Bread The shelf life of bread is generally limited due to staling and microbial growth (Leuschner and others 1999). Staling is a general term that describes the time-dependent loss in quality of flavor and texture of bread. Bread staling is a complex phenomenon in which multiple mechanisms operate (Gray and Bemiller 2003).
Applications of chitosan for extension of shelf life of bread by retarding starch retrogradation and/or by inhibiting microbial growth have been documented. Park and others (2002b) investigated the effect of chitosan (493 kDa) coating on shelf life of baguette. The surface of dough was coated with 0.5%, 1.0%, or 1.5% chitosan in 1.0% acetic acid using a brush after molding. Baguette coated with chitosan, especially with 1% chitosan, showed less weight loss, hardness, and retrogradation than the control during storage for 36 h at 25 °C. This is likely due to the moisture barrier properties of chitosan (Butler and others 1996; Nadarajah and others 2006). Chitosan coating may offer a protective barrier for moisture transfer through the bread surface, thus reducing weight loss and retarding hardness and retrogradation. By this, the shelf life (36 h) of 1% chitosan-treated baguette was extended by 24 h compared with that (12 h) of the control. In a separate experiment, Park and others (2002c) also found that the shelf life of baguette coated with 1% chitosan oligomer (2 kDa) dissolved in distilled water could be extended by 24 h compared with that (12 h) of the control, as similarly observed by Park and others (2002b) with 493-kDa chitosan.
Ahn and others (2003) reported that chitosan coating improved the shelf life and quality of bread by inhibiting microbial growth and by retarding antioxidation and retrogradation. Bread coated with 1% and 2% chitosan (120 kDa, DD = 85%) dissolved in 0.3% lactic acid showed lower total bacterial counts and thiobarbituric acid-reactive substances (TBARS) and higher water content than those of the control after 8 d of storage at room temperature. Mold growth was detected in the control after 4 d of storage, but was not detected in bread coated with 1% and 2% chitosan throughout 8 d of storage. In other experiments, Lee and Lee (1997) reported that the shelf life of fermented pan bread containing carboxymethyl chitosan was extended by retarding retrogradation and by inhibiting growth of microorganisms. Therefore, the improved shelf life and quality of bread by chitosan coating or addition is attributed to moisture barrier property and ability to retard retrogradation and microbial growth of chitosan.
Molecular weight and concentration of chitosan may affect the shelf life of bread. Lee and others (2002a) investigated the effects of chitosans with different molecular weights (Mw = 1, 5, 30, and 120 kDa) at various concentrations on the shelf life of wheat bread. In these studies, lower Mw (1 and 5 kDa; both DD = 95%) and higher Mw (30 and 120 kDa; DD = 92% and 85%, respectively) chitosans were dissolved in 0.3% lactic acid, and added to dough at 0.01%, 0.1%, 0.3%, and 0.5% concentrations. Chitosans with higher MW (30 and 120 kDa) were better than those with lower Mw (1 and 5 kDa) in extending the shelf life of bread. With 30 and 120 kDa chitosans, increase in chitosan concentration generally showed increased bacterial growth inhibition in bread. The bread containing 30 or 120 kDa chitosan at above 0.1% concentration showed 101 to 103 CFU/mL viable cells while the control bread containing no chitosan revealed 106 CFU/mL after storage for 8 d at room temperature.
Egg Several problems are encountered during storage of eggs, such as weight loss, interior quality deterioration, and microbial contamination (Bhale and others 2003; De Reu and others 2006). The movement of carbon dioxide and moisture from the albumen through the shell governs quality changes in albumen and yolk, and weight loss of eggs (Murray and Rutherford 1963; Stadelman 1986b). To overcome these problems, considerable attention has been given to the development of coating materials from synthetic polymers (Meyer and Spencer 1973), polysaccharides (Xie and others 2002), proteins (Herald and others 1995; Cho and others 2002; Xie and others 2002; Rhim and others 2004), and oils (Knight and others 1972; Obanu and Mpieri 1984). Chitosan has been documented to possess a film-forming property for use as edible films or coatings (Butler and others 1996; Jeon and others 2002; Nadarajah and others 2006). Thus, chitosan coating may offer a protective barrier for moisture and gas transfer from the albumen through the egg shell, thus extending the shelf life of eggs (Lee and others 1996; Bhale and others 2003).
Yolk index and Haugh unit values are indices of freshness of yolk and albumen quality of egg. The Haugh unit is an expression relating egg weight and height of the thick albumen. The higher the Haugh value, the better the albumen quality of the eggs. The spherical nature of egg yolk can be expressed as a yolk index value by measuring the yolk height and width (Stadelman 1986a). A decrease in a yolk index value during storage indicates a progressive weakening of the vitelline membranes and liquefaction of the yolk caused mainly by diffusion of water from the albumen (Obanu and Mpieri 1984).
Several workers (Lee and others 1996; Bhale and others 2003; Caner 2005) have reported that chitosan coating is effective in preserving the internal quality of eggs without affecting consumer acceptance. In these studies, eggs were coated with chitosan dissolved in acetic acid with (Caner 2005) and without (Lee and others 1996; Bhale and others 2003) glycerol as a plasticizer using a sponge brush (Lee and others 1996; Bhale and others 2003) or by immersing (Caner 2005). For example, Lee and others (1996) reported that chitosan coating increased shelf life of eggs. The Haugh unit value was significantly higher for 2% chitosan (dissolved in 2% acetic acid) coated eggs (45.87) than 1% chitosan (dissolved in 1% acetic acid) coated eggs (24.28) and noncoated eggs (9.38) after 30 d of storage at 20 °C. Bhale and others (2003) evaluated internal and sensory quality of eggs coated with 1% and 2% chitosans (1100, 746, or 470 kDa) dissolved in 1% and 2% acetic acid, respectively, during a 5-wk storage at 25 °C. Use of 2% 470 kDa chitosan as a coating material was advantageous in reducing weight loss and obtaining more desirable albumen (higher Haugh unit) and yolk (higher yolk index) quality during a 5-wk storage period. The Haugh unit and yolk index values indicated that the albumen and yolk quality of chitosan-coated eggs could be preserved up to 5 wk at 25 °C, which was at least 3 wk longer than observed for the control noncoated eggs. Bhale and others (2003) investigated whether consumers have negative perception toward chitosan-coated eggs. Regarding sensory perception on external quality, these authors found that consumers could not differentiate the coated eggs from the control noncoated eggs. Overall sensory acceptability for the external quality of all chitosan-coated eggs also was not different from the control and commercial eggs. Caner (2005) also found that the shelf life of eggs coated with chitosan (3% in 1% acetic acid) could be extended by at least 2 wk longer compared with that of control at 25 °C, and that overall sensory acceptability for the external quality of chitosan-coated eggs was not different from the control eggs.
