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Chitosan as A Preservative for Fruits and Vegetables: A Review on Chemistry and Antimicrobial Properties

Authors:

Abstract

Chitosan, derived from chitin, a major constituent (in quantity) of crustaceans, is a unique aminopolysaccharide with emerging commercial potential in agriculture, food, pharmaceuticals and nutraceuticals due to its nontoxic, biodegradable and biocompatable properties. Chitosan coating on fruits and vegetables has been found to be effective for the reduction of a variety of harmful micro-organims and extend the shelf-life of these products. In this review, our focus is on the antimicrobial properties of chitosan and its application as a natural preservative for fresh products. We detailed the key properties that are related to food preservation, the molecular mechanism of the antimicrobial activity of chitosan on fungi, gram-positive and gram-negative bacteria, coating methods for using chitosan and its formulation for preserving fruits and vegetables, as well as the radiation method of producing chitosan from chitin. Understanding the economic and scientific factors of chitosan's production and efficiency as a preservative will open its practical application for fruits and vegetable preservation.
Journal of Bioresources and Bioproducts. 2019, 4(1): 1121
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Review
DOI: 10.21967/jbb.v4i1.189
Chitosan as A Preservative for Fruits and Vegetables: A Review on Chemistry
and Antimicrobial Properties
Chao DUAN1, Xin MENG1, Jingru MENG1, Md. Iqbal Hassan KHAN2, Lei DAI1, Avik KHAN2, Xingye AN2, 3,
Junhua ZHANG2, 4, Tanzina HUQ2, Yonghao NI1, 2*
1 College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science & Technology, Xi’an 710021,
China;
2 Department of Chemical Engineering, University of New Brunswick, Fredericton, NB E3B 5A3, Canada;
3 Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, China;
4 Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China
*Corresponding author: Yonghao NI, yonghao@unb.ca
Received 21 October 2018; Accepted 10 December 2018
Abstract: Chitosan, derived from chitin, a major constituent (in quantity) of crustaceans, is a unique aminopolysaccharide
with emerging commercial potential in agriculture, food, pharmaceuticals and nutraceuticals due to its nontoxic,
biodegradable and biocompatable properties. Chitosan coating on fruits and vegetables has been found to be effective for
the reduction of a variety of harmful micro-organims and extend the shelf-life of these products. In this review, our focus
is on the antimicrobial properties of chitosan and its application as a natural preservative for fresh products. We detailed
the key properties that are related to food preservation, the molecular mechanism of the antimicrobial activity of chitosan
on fungi, gram-positive and gram-negative bacteria, coating methods for using chitosan and its formulation for preserving
fruits and vegetables, as well as the radiation method of producing chitosan from chitin. Understanding the economic and
scientific factors of chitosan’s production and efficiency as a preservative will open its practical application for fruits and
vegetable preservation.
Keywords: chitosan; food preservative; oligochitosan; coating; antimicrobial property
Citation: Duan Chao, Xin Meng, Jingru Meng, et al., 2019. Chitosan as a preservative for fruits and vegetables: a review
on chemistry and antimicrobial properties. Journal of Bioresources and Bioproducts, 4(1): 11–21.
1 Introduction
Freshness of fruits and vegetables (F&V) is an important
criterion that dictates which product a consumer prefers
to buy in the market. Supermarkets face challenges to
keep the F&V fresh and offer consumers better quality
products. The F&V are biodegradable and prone to
microbial attack. Challenges involving natural rippening
and the degradation process of the F&V, mainly through
enzymatic reaction, are an important concern for food
industries. The F&V are sensitive to decay and perish,
due to rapid ripening and softening, which limits their
storage, handling and transport potential (Hu et al., 2017).
