic acid by
the spores recovered from the SSF medium. The inter-
esting aspect about this work was that the spores were
not allowed to germinate as the bioconversion medium
did not contain any nitrogen source. The spores acted as
a biocatalyst, producing 200 g/L of gluconic acid with a
yield of 1.06 g per mass of glucose, very close to the
stoichiometric value.
Production of gluconic acid by bacteria
Acetic acid bacteria and Pseudomonas savastanoi were
the cultures initially observed to produce g luconic acid.
Unlike in fungi, in bacteria the reaction is carried out by
glucose dehydrogenase (GDH, E.C. that oxi
dises glucose to gluconic acid, which is further oxidised
to 2-ketogluconate by gluconic acid dehydrogenase
(GADH). The final oxidation step to 2,5-diketogluconic
acid (DKG) is mediated by 2-ketogluconate dehydroge
nase (KGDH). The reaction steps are shown in Fig. 4.
All three enzymes are localised in the membranes of the
cells and are induced by high glucose concentrations (>15
mM) (26). GDH is an extracellular protein and has PQQ
(pyrroloquinoline quinine) as a coenzyme. Also, there is
an intracellular enzyme, an NADP
-dependent glucose
dehydrogenase, which is less involved in the gluconic
acid formation when compared to the extracellular en
zyme. Gluconic acid produced is exported to the cell
and further catabolised via the reactions in pentose pho
sphate pathway. When the glucose concentration in the
medium is greater than 15 mM, pentose phosphate path
way is repressed and thus gluconic acid accumulation
takes place.
S. RAMACHANDRAN et al.: Gluconic Acid: A Review, Food Technol. Biotechnol. 44 (2) 185–195 (2006)
Gluconobacter oxydans is an obligate aerobic bacteri
um that oxidises glucose via two alternative pathways.
The first pathway requires an initial phosphorylation fol
lowed by oxidation via the pentose phosphate pathway.
The second is the »direct glucose oxidation« pathway,
which results in the formation of g luconic acid and ke-
togluconic acid (29). G. oxydans converts
D-glucose into
2,5-diketogluconic acid by the action of three membra
ne-bound NADP
-independent dehydrogenases as men-
tioned in Fig. 4. The acidotolerant acetic acid bacterium,
Acetobacter diazotrophicus, exhibited high rates of gluco-
nic acid formation. Glucose oxidation by the organism
was less sensitive to low pH values than glucose oxida-
tion by G. oxydans. Both the phosphorylative and direct
oxidative pathways of glucose metabolism appeared to
be operative. In addition to a pyridine nucleotide (strict-
ly NAD
)-dependent glucose dehydrogenase, A. diazotro-
phicus contained a PQQ-dependent glucose dehydroge-
nase, which was primarily responsible for gluconic acid
formation. Bacterial gluconic acid production has limi
ted success at industrial scale, as the oxidation proceeds
with the secondary reactions leading to oxogluconic ac
ids. The ability of Pseudomonas and Gluconobacter spp. to
produce gluconolactone and gluconic acid has been ex
ploited and the process is used commercially mainly in
the production of lactone.
Acetobacter methanolicus is also used to catalyze the
conversion of glucose into gluconic acid. The key advan
tage of using this facultatively methylotrophic microor
ganism as catalyst is that the gluconic acid formed is a
metabolic dead-end product, and unlike in other bacte
rial fermentation processes, organism uses methanol, a
cheap raw material as a substrate. Further in the process
glucose is not assimilated or consumed for growth, so
consequently the maximum theoretical yield coefficient
is achieved (80). A patent was filed by Currie and Carter
(81) in which the medium containing 200 g/L of glucose
with other nutrients and a neutralising agent was al
lowed to flow through a tower packed with wood shav
ings or coke, which had been inoculated with Acetobacter
suboxydans, while air was passed upwards through the
packing. Tsao and Kempe (82), working with Pseudomo
nas ovalis found that a particular strain could convert
glucose to gluconic acid with a yield of 99 %, and the
rate was directly related to the efficiency of aeration.
