The effect of pH on the antimicrobial activity of chitosans was evaluated with five different
molecular weight chitosans (DMPAC: M
W 152; DMPCA: M
W 338; DPMCA
(2): M
W 418; DMCPA: M
W550 and
DCMPA,M
W 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± 2
0C for 72h and enumerated on
sabouraud dextrose agar. The viable cell count of
Staphylococcus aureus
at pH 3.0 for all chitosans ranged between
1.23-1.76Log
10CFU/ml while at pH 5.8 viable cell numbers was 2.38-5.26log
10CFU/ml compared to the initial inoculum
number of 7.06Log
10CFU/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 log
10 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.
