Environmental factors and inoculation methods impact transfer of Salmonella Tennessee on surfaces to nonfat dried milk.

Cross-contamination during low-moisture food (LMF) processing poses a serious food safety risk, especially when pathogens like Salmonella persist on surfaces such as equipment and tools. Environmental factors, particularly relative humidity (RH), may significantly influence this risk. This study evaluated the persistent bacterial populations (PBP) of Salmonella Tennessee on stainless steel surfaces and its transfer to nonfat dry milk (NFDM) under varying RH conditions. Stainless steel coupons were inoculated with either high (7-8 log CFU/cm) or low (3-4 log CFU/cm) concentrations of Salmonella Tennessee using two contamination methods: (1) a premixed slurry of NFDM and inoculum, or (2) NFDM pressed onto the inoculum. Coupons were then stored at 25 °C and either 30 % or 70 % RH. Cross-contamination was simulated on day 3 or 7 by placing each coupon into a tube with 10 g of non-inoculated NFDM and vortexing for 2 min. Bacteria recovered from both the coupon and NFDM, and percent transfer (PT) was calculated and log transformed. ANOVA and Tukey's HSD revealed that PT decreased with higher initial inoculum and varied significantly by RH, matrix, and contamination method (P < 0.05). The highest levels of Salmonella transferred to the NFDM was 5.3 log CFU/g in the high inoculum group and 3.5 log CFU/g in the low inoculum group, both using the slurry method at 30 % RH after 7 days. These findings highlight how RH, contamination method, and bacterial load influence cross-contamination in LMF systems, suggesting the need for tailored hygiene interventions in food processing environments.