464 ◾ Fundamental Food Microbiology
fish). For optimum refrigeration in commercial facilities along with low temperatures, the relative
humidity and proper spacing of the products are also controlled.
Raw and processed foods of plant and animal origin, as well as many prepared and readyto-eat foods, are now preserved by refrigeration. Their volume is increasing rapidly because
consumers prefer such foods. Some of these foods are expected to have a storage life of 60
days or more.
For refrigerated products expected to have a long shelf life, additional preservation methods are
combined with the lowest possible temperature that can be used (close to –1°C). However, as the
products are nonsterile, even a very low initial microbial population (e.g., ≤10 cells or spores per
10 g) capable of growing (or germinating) under the storage condition can multiply to reach hazard (for pathogen) or spoilage levels, thereby reducing the safety and stability of the product. Any
fluctuation in temperature or other abuse (e.g., a leak in a vacuum or modified atmosphere package or oxygen permeation through the packaging materials) can greatly accelerate their growth.
The processing and storage conditions may provide environments in which different types of spoilage and pathogenic microorganisms grow advantageously. This may increase spoilage and wastage
of foods unless appropriate control measures are installed quickly.
Freezing
The minimum temperature used in home freezers (in the refrigerator) is –20°C (–4°F), a temperature at which most of the free water in a food remains in a frozen state. Dry ice (–78°C) and liquid
nitrogen (–196°C) can also be used for freezing; they are used for rapid freezing (instant freezing)
and not for only freezing a food to that low temperature. Following freezing, the temperature of
the food is maintained at approximately –20°C (–4°F) to –30°C (–22°F). Depending on the type,
foods can be stored in a freezer for months or even more than a year. Many raw produce (vegetables, fruits), meat, fish, processed products, and cooked products (ready-to-eat) are preserved
by freezing.
Microorganisms do not grow at –20°C (–4°F) in frozen foods. Instead, microbial cells die
during frozen storage. However, the survivors can multiply in the unfrozen foods. Accidental
thawing or slow thawing can facilitate growth of survivors (spoilage and pathogenic microorganisms); spores can also germinate and outgrow, depending on the temperature and time
following thawing. Enzymes released by the dead microbial cells can reduce the acceptance
quality of the food.
Conclusion
Microbial growth in a food depends on the storage temperature and the microbial types. On
the basis of these two factors and the specific need, foods are stored below room temperature,
refrigerated, or frozen. Although microorganisms cannot grow in frozen foods, some can grow in
refrigerated foods. In addition to preventing and retarding growth, microbial cells are also injured
and killed at frozen and refrigerated temperatures. Because refrigeration maintains the acceptance
quality of foods, lower temperatures (~ –1° to 5°C, i.e., 30.2°F–41°F) are used in combination
with other factors (low A W , pH, etc.) to extend the shelf life of foods for 60 days or more. Like
low temperature, reduced A W is also used to control microbial growth in food and is discussed in
Chapter 35.
fish). For optimum refrigeration in commercial facilities along with low temperatures, the relative
humidity and proper spacing of the products are also controlled.
Raw and processed foods of plant and animal origin, as well as many prepared and readyto-eat foods, are now preserved by refrigeration. Their volume is increasing rapidly because
consumers prefer such foods. Some of these foods are expected to have a storage life of 60
days or more.
For refrigerated products expected to have a long shelf life, additional preservation methods are
combined with the lowest possible temperature that can be used (close to –1°C). However, as the
products are nonsterile, even a very low initial microbial population (e.g., ≤10 cells or spores per
10 g) capable of growing (or germinating) under the storage condition can multiply to reach hazard (for pathogen) or spoilage levels, thereby reducing the safety and stability of the product. Any
fluctuation in temperature or other abuse (e.g., a leak in a vacuum or modified atmosphere package or oxygen permeation through the packaging materials) can greatly accelerate their growth.
The processing and storage conditions may provide environments in which different types of spoilage and pathogenic microorganisms grow advantageously. This may increase spoilage and wastage
of foods unless appropriate control measures are installed quickly.
Freezing
The minimum temperature used in home freezers (in the refrigerator) is –20°C (–4°F), a temperature at which most of the free water in a food remains in a frozen state. Dry ice (–78°C) and liquid
nitrogen (–196°C) can also be used for freezing; they are used for rapid freezing (instant freezing)
and not for only freezing a food to that low temperature. Following freezing, the temperature of
the food is maintained at approximately –20°C (–4°F) to –30°C (–22°F). Depending on the type,
foods can be stored in a freezer for months or even more than a year. Many raw produce (vegetables, fruits), meat, fish, processed products, and cooked products (ready-to-eat) are preserved
by freezing.
Microorganisms do not grow at –20°C (–4°F) in frozen foods. Instead, microbial cells die
during frozen storage. However, the survivors can multiply in the unfrozen foods. Accidental
thawing or slow thawing can facilitate growth of survivors (spoilage and pathogenic microorganisms); spores can also germinate and outgrow, depending on the temperature and time
following thawing. Enzymes released by the dead microbial cells can reduce the acceptance
quality of the food.
Conclusion
Microbial growth in a food depends on the storage temperature and the microbial types. On
the basis of these two factors and the specific need, foods are stored below room temperature,
refrigerated, or frozen. Although microorganisms cannot grow in frozen foods, some can grow in
refrigerated foods. In addition to preventing and retarding growth, microbial cells are also injured
and killed at frozen and refrigerated temperatures. Because refrigeration maintains the acceptance
quality of foods, lower temperatures (~ –1° to 5°C, i.e., 30.2°F–41°F) are used in combination
with other factors (low A W , pH, etc.) to extend the shelf life of foods for 60 days or more. Like
low temperature, reduced A W is also used to control microbial growth in food and is discussed in
Chapter 35.