Chitosan production typically involves deproteinization (DP), demineralization (DM), decolorization, and deacetylation. Simplification of chitosan production by elimination of the DP or DM step, or reduction of the reaction time required for DP and DM, would considerably reduce production cost due to reduction in chemical usage, process time, and voluminous wastewater discharge. Chitosans prepared by the simplified process may be used for particular usages, such as coating of eggs that does not require a pure chitosan product (No and others 2005). The internal quality of eggs coated with chitosans prepared in a laboratory scale under various deproteinization (DP at 0, 5, 15 min) and demineralization (DM at 0, 10, 20, 30 min) times was evaluated by No and others (2005). Eggs were coated with chitosan (2% in 1% acetic acid) without a plasticizer using a sponge brush. Chitosans prepared under DP 0 min/DM 30 min (viscosity of 1% chitosan in 1% acetic acid = 142.2 cP, DD = 82.3%), DP 5 min/DM 30 min (39.7 cP, DD = 84.7%), and/or DP 15 min/DM 20 min (28.2 cP, DD = 85.7%) conditions could be effectively used as an egg-coating material in preserving the internal quality of eggs compared with chitosan under the DP 15 min/DM 30 min (26.3 cP, DD = 85.0%) condition. The differences in weight (54.58 g compared with 54.59 g) and shell thickness (0.36 mm compared with 0.36 mm) of eggs before and after chitosan coating were very insignificant. The Haugh unit and yolk index values suggested that the chitosan-coated eggs could be preserved for at least 2 wk longer than the control noncoated eggs during 5 wk of storage at 25 °C.
Barrier properties of edible films may depend on their composition, such as plasticizers and solvents. In preparation of edible films, plasticizer is often incorporated into films to induce flexibility, and glycerol is one of the most widely used plasticizers (Wan and others 2005). Chitosan is insoluble in water or organic solvents, but is soluble in diluted organic acids and forms viscous solutions. The viscosity property of chitosan solution may differ with organic acid types used as a dissolving solvent, thus affecting the properties of resultant films or coatings (Kienzle-Sterzer and others 1982; Rhim and others 1998; Park and others 2002d). Most of the previous works on chitosan-coated eggs (Lee and others 1996; Bhale and others 2003; No and others 2005) have been done with chitosan dissolved in acetic acid without plasticizer. More recently, Kim and others (2006) evaluated the effects of plasticizer concentrations (0%, 0.5%, 1.0%, 1.5%, and 2.0% glycerol) and solvent types (1% acetic and 1% lactic acid) on internal quality of eggs coated with 2% chitosan (440 kDa) solution using a sponge brush. In comparison with plasticizer concentrations, eggs coated with chitosan dissolved in acetic acid containing 2% glycerol showed a significant reduction in weight loss and had at least 3 wk longer shelf life extension compared with the noncoated eggs during a 5-wk storage at 25 °C. Use of acetic acid rather than lactic acid as a chitosan solvent was more advantageous in view of shelf life extension of eggs.
In addition to weight loss, Haugh unit, and yolk index, albumen pH also can be used as a quality index (Scott and Silversides 2000). During storage, the carbon dioxide escapes and this increases the pH up to 9.4 (Scott and Silversides 2000; Silversides and Scott 2001; Caner 2005). Egg yolks have a pH of about 6.0, which stays relatively constant during storage as long as there is no carbon dioxide loss (Caner 2005). Scott and Silversides (2000) and Silversides and Scott (2001) evaluated egg (uncoated) quality during 10-d storage at room temperature and found that with storage, albumen pH increased from 7.34 to 9.37 (Scott and Silversides 2000) and 7.43 to 9.32 (Silversides and Scott 2001), with decreasing the albumen height. Caner (2005) observed that albumen pH of chitosan (3% in 1% acetic acid) coated eggs increased from 7.49 to 8.83 while that of uncoated eggs increased from 7.48 to 9.3 after 4-wk storage at 25 °C. These results indicate that chitosan coating decreased carbon dioxide release through the shell by acting as a gas barrier. This also provides an evidence that the improved quality and extended shelf life of eggs by chitosan coating is attributed to the protective barrier properties of chitosan film for moisture and gas transfer through the egg shell.
Fruits and vegetables The major postharvest losses of fruits are due to fungal infection, physiological disorders, and physical injuries (El Ghaouth and others 1991, 1992b). One of the potential approaches to extend the storability of these perishable commodities is to apply edible coatings on the surface, followed by a cold storage (Park and others 2005). Edible coatings can be used as a protective barrier to reduce respiration and transpiration rates through fruit surfaces, retard microbial growth and color changes, and improve texture quality of fruits (Kester and Fennema 1986).
Coating fruits with semipermeable film has generally been shown to retard ripening by modifying the endogenous CO2, O2, and ethylene levels of fruits (El Ghaouth and others 1991). Chitosan coating is likely to modify the internal atmosphere without causing anaerobic respiration, since chitosan films are more selectively permeable to O2 than to CO2 (Bai and others 1988). Therefore, chitosan coating with its ability to modify internal atmosphere in the tissue and fungistatic property has a potential to prolong storage life and control decay of fruits.
There is ample evidence that chitosan coating has the potential to prolong the storage life and control decay of fruits. Strawberry is among the most perishable fruits and is vulnerable to physical injuries and fungal infection caused by Botrytis cinerea and Rhizopus sp. (El Ghaouth and others 1991, 1992a; Park and others 2005). El Ghaouth and others (1991, 1992a) investigated the effect of chitosan coating on decay and quality of strawberries at 13 °C. Strawberry fruits were inoculated with spore suspension of Botrytis cinerea (El Ghaouth and others 1991, 1992a) or Rhizopus stolonifer (El Ghaouth and others 1992a) and subsequently dipped in chitosan solutions (1.0% and 1.5% in 0.25 N HCl). In both studies, chitosan coating significantly reduced the decay of strawberries compared to the control. However, there was no added benefit to decay control by increasing concentration of chitosan from 1.0% to 1.5%. During storage at 4 °C, chitosan-coated berries were firmer, had higher titratable acidity, and synthesized anthocyanin at a slower rate than the control and the fungicide, Rovral®-treated berries (El Ghaouth and others 1991). Chitosan coating decreased the respiration rate of strawberries with a greater effect at higher concentration (El Ghaouth and others 1991). The improved storability of fresh strawberries by chitosan-based coating also has been documented by Reddy and others (2000), Han and others (2004), Park and others (2005), Hernández-Muñoz and others (2006), and Vargas and others (2006).
