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.