Characteristics that lower the products quality, such as
browning, off-flavour deve l o pme nt an d tex t ur e
breakdown, are commonly seen on microbiologically
sp oiled food. Therefore, accep t able met hod s of
preservation are top priority in the food industry. Coating
the F&V with biocompatable nonallergic polymers is a
good choice for preservation. Inadequate and costly
solutions for food preservation has led scientists to create
natural preservatives which are safe, effective, and
acceptable (Huq et al., 2015). Keeping in mind relatively
long time storage and transportation, use of biologically
d e r i v e d p r e s e r v a t i v e s
with compliance to health and safety regulations can
bring a great solution for preservation of the F&V
(Romanazzi et al., 2017).
One such strategy that has been of recent interest is the
edible coatings in the fresh fruit industry to reduce the
deleterious effects that could take place on intact
vegetable tissues which are usually subject to minimal
processing. Preservatives, either chemically synthesized
or originating from nature, should meet the following
criterion: 1) efficient against a broad range of spoilage
organisms; 2) tasteless and odorless; 3) non-toxic; 4) safe;
and 5) inexpensive. The general perishability of the F&V,
varying from their sensitivity to decay induced by
enzymatic actions, is mainly due to the rapid ripening
process thus limiting the storage, handling and transport
potential of the F&V (Lee et al., 2015).
Edible coating, an innovative method of food
preservation, produces physical barriers on the surface of
the F&V that cause moisture and solute to migration, gas
exchange, respiration and oxidative reaction rates reduced
for extending the shelf-life (Arnon et al., 2014).
Biopolymer coating materials are formulated to carry
active ingredients such as antibrowning agents, colorants,
This is an Open Access article under the CC-BY-NC license.
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flavours, nutrients, spices and antimicrobial compounds
to extend product shelf life and reduce the risk of
pathogen growth on food surfaces (Pranoto et al., 2005).
Chitosan, a partially deacetylated derivative of chitin,
is a hetero-polysaccharide composed of 2-amino-deoxy-
β-D-glucopyranose and 2-acetamido-deoxy-β-D-glu-
copyranose (chitin) residue (Khan et al., 2014). The major
property of chitosan is dictated by the presence of three
different functional groups (primary —OH, secondary
—OH and —NH2) and its water solubility in acidic pH.
Due to the presence of reactive groups, it inhibits
the growth of a wide variety of bacteria and fungi
(Hosseinnejad and Jafari, 2016). Chitosan has many
different applications and can be utilized for developing
various formulations (Fatehi et al., 2010). Chitosan-based
edible coatings can also be used as carriers of food
ingredients (antimicrobials, texture enhancers and
nutraceuticals) to improve the safety, quality and
functionality of the F&V. Edible coating without
disturbing sensory quality and nutritional value of the
F&V needs further scientific research (Martins et al.,
2014).
Chitosan/chitin refers to one of the most abundant
natural polysaccharides in nature (Lavall et al., 2007). It
can be obtained from several different sources, but the
main source of chitosan is usually marine crustacean’s
shells (Arbia et al., 2013). Atlantic Canada, with its long
coastline, offers a great source of different marine crusta-
ceans, i.e., shrimp, lobster, crab, etc. that can be utilized
for the extraction of chitosan. Aquaculture industries in
Atlantic Canada are growing fast to meet current demand.
However, disposal of crustacian shells is an environ-
mental concern for aquaculture industries and recovery of
these shells from various sources can leverage industries for
producing chitosan and its derivatives at competitive costs.
2 Properties of Chitosan
2.1 Solubility and deacetylation of chitosan
Chitosan is derived by modifying chitin structure through
removal of the acetyl groups, which are bonded to amine
radicals in the C2 position on the glucan ring (Fig. 1).
Chemical hydrolysis in concentrated alkaline solution is
performed at elevated temperatures to produce a partially
deacetylated form of chitin referred to as chitosan. Chito-
san preparations differ by the degree of deacetylation.