Research carried out by several authors (36,37,83,84)
utilised Aureobasidium pullulans, a yeastlike form of the
dimorphic fungi, for the production of gluconic acid. Va
rious process parameters for the continuous and dis
continous production of gluconic acid such as pH, oxy
gen, temperature and medium composition, air saturation,
etc. were studied (37,83,84). The highest glucose conver
sion of 94 % and product yield of 87.1 % was achieved
at an optimum pH of 6.5. At pH=4.5, the product selec
tivity and yield were very poor, reaching 67.8 and 20.7
%, respectively. Temperature range of 29 to 31 °C was
found to be suitable for the production of gluconic acid
by the yeast. Increase of temperature by 1 °C, namely to
32 °C, dramatically influenced the reduction in steady
state concentration of biomass and product.
Immobilisation techniques are involved where the
biomass is immobilised onto the support and, in some
cases, the enzyme isolated from the culture is immobi
lised. It enables repetitive use of the high biomass to
carry out biochemical reactions rapidly leading to pro
cess economy and stability. Immobilisation seems to be
an attractive method for accomplishing high cell densi
ties in order to achieve rapid carbohydrate conversion
to organic acids (66). Matrix immobilization is a simple
and easy technique by which mycelia are retained on a
matrix by mycelial entanglement. The type of support,
cell retention, stabilization of enzyme or the mycelia and
the quantum of biomass, etc. play important roles.
In the past, there were several investigations related
to the production of gluconic acid with immobilised cells
of A. niger. There are also reports of the immobilisation
of A. niger pellets by flocculation with polyelectrolytes
(85), calcium alginate (86), glycidyl ester copolymers (87)
and entrapment in gels (88). Glass rings were used to
immobilise A. niger for the production of gluconic acid
by Heinrich and Rehm (64). Sakurai et al. (55) adopted a
novel method for the immobilisation of A. niger using a
support of nonwoven fabric. Vassilev et al. (66) and Mu
khopadhyay et al. (68) reported the immobilisation of
the same filamentous fungi on polyurethane foam. Dif
ferent carriers such as calcium alginate agar, polyure
thane sponge, pearlite, and activated carbon were used
for the immobilisation of Penicillium variabile by Petru
ccioli et al. (32).
Free gluconic acid was continuously produced in an
aerated tubular immobilized cell bioreactor using G. oxy
dans for at least 6 months, with a volumetric productivi
ty of at least 5 g/(L·h) per 100 g/L of glucose substrate
and the concentration of produced gluconic acid of about
80 g/L (89). Spores of A. niger were immobilised on sin
tered glass, pumice stones and polyurethane foams, and
mycelia which developed on the pumice stone carrier
produced high extracellular glucose oxidase (80 %) when
compared to the enzyme activity on free cells (90).
An attempt was made by Sankpal et al. (91) to study
the bioconversion of glucose to gluconic acid using A. ni
ger immobilized on cellulosic fabric as a support matrix.
Glucose solution (100 g/L) was made to flow through
capillaries of a vertical fabric support, used for immobi
S. RAMACHANDRAN et al.: Gluconic Acid: A Review, Food Technol. Biotechnol. 44 (2) 185–195 (2006)
Fig. 4. Specific pathway for oxidation of glucose by Gluconobacter
lization, and was oxidized to gluconic acid at the inter
face. The system was found to run continuously for a
period of 61 days utilizing the entire available glucose.
The emerging broth contained a product concentration
of 120–140 g/L of gluconic acid, which was higher than
expected (maximum of 109 g/100 g of glucose), as a re
sult of evaporative concentration during the downward
flow. Sankpal and Kulkarni (92) found that the optimum
biomass requirement on a porous cellulose support was
0.234 mg/cm
for efficient bioconversion. Increasing the
quantum of biomass beyond this value resulted in an
overgrown biofilm which affected productivity adversely.
