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

Abstract

<i>Salmonella</i> spp. exhibit prolonged survivability and high tolerance to heat in low-moisture foods. The reported thermal resistance parameters of <i>Salmonella</i> spp. in low-moisture foods appear to be unpredictable due to various unknown factors. We report here that temperature-dependent water activity (a_{w, treatment temperature}) plays an important role in the sharply increased thermal resistance of <i>Salmonella enterica</i> serovar Enteritidis PT 30 and its potential surrogate <i>Enterococcus faecium</i> 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_w,80°C 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 (<i>R</i>², 0.83 to 0.97). Reductions in the a_w,80°C values of bacterial cells exponentially increased the <i>D</i>_80°C (the time needed to achieve a 1-log reduction in a bacterial population at 80°C) values for <i>S</i> Enteritidis and <i>E. faecium</i> on silicon dioxide. The log-linear relationship between the <i>D</i>_80°C values for each strain in silicon dioxide and its a_w,80°C values was also verified for organic wheat flour. <i>E. faecium</i> showed consistently higher <i>D</i>_80°C values than <i>S</i> Enteritidis over the a_w,80°C range tested. The estimated z_aw (the change in a_w,80°C needed to change <i>D</i>_80°C by 1 log) values of <i>S</i> Enteritidis and <i>E. faecium</i> were 0.31 and 0.28, respectively. This study provides insight into the interpretation of <i>Salmonella</i> thermal resistance that could guide the development and validation of thermal processing of low-moisture foods.<b>IMPORTANCE</b> In this paper, we established that the thermal resistance of the pathogen <i>S</i> Enteritidis and its surrogate <i>Enterococcus faecium</i>, as reflected by <i>D</i> values at 80°C, increases sharply with decreasing relative humidity in the environment. The log-linear relationship between the <i>D</i>_80°C values of each strain in silicon dioxide and its a_w,80°C 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.