While chitin is insoluble in most organic solvents,
chitosan is readily soluble in diluted acidic solutions
below pH 6.0. This is because chitosan can be considered
a strong base as it possesses primary amino groups with a
pKa value of 6.3. The presence of the amino groups
indicates that pH substantially alters the charged state and
Chao DUAN et al.: Chitosan as A Preservative for Fruits and Vegetables: A Review on Chemistry and Antimicrobial Properties
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Fig. 1 Schematic flow chart of conventional process for chitosan production from shellfish
properties of chitosan (Elsabee and Abdou, 2013). At low
pH, these amines get protonated and become positively
charged and that makes chitosan a water-soluble cationic
polyelectrolyte. On the other hand, as the pH increases
above 6, chitosan’s amines become deprotonated and the
polymer loses its charge and becomes insoluble. The
soluble-insoluble transition occurs at its pKa value around
pH between 6.0 and 6.5. The solubility of chitosan is
dependent on the degree of deacetylation and the method
of deacetylation used (Cho et al., 2000). Whereas the
degree of ionization depends on the pH and the pKa of
the acid (acetic acid and hydrochloric acid) which causes
protonation of chitosan in the presence of acetic acid and
hydrochloric acid. Chitosan can easily form quaternary
ammonium salts at low pH values. So, organic acids such
as acetic, formic, and lactic acids can dissolve chitosan
(Cruz-Romero et al., 2013). The best solvent for chitosan
was found to be formic acid although the most commonly
used solvent is 1% acetic acid (as a reference) at about pH
4.0. Chitosan is also soluble in 1% hydrochloric acid and
dilute nitric acid but insoluble in sulfuric and phosphoric
acids. Concentrated acetic acid solutions at high
temperatures can cause depolymerization of chitosan
(Pillai et al., 2009). Combination of studies on intrinsic
viscosity, Fourier transform infrared spectroscopy (FT-IR),
and powder X-ray diffraction (XRD) showed that the
molecular weight and degree of deacetylation are
collectively responsible for the solubility which could be
due to intermolecular force (Shrinivas et al., 2007). It was
concluded that the solution properties of chitosan depend
not only on its average deacetylation, but also on the
distribution of the acetyl groups in the main chain
(Younes and Rinaudo, 2015). However, apart from the
deacetylation, the molecular weight is also an important
parameter that significantly controls the solubility and
antimicrobial properties of chitosan (Chang et al., 2015).
2.2 Molecular weight and depolymerization of chitosan
Although the chemical and physical processes control
some of the applications of chitosan and its derivatives,
considerable evidence has been gathered indicating that
most of their physiological activities and functional
properties depend on their molecular weight (MW) (Cota-
Arriola et al., 2013). It has been reported that the
distribution of chitosan prepar ations of different
molecular weight is influenced by the conditions
employed in the deacetylation process. The molecular
weight ranges from several hundred to over one million
Dalton (Mu) are common, with a mean molecular mass of
up to 1 Mu, corresponding to a chain length of
approximately 5000 U (Rhoades and Roller, 2000). There
are methods for determining the MW of chitosan such as
light scattering spectrophotometry, gel permeation
c h r o m a t o g r a p h y a n d
viscometry, and gel permeation chromatography is the
most widely used method (Niebel et al., 2014).
Depolymerization of chitosan using gamma irradiation is
a recent and useful approach. However, high doses of
gamma irradiation cause degradation of chitosan and
produces chitosan with relatively lower molecular weight
(Martínez- Morlanes et al., 2011). Degradation of chitosan
due to main-chain scission leads to the opposite effect on
the mechanical properties, and eventually produces soft,
gummy or tar like materials. When chitosan is irradiated,
both crosslinking and degradation often occur
simultaneously. Irradiation also brings about significant
changes in physicochemical, thermal and morphological
properties of chitosan which provides great potential for
many applications (Rashid et al., 2012). Among various
techniques used for the modification of polymer
properties, the use of ionizing radiation either in photonic
(gamma radiation, X-rays) or particulate forms
(accelerated electrons, ion beams) has proven to be a very
convenient technique (Senna et al., 2010). Since
molecular weight dictates physicochemical properties of
polymers, the control of molecular weight and its
distribution are of great importance in determining the
technical specifications required for an end-use (Huq et
al., 2012). Low MW chitosan is degraded at lower
temperatures than the higher MW. The interactions
between the molecules become weaker due to irradiation
and less energy is required for the thermal movement
(García et al., 2015). Irradiation also has an effect on
mechanical properties of chitosan films. It has been
reported that decreasing the MW increased the tensile
properties whereas the elongation breaks decreased
(García et al., 2015). Extensive study on the relationship
among molecular weight, water-solubility and
antimicrobial activity of chitosan is lacking until now.
