Investigation of the potential direct and cross protection effects of sublethal injured Salmonella Typhimurium induced by radio frequency heating stress

Effects of Cross-Resistance of Salmonella Enterica Serovar Enteritidis Induced by Sodium Hypochlorite to Environmental Stress

To investigate the effects of repeated sodium hypochlorite stress on the resistance of Salmonella enterica Serovar Enteritidis (S. Enteritidis) LWCC1051. LWCC1051 was exposed to Trypticase Soy Broth (TSB) containing sodium hypochlorite concentrations of 9 mmol/L, 10 mmol/L, and 11 mmol/L. After 13 repeated transfers and incubations, three sodium hypochlorite resisted LWCC1051 strains were obtained. The D-values and colony morphologies of these strains were assessed. Their survival rates at 60 °C, 65 °C, 70 °C, 75 °C, and - 20 °C were determined and lethality curves at these temperatures were fitted using the Weibull model. Additionally, the Minimal Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) for these strains in various chemicals, including malic acid, citric acid, ascorbic acid, acetic acid, lactic acid, hydrochloric acid, sodium hydroxide, potassium hydroxide, calcium chloride, sodium chloride, and potassium chloride were ascertained. Sodium hypochlorite concentrations of 9 mmol/L, 10 mmol/L, and 11 mmol/L in TSB induced sodium hypochlorite resistance in S. Enteritidis. D-value increased with the frequency of stress exposure. Higher concentrations of sodium hypochlorite resulted in greater D-values and noticeable differences in colony morphologies. The Weibull model accurately represented the temperature resistance curves of LWCC1051 at the specified temperatures. With increasing sodium hypochlorite stress, both high and low-temperature resistances of LWCC1051 improved. Furthermore, under acetic acid stress, the MIC and MBC values of LWCC1051 strains, post exposure to 9 mmol/L, 10 mmol/L, and 11 mmol/L sodium hypochlorite, doubled. Sodium hypochlorite stress enhances the cross-resistance of LWCC1051 to high temperature, low temperature, and acetic acid treatments.

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).

Salmonella spp. in low water activity food: Occurrence, survival mechanisms, and thermoresistance

The occurrence of disease outbreaks involving low-water-activity (a_w ) foods has gained increased prominence due in part to the fact that reducing free water in these foods is normally a measure that controls the growth and multiplication of pathogenic microorganisms. Salmonella, one of the main bacteria involved in these outbreaks, represents a major public health problem worldwide and in Brazil, which highlights the importance of good manufacturing and handling practices for food quality. The virulence of this pathogen, associated with its high ability to persist in the environment, makes Salmonella one of the main challenges for the food industry. The objectives of this article are to present the general characteristics, virulence, thermoresistance, control, and relevance of Salmonella in foodborne diseases, and describe the so-called low-water-activity foods and the salmonellosis outbreaks involving them.

Recent developments in low-moisture foods: microbial validation studies of thermal pasteurization processes

Outbreaks associated with low-moisture foods (e.g., wheat flour, nuts, and cereals) have urged the development of novel technologies and re-validation of legacy pasteurization process. For various thermal pasteurization processes, they share same scientific facts (e.g., bacterial heat resistance increased at reduced water activity) and guidelines. However, they also face specific challenges because of their different heat transfer mechanisms, processing conditions, or associated low-moisture foods' formulations. In this article, we first introduced the general structural for validating a thermal process and the shared basic information that would support our understanding of the key elements of each thermal process. Then, we reviewed the current progress of validation studies of 7 individual heating technologies (drying roasting, radiofrequency-assisted pasteurization, superheated steam, etc.) and the combined treatments (e.g., infrared and hot air). Last, we discussed knowledge gaps that require more scientific data in the future studies. We aimed to provide a process-centric view point of thermal pasteurization studies of low-moisture foods. The information could provide detailed protocol for process developers, operators, and managers to enhance low-moisture foods safety.