Morphological characteristics of immobilized A. niger have
also been investigated.
The recovery process depends on the method fol
lowed for broth neutralisation and the nature of carbon
sources used. Generally, the downstream process is simi
lar for the fermentation processes using fungal and bac
terial species. Gluconic acid, glucono-d-lactone, calcium
gluconate, and sodium gluconate are some of the impor
tant products and their extraction process is briefly men
tioned below.
For the recovery of free gluconic acid from calcium
gluconate the broth is clarified, decolorized, concen-
trated and exposed to –10 °C in the presence or absence
of alcohol. Thus the calcium salt of gluconic acid crystal-
lizes, then it is recovered and further purified. Gluconic
acid can also be obtained by precipitating the calcium
gluconate from hypersaturated solutions in the cold and
released subsequently by adding sulphuric acid stoichio-
metrically, removing the calcium as calcium sulphate.
Another method of passing the solution through a col-
umn containing a strong cation exchanger is also prac
tised where the calcium ions are absorbed.
For obtaining calcium gluconate as a product, cal
cium hydroxide or calcium carbonate is used as the neu
tralising agent. They are added to the nutritive broth ac
companied by heating and vigorous stirring. The broth
is concentrated to a hot supersaturated solution of cal
cium gluconate, followed by cooling at 20 °C, and add
ing water miscible solvents, which crystallises the com
pound. A treatment with activated carbon facilitates the
crystallisation process. Finally they are centrifuged,
washed several times and dried at 80 °C.
Sodium gluconate, the principal manufactured form
of gluconic acid, is prepared by ion exchange. In the pro
cess developed by Blom et al. in 1952 (15), the sodium
gluconate from the filtered fermented broth is concen
trated to 45 % (mass per volume), followed by the addi
tion of sodium hydroxide solution raising the pH to 7.5,
and drum drying. Carbon treatment of the hot solution
before drying process is practised for obtaining a re
fined product. Glucono-d-lactone recovery is a very sim
ple process. Aqueous solutions of gluconic acid are an
equilibrium mixture of glucono-d-lactone, glucono--lac
tone and gluconic acid. At temperature between 30–70
°C the crystal which is separated from the supersatura
ted solution is glucono-d-lactone. At temperature below
30 °C, gluconic acid results even above 70 °C, and the
resulting product would be glucono--lactone.
Molecular Biology
The molecular genetics of gluconic acid overproduc
tion is not very well investigated. It is well known that
the enzyme is actively induced by glucose concentration
and high aeration and pH above 4.0. The gene encoding
glucose oxidase of A. niger (gox A) has been cloned, and
its amplification resulted in a 2–3-fold increase in activi
ties (93–95). A. niger secretes multiple forms of catalases
to shield itself against the arising hydrogen peroxide (80),
among which one has been cloned and characterised (94).
Swart et al. (95) described nine different complementa
tion groups of glucose oxidase overproduction mutants.
Gox B, gox C, and gox F belong to linkage group 11, gox
1 to linkage group 111, gox D and gox G to linkage group
V, gox A and gox E to linkage group VII, and the linkage
of gox H is unknown. Their study also indicates that gox
A overproduction is regulated by the carbon source and
oxygen in an independent manner. Knowledge about
gene encoding lactonase is very narrow.
The gox-encoding gene of Penicillium variabile P16
was isolated and characterized to identify the molecular
bases of its high level of expression and in view of im-
proving enzyme production by developing a process
based on heterologous expression (96).