Though it has been reported that molecular weight could
affect the antimicrobial properties of chitosan (Kim et al.,
2011; Wang et al., 2017).
3 Mechanism of Antimicrobial Activity of Chitosan
Chitosan is cationic biopolymer, having antimicrobial
properties which can be affected by pH, concentration,
molecular weight, degree of polymerization and cross-
linking (Elsabee and Abdou, 2013). Chitosan solution is
highly stable over a long period of time, however its
stability in neutral pH is highly important for exhibiting
antimicrobial activity against a wide variety of foodborne
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pathogens (Alishahi and Aïder, 2012). The major
mechanism of action in antimicrobial activity involves
interaction with bacterial cell wall, cell membrane and
cytoplasmic constituents via electrostatic interactions.
Chitosan has been found to be effective against both
gram-positive and gram-negative bacteria. The outer
membrane (OM) of gram-negative bacteria such as,
Escherichia coli, is composed of an asymmetric lipid-
protein bilayer (lipopolysaccharide, LPS). The divalent
cations (i.e., Ca2+, Mg2+) present in the OM play an
important role in the stabilization of the core anionic
charges of the LPS molecules (Terry, 1999; Khan et al.,
2015). It can be hypothesized that chitosan replaces the
divalent cations from their binding sites and reduces the
interaction between the LPS molecules, causing
membrane disruption and cell lysis due to penetration
(through electrostatic interaction) of positively charged
chitosan through cell membrane of gram-negative
bacteria (Fig. 2). Unlike gram-negative bacteria, the
gram-positive bacteria do not have an outer membrane.
Hence, chitosan as a poly- cationic long chain molecule
can adhere better with gram-positive bacterial members
such as Staphylococcus aureus. For this reason, the
inhibition effort from chitosan is more effective against
gram-positive bacteria than gram-negative bacteria (Fig.
3). In the literature, it was reported that gram-positive
bacteria containing teichoic acid and lipoteichoic acid that
are poly-anionic surface polymers, interact with
intracellular substances, so that the vital bacterial
activities are impaired (Aranda-Martinez et al., 2016).
Fig. 2 Cell wall architecture and antibacterial activity of
chitosan against gram-negative bacteria
Chao DUAN et al.: Chitosan as A Preservative for Fruits and Vegetables: A Review on Chemistry and Antimicrobial Properties
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Fig. 3 Cell wall architecture and antibacterial activity of
chitosan against gram-positive bacteria
Raafat et al. (2008) reported simultaneous permeation
of the cell membrane to small cellular components,
coupled to a significant membrane depolarization. No
concomitant cell wall biosynthesis was observed. Later
they analyzed multiple changes in the expression profile
of S. aureus SG11 genes which are involved with the
regulation of stress and autolysis and the genes involved
with energy metabolism and postulated a possible
mechanism for chitosan’s activity. Chung and Chen (2008)
found that the removal ratios of chitosan for S. aureus
protoplasts and E. coli spheroplasts were significantly
higher than those for intact cells during the first 3–4 h of
contact time, indicating chitosan-cell wall interaction is
more intense than other cell membranes.
