462 ◾ Fundamental Food Microbiology
As the temperature is reduced enough to cause a large portion of the water to freeze, the growth
of most microorganisms stops except for some psychrophilic bacteria, yeasts, and molds. Although
there are conflicting reports, slow growth probably can occur up to –10°C (14°F), especially by some
molds. As the temperature drops further, to approximately –20°C (–4°F), and water in a food freezes
completely, more cells will have sublethal and lethal injury. The rate of freezing and the lowest temperature of freezing dictate the extent of microbial damage from ice crystals. Damage and death are
more extensive at a slower rate of freezing and at –20°C than at a rapid rate of freezing and at –78°C
or –196°C (temperatures of solid CO 2 and liquid N 2 , respectively). Death and sublethal injury are very
high during initial storage (ca. seven days) but subsequently slow down.
Fluctuation of temperature of a food during low-temperature storage has great impact on
growth, sublethal injury, and death of microorganisms. This quite readily happens to foods during
storage, transport, retail display, and at home. A fluctuation of temperature of food from ≤4.4°C to
10°C–12°C not only stimulates rapid growth of psychrotrophic pathogenic and spoilage bacteria,
but many mesophilic spoilage and pathogenic bacteria start to grow and their spores germinate at
this range. Just from the spoilage aspect, six- to eight-hour temperature abuse (12°C) of a vacuumpackaged, refrigerated, low-heat-processed meat product can reduce its expected shelf life of eight
weeks by approximately 7–10 days. A fluctuation in temperature of a frozen food increases microbial death and injury because of a repeated damaging solution effect and mechanical damage from
larger ice crystals that form during repeated freezing and thawing. Dead microbial cells can also
lyse, releasing intracellular enzymes, many of which (e.g., proteinases and lipases) can act on food
components and reduce the acceptance quality of food (see Chapter 22).
The rate of cooling of a food is also very important for effective control of the growth of pathogenic and spoilage microorganisms. A slow rate of cooling of foods has been implicated as a major
cause of foodborne diseases (Chapter 24). This can occur by trying to cool a large volume of hot or
warm food in a big (deep) container in a refrigerator or overstuffing refrigerators with hot or warm
foods. During thawing of a frozen food (such as an uncooked chicken), rapid thawing is desirable
in order to control microbial growth, especially growth of pathogens. If the food is thawed slowly,
the temperature on the food surface will soon increase, thereby allowing microbial growth, even
when the inside is still frozen.
Refrigerated foods have limited shelf life, and, with time, microorganisms grow and spoil the
products. In frozen foods, microorganisms (only cells, not spores) slowly die. However, even after
long storage, some survive in frozen foods.
Nature of Food
Composition, pH, A W , and presence of microbial inhibitors (preservatives) in a food can greatly
influence growth, sublethal injury, and viability of microorganisms during storage at low temperatures. A food with higher solid content (especially high proteins, carbohydrates, and lipids, but
low ions), pH closer to 7.0, higher A W , and the absence of microbial inhibitors facilitate growth
and survival of microorganisms at refrigeration temperatures and inflict less injury and cause less
death at frozen temperatures. Thus, the shelf life of refrigerated foods can be increased by using
one or more of these factors, such as low pH, low A W , incorporation of suitable microbial inhibitors, and, when possible, vacuum or modified air packaging (see Chapter 41).
In packaged frozen foods, ice may form in the packages (package ice), especially if the storage
temperature fluctuates. During thawing, the ice melts and is absorbed by the food, resulting in
an increase in the A W in a localized area (e.g., in a bread) and making it susceptible to microbial
growth after thawing.
As the temperature is reduced enough to cause a large portion of the water to freeze, the growth
of most microorganisms stops except for some psychrophilic bacteria, yeasts, and molds. Although
there are conflicting reports, slow growth probably can occur up to –10°C (14°F), especially by some
molds. As the temperature drops further, to approximately –20°C (–4°F), and water in a food freezes
completely, more cells will have sublethal and lethal injury. The rate of freezing and the lowest temperature of freezing dictate the extent of microbial damage from ice crystals. Damage and death are
more extensive at a slower rate of freezing and at –20°C than at a rapid rate of freezing and at –78°C
or –196°C (temperatures of solid CO 2 and liquid N 2 , respectively). Death and sublethal injury are very
high during initial storage (ca. seven days) but subsequently slow down.
Fluctuation of temperature of a food during low-temperature storage has great impact on
growth, sublethal injury, and death of microorganisms. This quite readily happens to foods during
storage, transport, retail display, and at home. A fluctuation of temperature of food from ≤4.4°C to
10°C–12°C not only stimulates rapid growth of psychrotrophic pathogenic and spoilage bacteria,
but many mesophilic spoilage and pathogenic bacteria start to grow and their spores germinate at
this range. Just from the spoilage aspect, six- to eight-hour temperature abuse (12°C) of a vacuumpackaged, refrigerated, low-heat-processed meat product can reduce its expected shelf life of eight
weeks by approximately 7–10 days. A fluctuation in temperature of a frozen food increases microbial death and injury because of a repeated damaging solution effect and mechanical damage from
larger ice crystals that form during repeated freezing and thawing. Dead microbial cells can also
lyse, releasing intracellular enzymes, many of which (e.g., proteinases and lipases) can act on food
components and reduce the acceptance quality of food (see Chapter 22).
The rate of cooling of a food is also very important for effective control of the growth of pathogenic and spoilage microorganisms. A slow rate of cooling of foods has been implicated as a major
cause of foodborne diseases (Chapter 24). This can occur by trying to cool a large volume of hot or
warm food in a big (deep) container in a refrigerator or overstuffing refrigerators with hot or warm
foods. During thawing of a frozen food (such as an uncooked chicken), rapid thawing is desirable
in order to control microbial growth, especially growth of pathogens. If the food is thawed slowly,
the temperature on the food surface will soon increase, thereby allowing microbial growth, even
when the inside is still frozen.
Refrigerated foods have limited shelf life, and, with time, microorganisms grow and spoil the
products. In frozen foods, microorganisms (only cells, not spores) slowly die. However, even after
long storage, some survive in frozen foods.
Nature of Food
Composition, pH, A W , and presence of microbial inhibitors (preservatives) in a food can greatly
influence growth, sublethal injury, and viability of microorganisms during storage at low temperatures. A food with higher solid content (especially high proteins, carbohydrates, and lipids, but
low ions), pH closer to 7.0, higher A W , and the absence of microbial inhibitors facilitate growth
and survival of microorganisms at refrigeration temperatures and inflict less injury and cause less
death at frozen temperatures. Thus, the shelf life of refrigerated foods can be increased by using
one or more of these factors, such as low pH, low A W , incorporation of suitable microbial inhibitors, and, when possible, vacuum or modified air packaging (see Chapter 41).
In packaged frozen foods, ice may form in the packages (package ice), especially if the storage
temperature fluctuates. During thawing, the ice melts and is absorbed by the food, resulting in
an increase in the A W in a localized area (e.g., in a bread) and making it susceptible to microbial
growth after thawing.