There are some works carried out on the bacterial
enzyme. A Tn5-induced glucose dehydrogenase (GDH)
deficient mutant of Gluconobacter oxydans IFO 3293 was
characterized. DNA sequencing showed that the inser-
tion site occurred in an open reading frame with homo-
logy to the pqqE gene. It was shown that acid produc-
tion could be restored by addition of the coenzyme PQQ
to the medium. The pqq cluster of G. oxydans ATCC 9937
was cloned and sequenced. It has five genes, pqqAE. The
cluster could complement the Tn5-induced mutation in
IFO 3293. Pulsed-field gel electrophoresis suggested that
the pqq genes are not closely linked to the ribF gene that
produces the riboflavin cofactor for the gluconic acid de
hydrogenase (97).
Although the production of gluconic acid is a sim
ple oxidation process that can be carried out by electro
chemical, biochemical or bioelectrochemical methods,
production by fermentation process involving fungi and
bacteria is well established commercially. Considerable
progress has been made in understanding the mecha
nism of fermentation process by different microorgan
isms, and highly efficient production process, which
dates back to five decades, has been developed. How
ever, development of novel, more economical process
for the conversion of glucose to gluconic acid with lon
ger shelf life would be promising. These requirements
could be met by enzymatic system. Another way of im
provement is to use cheap substrates, such as methanol
instead of glucose.
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Glukonska kiselina: svojstva, primjena i proizvodnja s pomoću
Glukonska kiselina je blaga organska kiselina dobivena iz glukoze jednostavnim
procesom oksidacije, koji pospješuju enzimi glukoza-oksidaza (iz plijesni) i glukoza-
dehidrogenaza (iz bakterija roda Gluconobacter). Posljednjih desetljeća prevladava postupak
proizvodnje glukonske kiseline s pomoću mikrooganizama, a od toga je najistraženija i
najraširenija biotehnološka proizvodnja s pomoću plijesni Aspergillus niger. Glukonska
kiselina i njezini derivati, od kojih je najvažniji natrijev glukonat, imaju raznovrsnu primjenu
u prehrambenoj i farmaceutskoj industriji. Ovaj rad donosi pregled proizvodnje glukonske
kiseline s pomoću mikroorganizama, njezina svojstva i primjenu.
... However, the production of this acid was already reported in several bacterial strains such as Pseudomonas and Acetobacter. By bacteria, the production of gluconic acid is induced by high glucose concentrations (Ramachandran et al., 2006). To date, no studies have reported the ability of Halomonas spp. to produce gluconic acid. ...
... However, no genes were found related to the expression of 6-phosphogluconate dehydrogenase which catalyzes the conversion of 6-phospho-d-gluconate into ribulose-5-phosphate through the PP pathway ( Figure 5). For this reason, 6-phosphogluconate is probably transformed into 2-keto-3-deoxy-6P-gluconate and subsequently cleaved into pyruvate and glyceraldehyde-3phosphate by the E-D pathway (Ramachandran et al., 2006). ...
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Polyhydroxyalkanoate (PHA) production using halophilic bacteria has been revisited because less severe operational conditions with respect to sterility can be applied, also alleviating production costs. Halomonas boliviensis was selected because it is a moderate halophile able to grow and attain high poly-3-hydroxybutyrate (P3HB) contents under 5–45 g/L NaCl concentrations, conditions that discourage microbial contamination. Industrial residues of the red alga Gelidium corneum after agar extraction were used as sugar platform to reduce costs associated with the carbon source. These residues still comprise a high carbohydrate content (30–40% w/w) of mainly cellulose, and their hydrolysates can be used as substrates for the bioproduction of value-added products. Preliminary assays using glucose were carried out to determine the best conditions for growth and P3HB production by H. boliviensis in bioreactor fed-batch cultivations. Two strategies were addressed, namely nitrogen or phosphorus limitation, to promote polymer accumulation. Similar P3HB cell contents of 50% (gpolymer/gCDW) and yields Y P3HB/glucose of 0.11–0.15 g polymer/g glucose were attained under both conditions. However, higher specific productivities were reached under P-limitation, and thus, this strategy was adopted in the subsequent study. Two organic acids, resulting from glucose metabolism, were identified to be gluconic and 2-oxoglutaric acid. Reducing the oxygen concentration in the cultivation medium to 5% sat was found to minimize organic acid production and enhance the yield of polymer on sugar to 0.20 gP3HB/gglucose. Finally, fed-batch cultivations using G. corneum hydrolysates as the only C-source achieved an overall volumetric productivity of 0.47 g/(L.h), 40% polymer accumulation, and negligible gluconic acid production.