In another study, chitosan in gel form was used in the
antibacterial tests carried out by turbidity and well
inhibition zone showing chitosan consistently more active
against the gram-positive S. aureus than gram-negative E.
coli (Goy et al., 2016). Morimoto et al. (2001) reported
that chitosan derivatives have better specific binding
activity on the cell wall of Pseudomonas aerogenosa. Lal
et al. (2013) reported the interaction of chitosan with cell
surface polymers such as teichoic acid of gram-positive
bacteria, which is consistent with the fact that binding of
chitosan with the lipopolysaccharide layer of a gram-
negative bacterial cell wall, would not significantly affect
the susceptibility. However, adherence due to electrostatic
interaction may cause secondary effects on cytoplasmic
membrane such as disruption of cell membrane, which
finally results in leakage of small cellular components.
The similarity between the antibacterial profiles and
patterns of chitosan and those of two other control
substances, polymyxin and ethylene diamine tetraacetic
acid (EDTA), verified amino group assisted mechanism
of chitosan. Helande et al. (2001) detailed the specific
binding mechanism of chitosan on gram-negative bacteria
that relates weakening of barrier function of outer
membrane. Chemical and electrophoretic analyses of free
cell supernatants of chitosan-treated cell suspensions
showed that interaction of chitosan with E. coli and S.
typhimurioum involved no release of the LPS or other
membrane lipids. This was further evidenced by using
highly cationic mutants of S. typhimurioum which was
more resistant to chitosan than parent strains. In the same
study, they found chitosan caused extensive cell surface
alterations and covered the outer membrane with
vesicular structures as shown in their electron microscopy
study (Helander et al., 2001). The activity of chitosan on
gram-positive and gram-negative bacteria was again
evidenced, and established that chitosan in acid pH is
extensively protonated and bound with carboxyl and
phosphate groups of the bacterial surface which offers
potential sites for electrostatic binding of chitosan (Li et
al., 2016). It should be noted that chitosan shows broad
spectrum activity on microorganisms except the fungi
which contain chitosan as wall constituent. Chitosan’s
antimicrobial activity (as preservative) is often limited to
food, such as the F&V products with low protein and
NaCl content (Roller and Covill, 1999).
4 Application of Chitosan as a Food Preservative
4.1 Edible coatings and films
The use of chitosan is widely investigated as an edible
coating, which is defined as the formation of a thin film
directly on the surface of the product they are intended to
protect. Edible coatings/films form a protective barrier
around the F&V and can be consumed along with the
coated product (Kerch, 2015). In the F&V preservative
applications, the creation of a moisture and gas barrier may
lead to weight loss and respiration rate reductions with a
consequent general delay in spoilage, which will extend
the shelf-life of the product (Chien et al., 2007). Water
vapor permeability (WVP) and oxygen permeability (OP)
are the barrier properties commonly studied to determine
the ability to protect foods from the environment
(Valencia-Chamorro et al., 2011). Edible chitosan films
are extremely good barriers for permeation of oxygen,
while exhibiting relatively low water vapor barrier
characteristics (Khan et al., 2012). Films and coatings
develop selective permeability characteristics, especially
to O2, CO2 and ethylene and allow some control of fruits
respiration and reduce growth of microorganisms
(Dominguez-Martinez et al., 2017). Coating practice has
long been followed for preservation of citrus, apples
(shellac and carnauba wax), tomatoes (mineral oil) and
cucumbers (various waxes). Schematic diagram of the
effect of chitosan coating on the physiological properties
of fruits and vegetables is shown in Fig. 4.
According to El Ghaouth et al. (1992), chitosan coated
tomatoes were prevented from attacks of Penicillium spp.,
Aspergillus spp., Rhizopus stolonifer and Botrytis cinerea.
Moreover, Chitosan has itself, the ability to control some
fungal diseases, which deteriorate fruits quality during
storage (Romanazzi et al., 2013). Studies were also carried
out on papaya (Carica papaya L.), which incur 20%–30%
loss post-harvest stage (Romanazzi et al., 2017). Post-
harvest deterioration of papaya is a microbiological
process; the fruits become a target of several pathogens in
the market thus decreasing its acceptability and shelf life.
