The antimicrobial mode of action for chitosan is still under investigation, but there is consensus that its activity is directly related to the degree of deacetylation, the molecular weight, concentration in solution, pH of the chitosan solution, temperature and food matrix in which the chitosan is applied (Dutta et al., 2009; Aider, 2010; Elsabee and Abdou, 2013). The theorized mechanisms of action include: interaction of the positively charged amino group of the chitosan disrupting the negatively charged cell membrane which will lead to leaking of cellular contents (Young et al., 1982; Shahidi et al., 1999; Kim and Thomas, 2003), chitosan may stimulate the production of chitinases and other defense proteins in host cells (Ghaouth et al., 1992), chitosan is able to chelate trace metals leading to inhibition of microbial growth (Cuero et al., 1991) and lastly, if chitosan penetrates the cell wall it disrupts protein synthesis by binding to host DNA (Sudarshan et al., 1992). It is reported in the literature that the more highly deacetylated chitosan has a greater antimicrobial effect and, in addition, the lower the pH of the chitosan the more effective it is (Sekiguchi et al., 1993; Dutta et al., 2009). More specifically, Zheng and colleagues (Zheng and Zhu, 2003) investigated the differing antimicrobial mechanisms of chitosan against Gram-positive and Gram-negative bacteria and concluded that as the molecular weight of chitosan increased it was more bactericidal towards Gram-positives and the direct opposite was true for Gram-negative
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organisms. It is their theory that for Gram-positive bacterial the chitosan forms a barrier that block nutrients from entering the cell and for the Gram-negative organism chitosan enters the cell and disrupts normal cellular functions. The utilization of chitosan to inhibit the growth of foodborne disease causing bacteria has received a great amount of attention and chitosan has been shown to be very effective against S. aureus, E.coli, non-typhoidal Salmonella, Listeria spp., Vibrio spp., B.
cereus and Campylobacter spp. (Chhabra et al., 2006; Beverlya et al., 2008; Ganan, 2009;
Friedman and Juneja, 2010). Research into the application of chitosan onto raw chicken skin has produced positive results—Menconi and colleagues (2013) were able to reduce Salmonella Typhimurium and extend the shelf-life of skin treated with a 0.5% chitosan solution, thus demonstrating the role for chitosan in enhancing food safety and improving quality. In addition to studies on planktonic cells, chitosan and its derivatives were tested on mature biofilms of Listeria
monocytogenes and Salmonella enterica and produced significant reductions in the attached
populations of the pathogens (Orgaz et al., 2011). 2.11.3 Chitosan Coating of Poultry Products
Research and development into the utilization of chitosan in the food industry is directly related to the increase in consumer interest for fresh, microbiologically safer, minimally processed food products—and chitosan films and coating are especially useful because of their antimicrobial and antifungal properties plus their functional capabilities as carriers of other inhibitory compounds, including spices, organic acids, essential oils, and nutraceuticals (Ouattara et al., 1997; Appendini and Hotchkiss, 2002; Tapia and Rojas Graü, 2007; Petrou et al., 2012; Vasilatos and Savvaidis, 2013; Fernández-Pan et al., 2014). Many natural compounds including plant derived antimicrobials and essential oils are themselves inhibitory to foodborne pathogens, however to obtain maximum effect high concentrations are sometimes needed, potentially
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affecting organoleptic properties of the food (Ntzimani et al., 2011; Sánchez-González et al., 2011; Petrou et al., 2012). There is much interest in the combination of chitosan and these natural compounds—to reduce the concentrations of natural compounds necessary for efficacy and to evaluate potential synergism (Elsabee and Abdou, 2013). The use of chitosan and its incorporation with natural compounds for the treatment of poultry products to extend shelf-life and reduce pathogens has generated much positive data. Petrou and colleagues (2012) successfully extended the shelf-life of MAP packaged chicken breast fillets by 6–12 days with the application of 1.5% chitosan or 1.5% chitosan plus oregano oil. The combination of chitosan and thyme essential oil was able to significantly reduce spoilage organisms, Enterobacteriacae and lactic acid bacteria on packaged chicken kebabs after 12 days of storage as compared to the controls (Giatrakou et al., 2010). Ready-to-eat products are an especially important product for the control of foodborne pathogens. When chitosan and mixtures of chitosan plus lauric arginate or nisin were applied to turkey deli meat Listeria innocua was reduced by up to 4 log CFU/cm2 (Guo et al., 2014). Similarly, Zheng and co-workers (2011) significantly reduced Listeria monocytogenes on roasted turkey meat by treatment with chitosan coatings incorporated with sodium lactate or sodium diacetate. The liquid purge that accumulates in the packaging of raw poultry products is another potential source for microbial growth of spoilage organisms and pathogens. The treatment of chicken purge, artificially inoculated with E. coli, by a chitosan-arginine solution was able to reduce both the actual microbial counts and the metabolic activity of E.coli (Lahmer et al., 2012). 2.12 Caprylic Acid in the Food Industry
The increased consumer preference for less chemically treated foods has turned more attention to plant derived antimicrobials as an alternative. It has been reported that the possible benefits for the use of antimicrobials derived from plants instead of organic chemicals include lack
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of inciting bacterial resistance and less potential for negative environmental effects (Wyk and Gericke, 2000; Ohno et al., 2003; Ali et al., 2005; EFSA BIOHAZ Panel, 2014). Caprylic acid— a medium chain fatty acid found in breast milk, cow milk, palm kernel oil and coconut oil—has many positive characteristics for its use as a decontaminant on edible products (Jensen and Ferris, 1990; Sprong et al., 2001; Al Shahib and Marshall, 2003). Caprylic acid has reported broad spectrum antibacterial activity, and specific activity against the foodborne illness causing organisms Listeria monocytogenes, E.coli O157:H7, Staphylococcus aureus, Salmonella spp. and
Campylobacter spp. (Kabara et al., 1972; Wang and Johnson, 1992; McLay et al., 2002; Nair and
Vasudevan, 2004; Skrivanova, 2007).