Fundamental Food Microbiology, Third Edition - Fuad Fathir

FACTORS INFLUENCING MICROBIAL GROWTH IN FOOD 73
Figure 6.1 Water sorption isotherm showing hysterisis. At the same percentage of water, A w
is lower by desorption than by adsorption.
minimum A w for growth of a bacterial strain at pH 6.8 is 0.91, then at pH 5.5, it can
be 0.95 or more. When the A w is reduced below the minimum level required for
growth of a microorganism, the cells remain viable for a while. But if the A w is
reduced drastically, microbial cells in a population lose viability, generally rapidly
at first and then more slowly. This information is used to control spoilage and
pathogenic microorganisms in food as well as enhance the growth of desirable types
in food bioprocessing (such as adding salt in processing of cured ham; see Chapter
34) and in laboratory detection of microorganisms (adding salt to media to enumerate
Sta. aureus).
D. pH and Growth
1. Principle
pH indicates the hydrogen ion concentrations in a system and is expressed as –log
[H + ], the negative logarithm of the hydrogen ion or proton concentration. It ranges
from 0 to 14, with 7.0 being neutral pH. [H + ] concentrations can differ in a system,
depending on what acid is present. Some strong acids used in foods, such as HCl
and phosphoric acid, dissociate completely. Weak acids, such as acetic or lactic
acids, remain in equilibrium with the dissociated and undissociated forms:
\
% Water
30
25
20
15
10
5
Desorption
Adsorption
0.12
0 0 0.2 0.4 0.4 0.6 0.8 1
Water Water activity activity (A (Aw)
[HCl] Æ [H + ] + [Cl – ], pH of 0.1 N HCl is 1.1