450 ◾ Fundamental Food Microbiology
and foodborne diseases and to determine the temperature and time requirements for their destruction. Studies were also conducted to identify the time-temperature relationships for the destruction of less heat-resistant microorganisms (vegetative cells). From these results, mathematical
expressions were developed to accurately predict time-temperature relationships to destroy microorganisms (also some enzymes and toxins) by heating foods at different temperatures and times.
objectives
The main objective (microbiological) of heating food is to destroy vegetative cells and spores
of microorganisms that include molds, yeasts, bacteria, and viruses (including bacteriophages).
Although very drastic heat treatment (sterilization) can be used to kill all the microorganisms
present in a food, most foods are heated to destroy specific pathogens and some spoilage microorganisms, which are important in a food. This is necessary in order to retain the acceptance and
nutritional qualities of a food. To control growth of surviving microorganisms in the food, other
control methods are used following heat treatment.
Heating of foods also helps destroy undesirable enzymes (microbial and food) that would
otherwise adversely affect the acceptance quality of food. Some microorganisms also produce heatstable proteinases and lipases in food. Heating a food to a desired temperature for a specific time
can help destroy or reduce the activity of these enzymes. 2 This is especially important in foods
stored for a long time at room temperature.
Some microorganisms can release toxins in food; also, some foods can have natural toxins.
If a toxin is heat sensitive, sufficient heating will destroy it, and consumption of such a food will
not cause health hazards. It is also important to recognize that microbial (and natural) heat-stable
toxins are not completely destroyed even after high heat treatment.
Heating (warming) of ready-to-eat foods before serving is also usually used to prevent growth
of pathogenic and spoilage microorganisms. A temperature above 50°C (122°F), preferably 60°C
(140°F), is important to control growth of many microorganisms in such foods during storage
before serving.
Finally, heating of raw materials, such as milk, is done before adding starter culture bacteria for
fermentation to kill undesired microorganisms (including bacteriophages) and to allow growth of
the starter cultures without competition.
Mechanism of thermal inactivation
Depending on the temperature and time of heating, microbial cells and spores can be heat
shocked, sublethally injured, or dead. Heat-shocked cells acquire some resistance to subsequent
heating, and sublethally injured cells and spores retain the ability to repair and multiply. These are
discussed in Chapter 10.
Results of different studies have shown that following heat injury, bacterial cells show a loss of
permeability and increased sensitivity to some compounds to which they are normally resistant.
Sublethally injured cells seem to suffer injury in the cell membrane, cell wall, DNA (strand break),
ribosomal RNA (degradation), and some important enzymes (denaturation). Death occurs from
damages in some vital functional and structural components. Bacterial spores, following heating,
were found to lose structural components from the spore coat, suffer damage to the structures that
are destined to become membrane and wall, and develop an inability to use water for hydration
and foodborne diseases and to determine the temperature and time requirements for their destruction. Studies were also conducted to identify the time-temperature relationships for the destruction of less heat-resistant microorganisms (vegetative cells). From these results, mathematical
expressions were developed to accurately predict time-temperature relationships to destroy microorganisms (also some enzymes and toxins) by heating foods at different temperatures and times.
objectives
The main objective (microbiological) of heating food is to destroy vegetative cells and spores
of microorganisms that include molds, yeasts, bacteria, and viruses (including bacteriophages).
Although very drastic heat treatment (sterilization) can be used to kill all the microorganisms
present in a food, most foods are heated to destroy specific pathogens and some spoilage microorganisms, which are important in a food. This is necessary in order to retain the acceptance and
nutritional qualities of a food. To control growth of surviving microorganisms in the food, other
control methods are used following heat treatment.
Heating of foods also helps destroy undesirable enzymes (microbial and food) that would
otherwise adversely affect the acceptance quality of food. Some microorganisms also produce heatstable proteinases and lipases in food. Heating a food to a desired temperature for a specific time
can help destroy or reduce the activity of these enzymes. 2 This is especially important in foods
stored for a long time at room temperature.
Some microorganisms can release toxins in food; also, some foods can have natural toxins.
If a toxin is heat sensitive, sufficient heating will destroy it, and consumption of such a food will
not cause health hazards. It is also important to recognize that microbial (and natural) heat-stable
toxins are not completely destroyed even after high heat treatment.
Heating (warming) of ready-to-eat foods before serving is also usually used to prevent growth
of pathogenic and spoilage microorganisms. A temperature above 50°C (122°F), preferably 60°C
(140°F), is important to control growth of many microorganisms in such foods during storage
before serving.
Finally, heating of raw materials, such as milk, is done before adding starter culture bacteria for
fermentation to kill undesired microorganisms (including bacteriophages) and to allow growth of
the starter cultures without competition.
Mechanism of thermal inactivation
Depending on the temperature and time of heating, microbial cells and spores can be heat
shocked, sublethally injured, or dead. Heat-shocked cells acquire some resistance to subsequent
heating, and sublethally injured cells and spores retain the ability to repair and multiply. These are
discussed in Chapter 10.
Results of different studies have shown that following heat injury, bacterial cells show a loss of
permeability and increased sensitivity to some compounds to which they are normally resistant.
Sublethally injured cells seem to suffer injury in the cell membrane, cell wall, DNA (strand break),
ribosomal RNA (degradation), and some important enzymes (denaturation). Death occurs from
damages in some vital functional and structural components. Bacterial spores, following heating,
were found to lose structural components from the spore coat, suffer damage to the structures that
are destined to become membrane and wall, and develop an inability to use water for hydration