Models of the water activity effect on microbial growth rate and initiation

The role water activity, a_w, plays in microbial growth by itself or in conjunction with other factors, notably temperature and pH, has been described mathematically by different algebraic models obtained by fitting experimental growth rate vs. a_w relationships. Many of these models have one, two, or all three cardinal parameters, namely the minimal, optimal, and maximal a_w, in their formulation. Although they all have good fit as judged by statistical criteria, their different mathematical structures have different ramifications concerning the threshold a_w for growth initiation, and the growth pattern around and beyond the optimal a_w level where it exists. The focus of this review is on the biological implications of the different growth rate vs. a_w models inferred exclusively from their mathematical properties, leaving out any statistical fit considerations. It also describes a recently proposed single-parameter model of monotonic or the monotonic part of experimental growth rate vs. a_w curves, which can be combined with a decay term to produce a general conceptual model of peaked and monotonic microbial growth rate vs. a_w relationships over the entire a_w range. KEY POINTS: • Traditional and new growth rate vs. a_w models are presented and their implications compared. • Analogy between a_w and the temperature or pH effect on microbial growth rate is reassessed. • Cardinal parameters alone do not establish a unique growth rate vs. a_w relationship.