The storage life of papaya was extended up to 33 through
the use of calcium chloride (2%) with chitosan coatings on
fruits. Kim et al. (2011) reported that low MW chitosan
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showed better antimicrobial activity against E. coli
whereas chitosan samples with broad MW range (1–1671 ku)
were effective to inhibit Listeria monocytogenes. Wang
et al. (2017) also reported strong in vitro and in vivo
antifungal activity of chitosan samples with low MW (50
ku). The in vivo antifungal activity was tested on pear
fruits. It was reported that the fungal proliferation was
much lower for the pear fruits coated with chitosan. The
coating also reduced the rate of browning and
demonstrated a dose dependent reduction in browning
incidence over a 34-day storage. Similarly, chitosan
coating was found to be effective to enhance the firmness,
delay ripening and reduce browning of peach fruits (Ma
et al., 2013).
Raafat et al. (2008) demonstrated and performed
challenge tests to establish chitosan’s activity against
potential bacterial contaminants for up to 28 d. Another
good example of chitosan’s use in coating application is in
the preservation of mango (Abbasi et al., 2009).
Biochemical reactions are involved with the ripening
process and softening of mango texture which results in a
series of consequences such as increased respiration,
ethylene production, change in structural polysaccharides
causing softening, degradation of chlorophyll, developing
pigments by carotenoids biosynthesis, change in
carbohydrates or starch conversion into sugars, organic
acids, lipids, phenolics and volatile compounds (Abbasi et
al., 2009). The chitosan-based coating can effectively slow
down these processes.
As discussed, coating and film are the most popular
Fig. 4 Effect chitosan coating on fruits and vegetables
methods of using chitosan for food/fruit preservation.
With the continuing increased demands of using natural
preservatives, chitosan may be added as an ingredient to
food/fruits. Literature review on this topic is available.
For example, Sharif et al. (2017) published an excellent
review with detailed preservation methods, focusing on
the use of natural preservatives as alternative to artificial
preservatives.
4.2 Chitosan derivatives
Amino groups of chitosan could be suitably modified to
impart desired properties and distinctive biological
Chao DUAN et al.: Chitosan as A Preservative for Fruits and Vegetables: A Review on Chemistry and Antimicrobial Properties
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functions to chitosan. Chemical modification of chitosan
has been discussed in the context of functionality (Islam
et al., 2017). The unique amino group’s functionality
involves chemical reactions such as acetylation,
quaternization, reactions with aldehydes and ketones,
alkylation, grafting, chelation of metals, etc. resulting in a
variety of products exhibiting the properties such as
antibacterial, anti-fungal, anti-viral, anti-acid, non-toxic,
non-allergenic, biocompatibility and biodegradability, etc.
On the other hand, the hydroxyl functional groups also
give various reactions such as O-acetylation, H-bonding
with polar atoms, grafting, etc. (Oyervides-Muñoz et al.,
2017).
One such chitosan derivative, carboxymethyl chitosan,
of different molecular weights was prepared and was
applied on peaches using a dipping treatment. The authors
found that low molecular weight chitosan has better
preservative and antioxidant activity than that of high
molecular weight (Elbarbary and Mostafa, 2014). Since
chitosan and its derivatives have solubility in acetic acid
which has inherent antimicrobial activity, it is important
to determine the allowable concentration of acetic acid.
Liu et al. (2006) determined that acetic acid concentration
more than 0.02% (i.e., 0.05%–0.10%) is bactericidal
against E. coli whereas at low concentrations below
0.02% acetic acid had no antibacterial activities. The low
molecular weight chitosan concentration (more than
0.005%) showed exceeding activity over acetic acid and
chitosan’s concentration over 0.02%, and it killed almost
all bacteria.
