Protection of Foods with Low-Temperatures
403
virus and the causative agent of trichinosis (Trichinella spiralis). Protozoa are generally killed when
frozen below −5
◦ C or −10
◦ C, if protective compounds are not present.
41
Effect of Thawing
Of great importance in the freezing survival of microorganisms is the process of thawing. Repeated
freezing and thawing will destroy bacteria by disrupting cell membranes. Also, the faster the thaw, the
greater the number of bacterial survivors. Why this is so is not entirely clear. From the changes listed
above that occur during freezing, it can be seen that the thawing process becomes complicated if it is
to lead to the restoration of viable activity. It has been pointed out that thawing is inherently slower
than freezing and follows a pattern that is potentially more detrimental. Among the problems attendant
on the thawing of specimens and products that transmit heat energy primarily by conduction, are the
following:
18
1. Thawing is inherently slower than freezing when conducted under comparable temperature
differentials.
2. In practice, the maximum temperature differential permissible during thawing is much less than
that which is feasible during freezing.
3. The time–temperature pattern characteristic of thawing is potentially more detrimental than that
of freezing. During thawing, the temperature rises rapidly to near the melting point and remains
there throughout the long course of thawing, thus affording considerable opportunity for chemical
reactions, recrystallization, and even microbial growth, if thawing is extremely slow.
It has been stated that microorganisms die not upon freezing but, rather, during the thawing process.
As to why some organisms are able to survive freezing while others do not, Luyet
39 suggested that it
is a question of the ability of an organism to survive dehydration and to undergo dehydration when
the medium freezes. With respect to survival after freeze-drying, Luyet has stated that it may be due
to the fact that bacteria do not freeze at all but merely dry up. (See Chapter 18 for further discussion
of the effect of freeze-drying on microorganisms.)
It is fairly well established that the freeze-thaw cycle leads to: (1) ice nucleation, (2) dehydration,
and (3) oxidative damage. During thawing, an oxidative burst has been shown to occur and superoxide
dismutase (SOD) provides resistance to the deleterious oxidative effects. In a study of Campylobacter
coli, it was found that SOD, but not catalase, formed during the freeze–thaw.
62 SOD is also important
in the resistance of Campylobacter cells to the oxidative stress that occurs during their survival in foods,
during intestinal colonization of poultry, and during their survival in macrophages (see reference 62).
Most frozen-foods processors advise against the refreezing of foods once they have been thawed.
Although the reasons are more related to the texture, flavor, and other nutritional qualities of the frozen
product, the microbiology of thawed frozen foods is pertinent. Some investigators have pointed out
that foods from the frozen state spoil faster than similar fresh products. There are textural changes
associated with freezing that would seem to aid the invasion of surface organisms into deeper parts
of the produce and, consequently, facilitate the spoilage process. Upon thawing, surface condensation
of water is known to occur. There is also, at the surface, a general concentration of water-soluble
substances such as amino acids, minerals, B vitamins, and, possibly, other nutrients. Freezing has the
effect of destroying many thermophilic and some mesophilic organisms, making for less competition
among the survivors upon thawing. It is conceivable that a greater relative number of psychrotrophs
on thawed foods might increase the spoilage rate. Some psychrotrophic bacteria have been reported to
403
virus and the causative agent of trichinosis (Trichinella spiralis). Protozoa are generally killed when
frozen below −5
◦ C or −10
◦ C, if protective compounds are not present.
41
Effect of Thawing
Of great importance in the freezing survival of microorganisms is the process of thawing. Repeated
freezing and thawing will destroy bacteria by disrupting cell membranes. Also, the faster the thaw, the
greater the number of bacterial survivors. Why this is so is not entirely clear. From the changes listed
above that occur during freezing, it can be seen that the thawing process becomes complicated if it is
to lead to the restoration of viable activity. It has been pointed out that thawing is inherently slower
than freezing and follows a pattern that is potentially more detrimental. Among the problems attendant
on the thawing of specimens and products that transmit heat energy primarily by conduction, are the
following:
18
1. Thawing is inherently slower than freezing when conducted under comparable temperature
differentials.
2. In practice, the maximum temperature differential permissible during thawing is much less than
that which is feasible during freezing.
3. The time–temperature pattern characteristic of thawing is potentially more detrimental than that
of freezing. During thawing, the temperature rises rapidly to near the melting point and remains
there throughout the long course of thawing, thus affording considerable opportunity for chemical
reactions, recrystallization, and even microbial growth, if thawing is extremely slow.
It has been stated that microorganisms die not upon freezing but, rather, during the thawing process.
As to why some organisms are able to survive freezing while others do not, Luyet
39 suggested that it
is a question of the ability of an organism to survive dehydration and to undergo dehydration when
the medium freezes. With respect to survival after freeze-drying, Luyet has stated that it may be due
to the fact that bacteria do not freeze at all but merely dry up. (See Chapter 18 for further discussion
of the effect of freeze-drying on microorganisms.)
It is fairly well established that the freeze-thaw cycle leads to: (1) ice nucleation, (2) dehydration,
and (3) oxidative damage. During thawing, an oxidative burst has been shown to occur and superoxide
dismutase (SOD) provides resistance to the deleterious oxidative effects. In a study of Campylobacter
coli, it was found that SOD, but not catalase, formed during the freeze–thaw.
62 SOD is also important
in the resistance of Campylobacter cells to the oxidative stress that occurs during their survival in foods,
during intestinal colonization of poultry, and during their survival in macrophages (see reference 62).
Most frozen-foods processors advise against the refreezing of foods once they have been thawed.
Although the reasons are more related to the texture, flavor, and other nutritional qualities of the frozen
product, the microbiology of thawed frozen foods is pertinent. Some investigators have pointed out
that foods from the frozen state spoil faster than similar fresh products. There are textural changes
associated with freezing that would seem to aid the invasion of surface organisms into deeper parts
of the produce and, consequently, facilitate the spoilage process. Upon thawing, surface condensation
of water is known to occur. There is also, at the surface, a general concentration of water-soluble
substances such as amino acids, minerals, B vitamins, and, possibly, other nutrients. Freezing has the
effect of destroying many thermophilic and some mesophilic organisms, making for less competition
among the survivors upon thawing. It is conceivable that a greater relative number of psychrotrophs
on thawed foods might increase the spoilage rate. Some psychrotrophic bacteria have been reported to