Growth and survival of Listeria monocytogenes during the manufacture and storage of artisanal kefir

This study aimed to evaluate the behavior of Listeria monocytogenes during the manufacture and storage of artisanal kefir. A five-strain cocktail of L. monocytogenes was introduced (at ca. 3.0 log(10) CFU/mL) into UHT milk as a pre-fermentation contaminant. The growth of L. monocytogenes, lactic acid bacteria and yeasts was monitored during the 24-h fermentation period of milk with two types of kefir grains at 20 degrees C and the subsequent storage of kefir under adequate (4 degrees C) and inadequate (10 degrees C) refrigerated-storage conditions. L. monocytogenes grew into much higher cell densities (5.3-5.8 log(10) CFU/mL) by the end of the fermentation period, but the final (24-h) counts were substantially lower than those obtained in the corresponding control trials with unfermented milk (7.2 log(10) CFU/mL). The growth rate of L. monocytogenes during fermentation differed depending on the type of kefir grain used. During the subsequent 11-day storage of the kefir filtrates at 4 degrees C or 10 degrees C, the behavior of L. monocytogenes varied from no growth to further population increases (of up to 1.1 log(10) CFU/mL), depending on the storage temperature, but also depending on the type of kefir grain and the grain surface-to-weight ratio that had been used to ferment the milk. This study highlights the need for strict adherence to good hygiene practices during artisanal kefir making and shows that L. monocytogenes can sustain considerable growth during milk fermentation and further grow during the subsequent cold storage of kefir, particularly under inadequate refrigerated storage conditions.