activity. Microorganisms vary widely in the water activity that they can
tolerate. Generally, the lowestawfor growth is for
Bacteria 0.98–0.9, halophilic ones down to*0.75
Yeasts about 0.9, osmophilic ones down to*0.6
Molds 0.92–0.8, xerophilic ones down to*0.65Osmotic Pressure. The lowestawfor growth is generally much
higher than the lowest one for most chemical reactions. This points to a very
different inhibitory mechanism. The simplest explanation is that a lowaw
means a high osmotic pressure (P), and that the organism cannot tolerate
this. IfPis high, it tends to draw water from the cell, thereby concentrating
the cell contents and damaging the metabolic system. The organism tries to
keep water in the cell, and it has several mechanisms for this, but they all fail
at very highP. From Eq. (8.3) we can calculate the relation
P
Pa& 135? 106 ð 1 awÞð 8 : 9 Þat room temperature and for not too lowaw. This implies that already at
aw¼ 0 : 92 ;P>100 bar. It may also be noted that a highPneed not kill the
organisms, and activity of bacterial enzymes has often been observed.
Bacterial spores can survive at very highP, though not germinate.
There are, however, severalcomplications. The terms halophilic and
osmophilic already suggest that different substances have different effects.
Halophilic bacteria can tolerate a high concentration of salts, but not of
sugars, and it is the other way round for osmophilic yeasts. Table 8.1 gives
some examples of the lowest aw tolerated, when caused by various
components. It is seen that the variation is considerable, by a factor of 5
inð 1 awÞ. This implies that the components have specific effects. Ethanol
and glycerol cannot be kept out of the cell by most organisms, and thus do
not cause an osmotic pressure difference over the cell membrane.
TABLE8.1 Minimum Water Activity for Growth of Two Bacteria, Where the
Water Activity Is Lowered by Addition of Various Components
Component Staphylococcus aureus Lactococcus lactis
Ethanol 0.973 —
Polyethylene glycol 0.927 —
Glycerol 0.89 0.924
Sucrose 0.87 0.949
Sodium chloride 0.86 0.965