Water soluble chitosan derivatives such as ethylamine
hydroxyethyl chitosan, chitosan lactate, chitosan hyd-
roglutamate were prepared and their activity was tested
against S. aureus, Listeria monocytogenes, Bacillus
cereus, E. coli, Shigella dysenteriae and Salomonella
typhimurium (Chung et al., 2011). The authors found that
chitosan-glucosamine derivative showed relatively higher
activity than the acid soluble chitosan. Their results
showed that these derivatives may be a promising
commercial substitute for acid-soluble chitosan. Schiff
base types of chitosan-saccharide derivatives, were
identified as good performers against various bacteria
(Ying et al., 2011). Benhabiles et al. (2013) developed N,
O-carboxymethyl chitosan (NOCC), and found that
tomatoes coated with the NOCC solutions had extended
shelf life. Several oligochitosan based formulations have
shown potential antimicrobial activity. Essential oils,
organic acids, inorganic compounds, inorganic
nanoparticles and composites enhance the antimicrobial
activity of chitosan. Lemon essential oils enhanced
chitosan’s activity against fungi on strawberries
inoculated with a spore suspension of Botrytis cinerea
(Perdones et al., 2012).
4.3 Chitosan Based Formulations
Chitosan was used to determine the synergistic activity
with sulfamethoxazole, a sulfonamide antimicrobial agent
(Lal et al., 2013). They used and compared wild and
mutant P. aeruginosa to unveil the efflux mechanism of
chitosan in combination with Sulphamethoxazole. Other
than benzoate, Chitosan has been reported to potentiate
the antimicrobial activity for a number of other
preservatives such as phenethyl alcohol, benzoic acid and
phenylmercuric acetate against a number of test strains
(Lei et al., 2014).
The effect of chitosan coating in combination with
phytic acid in fresh cut lotus root preservation has been
investigated and the results showed decreased weight loss,
postponed browning, restrained activities of peroxidase
(POD), polyp h enol oxidas e , an d phe n yl alanine
ammonia-lysae and increased content of vitamin C and
polyphenol (Yu, 2012a). Romanazzi et al. (2017) reported
that chitosan mixed with ethanol, wax and similar types
of organic materials, improved the protecting effect on
grapes from gray mold compared to the application
chitosan alone. Inorganic compounds such as calcium
ions (calcium gluconate) in combination with chitosan
helped the structural integrity of fruits and vegetable
membranes thus maintaining firmness of fruits skin in
addition to reducing fungal incidences (Kou et al., 2014).
Calcium gluconate at concentration of 0.5% with 1.5% or
1.0% with chitosan showed better antifungal activity than
chitosan only. The incorporation of calcium ions in fruits
tissue promo tes new crosslinks between anionic
homo-galacturonans, strengthening the cell wall and
particularly the middle lamella. The complex of zinc and
cerium in combination with chitosan was used for shelf
life extension of Chinese jejube fruits (Wu et al., 2010).
Addition of nano-silicon dioxide (0.04%) in 1%
chitosan solution decreased weight loss and respiration
rates, indicating that chitosan’s filmogenic property was
increased (Yu et al., 2012b). Reports are available on
combination treatment i.e., chitosan-heat, chitosan-1-met-
hylcycloprpene (1-MCP, ethylene inhibitor), chitosan and
modified packaging etc. (Zhong and Xia, 2007). In all
these cases, chitosan’s activity was increased compared to
the activity of chitosan alone. Coating on citrus fruits
following a layer by layer approach such as using
cellulose derivatives, methylcellulose, hydroxypropyl
cellulose, carboxymethyl cellulose and chitosan coatings,
were investigated, and the results showed that
carboxymethyl cellulose as internal layer and chitosan as
external layer gave the best performance for keeping
mandarins unaffected (Arnon et al., 2015). They also used
several formulations of carboxymethyl cellulose, with
steric acid, oleic acid, glycerol, and the results were
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compared with commercial wax. Several parameters of
fruits such as firmness, weight loss, ethanol concentration,
external appearance, ripening progression, sensory
evaluations, disease incidence etc. were considered and
they found the presence of chitosan coating contributed to
fruits preservation very effectively (Zhong and Xia, 2007).
