Control by Reduced Water Activity and Drying ◾ 469
as compared with solutes that enter freely in cells (e.g., glycerol), which are required in higher
amounts for similar inhibition.
Although microorganisms can be injured and killed at reduced A W , a lower A W value is less
detrimental. For a 90% reduction of salmonellae population in a product at 15°C (59°F), it took
27 days at an A W of 0.71 and 67 days at an A W of 0.34.
Studies on minimal A W values to support the growth of specific microorganisms have generated conflicting data. This could be a result of the inherent problems with different techniques
used to measure A W . However, with the modern electronic hygrometers, this problem is expected
to be minimized. 1,3,4
Nature of Foods
Minimal A W values for growth of microorganisms, as well as influence of A W on viability loss, vary
with the food characteristics and the food environment. In a homogeneous food, A W will remain
unchanged provided other factors do not change. However, a heterogeneous food with ingredients
or items of different A W (e.g., a sandwich or a meal with different items in the same package) will
generate a gradient. This can lead to microbial growth in an item preserved by reduced A W alone
and stored with an item of high A W containing a preservative. Also, condensation of water during
storage with temperature fluctuation, followed by dripping of moisture in some areas of food, can
alter a safe A W level in these areas to an unsafe state.
The minimum A W for growth of microorganisms in a food can be higher than that in a broth.
Thus, Sta. aureus has a minimal A W for growth of 0.86 in a broth, but it does not grow in shrimp
at an A W of 0.89. As the A W is reduced, anaerobic bacteria will require more of an anaerobic environment for growth. Clostridium perfringens grew in a broth with an A W of 0.995 at an O-R potential of +194 mV; when the A W was reduced to 0.975, an O-R potential of +66 mV was required
for growth. As the incubation temperature is moved in either direction from optimum without
changing the A W , the microorganisms require a longer time to grow. In a broth of A W of 0.975, a
Clo. botulinum E strain grew in six days at 30°C, in 19 days at 15°C, and in 42 days at 7.2°C. The
minimum A W for growth of Sta. aureus was 0.865 at 30°C but changed to 0.878 at 25°C. Reduced
A W and low pH interact favorably in inhibiting microbial growth. A Clo. botulinum B strain grew
at an A W of 0.99 up to pH 5.3 and at an A W of 0.97 up to pH 6.0, but at an A W of 0.95, it failed to
grow even at pH 7.0. Similarly, a spoilage strain of Clo. butyricum grew at an A W of 0.98 up to pH
3.8, but at an A W of 0.97, it failed to grow at pH 4.5 even after 30 days at 30°C. Many chemical
preservatives enhance the inhibitory effect of lower A W on microbial growth. In the presence of
low concentrations of sorbate, citrate, and phosphate, different microorganisms were found not to
grow at the lowest A W in which they grew in the absence of these chemicals.
Food composition can influence microbial death rate even at the same A W . At an A W of 0.33,
Escherichia coli counts reduced by log 10 2.8 in ice cream powder and log 10 4.8 in dried potatoes but by
more than log 10 6 in coffee. Under the same conditions, the death rate of Enterococcus faecalis was much
less. Although the survivors remain dormant in a low-A W food, as soon as it is rehydrated, the microorganisms regain the ability to metabolize and multiply. Thus, a rehydrated food should be treated as a
perishable food that, unless effective control methods are used, can be unsafe and spoiled. 3
Nature of Microorganisms
Microorganisms differ greatly in their minimal A W requirement for growth, sporulation, and
germination (Table 35.1). In general, molds and yeasts can grow at lower A W values than bacteria;
as compared with solutes that enter freely in cells (e.g., glycerol), which are required in higher
amounts for similar inhibition.
Although microorganisms can be injured and killed at reduced A W , a lower A W value is less
detrimental. For a 90% reduction of salmonellae population in a product at 15°C (59°F), it took
27 days at an A W of 0.71 and 67 days at an A W of 0.34.
Studies on minimal A W values to support the growth of specific microorganisms have generated conflicting data. This could be a result of the inherent problems with different techniques
used to measure A W . However, with the modern electronic hygrometers, this problem is expected
to be minimized. 1,3,4
Nature of Foods
Minimal A W values for growth of microorganisms, as well as influence of A W on viability loss, vary
with the food characteristics and the food environment. In a homogeneous food, A W will remain
unchanged provided other factors do not change. However, a heterogeneous food with ingredients
or items of different A W (e.g., a sandwich or a meal with different items in the same package) will
generate a gradient. This can lead to microbial growth in an item preserved by reduced A W alone
and stored with an item of high A W containing a preservative. Also, condensation of water during
storage with temperature fluctuation, followed by dripping of moisture in some areas of food, can
alter a safe A W level in these areas to an unsafe state.
The minimum A W for growth of microorganisms in a food can be higher than that in a broth.
Thus, Sta. aureus has a minimal A W for growth of 0.86 in a broth, but it does not grow in shrimp
at an A W of 0.89. As the A W is reduced, anaerobic bacteria will require more of an anaerobic environment for growth. Clostridium perfringens grew in a broth with an A W of 0.995 at an O-R potential of +194 mV; when the A W was reduced to 0.975, an O-R potential of +66 mV was required
for growth. As the incubation temperature is moved in either direction from optimum without
changing the A W , the microorganisms require a longer time to grow. In a broth of A W of 0.975, a
Clo. botulinum E strain grew in six days at 30°C, in 19 days at 15°C, and in 42 days at 7.2°C. The
minimum A W for growth of Sta. aureus was 0.865 at 30°C but changed to 0.878 at 25°C. Reduced
A W and low pH interact favorably in inhibiting microbial growth. A Clo. botulinum B strain grew
at an A W of 0.99 up to pH 5.3 and at an A W of 0.97 up to pH 6.0, but at an A W of 0.95, it failed to
grow even at pH 7.0. Similarly, a spoilage strain of Clo. butyricum grew at an A W of 0.98 up to pH
3.8, but at an A W of 0.97, it failed to grow at pH 4.5 even after 30 days at 30°C. Many chemical
preservatives enhance the inhibitory effect of lower A W on microbial growth. In the presence of
low concentrations of sorbate, citrate, and phosphate, different microorganisms were found not to
grow at the lowest A W in which they grew in the absence of these chemicals.
Food composition can influence microbial death rate even at the same A W . At an A W of 0.33,
Escherichia coli counts reduced by log 10 2.8 in ice cream powder and log 10 4.8 in dried potatoes but by
more than log 10 6 in coffee. Under the same conditions, the death rate of Enterococcus faecalis was much
less. Although the survivors remain dormant in a low-A W food, as soon as it is rehydrated, the microorganisms regain the ability to metabolize and multiply. Thus, a rehydrated food should be treated as a
perishable food that, unless effective control methods are used, can be unsafe and spoiled. 3
Nature of Microorganisms
Microorganisms differ greatly in their minimal A W requirement for growth, sporulation, and
germination (Table 35.1). In general, molds and yeasts can grow at lower A W values than bacteria;