... These sugar acids, being chemically similar to gluconic acid, are valuable raw materials for the cosmetic, pharmaceutical, food and construction industries and are of great economic value. (Ramachandran et al. 2006; Zhou et al. 2018). Microbial and enzymatic production of other sugar acids such as D-xylonate, Dgalactonate and L-arabonate has not been commercialized yet. ...
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The first hyperthermophilic L-arabinose/D-galactose 1-dehydrogenase ( Tm AraDH) from Thermotoga maritima was heterologously purified from Escherichia coli . It belongs to the Gfo/Idh/MocA protein family with NAD ⁺ /NADP ⁺ as a cofactor. The purified enzyme exhibited maximum activity toward L-arabinose at 75°C and pH 8.0, and retained 63.7% of its activity after 24 h at 60°C, and over 60% of its activity after holding a pH ranging from 7.0 to 9.0 for 1 h. Among all tested substrates, the purified Tm AraDH had highest specific activity towards L-arabinose, D-galactose and D-fucose. The catalytic efficiency ( k cat / K m ) towards L-arabinose and D-galactose was 123.85, 179.26 min − 1 mM − 1 for NAD ⁺ , and 56.06, 18.19 min − 1 mM − 1 for NADP ⁺ , respectively, and those for NAD ⁺ and NADP ⁺ were 374.64 and 2001.64 min − 1 mM − 1 using D-galactose, and 227.98 and 2661.27 min − 1 mM − 1 using L-arabinose, respectively. Tm AraDH exhibited complete conversion in 12 h at 70 o C to D-galactonate with 5 mM D-galactose. Modelling provides structural insights into the cofactor and substrate recognition specificity. Our results suggest that Tm AraDH have great potential for the conversion of L-arabinose and D-galactose into rare sugar acids.
... The products of glucose oxidation (gluconic and glucaric acids) are in-demand substances. In particular, gluconic acid is widely used in the food, pharmaceutical, metallurgical, and textile industries [1]. The demand for gluconic acid is constantly growing, and it is expected that by 2024, its consumption will reach 120 thousand tons per year [2]. ...
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Studies of the processes of the hydrolytic oxidation of disaccharides are the first step towards the development of technologies for the direct conversion of plant polysaccharides, primarily cellulose, into aldonic and aldaric acids, which are widely used in chemical synthesis and various industries. In this study, heterogeneous catalysts based on a porous matrix of hypercrosslinked polystyrene (HPS) and noble metals (Pt, Au, Ru, and Pd) were proposed for the hydrolytic oxidation of cellobiose to gluconic and glucaric acids. The catalysts were characterized using low-temperature nitrogen adsorption, hydrogen chemisorption, electron microscopy, and other methods. In particular, it was shown that the Pt-containing catalyst contained, on average, six times more active centers on the surface, which made it more promising for use in this reaction. At a temperature of 145 °C, an O2 pressure of 5 bars, and a substrate/catalyst weight ratio of 4/1, the yields of gluconic and glucaric acids reached 21.6 and 63.4%, respectively. Based on the data obtained, the mathematical model of the cellobiose hydrolytic oxidation kinetics in the presence of 3% Pt/HPS MN270 was developed, and the parameter estimation was carried out. The formal description of the kinetics of cellobiose hydrolytic oxidation was obtained.
