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

  • Corresponding author: Yonghao NI, yonghao@unb.ca
  • Received Date: 2018-10-21
  • 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.
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Chitosan as A Preservative for Fruits and Vegetables: A Review on Chemistry and Antimicrobial Properties

    Corresponding author: Yonghao NI, yonghao@unb.ca
  • 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

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.

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 development and texture breakdown, are commonly seen on microbiologically spoiled food. Therefore, acceptable methods 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 derived preservatives 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, 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
  • 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.

    Figure 1.  Schematic flow chart of conventional process for chitosan production from shellfish

    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 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).

  • 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 preparations 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 chromatography and 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 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).

    Figure 2.  Cell wall architecture and antibacterial activity of chitosan against gram-negative bacteria

    Figure 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
  • 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.

    Figure 4.  Effect chitosan coating on fruits and vegetables

    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 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 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.

  • Amino groups of chitosan could be suitably modified to impart desired properties and distinctive biological 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 hydroglutamate 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).

  • 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), polyphenol oxidase, and phenylalanine 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 promotes 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-methylcycloprpene (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 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).

  • 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.

  • 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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