Factors Influencing Microbial Growth in Food ◾ 69
most molds are strict aerobes, a few can tolerate less aerobic conditions. Similarly, yeasts are basically aerobic, but some can grow under low Eh (below +300 mV). Many clostridial species can
grow at Eh +100 mV, but some need –150 mV or less.
The presence or absence of oxygen and the Eh of food determine the growth capability of a
particular microbial group in a food and the specific metabolic pathways used during growth to
generate energy and metabolic byproducts. This is important in microbial spoilage of a food (such
as putrification of meat by Clostridium spp. under anaerobic conditions) and to produce desirable
characteristics of fermented foods (such as growth of Penicillium species in blue cheese under
aerobic conditions). This information is also important to isolate microorganisms of interest from
foods (such as Clostridium laramie, a strict anaerobe from spoiled meat) in the laboratory.
extrinsic Factors
Extrinsic factors important in microbial growth in a food include the environmental conditions in
which it is stored. These are temperature, relative humidity, and gaseous environment. The relative
humidity and gaseous condition of storage, respectively, influence the A W and Eh of the food. The
influence of these two factors on microbial growth has been discussed previously. In this section,
the influence of the storage temperature of food on microbial growth is discussed.
Temperature and Growth
Principle
Microbial growth is accomplished through enzymatic reactions. It is well known that within a
certain range with every 10°C (50°F) rise in temperature, the catalytic rate of an enzyme doubles.
Similarly, the enzymatic reaction rate is reduced to half by decreasing the temperature by 10°C.
This relationship changes beyond the growth range. Because temperature influences enzyme reactions, it has an important role in microbial growth in food.
Food and Temperature
Foods are exposed to different temperatures from the time of production until consumption.
Depending on processing conditions, a food can be exposed to high heat, from 65°C (149°F)
(roasting of meat) to more than 100°C (in ultrahigh temperature processing). For long-term storage, a food can be kept at 5°C (refrigeration) to –20°C or below (freezing). Some relatively stable
foods are also kept between 10 and 35°C (cold to ambient temperature). Some ready-to-eat foods
are kept at warm temperature (50–60°C) for several hours (e.g., in the supermarket deli). Different
temperatures are also used to stimulate desirable microbial growth in food fermentation. 11
Microbial Growth and Viability
Microorganisms important in foods are divided into three groups on the basis of their temperature of growth, each group having an optimum temperature and a temperature range of growth:
(1) thermophiles (which grow at relatively high temperatures) with an optimum at approximately
55°C (131°F) and a range of 45°C–70°C; (2) mesophiles (which grow at ambient temperatures),
with an optimum at 35°C and a range of 10°C–45°C; and (3) psychrophiles (which grow at cold
most molds are strict aerobes, a few can tolerate less aerobic conditions. Similarly, yeasts are basically aerobic, but some can grow under low Eh (below +300 mV). Many clostridial species can
grow at Eh +100 mV, but some need –150 mV or less.
The presence or absence of oxygen and the Eh of food determine the growth capability of a
particular microbial group in a food and the specific metabolic pathways used during growth to
generate energy and metabolic byproducts. This is important in microbial spoilage of a food (such
as putrification of meat by Clostridium spp. under anaerobic conditions) and to produce desirable
characteristics of fermented foods (such as growth of Penicillium species in blue cheese under
aerobic conditions). This information is also important to isolate microorganisms of interest from
foods (such as Clostridium laramie, a strict anaerobe from spoiled meat) in the laboratory.
extrinsic Factors
Extrinsic factors important in microbial growth in a food include the environmental conditions in
which it is stored. These are temperature, relative humidity, and gaseous environment. The relative
humidity and gaseous condition of storage, respectively, influence the A W and Eh of the food. The
influence of these two factors on microbial growth has been discussed previously. In this section,
the influence of the storage temperature of food on microbial growth is discussed.
Temperature and Growth
Principle
Microbial growth is accomplished through enzymatic reactions. It is well known that within a
certain range with every 10°C (50°F) rise in temperature, the catalytic rate of an enzyme doubles.
Similarly, the enzymatic reaction rate is reduced to half by decreasing the temperature by 10°C.
This relationship changes beyond the growth range. Because temperature influences enzyme reactions, it has an important role in microbial growth in food.
Food and Temperature
Foods are exposed to different temperatures from the time of production until consumption.
Depending on processing conditions, a food can be exposed to high heat, from 65°C (149°F)
(roasting of meat) to more than 100°C (in ultrahigh temperature processing). For long-term storage, a food can be kept at 5°C (refrigeration) to –20°C or below (freezing). Some relatively stable
foods are also kept between 10 and 35°C (cold to ambient temperature). Some ready-to-eat foods
are kept at warm temperature (50–60°C) for several hours (e.g., in the supermarket deli). Different
temperatures are also used to stimulate desirable microbial growth in food fermentation. 11
Microbial Growth and Viability
Microorganisms important in foods are divided into three groups on the basis of their temperature of growth, each group having an optimum temperature and a temperature range of growth:
(1) thermophiles (which grow at relatively high temperatures) with an optimum at approximately
55°C (131°F) and a range of 45°C–70°C; (2) mesophiles (which grow at ambient temperatures),
with an optimum at 35°C and a range of 10°C–45°C; and (3) psychrophiles (which grow at cold