Devlieghere and Debevere [154] investigated the antimicrobial effect
of chitosan, with high deacetylation degree and low molecular
weight against several psychrotrophic food-borne pathogens and
spoilage micro-organisms and compared to those known from the literature.
They found that Gram-negative bacteria were more susceptible,
while the sensitivity of the Gram-positive bacteria was highly
variable: Brochotrix thermosphacta and B. cereus were very sensitive
to the applied chitosan while L. monocytogenes and different lactic
acid bacteria were less susceptible. Yeasts, represented by Candida
lambica and Cryptococcus humicolus, showed an intermediate sensitivity,
the effect of pH on the antimicrobial activity of chitosan on
C. lambica is illustrated in Fig. 21. The growth of C. lambica in
Sabouraud medium (Oxoid, CM147) was not influenced by the pH
in the absence of chitosan. In a medium containing 0.005% (w/v) chitosan
the growth was completely inhibited at pH 4.0, while at pH 6.0,
the same chitosan concentration led to a significant extension of the
lag phase and a rather small decrease in growth rate.
Devlieghere and Debevere [154] used soluble starch, which caused a
pH decrease in the growthmedium. Three different chitosan concentrations
(0%, 0.005% and 0.01% (w/v)) and starch concentrations (0%, 1%
and 30% (w/v)) were tested. Without starch, 0.005% chitosan was
enough to cause a significant retardation of the growth of C. lambica at
pH 5.0 and a higher concentration of chitosan even caused inactivation
of the yeast. Neither the lag phase nor the growth rate of C. lambica was
influenced by low amounts of starch (1% (w/v)).
On the other hand, the activity of chitosanwas strongly decreased by
high amounts (30% (w/v)) of starch, leading to a significantly shorter lag
phase and a significantly higher growth rate (Fig. 22), demonstrating the