... In addition, genes involved in the production of gluconic acid, an important organic acid responsible for mineral phosphate solubilization, were found in the beneficial strains. Such genes include glucose dehydrogenase (gcd) and those related to pyrroloquinoline quinine synthesis, a coenzyme for gcd activity [109], reported in ANP8, C1, and P5 strains [106][107][108]. Interestingly, ANP8 harbors several genes involved in regulating potassium concentrations, which the authors associated with the salinity tolerance potential of this strain [106]. ...
Full-text available
Multifaceted microorganisms such as the bacterium Pantoea colonize a wide range of habitats and can exhibit both beneficial and harmful behaviors, which provide new insights into microbial ecology. In the agricultural context, several strains of Pantoea spp. can promote plant growth through direct or indirect mechanisms. Members of this genus contribute to plant growth mainly by increasing the supply of nitrogen, solubilizing ammonia and inorganic phosphate, and producing phytohormones (e.g., auxins). Several other studies have shown the potential of strains of Pantoea spp. to induce systemic resistance and protection against pests and pathogenic microorganisms in cultivated plants. Strains of the species Pantoea agglomerans deserve attention as a pest and phytopathogen control agent. Several of them also possess a biotechnological potential for therapeutic purposes (e.g., immunomodulators) and are implicated in human infections. Thus, the differentiation between the harmful and beneficial strains of P. agglomerans is mandatory to apply this bacterium safely as a biofertilizer or biocontroller. This review specifically evaluates the potential of the strain-associated features of P. agglomerans for bioprospecting and agricultural applications through its biological versatility as well as clarifying its potential animal and human health risks from a genomic point of view.
... C 6 H 10 O 7 may be related to glucono-delta-lactone. This occurs due to the presence of glucose that reacts with atmospheric O 2 and oxidizes it to gluconic acid (present in up to 1% in honey), the amounts are greater due to the presence of the enzyme glucose oxidase, where oxidation of the aldehyde group on C-1 β -D-glucose to a carboxyl group causes the production of glucono-delta-lactone (C 6 H 10 O 6 ) and H 2 O 2 [85,86] (Table 5). ...
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Honeys can be classified as polyfloral or monofloral and have been extensively studied due to an increased interest in their consumption. There is concern with the correct identification of their flowering, the use of analyses that guarantee their physicochemical quality and the quantification of some compounds such as phenolics, to determine their antioxidant and antimicrobial action. This study aims at botanical identification, physicochemical analyses, and the determination of total polyphenols, chromatographic profile and antiradical and antimicrobial activity of honey from different regions of Minas Gerais. Seven different samples were analyzed for the presence of pollen, and color determination. The physicochemical analyses performed were total acidity, moisture, HMF, reducing sugar, and apparent sucrose. The compound profile was determined by UHPLC/MS, the determination of total phenolics and antiradical activity (DPPH method) were performed by spectrophotometry, and minimum inhibitory and bacterial concentrations were determined for cariogenic bacteria. All honey samples met the quality standards required by international legislation, twenty compounds were detected as the main ones, the polyfloral honey was the only honey that inhibited all of the bacteria tested. Sample M6 (Coffee) was the one with the highest amount of total polyphenols, while the lowest was M4 (Cipó-uva). Regarding the antioxidant activity, M5 (Velame) had the best result and M4 (Cipó-uva) was the one that least inhibited oxidation. Of the polyfloral honeys, there was not as high a concentration of phenolic compounds as in the others. Coffee, Aroeira, Velame and Polyfloral have the best anti-radical actions. Betônica, Aroeira, Cipó-uva and Pequi inhibited only some bacteria. The best bacterial inhibition results are from Polyfloral.
... It is an acid that has low toxicity and caustic action and is therefore capable of forming complexes with divalent and trivalent metals in aqueous solutions, especially iron, aluminum, zinc and calcium. used as chelators in the metallurgical and paper industries (Mafra 2013;Ramachandran et al. 2006). Gluconic acid and its derivatives are considered safe by the Food and Drug Administration (FDA) (Wong, Wong, and Chen 2008). ...
