Le Chatelier’s Principle
This principle considers the changes to the equilibrium as a result of pressure application. It states that any phenomenon (phase transition, change in molecular configuration, chemical reaction) accompanied by a decrease in volume is enhanced by pressure.
If pressure (extensive variable) changes, the equilibrium shifts in the direction that tends to reduce the change in the corresponding intensive variable (volume). Thus, pressure shifts the system to that of lowest volume.
This considers the uniform application of pressure, act equally in all directions. Unlike heat treatment, true hydrostatic condition should be independent of time, and space. Since air and water differ in compressibility under pressure, the structure and shape of the foods containing air pockets (as in the case of marshmallows) may be altered upon pressure treatment, unless the food is perfectly elastic and consists of closed-cell foam from which air cannot escape.
Pressure leads to increased ionization because water molecules arrange more compact around electric charges. This results in more or less pronounced negative and reversible pH shifts dependent on the chemical nature of the buffer.
Principle of microscopic ordering
At constant temperature, an increase in pressure increases the degree of ordering of molecules of a given substance. Therefore, pressure and temperature exert antagonistic forces on molecular structure and chemical reactions.
Like thermal processing, various reaction rates during HPP are also influenced by thermal effects during pressure treatment. The net pressure-thermal effects can be synergistic, additive or antagonistic.