Water Activity Relationships for Selected Mesophiles and Psychrotrophs at Refrigeration Temperature

Abstract

Growth rates and lag phases of Pseudomonas fluorescens, Brochothrix thermosphacta, Salmonella typhimurium, Enterococcus faecalis and Staphylococcus aureus were studied in liquid media as a function of temp. [between 4 and 26°C], aw and solute type [glycerol, NaCl or sucrose]. The lag phase lengthened and growth rate decreased when the temp. was lowered or the aw reduced; these variations depended on the aw-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 aw-controlling solute as the lag phase. A linear extrapolation method was a reliable and convenient method to estimate the min. aw for microbial growth.