Study On Applications Of Chitosan In Antimicrobial Food Packaging

The demand for minimally processed, easily prepared and ready-to-eat ‘fresh’ food products, globalization of food trade, and distribution from a centralized processing area pose major challenges for food packaging, and promotes the research and development of antimicrobial food packaging. Antimicrobial packaging encompasses any packaging technique(s) used to control the growth of pathogenic and spoilage bacteria in food products. This allows for the enhancement of the quality of food products and the extension of their shelf life. An antimicrobial packaging system includes packaging materials, antimicrobial agents, and also various methods which allow for the application of packaging materials and antimicrobial agents on the surface of foods. To meet the growing needs of an antimicrobial packaging system with natural, biodegradable, environmentally-friendly packaging materials and safe and effective antimicrobials, new materials and novel technologies have been and continue to be used or developed. Chitosan is a natural polymer and has great potential for a wide range of food applications due to its biodegradability, biocompatibility, antimicrobial activity, non-toxicity and film-forming properties. Chitosan-based edible films and coatings have been widely studied. Therefore, the objective of this study is to develop chitosan-based novel food packaging systems that are safe, effective, and affordable. In this study, the current applications of chitosan in food packaging have been deeply reviewed; unique formulations with chitosan and other chemicals for antimicrobial coatings and films and their application methods are developed; the antimicrobial properties of the developed chitosan coating and films alone or in combination with other intervention technologies against pathogenic and spoilage microorganisms are investigated in culture media and foods; the physiochemical properties and storage stability of the coatings and films are evaluated; and applications of high pressure homogenization and natural plant emulsifiers in antimicrobial packaging are also explored.The antimicrobial activity of chitosan is well documented. However, when chitosan is used alone, its antimicrobial activity is limited, particularly when low concentrations are used, is able to reduce bacterial populations by less than one log. Therefore, a combination of chitosan with organic acids and other antimicrobials is needed to achieve greater reductions in bacterial populations.The antimicrobial activities of organic acids and their salts are also well documented. However, there is limited information on their antimicrobial effect when used in combination for film coatings. Most of the other studies have used 1-2% acetic acid for making chitosan solutions. In this study, the combinations of lactic acid(LA), levulinic acid(Lev A), acetic acid(AA), and citric acid(CA) are evaluated for their antimicrobial activity. LA+Lev A and LA+Lev A+AA in coating solutions exhibited the highest antimicrobial activity among the test formulas. The addition of citric acid into coating solutions did not make them more effective against inhibiting the growth of Listeria.Sodium lactate and sorbic acid have been used as antimicrobials. Lauric arginate ester(LAE) has a broad spectrum of antimicrobial activity and has received GRAS(Generally Recognized as Safe) status for many food applications in the United States. It has received approval by the USDA/FSIS to be used as a food preservative on the surface of ready-to-eat(RTE) meat and poultry products at concentrations of up to 200 ppm. However, few studies have reported the combination of multiple organic acids, lauric arginate ester, sodium lactate, and sorbic acid in antimicrobial packaging. In this study, edible chitosan-acid solutions incorporating lauric arginate ester, sodium lactate and sorbic acid alone or in various combinations were developed and their antimicrobial effects against Listeria innocua, Listeria monocytogenes and Salmonella typhimurium in culture medium and on the surface of meat samples were investigated. The results reveal that antimicrobial films containing lauric arginate ester are the most effective against both Listeria and Salmonella in tryptic soy broth(TSB) and reduced the populations of both species of bacteria to undetectable levels(< 0.69 log CFU/m L). These films also significantly(p<0.05) reduced the growth of L. innocua, L. monocytogenes and S. Typhimurium on RTE meat stored at 10°C over a three-week period and a five-week period. Specifically, these films caused a 2-3 log reduction in Listeria populations and a 1-1.5 log reduction in Salmonella populations as compared to the controls. Films containing sodium lactate and sorbic acid significantly reduced the growth of L. innocua but were less effective against the growth of Salmonella. The combination of sodium lactate and sorbic acid with lauric arginate ester did not generate additional or synergetic antimicrobial effect against Listeria or Salmonella on RTE meat surfaces.Antimicrobial films can be applied onto the surface of foods by two methods. In the direct coating method, antimicrobial coating solutions can be applied directly on the surface of food. In the indirect coating method, the films can be made and applied to the surface of food. In this study, there was no significant difference in antimicrobial effectiveness between antimicrobial films and coatings using the same antimicrobial coating formula. Therefore, food processors can make a choice