Pressure Effects on the Rate of Chemical Reactions Under the High Pressure and High Temperature Conditions Used in Pressure-Assisted Thermal Processing

Thermal processing technologies are widely used to reduce microbial safety risks and extend the shelf life of foods. In spite of continuous improvements, the development of new technologies has become a necessary response to consumer demands for safer foods with closer-to-fresh quality, higher retention of nutrients, and higher bioavailability of phytochemicals promoting health. High-pressure processing (HPP) has become a well-established food pasteurization alternative with numerous and worldwide applications since the first products were first commercialized in Japan in the early 1990s. Efforts in the early 2000s to combine multiple pressure pulses and pressure shifting of pH with HPP treatments at moderate temperatures (<100 °C) failed in achieving the inactivation of bacterial spores at acceptable levels. Pressure-assisted thermal processing (PATP), called pressure-assisted sterilization (PATS) when the process yields shelf-stable foods, is an emerging technology combining the application of high temperature (>100 °C) and high pressure (>600 MPa). Under these conditions, bacterial spores can be inactivated, but the extent of chemical changes must be determined from a quality and safety point of view. However, adiabatic heating during pressurization increases temperature to lethal levels for microorganisms, which in combination with fast decompression cooling can lower the extent of chemical changes to levels below conventional thermal processing. Development of enzyme and microbial inactivation models and calculation methods to assist the design of PATP processes and equipment are advancing rapidly. However, elucidating reaction mechanisms in PATP-treated foods is challenging due to limitations when performing in situ measurements under high pressure. In this chapter, current knowledge of reaction kinetics at high pressure and elevated temperature and information on specific chemical reactions at the temperature and pressure levels required for the pasteurization and sterilization of foods, particularly those reactions known to yield toxic compounds, are presented. Predicting the direction of pressure effects on chemical reaction cannot be predicted unless its activation volume value (V _a) is experimentally determined. Reactions are accelerated or slowed by pressure if V _a is negative or positive, respectively. For example, acrylamide can be formed when foods are subjected to conventional thermal treatments above 100 °C. Under PATP treatment conditions, its formation in model systems was inhibited by pressure suggesting a positive V _a for this reaction, but this must be confirmed with experiments in foods.