The pressure sensitivity of microorganisms may vary
between species and probably among the strains of the same
species. Microorganisms can be divided into those that are
relatively pressure sensitive and those that are pressure
resistant. Generally Gram-positive bacteria are more resistant
to pressure than Gram-negative bacteria, moulds and yeasts.
Most resistant to high hydrostatic pressure are bacterial
spores. The sensitivity of microbial cells depends on the stage
of the growth cycle at which the organisms are subjected to
high hydrostatic pressure treatment. In general, cells in the
exponential phase are more sensitive to pressure treatments
than cells in the log or stationary phases of growth (16, 42).
The grater resistance to pressure when the cell metabolism is
slowed down may be due to accumulation of cell components
that reduce the effect of high pressure (31). Rich nutrient media
such as meat reinforce the resistance of the microorganisms to
HPP. Carbohydrates, proteins and lipids also have a protective
effect (35). A low water activity protects microorganisms
against pressure and tends to inhibit pressure inactivation with
noticeable retardation as water activity falls below 0.95 (46).