Control by Low Temperature ◾ 463
Nature of Microorganisms
Although some microorganisms can grow at as low as –10°C (14°F), many mesophilic and thermophilic bacterial cells can be sublethally injured and may die with time at low temperatures above
freezing. At temperatures below –10°C, vegetative cells of microorganisms can sustain sublethal
injury and die. In general, gram-negative or rod-shaped bacteria are more susceptible to the damaging effect of freezing than gram-positive or spherical-shaped bacteria. Also, cells from the early
exponential phase of growth are more susceptible to freezing than those from the early stationary
phase. Species and strains of microorganisms also differ greatly in sensitivity and resistance to
freezing damage. Germination and outgrowth of spores of some Clostridium spp. can occur at
as low as 2°C (35.6°F) and maybe at a slightly higher temperature for some Bacillus spp. spores.
Spores do not lose their viability in frozen foods. Some microbial enzymes, either heat stable or
released by the dead and lysed cells, can catalyze reactions at temperatures above –20°C (–4°F) but
at a slow rate and can reduce the acceptance quality of a food. 4
Methods
Foods are stored at low temperatures in different ways to extend their shelf life. Many fresh fruits
and vegetables are kept at temperatures between 10°C (50°F) and 20°C (68°F) or lower, mainly
to reduce their metabolic rate. Microorganisms to which these products are susceptible, namely
yeasts and molds (and some bacteria), can grow at this temperature. Maintaining a low relative humidity to prevent moisture buildup on the food surface is very important to reduce their
growth. Highly perishable products are generally stored at a low temperature, below 7°C (44.6°F),
often in combination with other preservation methods. The importance of rapid cooling of a food
for microbiological safety has been mentioned before. The methods used for low-temperature
preservation of foods and in food safety are briefly discussed. 5
Ice Chilling
This is used in retail stores where the foods are kept over ice; the surface in contact with the ice
can reach between 0°C and 1°C (33.8°F). 6 Fresh fish, seafood, meats, cut fruits, vegetable salads
(in bags), different types of ready-to-eat salads (prepared at the retail store), salad dressings (high
pH, low caloric), sous vide, and some ethnic foods (e.g., tofu) are stored by this method. The trend
is increasing.
Temperature fluctuation (resulting from the size of the container or melting of ice), duration of
storage (fresh or several days), and cross-contamination (raw fish, shrimp, oysters, and ready-to-eat
fish salads in an open container in the same display case at the retail store) can cause microbiological problems, especially from foodborne pathogens.
Refrigeration
The temperature specification for refrigeration of foods has changed over time (Chapter 21). Previously,
7°C (44.6°F) was considered a desirable temperature. However, technological improvements have
made it economical to have domestic refrigeration units at 4°C–5°C (39.2°F–41°F). For perishable
products, ≤4.4°C (40°F) is considered a desirable refrigeration temperature. Commercial food processors may use as low as ~1°C (33.8°F) for refrigeration of perishable foods (such as fresh meat and
Nature of Microorganisms
Although some microorganisms can grow at as low as –10°C (14°F), many mesophilic and thermophilic bacterial cells can be sublethally injured and may die with time at low temperatures above
freezing. At temperatures below –10°C, vegetative cells of microorganisms can sustain sublethal
injury and die. In general, gram-negative or rod-shaped bacteria are more susceptible to the damaging effect of freezing than gram-positive or spherical-shaped bacteria. Also, cells from the early
exponential phase of growth are more susceptible to freezing than those from the early stationary
phase. Species and strains of microorganisms also differ greatly in sensitivity and resistance to
freezing damage. Germination and outgrowth of spores of some Clostridium spp. can occur at
as low as 2°C (35.6°F) and maybe at a slightly higher temperature for some Bacillus spp. spores.
Spores do not lose their viability in frozen foods. Some microbial enzymes, either heat stable or
released by the dead and lysed cells, can catalyze reactions at temperatures above –20°C (–4°F) but
at a slow rate and can reduce the acceptance quality of a food. 4
Methods
Foods are stored at low temperatures in different ways to extend their shelf life. Many fresh fruits
and vegetables are kept at temperatures between 10°C (50°F) and 20°C (68°F) or lower, mainly
to reduce their metabolic rate. Microorganisms to which these products are susceptible, namely
yeasts and molds (and some bacteria), can grow at this temperature. Maintaining a low relative humidity to prevent moisture buildup on the food surface is very important to reduce their
growth. Highly perishable products are generally stored at a low temperature, below 7°C (44.6°F),
often in combination with other preservation methods. The importance of rapid cooling of a food
for microbiological safety has been mentioned before. The methods used for low-temperature
preservation of foods and in food safety are briefly discussed. 5
Ice Chilling
This is used in retail stores where the foods are kept over ice; the surface in contact with the ice
can reach between 0°C and 1°C (33.8°F). 6 Fresh fish, seafood, meats, cut fruits, vegetable salads
(in bags), different types of ready-to-eat salads (prepared at the retail store), salad dressings (high
pH, low caloric), sous vide, and some ethnic foods (e.g., tofu) are stored by this method. The trend
is increasing.
Temperature fluctuation (resulting from the size of the container or melting of ice), duration of
storage (fresh or several days), and cross-contamination (raw fish, shrimp, oysters, and ready-to-eat
fish salads in an open container in the same display case at the retail store) can cause microbiological problems, especially from foodborne pathogens.
Refrigeration
The temperature specification for refrigeration of foods has changed over time (Chapter 21). Previously,
7°C (44.6°F) was considered a desirable temperature. However, technological improvements have
made it economical to have domestic refrigeration units at 4°C–5°C (39.2°F–41°F). For perishable
products, ≤4.4°C (40°F) is considered a desirable refrigeration temperature. Commercial food processors may use as low as ~1°C (33.8°F) for refrigeration of perishable foods (such as fresh meat and