Full-text available
Lignocellulosic Lignocellulosic wastes from agroindustry are sources of available carbohydrates to produce second-generation ethanol (E2G) and other high added values substances (as organic acids). Among these residues, sugarcane bagasse, which is obtained from grinding of sugarcane through the extraction of the juice and which is already applied for E2G production, emerges as one of the main wastes. In this work, enzymatic hydrolysates of samples of pretreated sugarcane bagasse were processed to purification with active charcoal and subjected to catalytic oxidation in a three-phase reactor (slurry), using Pd /Al2O3 as a catalyst, aiming production of gluconic acid (GA), a product with a high added value, used in the textile and food industries. The catalytic oxidation of the hydrolysates which were purified under these conditions yielded a conversion of 70% of glucose and produced GA in a yield close to 80% in 4 hours. A mathematical model (Langmuir-Hinshelwood) studied for the catalytic oxidation of glucose from enzymatic hydrolysates proved to be reasonably adequate with the experimental data and reveals that the presence of acetic acid from the buffer solution of enzymes did not change the reaction mechanism pathway. Also, it was made a brief economic discussion of the feasibility of the process steps studied in this work based on the raw material and final product. These results point out sugarcane bagasse as a very profitable raw material for gluconic acid.
... Relatively, GA production by microbial fermentation is the most cost-effective approach with multiple advantages, such as cheap raw materials and microbial cells being easy to recycle and reuse. So far, many fungi and bacteria with the ability to produce GA have been reported [8][9][10]. Of them, Aspergillus niger is the most commonly used microbe for GA production, which could achieve a final 200 g/L GA with a yield of 1.025 (g/g) through submerged fermentation. ...
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Gluconic acid is a widely used food and beverage additive, but its production suffers from low efficiency and high cost. In this study, a preferable gluconic acid biosynthesis method without repeated seed culture was proposed and developed using the superior performance of Gluconobacter oxydans. A high oxygen atmosphere satisfying the demand of bio-oxidation increased the productivity of gluconic acid up to ~ 32 g/L/h and the accumulation up to ~ 420 g/L within 24 h of fed-batch fermentation. However, the productivity remarkably decreased when the gluconic acid content was over 350 g/L. Therefore, a continuous fermentation was designed, which in combination with 5 runs of fed-batch fermentation resulted in the final production of 1700 g gluconic acid from 1750 g glucose within 60 h in a 3 L bioreactor. The results suggest that the validity of this model and can enable cost-competitive gluconic acid production in the industry.
The current approach to gluconic acid production is acetification at 30°C, a temperature which can be difficult to maintain in tropical countries. This study investigated the production of gluconic acid during acetification by Acetobacter aceti WK at high temperatures. An acid‐tolerant and thermotolerant species, A. aceti WK, was used for acetification at three different temperatures, namely, 30°C (normal temperature), 37°C, and 40°C (high temperature). Acetification was performed in a 100 L bioreactor with 0.15% CaCl2 for protection of the cells against high temperatures. The production of the organic acids, i.e., acetic acid, gluconic acid, 2‐keto gluconic acid, glucuronic acid, citric acid, succinic acid, lactic acid, and formic acid, was analyzed. Under acetification in the target total concentration of 80 g/L, the highest acetic acid content (39.3 g/L) was obtained at 37°C with an acetification rate of 0.3013 g/L/h, while the acetic acid content and acetification rate achieved at 30°C were 31 g/L and 0.3089 g/L/h, respectively. Additionally, gluconic acid presented at the highest concentration of 2.17 g/L. The rate of production of gluconic acid was 0.0169 g/L/h at 37°C. This acetification process at 37°C will be valuable as an alternative source for gluconic acid production for commercial applications. This article is protected by copyright. All rights reserved
Aldonic acids are valuable raw materials in the pharmaceutical, food, cosmetic, and construction industries. Herein, we report the biocatalytic synthesis of aldonic acids using pyranose 2-oxidase (P2O) to catalyze the key oxidation step of the sugar precursors. The reactions of P2O with monosaccharides in the presence of oxygen as an electron acceptor can result in the 2-keto, 2-keto sugar lactone, and 2-keto sugar acids, which can be further reduced to yield the corresponding aldonic acids. These chemo-enzymatic reactions can convert D-galactose and L-arabinose to galactonic acid and arabinoic acid, respectively, with a >99% conversion. We used methylation and high-resolution mass spectrometry to identify the product structures. Results of molecular docking and structural analysis explain how P2O controls the mechanism and regio-selectivity of the second oxidation of keto-sugars. The monosaccharides with OH at the C4 equatorial position favor C3 second oxidation, while the monosaccharides with C4-OH at the axial position favor C1 second oxidation. The data presented here explain for the first time at the molecular level how a sugar oxidase such as P2O produces two double oxidation products when different sugars are used as substrates.
