Intrinsic and Extrinsic Parameters of Foods That Affect Microbial Growth
45
can be affected by pH. The length of the hyphae of Penicillium chrysogenum has been reported to
decrease when grown in continuous culture where pH values increased above 6.0. Pellets of mycelium
rather than free hyphae were formed at about pH 6.7.
45 Extracellular H
+ and K
+ may be in competition
where the latter stimulates fermentation, for example, while the former represses it. The metabolism
of glucose by yeast cells in an acid medium was markedly stimulated by K
+ .
46 Glucose was consumed
83% more rapidly in the presence of K
+ under anaerobic conditions and 69% more under aerobic
conditions.
Other environmental factors interact with pH. With respect to temperature, the pH of the substrate
becomes more acid as the temperature increases. Concentration of salt has a definite effect on pH
growth rate curves, as illustrated in Figure 3–2, where it can be seen that the addition of 0.2 M NaCl
broadened the pH growth range of Alcaligenes faecalis. A similar result was noted for Escherichia
coli by these investigators. When the salt content exceeds this optimal level, the pH growth range
is narrowed. An adverse pH makes cells much more sensitive to toxic agents of a wide variety, and
young cells are more susceptible to pH changes than older or resting cells.
When microorganisms are grown on either side of their optimum pH range, an increased lag phase
results. The increased lag would be expected to be of longer duration if the substrate is a highly buffered
one in contrast to one that has poor buffering capacity. In other words, the length of the lag phase may
be expected to reflect the time necessary for the organisms to bring the external environment within
their optimum pH growth range. Analysis of the substances that are responsible for the adverse pH is
of value in determining not only the speed of subsequent growth, but also the minimum pH at which
salmonellae would initiate growth. Chung and Goepfert
14 found the minimum pH to be 4.05 when
hydrochloric and citric acids were used, but 5.4 and 5.5 when acetic and propionic acids were used,
respectively. This is undoubtedly a reflection of the ability of the organisms to alter their external
environment to a more favorable range in the case of hydrochloric and citric acids as opposed to the
other acids tested. It is also possible that factors other than pH come into play in the varying effects of
organic acids as growth inhibitors. For more information on pH and acidity, see Corlett and Brown.
17
Moisture Content
One of the oldest methods of preserving foods is drying or desiccation; precisely how this method
came to be used is not known. The preservation of foods by drying is a direct consequence of removal
or binding of moisture, without which microorganisms do not grow. It is now generally accepted that
the water requirements of microorganisms should be described in terms of the water activity (a w )
in the environment. This parameter is defined by the ratio of the water vapor pressure of food substrate
to the vapor pressure of pure water at the same temperature: a w = p/ p o , where p is the vapor pressure
of the solution and p o is the vapor pressure of the solvent (usually water). This concept is related to
relative humidity (RH) in the following way: RH = 100 × a w .
13 Pure water has an a w of 1.00, a 22%
NaCl solution (w/v) has an a w of 0.86, and a saturated solution of NaCl has an a w of 0.75 (Table 3–4).
The water activity (a w ) of most fresh foods is above 0.99. The minimum values reported for the
growth of some microorganisms in foods are presented in Table 3–5 (see also Chapter 18). In general,
bacteria require higher values of a w for growth than fungi, with Gram-negative bacteria having higher
requirements than Gram positives. Most spoilage bacteria do not grow below a w = 0.91, whereas
spoilage molds can grow as low as 0.80. With respect to food-poisoning bacteria, Staphylococcus
aureus can grow as low as 0.86, whereas Clostridium botulinum does not grow below 0.94. Just
as yeasts and molds grow over a wider pH range than bacteria, the same is true for a w . The lowest
reported value for foodborne bacteria is 0.75 for halophiles (literally, “salt-loving”), whereas xerophilic
45
can be affected by pH. The length of the hyphae of Penicillium chrysogenum has been reported to
decrease when grown in continuous culture where pH values increased above 6.0. Pellets of mycelium
rather than free hyphae were formed at about pH 6.7.
45 Extracellular H
+ and K
+ may be in competition
where the latter stimulates fermentation, for example, while the former represses it. The metabolism
of glucose by yeast cells in an acid medium was markedly stimulated by K
+ .
46 Glucose was consumed
83% more rapidly in the presence of K
+ under anaerobic conditions and 69% more under aerobic
conditions.
Other environmental factors interact with pH. With respect to temperature, the pH of the substrate
becomes more acid as the temperature increases. Concentration of salt has a definite effect on pH
growth rate curves, as illustrated in Figure 3–2, where it can be seen that the addition of 0.2 M NaCl
broadened the pH growth range of Alcaligenes faecalis. A similar result was noted for Escherichia
coli by these investigators. When the salt content exceeds this optimal level, the pH growth range
is narrowed. An adverse pH makes cells much more sensitive to toxic agents of a wide variety, and
young cells are more susceptible to pH changes than older or resting cells.
When microorganisms are grown on either side of their optimum pH range, an increased lag phase
results. The increased lag would be expected to be of longer duration if the substrate is a highly buffered
one in contrast to one that has poor buffering capacity. In other words, the length of the lag phase may
be expected to reflect the time necessary for the organisms to bring the external environment within
their optimum pH growth range. Analysis of the substances that are responsible for the adverse pH is
of value in determining not only the speed of subsequent growth, but also the minimum pH at which
salmonellae would initiate growth. Chung and Goepfert
14 found the minimum pH to be 4.05 when
hydrochloric and citric acids were used, but 5.4 and 5.5 when acetic and propionic acids were used,
respectively. This is undoubtedly a reflection of the ability of the organisms to alter their external
environment to a more favorable range in the case of hydrochloric and citric acids as opposed to the
other acids tested. It is also possible that factors other than pH come into play in the varying effects of
organic acids as growth inhibitors. For more information on pH and acidity, see Corlett and Brown.
17
Moisture Content
One of the oldest methods of preserving foods is drying or desiccation; precisely how this method
came to be used is not known. The preservation of foods by drying is a direct consequence of removal
or binding of moisture, without which microorganisms do not grow. It is now generally accepted that
the water requirements of microorganisms should be described in terms of the water activity (a w )
in the environment. This parameter is defined by the ratio of the water vapor pressure of food substrate
to the vapor pressure of pure water at the same temperature: a w = p/ p o , where p is the vapor pressure
of the solution and p o is the vapor pressure of the solvent (usually water). This concept is related to
relative humidity (RH) in the following way: RH = 100 × a w .
13 Pure water has an a w of 1.00, a 22%
NaCl solution (w/v) has an a w of 0.86, and a saturated solution of NaCl has an a w of 0.75 (Table 3–4).
The water activity (a w ) of most fresh foods is above 0.99. The minimum values reported for the
growth of some microorganisms in foods are presented in Table 3–5 (see also Chapter 18). In general,
bacteria require higher values of a w for growth than fungi, with Gram-negative bacteria having higher
requirements than Gram positives. Most spoilage bacteria do not grow below a w = 0.91, whereas
spoilage molds can grow as low as 0.80. With respect to food-poisoning bacteria, Staphylococcus
aureus can grow as low as 0.86, whereas Clostridium botulinum does not grow below 0.94. Just
as yeasts and molds grow over a wider pH range than bacteria, the same is true for a w . The lowest
reported value for foodborne bacteria is 0.75 for halophiles (literally, “salt-loving”), whereas xerophilic