Chitosan has the ability to inhibit growth of several fungi, to induce chitinase, a defense enzyme, and to elicit phytoalexin in pea pods (El Ghaouth and others 1991, 1992b). Thus, the control of decay in strawberries could be attributed either to the fungistatic property of chitosan per se or to its ability to induce defense enzymes (that is, chitinase and β-1,3-glucanase) or a combination (El Ghaouth and others 1991, 1992a). In studies on antifungal activity of chitosan on 2 postharvest pathogens (Botrytis cinerea and Rhizopus stolonifer) of strawberry fruits, El Ghaouth and others (1992a) observed that coating of intact fruits with chitosan did not stimulate chitinase, chitosanase, or β-1,3-glucanase activities in the tissue. Stimulation of chitinase activities was observed, however, when chitosan was applied directly on freshly cut strawberries. The inability of chitosan coating to stimulate chitinase in intact fruits could quite possibly be due to the limited intimate interaction between the coating material and the tissue. Strawberry cuticle, which is nonporous, may physically separate chitosan from the tissue and consequently prevent chitosan from inducing chitinases. Based on these observations, El Ghaouth and others (1992a) concluded that mechanisms by which chitosan coating reduced the decay of the whole intact strawberries appeared to be related to its fungistatic property rather than to its ability to induce defense enzymes such as chitinase, chitosanase, and β-1,3-glucanase.
For example, Han and others (2004) applied 3 chitosan (DD = 89.9%, 2% in 1% acetic acid) based coatings [chitosan, chitosan containing 5% Gluconal® CAL (a mixture of calcium lactate and calcium gluconate), and chitosan containing 0.2% DL-α-tocopheryl acetate] to strawberries (Fragaria ananassa) and red raspberries (Rubus ideaus). These coatings proved to extend the shelf life of strawberries and red raspberries by decreasing the decay incidence and weight loss, delaying changes in color, pH and titratable acidity during storage at 2 °C. Adding calcium or vitamin E into chitosan-based coatings did not significantly alter their antifungal and moisture barrier functions. In these studies, Han and others (2004) observed no significant difference in weight loss among the 3 types of coatings. However, results on the water vapor permeability (WVP) of the films made from the same coating solutions demonstrated that the moisture barrier properties of chitosan coating containing calcium or vitamin E were significantly improved. For this, these authors explained that, by adding a high concentration of calcium salts into the film matrix, counter ion interactions increase among adjacent molecular structures with small mineral ions acting as fillers, thus resulting in decreased diffusivity of water vapor through the film matrix and a decrease in the hydrophilic tendency of chitosan films. The influence of hydrophobic additives such as vitamin E on the WVP of the films is generally expected to improve the water vapor barrier properties by providing hydrophobicity and increasing film resistance to water transmission (Amarante and Banks 2001; Han and others 2004). To this end, Vargas and others (2006) added oleic acid to chitosan (DD = 82.7%) films to improve their moisture barrier properties. Addition of oleic acid not only enhanced chitosan antimicrobial activity but also improved water vapor resistance of chitosan (1% in 1% acetic acid) coated strawberries. However, it was recommended that oleic acid be incorporated at a ratio of less than 4:1 (oleic acid:chitosan) to avoid unpleasant changes in the sensory attributes.
In addition, Han and others (2004) observed that incorporation of calcium or vitamin E into chitosan coating significantly increased the content of these nutrients in strawberries and red raspberries; this increase was likely due to the diffusion of calcium and vitamin E during soaking. Similarly, Hernández-Muñoz and others (2006) observed that while addition of 1% calcium gluconate to the chitosan coating formulation (1.5% in 0.5% acetic acid) did not further extend the shelf life of strawberries, the amount of calcium (3079 g/kg dry matter) retained by the fruit was greater than that obtained with calcium dips alone (2340 g/kg), thus resulting in increased nutritional value of the strawberries.
Han and others (2005) evaluated sensory quality of fresh strawberries coated with three 1% chitosan (DD = 89.8%) solutions: chitosan in 0.6% acetic acid solution, in 0.6% lactic acid solution, and in 0.6% lactic acid solution plus 0.2% vitamin E. The 1% chitosan coating resulted in no perception of astringency. Lactic acid helped increase the glossiness of the coated strawberries, but the incorporation of vitamin E into the chitosan coating reduced the glossiness of coated strawberries; this could affect consumer acceptance.
Park and others (2005) evaluated the antifungal effect of 2% chitosan (DD = 89.9%; dissolved in 0.5% acetic acid) and 2% chitosan containing 0.3% potassium sorbate (PS) against Cladosporium sp. and Rhizopus sp. that were coated on fresh strawberries by dipping. Coating with chitosan containing PS did not show any advantage over the chitosan-alone coating for delay of fungal growth on fresh strawberries. However, significant synergistic inhibition activity was observed in the in vitro testing when PS was incorporated into chitosan coating. These different results may be explained by the amount of PS that spores were exposed to. In the in vitro test, spore suspensions on the surface of each agar plate were fully exposed to PS, that is, direct interactions between PS and the spores. However, in the coating application, PS was incorporated into the thin layer of the coatings and may have gradually diffused into the surface of the strawberries to interact with the spores, thus limiting antifungal action. In addition, the coating material may have interfered with the antifungal property of PS (Park and others 2005). The nonsignificant antifungal activity of chitosan with PS was also reported by Chen and others (1996), who suggested that the antimicrobial activity of chitosan films with PS could be limited by ionic interaction between PS and chitosan. Furthermore, Park and others (2005) observed that the addition of PS into chitosan coating significantly increased water vapor permeability (48.2 and 67.6 g.mm/m2.d.kPa for 2% chitosan and 2% chitosan containing 0.3% PS, respectively) of the coating without affecting thickness (44.5 and 46.2 μm, respectively) and, thus, may have compromised weight loss control.
Rapid postharvest browning of litchi fruit pericarp is the result of polyphenol oxidase activity, anthocyanin hydrolysis, and nonenzymatic polymerization of o-quinones into melanins (Caro and Joas 2005). Effects of chitosan coating on browning of litchi (Litchi chinensis Sonn.) fruit were investigated by several workers (Zhang and Quantick 1997; Caro and Joas 2005; Jiang and others 2005; Joas and others 2005). In these studies, coating was formed by dipping fruits in chitosan solution. Zhang and Quantick (1997) reported that chitosan coating, irrespective of concentration (1% and 2% dissolved in 2% glutamic acid), delayed changes in contents of anthocyanins, flavonoids, and total phenolics. It also delayed the increase in polyphenol oxidase (PPO) activity and partially inhibited the increase in peroxidase activity. Jiang and others (2005) also similarly observed that chitosan (2% in 5% acetic acid) coating delayed the decrease in anthocyanin content and the increase in PPO activity. Such effects of chitosan coating were also observed with peeled litchi fruit (Dong and others 2004), longan fruit (Jiang and Li 2001), and fresh-cut Chinese water chestnut vegetable (Pen and Jiang 2003). Dependence of browning rate of chitosan-coated litchi fruit on the initial pericarp water content (Caro and Joas 2005), pericarp pH, and dehydration rate during storage (Joas and others 2005) has been reported.
