Chitosan

Chitosan, (1^4)-2-amino-2-deoxy-^-D-glucan, is a natural constituent of fungal cell walls (Ruiz-Herrera, 1992). It is produced commercially from chitin, a by-product of shellfish processing, by alkaline deacetylation. Chitosan is the designated name for the series of polymers with different ratios of glucosamine (GlcN) and V-acetyl glucosamine (GlcNAc). Most commercial chitosans have less than 30% acetylated units (referred to as degree of acetylation less than 30%) and molecular weights between 100 and 1,200 kDa (Li et al., 1997; Onsoyen and Skaugrud, 1990).

Chitosan inhibits growth of foodborne molds, yeasts and bacteria including Aspergillus flavus, Saccharomyces cerevisiae, Zygosaccharomyces bailii, Mucor racemosus, Byssochlamys spp., Botrytis cinerea, Rhizopus stolonifer and Salmonella, Staphylococcus aureus, Escherichia coli, Yersinia enterocolitica, Listeria monocytogenes and Lactobacillus fructivorans (Roller and Covill, 2000; Sudarshan et al., 1992; Papineau et al., 1991; Wang, 1992). However, reported minimum inhibitory concentrations for both bacteria and yeasts vary widely from 0.01-5.0% depending on polymer characteristics and pH, temperature, and presence of interfering substances such as proteins and fats (Chen et al., 1998; Rhoades and Roller, 2000; Roller and Covill, 1999; Sudarshan et al., 1992; Tsai and Su, 1999; Tsai et al., 2000). Chitosan may directly affect the microbial cell by interaction with the anionic cell wall polysaccharides or components of the cytoplasmic membrane resulting in altered permeability or prevention of transport (Tsai and Su, 1999; Fang et al., 1994).

Darmadji and Izumimoto (1994) showed that 1% chitosan was necessary for reduction of only 1-2 logs of Pseudomonas, staphylococci, and total bacteria count in minced beef patties and lower concentrations (0.2 and 0.5%) had no effect on the microflora. In contrast, fresh strawberries and bell peppers dipped in acidic chitosan solutions and inoculated with B. cinerea or R. stolonifer were reported to have a shelf life equivalent to that of fruit treated with conventional fungicide (El-Ghaouth et al, 1991; El-Ghaouth, 1997). Roller and Covill (1999) reported that 0.1 to 5 g/l of chitosan glutamate inhibited growth of eight yeast species in apple juice at 250C. The most sensitive strain was Z. bailii, which was completely inactivated by chitosan glutamate at 0.1 g/l. For S. cerevisiae, the minimum inhibitory concentration was 0.4 g/l and no resumption of growth was observed after 32 days.

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