Review : High-pressure, microbial inactivation and food preservation

High pressure (1 to 10 kbars, i.e. 100-1000 MPa) affects biological constituents and systems. Several physicochemical properties of water are modified, such as the density, the ionic dissociation (and pH), and the melting point of ice. Pressure-induced unfolding, aggregation, and gelation of food proteins mainly depend on the effects of pressure on various noncovalent bonds and interactions. Enzyme inactivation (e.g., of ATPases) also results from similar effects, but some enzymes, including oxidative enzymes from fruits and vegetables, are strongly baroresistant. Chemical reactions, macromolecular transconformations, changes in membrane structure, or changes in crystal form and melting point that are accompanied by a decrease in volume are enhanced under pressure (and vice versa). Several of these phenomena, still poorly identified, are involved in the high inactivation ratio (5–6 logarithmic cycles) of most vegetative microbial cells: gram-negative bacteria, yeasts, complex viruses, molds, and gram-positive bacteria, in this decreasing order of sensitivity to pressure. Much variability is noted in the baroresistance of microorganisms, even within one single species or genus. Other parameters influence this resistance: pressure level, holding time (a two-phase kinetics of inactivation is often observed that prevents the calculation of decimal reduction times), temperature of pressure processing (temperatures above 50°C or between –30 and +5°C enhancing inactivation), composition of the medium or of the food (the pH having apparently little influence, but high salt or sugar concentrations, and low water contents, exerting very strong baroprotective effects). Taking into account the baroprotective effects of some food constituents and the strong resistance of some microbial strains, recent research aims at combined processes in which high pressure is associated with moderate temperature, CO2, other bacteriostatic agents, or to nonthermal physical processes such as ultrasounds, alternative currents, high-voltage electric pulses, and so forth. The safety and refrigerated shelf life of pressurized foods could be maintained or extended, while the sensorial quality should improve due to the reduced severity of thermal processing. Further research is, however, needed for the regulatory authorities to assess and accept these novel foods and processes.