Radio frequency inactivation of Salmonella Typhimurium and Listeria monocytogenes in skimmed and whole milk powder

Milk powder is a convenient, shelf-stable food ingredient used in a variety of food products. However, pathogenic bacteria can be present and survive during prolonged storage, leading to outbreaks of foodborne diseases and product recalls. Radio frequency (RF) heating is a processing technology suitable for bulk treatment of milk powder, aiming at microbial inactivation. This study investigates the RF inactivation of Salmonella Typhimurium and Listeria monocytogenes in two types of milk powder; skimmed and whole milk powder. Specifically, the aims were to (i) examine the influence of the powder's composition on bacterial inactivation, (ii) evaluate the response of bacteria with different Gram properties (Gram positive and Gram negative) and (iii) verify the use of Enterococcus faecium as a surrogate for the two microorganisms for the specific RF process. In order to examine exclusively the influence of RF, a non-isothermal temperature profile was used, employing solely different RF energy levels to heat the product to the target temperatures. A log-linear model with a Bigelow-type temperature dependency was fitted to the experimental data. S. Typhimurium was less susceptible to RF treatments in comparison to L. monocytogenes, demonstrating a higher inactivation rate (k) and higher percentage of sublethal injury. A higher k was also observed for both microorganisms in the whole milk powder, indicating that the increased fat content and decreased levels of lactose and protein in the milk powder had an adverse impact on the microbial survival for both pathogens. The surrogate microorganism E. faecium successfully validated the microbial response of the two microorganisms to RF treatments. In general, a low heating rate RF-only process was successful in inactivating the two foodborne pathogens in skimmed and whole milk powder by 4 log(CFU/g).