Besides the above-mentioned fruits, El Ghaouth and others (1992b) reported that chitosan coating (1% and 2% in 0.25 N HCl) reduced the respiration rate and ethylene production of tomato, with a greater effect observed at 2% than 1% chitosan. Chitosan-coated tomatoes were firmer, higher in titratable acidity, and less decayed, and they exhibited less red pigmentation than the control fruit after 4 wk of storage at 20 °C. Similarly, Kim and others (1999) also observed that chitosan (318 kDa; 2% in 0.2-M acetic acid) coated tomatoes showed less weight loss and higher flesh firmness than the noncoated control during storage at room temperature for 18 d. Reduction of the respiration rate and ethylene production, control of decay, and retention of firmness as a result of chitosan coating have also been reported for apples (Hwang and others 1998; Davies and others 1989), banana (Kittur and others 2001), citrus (Chien and Chou 2006; Chien and others 2007a), mango (Kittur and others 2001; Chien and others 2007b), peach (Li and Yu 2000), carrots (Durango and others 2006), and lettuce (Devlieghere and others 2004). Apples coated with N,O-carboxymethyl chitosan films and placed in cold storage could be kept in fresh condition for more than 6 mo (Davies and others 1989). Chitosan (43 kDa, DD = 94%) coating is effective in controlling decay of strawberry and lettuce; however, its applicability for lettuce may be hampered due to a pronounced bitter taste developed after treatment (Devlieghere and others 2004).
The above documents collectively demonstrate the ability of chitosan film or coating to prolong the storage life and to better control decay of fruits and vegetables by decreasing respiration rates, inhibiting fungal development, and/or delaying of ripening by reduction of ethylene and carbon dioxide evolution. More comprehensive information on effect of chitosan on horticultural commodities is available from Bautista-Baños and others (2006).
Juice Processing of clarified fruit juices commonly involves the use of clarifying aids, including gelatin, bentonite, silica sol, tannins, polyvinylpyrrolidone, or combinations of these compounds (Soto-Peralta and others 1989). Chitosan with a partial positive charge has been shown to possess acid-binding properties (Imeri and Knorr 1988) and to be effective in aiding the separation of colloidal and dispersed particles from food processing wastes (Knorr 1985; No and Meyers 2000). These properties make chitosan an attractive processing aid in fruit juice production.
Chitosan can be used as a clarifying agent in fruit juices. Clarification of fruit juices with chitosan has been attempted by Soto-Peralta and others (1989), Chatterjee and others (2004), and Rungsardthong and others (2006). Chatterjee and others (2004) applied water-soluble chitosan, hydrolyzed with 7% acetic acid, for clarification of apple, grape, lemon, and orange juices. Chitosan (2% dissolved in water) was more effective in reducing the turbidity of juices than bentonite and gelatin. The appearance and acceptability of the juices (measured on a 9-point hedonic scale), after chitosan treatment, significantly increased. Rungsardthong and others (2006) compared the effectiveness of chitosan prepared from Absidia glauca var. paradoxa (fungal chitosan; DD = 86%) with that prepared from shrimp shells as fining agents for apple juice. They found that the fungal chitosan treatment was more effective in reducing the apple juice turbidity and yielded lighter (higher Hunter L* value) juices than the sample treated with shrimp chitosan. Turbidity decreased gradually with increasing chitosan (dissolved in 2% malic acid) concentration from 0.1 to 0.7 g/L of juice, but it was then increased at 1.0 g/L, which may have been due to the saturation of the active chitosan adsorption sites. Fungal chitosan treatment with approximately 0.5 to 1.0 g/L resulted in higher L* values than the control and the juice treated with shrimp chitosan.
Chitosan may also be used to control acidity in fruit juices. Imeri and Knorr (1988) observed that chitosan (1.2% in 2% ascorbic acid) treatment had no effect on carrot and apple juice yield but significantly reduced titratable acidity due to its acid-binding properties. The effects of chitosan treatment on the reduction of titratable acidity provide a potential for acidity control in other food systems.
Enzymatic browning in apple and pear juices can be prevented by removing particulate matters by filtration or centrifugation (Sapers 1992). Sapers (1992) investigated whether the addition of chitosan (1% in 1% malic acid) to apple and pear juices would increase the effectiveness of filtration or centrifugation treatments in controlling browning. Results indicated that browning could be prevented in the McIntosh apple juice by addition of at least 200 ppm chitosan, irrespective of the type of chitosan (low or high viscosity) tested, followed by filtration with diatomaceous earth filter aid. Chitosan at 1000 ppm was required to prevent browning in juices prepared from ripen Bartlett and Bosc pears. The success of chitosan treatments in controlling enzymatic browning in filtered apple and pear juices is probably due to the ability of the positively charged polymer to coagulate suspended solids to which polyphenol oxidase (PPO) is bound (Sapers 1992).
Few attempts have been made to date to assess the antimicrobial properties of chitosan in juices (Roller and Covill 1999; Rhoades and Roller 2000). Roller and Covill (1999) reported that chitosan glutamate was an effective preservative against spoilage yeasts in apple juice. The presence of chitosan glutamate (DD = 75% to 85%) in apple juice (pH 3.4) at a level ranging from 0.1 to 5 g/L inhibited growth of all spoilage yeasts (Zygosaccharomyces bailii 906 and HP; Saccharomyces cerevisiae 3085, SD and 28; Schizosaccharomyces pombe; Saccharomyces exiguous; Saccharomycodes ludwigii) at 25 °C. The most sensitive strain was Z. bailii: it was completely inactivated by chitosan at 0.1 and 0.4 g/L for 32 d of storage at 25 °C. The most resistant strain was S. ludwigii, which required a level of 5 g/L of chitosan for complete inactivation and for maintaining yeast-free conditions in apple juice for 14 d at 25 °C. Rhoades and Roller (2000) also studied the antimicrobial activities of native and degraded (hydrolyzed by papaya latex) chitosan against the natural microbial flora in pasteurized apple-elderflower juice (pH 3.3) stored at 7 °C. At 0.3 g of chitosan/L addition, irrespective of chitosan types, yeasts were eliminated entirely during 13 d of storage; however, the total counts and the lactic acid bacterial counts increased at a slower rate than did in the control.
