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.