based on their preference, convenience, availability, and cost effectiveness. Either method has advantages and disadvantages. As antimicrobials must directly contact the surface of food to exert an antimicrobial effect, a direct coating method may be more suitable for foods with irregular shapes or rough surfaces, where films cannot directly contact food surfaces without vacuum-packaging. However, preparation of coating solutions and coating/drying procedures might slow down production speed. In contrast, antimicrobial films can be customized within current film manufacturing lines, pre-made by a packaging company, and used as regular packaging materials.Antimicrobial composite packaging films can be made by mixing antimicrobials with polymer materials(one step method). This study explores an alternative method by coating a layer of antimicrobial material onto the surface of pre-extruded films to make double-layer antimicrobial films. The antimicrobial activities of chitosan-coated polylactic acid(PLA) films containing multiple organic acids and other antimicrobials against the growth of L. innocua, L. monocytogenes, and S. typhimurium are clearly demonstrated in this study. The antimicrobial PLA films significantly inhibited the growth of both microorganisms on RTE meat during storage at 10°C for up to five weeks and indicated a potential application of this antimicrobial packaging system in meat products. The advantages of the double-layer films are minimization of the loss of antimicrobial activity during the thermal film making, reduction of the amount of antimicrobial compounds used during the packaging process because only the outer layer of the sample is coated with the antimicrobial material, and avoidance of the “filler effect” and minimization of the impact on physical or mechanical properties of the base materials. However, one challenge to the development of double-layer antimicrobial films is the binding or adhering capacity of the active layer on base films because of the different surface characteristics of the active layer and base films. Separation of the active layer from the base layer is often observed during storage, which restricts the commercial application of the packaging films. SEM images of the cross-section of each chitosan-coated PLA film indicate the binding or adhering capacity of coatings on base PLA films, as a gap between the coating and base PLA film were observed. Among the four coatings, their binding or adhering capacity showed the following order(strong to weak): chitosan only > chitosan+LAE > chitosan+nisin > chitosan+nisin+LAE. Therefore, in this study, corona discharge was used to treat PLA films. Corona discharge significantly improved the binding capacity and storage stability of double-layer antimicrobial films. The film samples which were packed in bags and stored at 4 and 10°C did not show separation of the active layer from the base layer for 210 days. Hence, these films are suitable for commercial application.High pressure homogenization(HPH) processing can enhance the antimicrobial efficacy of micro-emulsion coatings and films with the aid of an emulsifier. The composite films made from the micro-emulsions significantly reduced L. innocua populations by over 4 log CFU/m L in tryptic soy broth after two days of storage at 22°C and on the surface of RTE meat samples after 35 days of storage at 10°C. The composite films made from HPH processed micro-emulsions had a smooth surface and micro-channels around micro-particles in the cross-section, which facilitates the release of antimicrobials.The concept of food preservation as hurdle technology has been used to improve the safety, quality and stability of foods when a single process is not good enough to achieve these goals. Multiple intervention technologies have been used in this study in combination with the antimicrobial packaging systems including cryogenic freezing, ozone treatment, acid washing, and steam flash pasteurization. The combination of antimicrobial coatings or films with these technologies further enhance their antimicrobial efficacy against pathogenic and spoilage microorganisms, which allows a design flexibility of using antimicrobial packaging alone or in combination with other intervention technologies.It has been observed that the antibacterial effect of antimicrobials is reduced in foods as compared to growth media. This is because a food matrix provides a protective environment to microorganisms. This suggests that in-vivo studies are a necessary step to develop antimicrobial packaging for food applications. Therefore, in addition to the extensive in-vitro studies, the developed antimicrobial packaging systems were evaluated in real foods using raw and RTE shrimp, RTE meats, and fresh fruit as models, which provides very important and necessary information for further scale-up and commercial application of this novel technology.This study demonstrates an effective approach to developing chitosan-based antimicrobial food packaging. The outcome of this study would not only reduce foodborne pathogens in these food commodities and enhance food safety, but also improve the quality and extend the shelf-life of food products. A successful application of the developed edible coating systems could provide the food industry with a more efficient tool to produce high quality food products and reduce contamination and re-contamination in the production chain and marketing. This could also help the food industry avoid economic losses due to short shelf-life, product recalls and human illnesses.