Exponentially Increased Thermal Resistance of Salmonella spp. and Enterococcus faecium at Reduced Water Activity

Abstract

spp. exhibit prolonged survivability and high tolerance to heat in low-moisture foods. The reported thermal resistance parameters of spp. in low-moisture foods appear to be unpredictable due to various unknown factors. We report here that temperature-dependent water activity (a) plays an important role in the sharply increased thermal resistance of serovar Enteritidis PT 30 and its potential surrogate NRRL B-2354. In our study, silicon dioxide granules, as carriers, were separately inoculated with these two microorganisms and were heated at 80°C with controlled relative humidity between 18 and 72% (resulting in corresponding a values for bacteria between 0.18 and 0.72) in custom-designed test cells. The inactivation kinetics of both microorganisms fitted a log-linear model (, 0.83 to 0.97). Reductions in the a values of bacterial cells exponentially increased the (the time needed to achieve a 1-log reduction in a bacterial population at 80°C) values for Enteritidis and on silicon dioxide. The log-linear relationship between the values for each strain in silicon dioxide and its a values was also verified for organic wheat flour. showed consistently higher values than Enteritidis over the a range tested. The estimated z (the change in a needed to change by 1 log) values of Enteritidis and were 0.31 and 0.28, respectively. This study provides insight into the interpretation of thermal resistance that could guide the development and validation of thermal processing of low-moisture foods. In this paper, we established that the thermal resistance of the pathogen Enteritidis and its surrogate, as reflected by values at 80°C, increases sharply with decreasing relative humidity in the environment. The log-linear relationship between the values of each strain in silicon dioxide and its a values was also verified for organic wheat flour. The results provide new quantitative insight into the way in which the thermal resistance of microorganisms changes in low-moisture systems, and they should aid in the development of effective thermal treatment strategies for pathogen control in low-moisture foods.

Expression of DnaK and HtrA genes under high temperatures and their impact on thermotolerance of a Salmonella serotype isolated from tahini product

“…The main cause of the high values in the previously mentioned study could be due to lower water activity. In addition, Liu et al [4] showed that 70 °C is a critical temperature for Salmonella death, which is coherent with the results of the present study. Moreover, high thermotolerance of Salmonella in the aqueous solutions with high water activity was reported, where the heat treatment up to 85 °C for 1 min did not eliminate the naturally occurring contaminant from alfalfa seeds [34].…”

“…It has been reported that the D values at 60 °C and 65 °C of Salmonella spp. in beef meat were 8.6 min and 1.5 min, respectively [4], while according to [34–36], the D values of Salmonella spp. at the same temperatures and in the same product were 5.3 and 0.53 min, respectively [33], indicating variability in the D values of Salmonella spp.…”

“…In addition, the D value of Salmonella spp. at 70 °C was found to be 24 s at water activity of 0.72 in poultry meat [4] and this is a high thermotolerance degree for a member of Enterobacteriaceae .…”

Understanding water activity change in oil with temperature

“…It could be a useful tool in the design of thermal treatments for control of bacterial pathogens in oil-rich low-moisture foods, in which the high-temperature water activity of oil dominates the thermal resistance of pathogens of concern. As previously illustrated ( Liu et al., 2018 , Xu et al., 2019 ), it is necessary to elevate the water activity of a product to above 0.8 at 80 °C to achieve a 5-log reduction of Salmonella within 12 min thermal treatments at 80 °C. According to Fig.…”

“…Our recent studies on thermal inactivation of Salmonella and E. faecium suggest that the actual water activity at the treatment temperature is a key factor influencing the heat tolerance of bacteria in low-moisture foods. The thermal death times of these microorganisms in almond flour, wheat flour, whey protein powder, and sands (SiO 2 ) increase exponentially with decreasing water activities controlled at 80 °C ( Liu et al., 2018 , Xu et al., 2019 ). The water activities of different matrices respond differently to temperature changes.…”