402 Modern Food Microbiology
From the strict standpoint of food preservation, freezing should not be regarded as a means of
destroying foodborne microorganisms. The type of organisms that lose their viability in this state
differ from strain to strain and depend on the type of freezing employed, the nature and composition
of the food in question, the length of time of freezer storage, and other factors, such as temperature
of freezing. Low freezing temperatures of about −20
◦ C are less harmful to microorganisms than the
median range of temperatures, such as −10
◦ C. For example, more microorganisms are destroyed at
−4
◦ C than at −15
◦ C or below. Temperatures below −24
◦ C seem to have no additional effect. Food
constituents such as egg white, sucrose, corn syrup, fish, glycerol, and undenatured meat extracts
have all been found to increase freezing viability, especially of food-poisoning bacteria, whereas acid
conditions have been found to decrease cell viability.
21 Consider some of the events that are known
to occur when cells freeze:
1. The water that freezes is the so-called free water. Upon freezing, the free water forms ice
crystals. The growth of ice crystals occurs by accretion, so that all of the free water of a cell
might be represented by a relatively small number of ice crystals. In slow freezing, ice crystals
are extracellular; in fast freezing, they are intracellular. Bound water remains unfrozen. The
freezing of cells depletes them of usable liquid water and thus dehydrates them.
2. Freezing results in an increase in the viscosity of cellular matter, a direct consequence of water
being concentrated in the form of ice crystals.
3. Freezing results in a loss of cytoplasmic gases such as O 2 and CO 2 . A loss of O 2 to aerobic
cells suppresses respiratory reactions. Also, the more diffuse state of O 2 may make for greater
oxidative activities within the cell.
4. Freezing causes changes in pH of cellular matter. Various investigators have reported changes
ranging from 0.3 to 2.0 pH units. Increases and decreases of pH upon freezing and thawing have
been reported.
5. Freezing effects concentration of cellular electrolytes. This effect is also a consequence of the
concentration of water in the form of ice crystals.
6. Freezing causes a general alteration of the colloidal state of cellular protoplasm. Many of the
constituents of cellular protoplasm such as proteins exist in a dynamic colloidal state in living
cells. A proper amount of water is necessary to the well-being of this state.
7. Freezing causes some denaturation of cellular proteins. Precisely how this effect is achieved is
not clear, but it is known that upon freezing, some –SH groups disappear and such groups as
lipoproteins break apart from others. The lowered water content, along with the concentration
of electrolytes, no doubt affects this change in state of cellular proteins.
8. Freezing induces temperature shock in some microorganisms. This is true more for thermophiles
and mesophiles than for psychrophiles. More cells die when the temperature decline above
freezing is sudden than when it is slow.
9. Freezing causes metabolic injury to some microbial cells such as certain Pseudomonas spp.
Some bacteria have increased nutritional requirements upon thawing from the frozen state and
as much as 40% of a culture may be affected in this way.
Clearly, the effects of the freezing process on living cells such as bacteria and other microorganisms,
as well as on foods, are complex. According to Mazur
41 the response of microorganisms to subzero
temperatures appears to be largely determined by solute concentration and intracellular freezing,
although there are only a few cases of clear demonstration of this conclusion.
Why are some bacteria killed by freezing but not all cells? Some small and microscopic organisms
are unable to survive freezing as can most bacteria. Examples include the foot-and-mouth disease
From the strict standpoint of food preservation, freezing should not be regarded as a means of
destroying foodborne microorganisms. The type of organisms that lose their viability in this state
differ from strain to strain and depend on the type of freezing employed, the nature and composition
of the food in question, the length of time of freezer storage, and other factors, such as temperature
of freezing. Low freezing temperatures of about −20
◦ C are less harmful to microorganisms than the
median range of temperatures, such as −10
◦ C. For example, more microorganisms are destroyed at
−4
◦ C than at −15
◦ C or below. Temperatures below −24
◦ C seem to have no additional effect. Food
constituents such as egg white, sucrose, corn syrup, fish, glycerol, and undenatured meat extracts
have all been found to increase freezing viability, especially of food-poisoning bacteria, whereas acid
conditions have been found to decrease cell viability.
21 Consider some of the events that are known
to occur when cells freeze:
1. The water that freezes is the so-called free water. Upon freezing, the free water forms ice
crystals. The growth of ice crystals occurs by accretion, so that all of the free water of a cell
might be represented by a relatively small number of ice crystals. In slow freezing, ice crystals
are extracellular; in fast freezing, they are intracellular. Bound water remains unfrozen. The
freezing of cells depletes them of usable liquid water and thus dehydrates them.
2. Freezing results in an increase in the viscosity of cellular matter, a direct consequence of water
being concentrated in the form of ice crystals.
3. Freezing results in a loss of cytoplasmic gases such as O 2 and CO 2 . A loss of O 2 to aerobic
cells suppresses respiratory reactions. Also, the more diffuse state of O 2 may make for greater
oxidative activities within the cell.
4. Freezing causes changes in pH of cellular matter. Various investigators have reported changes
ranging from 0.3 to 2.0 pH units. Increases and decreases of pH upon freezing and thawing have
been reported.
5. Freezing effects concentration of cellular electrolytes. This effect is also a consequence of the
concentration of water in the form of ice crystals.
6. Freezing causes a general alteration of the colloidal state of cellular protoplasm. Many of the
constituents of cellular protoplasm such as proteins exist in a dynamic colloidal state in living
cells. A proper amount of water is necessary to the well-being of this state.
7. Freezing causes some denaturation of cellular proteins. Precisely how this effect is achieved is
not clear, but it is known that upon freezing, some –SH groups disappear and such groups as
lipoproteins break apart from others. The lowered water content, along with the concentration
of electrolytes, no doubt affects this change in state of cellular proteins.
8. Freezing induces temperature shock in some microorganisms. This is true more for thermophiles
and mesophiles than for psychrophiles. More cells die when the temperature decline above
freezing is sudden than when it is slow.
9. Freezing causes metabolic injury to some microbial cells such as certain Pseudomonas spp.
Some bacteria have increased nutritional requirements upon thawing from the frozen state and
as much as 40% of a culture may be affected in this way.
Clearly, the effects of the freezing process on living cells such as bacteria and other microorganisms,
as well as on foods, are complex. According to Mazur
41 the response of microorganisms to subzero
temperatures appears to be largely determined by solute concentration and intracellular freezing,
although there are only a few cases of clear demonstration of this conclusion.
Why are some bacteria killed by freezing but not all cells? Some small and microscopic organisms
are unable to survive freezing as can most bacteria. Examples include the foot-and-mouth disease