The antimicrobial activity and minimum inhibitory
concentration of chitosan glucose complex as a novel
preservative was estimated and found effective against
common spoilage microorganisms for fruits and
vegetables such as E. coli, Psudomonas, S. aureus and B.
cereus (Jiang et al., 2012). Chitosan-glucose complex was
found to have both antioxidant and antimicrobial activity
and thus a promising novel preservative for various fruits.
Combined treatment of UV and coating with 1.5%
Chitosan resulted in decrease of decay for Jejubes and
restrained increased respiration rate, weight loss,
malonaldehyde content and electrolyte leakage (Zhang et
al., 2014).
4.4 Irradiated chitosan
Different types of irradiated chitosan coatings were
studied for enhancing the shelf life and improving quality
of mangos. The effect of coating with irradiated crab and
shrimp chitosan (MW=5.14×104) and un-irradiated crab
chitosan (MW=2.61×105) on postharvest preservation of
mangos (Mangifera indica L.) was studied (Abbasi, 2009)
and results showed effectiveness at an appreciable level.
The effect of both control and irradiated chitosan was
observed on the fruits-spoiling fungi (Colletotrichum
gleosporioides). The percentages of spoiled fruits were
13.3% and 6.9% respectively, for untreated and treated
mangoes after 14 d of storage. At the end of storage, the
control fruits were fully spoiled. However, 75% of
irradiated chitosan coated fruits were not attacked by
diseases. In another study (Oyervides-Muñoz et al., 2017),
it was reported that the application of irradiated chitosan
was effective on preservation of fresh fruits, as well as
limiting the growth of fungi without affecting ripening
characteristics of fruits. In a recent study, the use of a
150ku MW chitosan for coating over papaya was
compared to chitosan of 300ku MW (Dotto et al., 2015)
and an increased shelf life of papayas at ambient storage
temperatures was found and the count of mesophilic
bacteria, yeasts and molds were substantially decreased.
They explained that chitosan of 150 ku has less organized
structure with lower crystallinity and a rough surface with
protuberances and cavities which improved its solubility
in acid solvent, forming a more homogeneous solution
and consequently a more homogeneous coating was
obtained.
4.5 Toxicity of chitosan
The toxicity of chitosan and its derivatives has been a
well- studied subject, and a thorough review paper on the
topic was competed by Kean and Thanou (2010).
Chitosan is a non-toxic, biologically compatible polymer.
Its use for dietary applications is well known in many
different countries and it has been approved by the Food
and Drug Administration for use in wound dressings.
5 Summary
Chitosan, due to having amino groups available to
interact with microbial cell walls when applied to fruits
and vegetables, causes ultimate death of bacteria and
fungi through cell lysis mechanisms. Chitosan can have
interactions with selective microorganisms, and the
chitosan based formulations have been studied for
augmenting the activity of chitosan for fruits and
vegetable preservation. In many reports, chitosan has
shown to be an effective natural antimicrobial agent based
on the electrostatic mechanism, and control of respiration
rate, weight loss and water loss, without affecting taste,
odor and palatability of skinned and fresh cut fruits and
vegetables. The polymeric nature of chitosan is
structurally tuned in utilizing antimicrobial properties
against microorganisms, it can also produce a protective
barrier on fruits and vegetables. Although both
gram-positive and gram-negative bacteria are sensitive to
the antimicrobial activity of chitosan, the former is more
sensitive than the latter due to the difference of molecular
architectural features in cell walls (negative charge
distribution and availability). Optimization of molecular
weight in relation to antimicrobial activity and further
understanding molecular mechanisms may pave the way
for commercial technologies to use chitosan for
preservation of fruits and vegetables. Irradiation is a good
choice for producing low MW chitosan that not only
gives oligo-chitosan, but also influences deacetylation
without leaving any chemical residues. Radiation has a
good sterilization effect as well. In this way, chitosan can
be produced cost effectively with a high degree of
deacetylation using green technology. Technological
parameters, costs, environmental impact or details about
producing of chitosan are presented in this review,
especially industrial methods or with radiation
application.
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