Effect of water activity on the thermal inactivation kinetics of Salmonella in milk powders

Abstract

Persistence of Salmonella in milk powders has caused several foodborne outbreaks. The determination of proper pasteurization processing conditions requires an understanding of the thermal inactivation kinetics of Salmonella in milk powders. However, there is a lack of knowledge related to the effects of water activity (a w ) and fat content on Salmonella inactivation in milk powder during thermal processing. Two types of milk powders, nonfat dry milk and whole milk powder, with different fat contents (0.62 and 29.46% wt/wt, respectively) were inoculated with a 5-strain cocktail of Salmonella and equilibrated to 3 a w levels (0.10, 0.20, and 0.30) for isothermal treatments at 75, 80, and 85°C to obtain D-values (the time required to achieve a 10-fold reduction of the bacteria at the isothermal treatment temperature) and z-values (the increase in temperature required to achieve a 90% reduction of the decimal reduction time D). Stability tests showed that the inoculation method used in this study provided a high and stable population of Salmonella for thermal inactivation studies. A moisture sorption isotherm was measured to understand the relationship between a w and moisture content of milk powders. The thermal resistance of Salmonella was found to significantly increase as a w decreased, which suggested that a higher temperature or longer processing time would be required at low a w to achieve the desired inactivation of Salmonella. The microbial inactivation kinetics were not significantly different for the 2 milk powders; therefore, data were combined to develop a universal model. A response surface model was compared with a modified Bigelow model. The modified Bigelow model performed well to predict D-values [root mean square error (RMSE) = 1.47 min] and log reductions (RMSE = 0.48 log cfu/g). The modified Bigelow model developed here could be used to estimate D-value as a function of water activity and temperature to design a thermal pasteurization system for milk powders.

Survival of Escherichia coli O157:H7 during Moderate Temperature Dehydration of Plant-Based Foods

“…With temperature-only and temperature and a w models resulting in similar RMSE values, this indicates that the addition of the a w term did not lower the model’s predictive power. This is in contrast to previous work where interaction effects were not identified [ 35 , 36 , 37 , 38 ]. Heating rate has been previously identified as a relevant factor in inactivation models, so the combination of a moderate treatment temperatures and acidic matrixes may by unique to dehydration in apples [ 39 , 40 ].…”

Influence of Hurdle Technology on Foodborne Pathogen Survival in the Human Gastrointestinal Tract

“…Typhimurium. Previous research concerning the acid responses of Salmonella after exposure to low a w foods is limited, yet the prolonged survival of Salmonella in low a w values has been observed to induce the tolerance of the pathogen against heat [ 31 , 32 , 33 ].…”

Evaluation of Enterococcus faecium NRRL B-2354 as a surrogate for Salmonella enterica in milk powders at different storage times and temperatures

“…The milk powders were stored inside polyethylene bags and held at ambient temperature (23 ± 2°C) before inoculation. On receiving the samples, the proximate analysis and background microorganism tests were conducted as described in Wei et al (2020a). A water activity meter (4TE, METER Group, Pullman, WA) was used to measure the water activity of milk powders at 25°C.…”

“…The mixed inoculated sample (5 mm thickness) was then placed on a sterile aluminum tray (230 × 300 × 15 mm) and transferred into a custom-made relative humidity chamber (Lau and Subbiah, 2020a) set to a target relative humidity of 10% to equilibrate the inoculated sample to a water activity of 0.10 at 25°C. Based on the previous study (Wei et al, 2020a), a low water activity of 0.10 was selected, which could represent the worstcase scenario where Salmonella exhibits high thermal resistance. This low water activity (0.10) represents a conservative study, as Salmonella resistance is higher at lower water activity.…”

“…The effect of storage time on the thermal resistance of Salmonella and E. faecium was investigated at 85°C and at the water activity of 0.1. The low water activity of 0.1 was selected in this study to represent the worst-case scenario, as increased thermal resistance of Salmonella has been reported at reduced water activity levels in both WMP and NFDM (Wei et al, 2020a). The isothermal treatment temperature of 85°C was chosen because there was a minimum quality impact on sensory properties of milk powders at this temperature.…”