Control by Heat (Thermal Processing) ◾ 451
during germination. Death results from the inability of a spore either to germinate or to outgrow
(see Chapter 10).
Exposure of microbial cells to approximately 45°C–50°C (104°F–122°F) for a short time,
which can occur while heating a large volume of a food, such as a large rare roast, may induce
production of heat-shock proteins (stress proteins) by the cells. In the presence of these proteins,
the microbial cells can develop relatively greater resistance to subsequent heating at higher temperatures (see Chapter 10). The implication of this phenomenon in the thermal destruction of
microbial cells in low-heat-processed foods is important. 3,4
influencing Factors
The effectiveness of heat in killing microbial cells and spores depends on many factors, some of
which are related to the inherent nature of the foods and others to both the nature of the microorganisms and the nature of processing. An understanding of these factors is important to develop
and adopt an effective heat-processing procedure for a food. 1
Nature of Food
Composition (the amount of carbohydrates, proteins, lipids, and solutes), A W (moisture), pH,
and antimicrobial content (natural or added) greatly influence microbial destruction by heat in a
food. In general, carbohydrates, proteins, lipids, and solutes provide protection to microorganisms
against heat. Greater microbial resistance results in higher concentrations of these components.
Microorganisms in liquid food and food containing small-sized particles suspended in a liquid are
more susceptible to heat destruction than in a solid food or a food with large chunks in liquid.
Microorganisms are more susceptible to heat damage in a food that has higher A W or lower pH. In
low-pH foods, heating is more lethal to microorganisms in the presence of acetic, propionic, and
lactic acids than phosphoric or citric acids at the same pH. In the presence of antimicrobials, not
inactivated by heat, microorganisms are destroyed more rapidly, the rate differing with the nature
of the antimicrobials.
Nature of Microorganisms
Factors that influence microbial sensitivity to heat include inherent resistance of species and
strains, stage of growth, previous exposure to heat, and initial load. In general, vegetative cells of
molds, yeasts, and bacteria are more sensitive than spores. Cells of molds, yeasts, and many bacteria (except thermoduric and thermophilic), as well as viruses, are destroyed within 10 minutes at
65°C (149°F). Most thermoduric and thermophilic bacterial cells important in foods are destroyed
in 5 to 10 minutes at 75°C–80°C (167°F–176°F). Yeast and most mold spores are destroyed at
65°C–70°C (149°F–158°F) in a few minutes, but spores of some molds can survive at as high
as 90°C (194°F) for four to five hours. Bacterial spores vary greatly in their sensitivity to heat.
Generally, heating at 80°C–85°C (176°F–185°F) for a few minutes does not kill them. Many are
destroyed at 100°C (212°F) in 30 minutes, but there are bacterial species whose spores are not
destroyed even after boiling (100°C) for 24 hours. All spores are destroyed at 121°C (249.8°F)
in 15 minutes (sterilization temperature and time). Below this temperature (and time), spores of
some bacterial species can survive; however, this depends on the initial number of spores and the
nature of the suspending medium. Species and strains of bacterial cells and spores also differ in
during germination. Death results from the inability of a spore either to germinate or to outgrow
(see Chapter 10).
Exposure of microbial cells to approximately 45°C–50°C (104°F–122°F) for a short time,
which can occur while heating a large volume of a food, such as a large rare roast, may induce
production of heat-shock proteins (stress proteins) by the cells. In the presence of these proteins,
the microbial cells can develop relatively greater resistance to subsequent heating at higher temperatures (see Chapter 10). The implication of this phenomenon in the thermal destruction of
microbial cells in low-heat-processed foods is important. 3,4
influencing Factors
The effectiveness of heat in killing microbial cells and spores depends on many factors, some of
which are related to the inherent nature of the foods and others to both the nature of the microorganisms and the nature of processing. An understanding of these factors is important to develop
and adopt an effective heat-processing procedure for a food. 1
Nature of Food
Composition (the amount of carbohydrates, proteins, lipids, and solutes), A W (moisture), pH,
and antimicrobial content (natural or added) greatly influence microbial destruction by heat in a
food. In general, carbohydrates, proteins, lipids, and solutes provide protection to microorganisms
against heat. Greater microbial resistance results in higher concentrations of these components.
Microorganisms in liquid food and food containing small-sized particles suspended in a liquid are
more susceptible to heat destruction than in a solid food or a food with large chunks in liquid.
Microorganisms are more susceptible to heat damage in a food that has higher A W or lower pH. In
low-pH foods, heating is more lethal to microorganisms in the presence of acetic, propionic, and
lactic acids than phosphoric or citric acids at the same pH. In the presence of antimicrobials, not
inactivated by heat, microorganisms are destroyed more rapidly, the rate differing with the nature
of the antimicrobials.
Nature of Microorganisms
Factors that influence microbial sensitivity to heat include inherent resistance of species and
strains, stage of growth, previous exposure to heat, and initial load. In general, vegetative cells of
molds, yeasts, and bacteria are more sensitive than spores. Cells of molds, yeasts, and many bacteria (except thermoduric and thermophilic), as well as viruses, are destroyed within 10 minutes at
65°C (149°F). Most thermoduric and thermophilic bacterial cells important in foods are destroyed
in 5 to 10 minutes at 75°C–80°C (167°F–176°F). Yeast and most mold spores are destroyed at
65°C–70°C (149°F–158°F) in a few minutes, but spores of some molds can survive at as high
as 90°C (194°F) for four to five hours. Bacterial spores vary greatly in their sensitivity to heat.
Generally, heating at 80°C–85°C (176°F–185°F) for a few minutes does not kill them. Many are
destroyed at 100°C (212°F) in 30 minutes, but there are bacterial species whose spores are not
destroyed even after boiling (100°C) for 24 hours. All spores are destroyed at 121°C (249.8°F)
in 15 minutes (sterilization temperature and time). Below this temperature (and time), spores of
some bacterial species can survive; however, this depends on the initial number of spores and the
nature of the suspending medium. Species and strains of bacterial cells and spores also differ in