The dependence of microbial inactivation by emergent nonthermal processing technologies on pH and water activity

pH and water activity (aw) are crucial for the control of microorganisms in foods. This review critically analyzed the influence of pH and water activity on the inactivation of microorganisms in foods and how the synergistic effect between them and how they may be adjusted in the foods influences the effectiveness of the processing technology. Generally, lower pH values (≤4.6) synergistically affect microorganisms' inactivation, requiring less intense thermal exposure to ensure food safety, compared to low-acid foods (pH >4.6) and this occurs for most technologies presented, like high pressure processing (for bacteria) but for pulsed electric field, cold plasma, ultrasound or ultraviolet radiation this relationship is not straightforward. Frequently, higher aw values cause higher inactivation, except for some irradiation studies. The interaction between pH and aw and which acids and solutes are employed may also affect microorganisms' inactivation rates.