Water Activity Relationships for Selected Mesophiles and Psychrotrophs at Refrigeration Temperature

The growth rate and lag phase of Pseudomonas fluorescens , Brochothrix thermosphacta , Salmonella typhlmurium , Enterococcus faecalis , and Staphylococcus aureus were studied in liquid media as a function of temperature, water activity (a_w) and solute type. The lag phase lengthened and the growth rate decreased when the temperature was lowered or the a_w reduced, and these variations depended on the a_w-controlling solute. In general, the magnitude order of the solute effect on the growth rate parameters was glycerol < NaCl < sucrose. This effect can be related to the ability of the solutes to permeate the cell and can be explained by the osmoregulatory mechanism. The specific growth rate was not as sensitive to the a_w-controlling solute as the lag phase. A linear extrapolation method was a reliable and convenient method to estimate the minimum a_w for microbial growth.