Abstract:
Listeria monocytogenes, a food-borne pathogen often found in milk and milk
products, causes listeriosis in pregnant women, newborns, elderly and immuno-suppreSsed people? There have been reports on survival of Listeria in various milk
products in the world, but comparable studies on the survival of Listeria in buffalo
curd have not been reported. Therefore, the present study investigated the effects of
fermenting buffalo milk and acidity on survival kinetics of L. monocytogenes. L.
monocytogenes FSTLC2 and lactic acid starter cultures were aseptically introduced
to boiled and cooled buffalo milk (fat 6%, protein 4%) and the mix was allowed to
ferment at ambient temperature ( 2 6 . °Q for 18 h. The Listeria count on Modified
Oxford Agar (MOX; Oxoid Ltd.), lactic acid bacteria count on MRS Agar (Oxoid
Ltd ), pH and titratable acidity were determined in the beginning and at 2 h intervals
during fermentation. The effect of pH/acidity on Listeria was determined by
introducing Listeria to buffalo milk with pH values (pH 4;0, 4.5, 5.0, 5.5) adjusted
using 88% lactic acid (BDH Chemicals) and enumerating Listeria on MOX Agar at
12 h intervals for 96 h. It was observed in the present study that Listeria count
decreased over time and after 16 h of fermentation of buffalo milk at ambient
temperature, Listeria cannot be detected on MOX Agar. The pH value decreased
from 6.8 to 4.1 and titratable acidity (lactic acid %) increased from 0% to 1.2 %
during fermentation. Fermentation appeared to be an effective preservation
technique in eliminating Listeria in buffalo curd. The pH value of 5.5 appeared to
be the critical pH for inactivation of Listeria as no growth of Listeria was observed
below pH 5.5. The total inactivation of Listeria in buffalo milk appeared to be due
to lowering of pH coupled with increasing titratable acidity as well as action of
bacteriocins, especially Nisin, produced by lactic acid bacteria during fermentation.
It can be concluded that fermentation is an effective tool in inactivating Listeria in
buffalo milk.
