REVIEW - HIGH-PRESSURE, MICROBIAL INACTIVATION AND FOOD PRESERVATION

High pressure (1 to 10 kbars, i.e. 100-1000 MPa) affects biological co nstituents and systems. Several physicochemical properties of water ar e 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, inc luding oxidative enzymes from fruits and vegetables, are strongly baro resistant. Chemical reactions, macromolecular transconformations, chan ges in membrane structure, or changes in crystal form and melting poin t that are accompanied by a decrease in volume are enhanced under pres sure (and vice versa). Several of these phenomena, still poorly identi fied, are involved in the high inactivation ratio (5-6 logarithmic cyc les) of most vegetative microbial cells: gram-negative bacteria, yeast s, complex viruses, molds, and gram-positive bacteria, in this decreas ing order of sensitivity to pressure. Much variability is noted in the baroresistance of microorganisms, even within one single species or g enus. Other parameters influence this resistance: pressure level, hold ing time (a two-phase kinetics of inactivation is often observed that prevents the calculation of decimal reduction times), temperature of p ressure processing (temperatures above 50 degrees C or between -30 and +5 degrees 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 ba roprotective 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 pres sure is associated with moderate temperature, CO2, other bacteriostati c agents, or to nonthermal physical processes such as ultrasounds, alt ernative currents, high-voltage electric pulses, and so forth. The saf ety and refrigerated shelf Life of pressurized foods could be maintain ed or extended, while the sensorial quality should improve due to the reduced severity of thermal processing Further research is, however, n eeded for the regulatory authorities to assess and accept these novel foods and processes.