Changes in pH affects bioactivity of chitosans from Callinectes sapidus

Abstract

The effect of pH on the antimicrobial activity of chitosans was evaluated with five different molecular weight chitosans (DMPAC: MW 152; DMPCA: MW 338; DPMCA₍₂₎: MW 418; DMCPA: MW550 and DCMPA,MW 558) at 500μg/ml concentration on different food-borne bacteria and fungi. Studies on pH was carried out at pH 3.0, 3.5, 4.0, 5.0 and 5.8 with 500μg/ml chitosan using an overnight broth of bacteria (0.05ml) sub-cultured in nutrient broth or MRS broth (for lactic acid bacteria). Fungi were incubated at 28+ 2⁰C for 72h and enumerated on sabouraud dextrose agar. The viable cell count of Staphylococcus aureus at pH 3.0 for all chitosans ranged between 1.23-1.76Log₁₀CFU/ml while at pH 5.8 viable cell numbers was 2.38-5.26log₁₀CFU/ml compared to the initial inoculum number of 7.06Log₁₀CFU/ml. The growth of Listeria monocytogenes was totally suppressed by 500μg/ml chitosan at or below pH 5.0. Bacillus subtilis was susceptible to inhibition at low pH and had no detectable viable cell counts at pH 3.0-4.0. The viable cell numbers of Escherichia coli 0157:pH7 were reduced by approximately 2 log₁₀ cycles at pH 5.8 and by more than 5 logs at pH3.0 with DMPAC chitosan. Rhizopus Stolonifer was reduced to non-detectable levels by DMPAC chitosan at pH 3-3.5.This mould was more sensitive to chitosan (500μg/ml) at all pH compared to Penicillium expansum and Aspergillu sniger. Saccharomyces cerevisiae and Pichia fermentans were similarly affected by low pH. The results of the present study show that application of chitosan to acidic foods such as fruit juices will enhance its effectiveness as a natural preservative.

Keywords: Antimicrobial, chitosans, Crab, Acidity, Microorganisms