On modeling the effect of water activity on microbial growth and mortality kinetics

Abstract

The effect of moisture on the growth of different kinds of bacteria, yeasts, and molds in or on foods has traditionally been depicted in the form of a rate vs. water activity (aw) plot. The rate curve of each organism type begins (or ends) at a point along the aw-axis. At lower aw levels, presumably, the particular type of organism does not grow, i.e., its growth rate is zero. While such plots can and have been successfully used to identify safe water activities for food storage, for example, they do not always tell the whole story. One reason is that the borderline between growth and no growth is rarely sharp. Another reason is that a plot of this kind does not fully reveal what happens above the marked water activity threshold. Moreover, in many foods, water activity control is accompanied by a drying process, in which case the temperature history may affect the type or number of organisms remaining in the food, and/or the introduction of a complementary agent such as salt or chemical preservative. To deal with these aspects of microbial growth inhibition, it is necessary to develop a kinetic model of growth whose parameters account for the role of all pertinent factors that affect a food’s biological stability and that of their interactions. The same can be said about chemical and physical changes that inevitably occur during food preservation, especially when heat is involved. Such changes are of two kinds: degradation, notably the loss of freshness and nutrients, or synthesis, notably the accumulation of Maillard reaction products, but also other compounds that impart off flavor. Water activity, and factors such as pH and salt concentration, affect the heat resistance not only of microorganisms, including bacterial spores, but also of enzymes. The subject has been amply studied and there is a large body of literature covering its various aspects.