Abstract
Milk preservation is closely related with its microbiological quality. The spoilage may occur at any stages from production till consumption and during milk processing. Many potent antimicrobials discovered recently but, it is important to test their efficacy against undesirable bacteria inside the food materials that help to improve food safety and validity. Our research was focused on comparative antibacterial activities between chitosan, lysozyme and their different mixtures against different undesirable bacterial strains by agar well diffusion assay. Then apply the selected antibacterial substances in raw cow milk contaminated with food borne pathogens and spoilage bacteria with monitoring the bacterial growth or inhibition. Our preliminary investigation showed that, chitosan at concentration 0.5% exhibited the largest inhibition zones diameter followed by LZHC complex against Salmonella enteritidis and Bacillus subtilus in-vitro by agar well diffusion method. During application in raw cow milk, LZHC complex exhibited powerful bactericidal effect followed by chitosan especially against Clostridium perfringens, Staphylococcus aureus and Listeria monocytogenes after 24h from cooling storage of raw cow milk. We found that, the bactericidal activity of LZ and LZH were greatly enhanced upon combination with chitosan. But, the bactericidal activity of LZHC complex exhibited great killing power than the conjugation between native LZ and chitosan (LZC) complex at same concentrations. This may be attributed to the chitosan oligomers and lysozyme peptides acting in a synergistic manner in penetrating and killing undesirable bacteria. Although chitosan was effective in inhibiting the growth of spoilage microorganisms but, It is induce changes in raw milk pH. Accordingly, we suggest LZHC complex will be a promising antibacterial additive to produce highly safe raw milk without heat treatment and recommended further future studies to explore its antibacterial mechanism. Also we recommend chitosan to be applied in fermented dairy products to produce safe and good quality product.

Key words: Chitosan, Lysozyme, Raw milk, Food borne pathogens, antibacterial activity.