Long-Term Survival of Cronobacter sakazakii, Salmonella enterica, Listeria monocytogenes, and Enterobacter spp. in Powdered Infant Formula Based on Relative Humidity

Powdered infant formula (PIF) has previously been shown to support the survival of bacterial foodborne pathogens including Cronobacter sakazakii, Listeria monocytogenes, and Salmonella enterica at various storage temperatures. However, the effect of long-term storage on bacterial survival under different relative humidity (RH) conditions is not well understood. In this study, milk-based powdered infant formula (water activity (a_w) = 0.27) was inoculated individually with cocktails of Salmonella enterica, Enterobacter spp., Cronobacter sakazakii, or Listeria monocytogenes and stored at 25 °C under controlled relative humidity (RH) levels of 23, 33, 43, 53, or 75%. Bacterial populations and PIF a_w were measured at intervals up to 360 days. The highest a_w of 0.39 was recorded for PIF stored under 75% RH after 90 d or at 53% RH after 360 d; however, these levels also resulted in significant bacterial population reductions of >1 log CFU/g after 60-120 d and 60-180 d, respectively, with L. monocytogenes being the least desiccation-resistant of those tested. RH levels between 23 and 43% did not result in a significant change in a_w over 360 d; however, bacterial populations did decrease, but to a lesser extent than for the higher RH levels. Population reductions for all bacteria were ≤3.50 log CFU/g after 360 d. All four bacteria examined in this study persisted in PIF for ≥90 d without a significant decrease in population regardless of RH storage conditions.