In an effort to extend the shelf life of kimchi, No and others (1995) soaked Chinese cabbage in a salt solution containing various concentrations of chitosan for 24 h at room temperature. The shelf life of kimchi, prepared with Chinese cabbage soaked in 10% salt solution containing 0.1% or 0.15% chitosan, was extended by approximately 10 d longer than that of the control kimchi. Similarly, Hur and others (1997) and Lee and Jo (1998) also observed that soaking the salted and washed Chinese cabbage in 0.5% or 1.0% chitosan solution resulted in the delay of the fermentation rate of kimchi.
Chitosan also was reported to delay softening of kimchi tissues during fermentation (Ahn and Lee 1995; Park and others 2002a) and to increase the chemopreventive effect of kimchi (Kim and others 2004). In some instances, chitosan has been applied with other preservatives, such as medicinal herb extracts (Lee and Cho 1998), Na-benzoate (Son and others 1996), and liquid calcium (Jang and Jeong 2005) for improved quality and shelf life of kimchi. For example, the color of kimchi prepared with medicinal herb extracts (3% of a 1:1 mixture of Lithospermum erythrorhizon and Glycyrrhiza uralensis) became more reddish in the presence of added chitosan (Lee and Cho 1998). The shelf life of kimchi with 0.5% chitosan-liquid calcium added was improved compared with that of kimchi with liquid calcium alone; this was due to growth inhibition of lactic acid bacteria at an early stage of fermentation (Jang and Jeong 2005).
Mayonnaise Mayonnaise is an oil-in-water emulsion. A few studies have been conducted on the use of chitosan to enhance emulsification in mayonnaise preparation. Lee (1996) reported that addition of chitosan (1500 kDa, 0.1% based on egg yolk weight) increased emulsifying capacity of egg yolk by about 10% and enhanced emulsion stability of mayonnaise by 9.4% compared with those of the control. Kim and Hur (2002) also suggested the use of chitosan as an emulsion stabilizer in commercial mayonnaise preparation. Unlike other polysaccharides, chitosan possesses a positive ionic charge and has both reactive amino and hydroxyl groups, which give it the ability to chemically bond with negatively charged protein. When pH is less than 6.5, chitosan solution carries a positive charge along its backbone. Because of its polar groups, chitosan also provides additional stabilization due to hydration forces (Del Blanco and others 1999). According to Filar and Wirick (1978), chitosan functions only in acid systems to show possible utility as a thickener and stabilizer.
Del Blanco and others (1999) investigated the effect of chitosan DD (73% to 95%) on emulsification properties. Chitosan solution was prepared in 1% acetic acid at a 1% concentration. Results indicated that all DD gave stable polydispersed water/oil/water emulsions with different viscosities. Two optimum DD values were found at 81% and 88%, giving complete emulsification without residual oil or sedimentation. Chitosans with intermediate DD were less effective emulsifiers while chitosans with higher DD gave poor emulsification. Rodríguez and others (2002) also studied emulsification properties of chitosans with 7 different DD (75% to 95%). In this study, chitosan solutions (0.2%, 1.0%, and 2.0%) were prepared in 0.1 M hydrochloric acid. They found that chitosan produced stable water/oil/water (w/o/w) emulsions and that the drop size distribution was unimodal at low DD (75%) and at high DD (95%) for all chitosan concentrations. Meanwhile, drop size distribution was independent of both chitosan solution viscosity and emulsion viscosity. The emulsion viscosity and emulsion stability were proportional to chitosan concentration (Rodríguez and others 2002).
Roller and Covill (2000) investigated the effectiveness of chitosan glutamate as a preservative against both bacteria and yeast in mayonnaise. Mayonnaise containing 3 g/L of powered chitosan combined with acetic acid (0.16%) or lemon juice (1.2% and 2.6%) was inoculated with log 5 to 6 CFU/g of Salmonella Enteritidis, Zygosaccharomyces bailii, or Lactobacillus fructivorans and stored at 5 and 25 °C for 8 d. They found that mayonnaise samples at the same pH (4.4 to 4.5) but containing acetic acid as the acidulant were less supportive of microbial growth and survival than those containing lemon juice. Especially in mayonnaise containing chitosan and 0.16% acetic acid, L. fructivorans was completely inactivated to a level below the detection limit of the plate counting method at both storage temperatures. In mayonnaise containing lemon juice at both 1.2% and 2.6%, no substantial differences were observed between the controls and the samples containing chitosan at either storage temperature.
Meat Meat or meat products are highly susceptible to lipid oxidation, which leads to rapid development of rancid or warmed-over flavor. Chitosan possesses antioxidant and antibacterial capacity (Kamil and others 2002; No and others 2002), and may retard the lipid oxidation and inhibit the growth of spoilage bacteria in meat during storage.
Effectiveness of chitosan addition on storage stability of meat has been reported by several workers. For example, Darmadji and Izumimoto (1994a) observed that addition of 1.0% chitosan to beef decreased the TBA value by about 70% compared to that of the control sample after 3 d of storage at 4 °C. After 10 d of storage, the TBA value of the beef sample containing 0.5% to 1.0% chitosan was almost the same as that at day 0, whereas the TBA value of the control sample increased sharply. Chitosan had a desirable effect on the development of the red color of beef during storage. Darmadji and Izumimoto (1994b) also found that the effectiveness of chitosan addition on storage stability of minced beef was enhanced when combined with nitrite and Lactobacillus plantarum. The use of 3 combined treatments (0.5% chitosan, 1.0%Lactobacillus plantarum starter culture, and 100 ppm nitrite) inhibited the growth of bacteria by about 2-log cycle, reduced TBA value by 36%, dissipated residual nitrite by 63%, and resulted in a better color of fermented meat. Youn and others (2004) noted that the shelf life of spicy beef added with 1.0% chitosan (120 kDa, DD = 85%) dissolved in 0.3% lactic acid was remarkably improved by reducing the total bacterial cell counts and inhibiting lipid oxidation during storage for 10 d at 4 °C. Sagoo and others (2002) demonstrated that chitosan was an effective inhibitor of microbial growth in chilled comminuted pork products and that the effect of chitosan was concentration dependent. Addition of 0.3% and 0.6% chitosan glutamate to an unseasoned minced pork mixture reduced total viable counts, yeasts and molds, and lactic acid bacteria by up to 3-log CFU/g for 18 d at 4 °C compared with the untreated control. Juneja and others (2006) reported that addition of 3% chitosan glutamate (DD = 86%) to ground beef or turkey may reduce the potential risk of Clostridium perfringens spore germination and outgrowth during abusive cooling from 54.4 to 7.2 °C in 12, 15, or 18 h.
