Purpose: The present study examined the survival of L. monocytogenes in 34% whey protein concentrate powder, nonfat dried milk and spray-dried cheese powder.
Methods: The three dairy powders were inoculated at a low (1-10 CFU/g) and high (10-100 CFU/g) levels using a three strain cocktail of freeze-dried L. monocytogenes. The inoculated products were stored at room temperature in sealed Whirlpak bags. A 4 dilution 3-tube most probable number (MPN) method was used to verify initial inoculation levels and to quantify L. monocytogenes at days 3, 7, 14, 21, and 28. Thereafter, 25 g of each powder at both inoculation levels was tested for the presence of L. monocytogenes at 30 day intervals using the Biomerieux VIDAS LMO2 assay.
Results: No significant differences (P > 0.05) were observed among the levels of L. monocytogenes in the low level inoculation of each dairy powder by day 28. No significant differences (P > 0.05) were also observed among the levels of L. monocytogenes in the high level inoculation of each dairy powder by day 28. L. monocytogenes was detected in the low level inoculation of the three dairy powders at days 58, 88, 118 and 148. L. monocytogenes was detected in the high level inoculation of the three dairy powders at days 58, 88, 118, 148 and 178. Results from this study indicate that L. monocytogenes can survive in these dairy powders for up to six months.
Significance: This data has significant implications for Listeria control strategies implemented during the manufacture of dairy powders.