Chemical Kinetics for the Microbial Safety of Foods Treated with High Pressure Processing or Hurdles

The application of chemical kinetics is well known in food engineering, such as the use of Arrhenius plots and D- and z-values to characterize linear microbial inactivation kinetics by thermal processing. The emergence and growing commercialization of nonthermal processing technologies in the past decade provided impetus for the development of nonlinear models to describe nonlinear inactivation kinetics of foodborne microbes. One such model, the enhanced quasi-chemical kinetics (EQCK) model, postulates a mechanistic sequence of reaction steps and uses a chemical kinetics approach to developing a system of rate equations (ordinary differential equations) that provide the mathematical basis for describing an array of complex nonlinear dynamics exhibited by microbes in foods. Specifically, the EQCK model characterizes continuous growth–death–tailing dynamics (or subsets thereof) for pathogens such as Staphylococcus aureus, Listeria monocytogenes, or Escherichia coli in various food matrices (bread, turkey, ham, cheese) controlled by “hurdles” (water activity, pH, temperature, antimicrobials). The EQCK model is also used with high pressure processing (HPP), to characterize nonlinear inactivation kinetics for E. coli (inactivation plots show lag times), baro-resistant L. monocytogenes (inactivation plots show slight lag times and protracted tailing), and Bacillus amyloliquefaciens spores (inactivation plots show protracted tailing; HPP also induces spore activation and spore germination). We invoke further chemical kinetics principles by applying transition-state theory (TST) to the HPP inactivation of L. monocytogenes and develop novel dimensionless secondary models for temperature and pressure (TST temperature and TST pressure) to estimate kinetics parameters (activation energy E a and activation volume ∆V ‡), thereby offering new insights into the inactivation mechanisms of pathogenic organisms by HPP.