Lee and others (2003) investigated storage stability of pork dipped in chitosan solution. Pork was dipped for 1 min in various concentrations (0.1%, 0.5%, 1.0%) of chitosans with different molecular weights (5, 30, 120 kDa) and then stored for 8 d at 10 °C. The results indicated that the shelf life and antioxidation of pork increased by dipping with 1.0% solution of 30 and 120 kDa chitosans. The 5 kDa chitosan treatment, irrespective of its concentrations, was ineffective in extending the shelf life and preventing lipid oxidation of pork. The external redness color of pork treated with 30 and 120 kDa chitosans remained unchanged during storage. The treatment of beef loins by dipping in 1% chitosan solution, followed by 5% trisodium phosphate, effectively inhibited the growth of aerobic spoilage microorganisms during storage at 10 °C (Cheong and others 2001).
Wu and others (2000) found that wrapping with chitosan (CH, DD = 85%; 2% in 1% formic acid), wheat gluten (WG), or soy protein (SP) film was not effective in controlling lipid oxidation of precooked beef patties. These authors explained that a higher O2 permeability may contribute to the higher thiobarbituric acid-reactive substances (TBARS) from these treatments. After being applied on patties and during the 3 d of storage, the CH, WG, and SP wrappings absorbed moisture from meat samples and swelled. The plasticizing effect of water molecules may have changed the barrier properties of the edible films, and resulted in higher O2 permeability or even loss of film integrity. The composition of the CH film may have also contributed to its higher O2 permeability. Formic acid was used as a solvent to form the CH film in this study. Formic CH film has been reported to have higher O2 permeating coefficient than lactic acid and acetic acid CH films (Caner and others 1998).
Radiation processing is an alternate technology to eliminate microbial contamination in meat and meat products; however, it accelerates lipid peroxidation (Kanatt and others 2004, 2005). Synthetic antioxidants such as butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and others are being currently used to prevent oxidative changes in foods (Rao and others 2005). Kanatt and others (2004) investigated the use of irradiated chitosan as a natural antioxidant to minimize lipid peroxidation of radiation-processed lamb meat. Irradiation of chitosan at 25-kGy dose of gamma radiation resulted in a 6-fold increase in its antioxidant activity as compared to the nonirradiated chitosan, as measured by 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging activity. The TBA values of irradiated lamb meat containing irradiated chitosan (4 mL of 1% chitosan in 1% acetic acid/100 g meat) decreased by 88% (leg) and 54% (rib) as compared to the corresponding sample without chitosan. Further development of postprocessing rancidity was reduced by 39% (leg) and 59% (rib) in the samples treated with chitosan as compared to nontreated controls even after 1 wk of storage at 0 to 3 °C.
Rao and others (2005) also demonstrated the potential of irradiated chitosan as an antioxidant that substantially retarded lipid peroxidation and could prevent the spoilage due to microbial growth in the irradiated intermediate-moisture (IM) meat products. Chitosan (1% in 1% acetic acid) was irradiated at a 4-kGy dose in a Cobalt-60 Gamma cell-220 for the purpose of decontamination. The antioxidant activity of chitosan increased upon irradiation, as reflected by an increase in reducing power and free radical scavenging activity, without significantly affecting its antimicrobial properety. The chitosan-coated irradiated (4 kGy) IM meat products, such as mutton kebabs and streaky bacon, showed lower TBARS values than the noncoated irradiated samples for up to 4 wk of storage at ambient temperature. The use of chitosan coating in place of polyethylene packaging for the preparation of safe and stable IM meat products was thus possible.
Rice cakeLee and others (2000) observed that chitosan treatment considerably extended the shelf life of white rice cake. These authors dipped white rice cake in either 95% alcohol, 1% lactic acid, or 1% and/or 2% chitosan (Mw = 37 kDa, dissolved in 1% lactic acid) for 10 s prior to vacuum packing. The total microbial counts for the control (no treatment), alcohol-treated, and 1% lactic acid-treated white rice cake exceeded the initial putrefactive criterion level of 1 × 106 CFU/g, respectively, at 6, 27, and 20 d of storage at 4 °C. On the other hand, the total microbial counts (3.3 × 105 and 1.4 × 105 CFU/g, respectively) of the white rice cake treated with 1% and 2% chitosan were less than the criterion level even after 76 d of storage. However, the dipping treatment, irrespective of solution types, was found to negatively affect sensory acceptability of the white rice cake. Prolonged shelf life of rice cake by addition of chitosan was also demonstrated by Park and Chong (2002). Water-soluble chitosan dissolved in water was added to rice cake at 0.0%, 0.05%, 0.1%, 0.3%, and 0.5% concentrations. During storage for 4 wk at 5 °C, the total microbial counts decreased with increasing chitosan concentrations. After 4 wk of storage, the total microbial counts of rice cake containing 0.3% and 0.5% chitosan were 2-log cycle lower than that (8.2 × 104 CFU/g) of the control.
Nam and Woo (2002) investigated the quality characteristics of jeung-pyun, the traditional Korean fermented rice cake leavened by yeast, as affected by the addition (0%, 2%, 4%, and 6%) of chitosan oligosaccharide. Results for sensory evaluation and rheological properties measured with rheometer indicated that addition of 2% chitosan oligosaccharide imparted positive effects on the quality of jeung-pyun.
Sausage In preparation of sausage in Korea, sodium nitrite is generally used as a curing agent for color and flavor development as well as preservative effect (Park and others 1999). However, nitrite reacts with amine in meat and may produce nitrosoamine, a strong toxicant detrimental to human health. Several workers (Park and others 1999; Youn and others 1999, 2000, 2001b) have investigated the possible role of chitosan, in lieu of sodium nitrite, as curing agent in sausage, and found that addition of chitosan could reduce or replace the use of nitrite without affecting preservative effect and color development.
Park and others (1999) reported that a mixture of 0.2% chitosan (120 kDa, DD = 85%, dissolved in 0.3% lactic acid) and 0.005% sodium nitrite or 0.5% chitosan alone exhibited the same preservative effect as did 0.01% sodium nitrite alone when added in meat sausage. Similarly, Youn and others (1999) reported that addition of 0.2% chitosan (30 kDa, DD = 92%) dissolved in 0.3% lactic acid could reduce the amount of sodium nitrite in half of the standard volume (150 ppm) without affecting quality and storage stability of sausage. In addition to the preservative effect, chitosan also notably reduced the concentration of residual nitrite in sausage (Youn and others 2001b). The concentration of residual nitrite in sausage decreased with increased molecular weights and concentrations of chitosan.
