Behavior of Listeria monocytogenes in Buttermilk, an Indian Fermented Milk Beverage

Abstract

The behavior of Listeria monocytogenes during preparation and storage of dahi, and in buttermilk made from pre- or post-fermentation inoculated dahi during refrigerated storage was determined. A five-strain L. monocytogenes cocktail was inoculated into milk mix prior to fermentation or buttermilk (prepared from non-inoculated dahi) to achieve ca. 3.0 or 6.0 log CFU/ ml. The pH, acidity and microbial populations in dahi and buttermilk were determined at different time intervals during fermentation and storage at 4 or 8°C for 10 days. Titratable acidity of dahi increased from 0.18 to 0.93%; and the pH declined from 6.53 to 4.53 within 5 h of fermentation. Presence of L. monocytogenes at either the low or high inoculum levels did not affect fermentation process (p>0.05). Titratable acidity of pre-fermentation inoculated dahi increased slowly (p≤0.05) from 0.98 to 1.14% during refrigerated storage at 4°C for 10 days, while  L. monocytogenes population decreased by 2.71 and 2.55 log CFU/ ml during the same period. Compared to dahi (stored at 4°C), minimal increase in acidity (0.03%) and decline in pH of buttermilk (pre-fermentation inoculated) was observed at 4°C for 10 days. A reduction in L. monocytogenes population (p≤0.05; ca. 1.5 log CFU/ml) was observed during storage of buttermilk, regardless of the initial inoculum level. Reductions in L. monocytogenes populations were lower (p>0.05) in buttermilk (post-fermentation) compared to dahi, subsequent to storage at either 4° or 8°C. As L. monocytogenes survives in dahi or buttermilk, it is essential that extreme care is required to avoid the post-heating contamination of milk by adopting good hygienic practices during manufacture of dahi and buttermilk. In addition, Post-preparation pasteurization should be adopted to assure its safety as reductions in L. monocytogenes populations in buttermilk either during processing or during refrigerated storage were minimal.