Soil salinization is a major stress affecting crop production on a global scale. Application of stress tolerant plant growth promoting rhizobacteria (PGPR) in saline soil can be an ideal practice for improving soil fertility. Rhizospheric microbiota of stress tolerant Eichhornia crassipes was screened for saline tolerant phosphate solubilizing bacteria, and the two isolates showing maximum solubilization index at 1 M NaCl were subjected to further analyses. The isolates were identified as Pantoea dispersa and Pseudomonas aeruginosa. Among the two isolates, P. dispersa PSB1 showed better phosphorus (P) solubilization potential under saline stress (335±30 mg/L) than P. aeruginosa PSB5 (200±24 mg/L). The mechanisms of P-solubilization, such as the production of organic acids and phosphatase were found to be influenced negatively by saline stress. The adaptive mechanisms of the isolates to overcome salt stress were analyzed by protein profiling which revealed salt stress induced modulations in protein expression involved in amino acid biosynthesis, carbon metabolisms, chemotaxis, and stress responses. Survival mechanisms such as protein RecA, LexA repressor and iron-sulfur cluster synthesis were upregulated in both the organisms under saline stress. P. dispersa PSB1 showed improved defense mechanisms such as the production of osmotolerants, redox enzymes, and quorum quenchers under saline stress, which may explain its better P solubilization potential than the P. aeruginosa PSB5. This study emphasizes the need for molecular approaches like proteome analysis of PGPR for identifying novel traits like stress tolerance and plant growth promotion before developing them as biofertilizers and biocontrol formulations.
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Enzymes are among the most important products, obtained for human needs through microbial sources. A large number of industrial processes in the areas of industrial, environmental and food biotechnology utilize enzymes at some stage or the other. Current developments in biotechnology are yielding new applications for enzymes. Solid state fermentation (SSF) holds tremendous potential for the production of enzymes. It can be of special interest in those processes where the crude fermented products may be used directly as enzyme sources. This review focuses on the production of various industrial enzymes by SSF processes. Following a brief discussion of the microorganisms and the substrates used in SSF systems, and aspects of the design of fermenter and the factors affecting production of enzymes, an illustrative survey is presented on various individual groups of enzymes such as cellulolytic, pectinolytic, ligninolytic, amylolytic and lipolytic enzymes, etc.
Gluconic acid, its delta -lactone, and sodium and calcium gluconates have been produced for 40 years by fermentation processes using strains of Aspergillus niger and Acetobacter suboxydans acting on glucose. The physical and chemical factors affecting these processes have been described and the necessary items of plant required outlined. The relatively simple dehydrogenation of D-glucopyranose to the intermediate D-glucono- delta -lactone by either glucose oxidase (A. niger) or glucose dehydrogenase (A. suboxydans) has been discussed and the chemical procedures used for producing 50% gluconic acid solution, gluconate salts, or glucono- delta -lactone, defined. The fields of usage of gluconic acid and its derivatives, which range from food to bottle washing, have been indicated.