The preservative effect of chitosan in sausage may differ with its molecular weight. According to Youn and others (2000), the preservative effect of chitosan increased with increased molecular weight of chitosans (1, 5, 30, and 120 kDa). No preservative effect of chitosan with a molecular weight of 1 kDa (Youn and others 2000, 2001b) and 5 kDa (Youn and others 2000, 2001b; Jo and others 2001) added at 0.2% or 0.5% concentration in pork sausage was observed. In studies with chitosan glutamate (DD = 75% to 85%), Sagoo and others (2002) found that dipping of standard and skinless pork sausages in chitosan solution (1% dissolved in water) reduced the native microflora (total viable counts, yeasts and molds, and lactic acid bacteria) by approximately 1- to 3-log CFU/g for 18 d at 7 °C and chitosan treatment increased the shelf life of chilled skinless pork sausages from 7 to 15 d.
Lin and Chao (2001) observed that addition of 0.1% chitosan (dissolved in 1% lactic acid) with a molecular weight of 150 (DD = 85%), 600 (DD = 85%), or 1250 kDa (DD = 87%) into the reduced-fat (approximately 22%) Chinese-style sausage had no adverse effect on the textural and sensory characteristics. Jo and others (2001) also found no differences in sensory characteristics (color, flavor, texture, and overall acceptance) between sausages prepared with and without water-soluble chitosan oligomer (5 kDa, 0.2%). Compared with the control sausage, lipid oxidation was, however, lower in the sausage containing chitosan oligomer stored in an aerobic packaging after 3-wk storage at 4 °C. Chitosan may provide antioxidative effect in emulsion-type sausages. According to Youn and others (2001a), antioxidative effect of chitosan in emulsion sausage increased with increased molecular weights (1, 5, 30, 120 kDa) and concentrations (0.2%, 0.35%, and 0.5%) of chitosan.
Seafoods and seafood products Seafood products are highly susceptible to quality deterioration due to lipid oxidation of unsaturated fatty acids, catalyzed by the presence of high concentrations of hematin compounds and metal ions in the fish muscle (Decker and Hultin 1992). Furthermore, seafood quality is highly influenced by autolysis, contamination by and growth of microorganisms, and loss of protein functionality (Jeon and others 2002).
Antioxidant activity of chitosans of different viscosities (360, 57, and 14 cP; corresponding molecular weight of 1800, 960, and 660 kDa) in cooked, comminuted flesh of herring (Clupea harengus) was investigated by Kamil and others (2002). The oxidative stability of fish flesh with added chitosans (50, 100, and 200 ppm) was compared with those added with conventional antioxidants, butylated hydroxyanisole + butylated hydroxytoluene (BHA + BHT, 200 ppm) and tert-butylhydroquinone (TBHQ, 200 ppm), during storage at 4 °C. Among the 3 chitosans, 14 cP chitosan was most effective in preventing lipid oxidation. The formation of TBARS in herring samples containing 200 ppm of 14 cP chitosan was reduced after 8 d of storage by 52% as compared to that of the control. At 200 ppm, 14 cP chitosan exerted an antioxidant effect similar to that of commercial antioxidants in reducing TBARS values in comminuted herring flesh. These studies indicate that antioxidant capacity of chitosan added to the fish muscle depended on the molecular weight and concentration of chitosan (Kamil and others 2002). Similarly, Kim and Thomas (2007) also observed that the antioxidative effects of chitosan in salmon depended on its molecular weight (30, 90, and 120 kDa) and concentrations (0.2%, 0.5%, and 1.0%). The 30 kDa chitosan showed the highest scavenging activity compared to 90 and 120 kDa chitosan. The scavenging activities of chitosans increased with increasing concentration, except for the 120 kDa chitosan, which showed no significant difference with concentration.
Chitosans may retard lipid oxidation by chelating ferrous ions present in the fish model system, thus eliminating prooxidant activity of ferrous ions or preventing their conversion to ferric ion. Amino groups in chitosans may participate in the chelation of metal ions (Peng and others 1998). The varying antioxidant effects of chitosans with different viscosities observed in cooked comminuted fish model systems (Kamil and others 2002) may be attributed to the molecular weight differences, which determine the extent of chelation of metal ions. In their charged state, the cationic amino groups of chitosans impart intramolecular electric repulsive forces, which increase the hydrodynamic volume by extended chain conformation. Perhaps this phenomenon may be responsible for lesser chelation by high viscosity (high molecular weight) chitosans (Kamil and others 2002). In addition, Xue and others (1998) reported that the antioxidant mechanism of chitosan could be by chelant action of ion metals and/or the combination with lipids. The protective action of chitosan is also effective when it is applied as a protective film, where it retards lipid oxidation and microbial spoilage by acting as a barrier against oxygen (Jeon and others 2002).
Jeon and others (2002) studied the effect of 3 different viscosities (360, 57, and 14 cP; corresponding molecular weight of 1800, 960, and 660 kDa) of chitosan on shelf life extension of fresh fillets of Atlantic cod (Gadus morhua) and herring (Clupea harengus) over a 12-d storage at refrigerated temperature (4 °C). Chitosan coating significantly reduced lipid oxidation, chemical spoilage (total volatile basic nitrogen, trimethylamine, and hypoxanthine), and growth of microorganisms in both fish compared to the uncoated samples. The preservative efficacy of chitosans with viscosities of 57 and 360 cP was found to be superior to that of chitosan with 14 cP viscosity. These authors concluded that chitosan as edible coating would enhance the quality of seafoods during storage. Similarly, Tsai and others (2002) also observed that the shelf life of salmon fillets dipped in chitosan (49.1 kDa) solution (1% in 0.1 N HCl) was extended from 5 to 9 d. Sathivel (2005) evaluated the effects of chitosan as edible coating on the quality of skinless pink salmon (Oncorhynchus gorbuscha) fillets during 3-mo frozen storage. Coating with chitosan was effective in reducing about 50% moisture loss of fillets compared to the control noncoated fillets, and in delaying lipid oxidation. There were no significant (P > 0.05) effects of chitosan coating on a*, b*, and whiteness values for cooked pink salmon fillets after 3-mo frozen storage. Chitosan coating applied on the surface of the pink salmon fillets may have acted as a barrier between the fillet and its surrounding, thus slowing down the diffusion of oxygen from the surrounding via the surface into the fillet (Sathivel 2005).
Cho and others (1998a) applied chitosan hydrolysate, hydrolyzed with chitosanase, as a natural food preservative for fish paste and found that the shelf life of the product containing 0.3% chitosan hydrolysate was extended by 6 d at 15 °C, 4 d at 20 °C, and 2 d at 30 °C. Ahn and Lee (1992) studied the preservative effect of chitosan film on quality of lightly salted and dried horse mackerel. The product was prepared by soaking the fresh horse mackerel in 15% salt solution for 30 min, coating with or without (control) chitosan, and drying for 3 h at 40 °C in a hot air dryer. During storage at 5 °C for 20 d, the chitosan-coated samples had lower volatile basic nitrogen (VBN), amino nitrogen, trimethylamine (TMA), thiobarbituric acid (TBA), and peroxide values as well as viable cell counts than those of the control. The chitosan-coated samples also obtained higher overall consumer acceptance scores than the control. As such, these authors concluded that chitosan film packing was an effective method for retaining the quality of lightly salted and dried horse mackerel. Similarly, López-Caballero and others (2005) also found that coating fish (cod) patties with a chitosan–gelatin blend retarded spoilage. Addition of chitosan in a powder form into fish patties had no effect on the prevention of spoilage due to the poor insolubility of chitosan at a neutral pH and the presence of a significant proportion of uncharged amino groups.
Soybean curd (tofu) Freshly prepared soybean curd is quite perishable due to its rich nutrients and high moisture content, with a typical maximum shelf life of 1 to 2 d even under commercial refrigeration (Dotson and others 1977; Kovats and others 1984). This spoilage is associated with bacterial growth (Dotson and others 1977).
Several workers (Chun and others 1997, 1999) applied water-soluble chitosan as a coagulant or as an immersion solution for extension of shelf life of tofu. Addition of chitosan to a coagulant CaCl2[chitosan:CaCl2 (g/g) = 1.0:8.0, 1.5:7.5, or 2.0:7.0] extended the shelf life of tofu by more than 7 d at 4 °C compared with tofu made with CaCl2 only (Chun and others 1999). Similarly, use of 0.5% water-soluble chitosan solution as an immersion solution extended the shelf life of tofu by more than 7 d at 4 °C (Chun and others 1997). The enhanced shelf life of tofu by chitosan was due to the antimicrobial activity of chitosan.
Lee and others (2001) prepared tofu products at a commercial scale using 4 different coagulants [1:1 (v/v) mixture of 1% acetic + 1% lactic acids (solution A); 1% chitosan dissolved in A; CaCl2; chitosan + CaCl2] and compared their shelf life during storage at 10 °C for 7 d. The tofu made with 1% chitosan dissolved in solution A as a coagulant had a shelf life of 4 d compared to 3 d when made with other 3 coagulants. In the comparison of 3 different molecular-weight chitosans (746, 224, and 28 kDa) as coagulants, use of lower molecular-weight chitosan resulted in higher tofu hardness and lower turbidity in the immersion solution during storage. Addition of chitosan also has been reported to increase the yield of soybean curd by decreasing solid materials present in whey (Lee and others 2002b).
No and Meyers (2004) extensively investigated the potential of chitosan as a coagulant in tofu preparation. They used 6 different molecular weights (1106, 746, 471, 224, 28, and 7 kDa) of chitosans under various treatment conditions and established optimum processing conditions as follows: chitosan with a molecular weight of 28 kDa; chitosan concentration and solvent, 1% chitosan in a 1:1 (v/v) mixture of 1% acetic acid and 1% lactic acid; chitosan solution to soymilk ratio, 1:8; coagulation temperature, 80 °C; coagulation time, 15 min. They also reported that that the chitosan tofu had a longer shelf life, about 3 d, than the tofu made with CaCl2. Increase in gel strength and shelf life of tofu by addition of 2% chitosan was observed by Chang and others (2003). The gel strength of tofu increased with increased molecular weight (189, 720, and 2780 kDa) of chitosan while the shelf life of tofu increased with increased degree of deacetylation (DD 54%, 73%, and 91%) of chitosan. It is possible that the higher molecular weight chitosan strengthened the gel structure of tofu more than did the lower molecular weight chitosan.
Lee and others (1999b) reported that soaking soybeans (Iksan cultivar) in chitosan solution (1% in 0.25% acetic acid) for 1 h and then in water for 11 h increased the total weight of soybean sprouts and the hypocotyl length, respectively, by 8.5% and by 25.4%, compared with the control after cultivation for 5 d at 20 °C. Choi and others (2000) observed that soaking soybeans (Joonjul cultivar) in chitosan solution (0.1% in 0.25% acetic acid) for 20 min increased the weight of soybean sprouts by 6.9% and the length by 9.5%, compared with the control after cultivation for 4 d at 25 °C. Lee and others (1999a) reported that soaking seeds in 1000 ppm chitosan (5 to 10 kDa) solution significantly reduced soybean sprout rot from 13.8% (of control) to 7.0%.
Recently, No and others (2003) extensively investigated the effects of molecular weights (22, 59, 224, 493, and 746 kDa), concentrations (0%, 0.01%, 0.05%, 0.10%, 0.25%, and 0.50%), and dissolving solvents (acetic and lactic acid) of chitosan, soaking times (0, 1, 3, 5, and 8 h), and soybean/chitosan solution ratios (1:4, 1:5, 1:7, and 1:10) on growth and quality of soybean sprouts. Among 5 chitosans, treatment with 493 kDa chitosan was the most effective in increasing total weight, vitamin C content, and hardness of soybean sprouts. Use of acetic acid rather than lactic acid as a chitosan solvent increased the hypocotyl weight and vitamin C content of soybean sprouts. Soaking of soybeans in chitosan solution for 8 h increased the total weight by 13% and vitamin C content by about 10% in soybean sprouts. No and others (2003) established the optimal cultivation conditions: soaking in 0.05% chitosan with 493 kDa in 0.05% acetic acid for 8 h and a soybean/chitosan solution ratio of 1:4. Under these optimal cultivation conditions established by No and others (2003), Park and Kim (2003) found that the total free amino acid content of chitosan-treated soybean sprouts was higher than that of the control after 5 d cultivation at 20 °C.
Furthermore, chitosan treatment effectively decreased the activity of lipoxygenase-2 and lipoxygenase-3, which are major causes in off-flavor formation, during cultivation of soybean sprouts (Lee and Rhee 1999). Increased shelf life of soybean sprouts treated with chitosan (200 kDa) and chitosan oligomer (20 kDa), compared with control, also was reported by Ji and others (2003).
More recently, No and others (2006) evaluated the effects of chitosans, prepared in a laboratory scale under various deproteinization (DP at 0, 5, 15 min) and demineralization (DM at 0, 10, 20, 30 min) times, on the growth of soybean sprouts in an effort to develop an ultimate economic chitosan production process for particular food applications. These authors observed that chitosan treatment increased the total weight of soybean sprouts by 10.7% to 13.8% compared with that of the control; the effectiveness of the chitosan treatment was independent of DP and DM times. The increase (12.7%) in the growth of soybean sprouts by chitosan prepared without DP and DM steps was noteworthy. Chitosan treatment may increase the production cost of soybean sprouts in commercial plants. However, the increased yield of soybean sprouts can certainly offset the increased production cost of soybean sprouts by chitosan treatment.