Protection of Foods with Low-Temperatures
405
in total phospholipids of vibrios as growth temperatures were lowered from 15 to −5
◦ C but not among
the pseudomonads over their range of growth.
6,7,27 A change from saturated to unsaturated lipids
would not be expected to occur in pseudomonads as growth temperatures are lowered because the
psychrotrophic strains contain between 59% and 72% unsaturated lipids, making them more versatile
than many other organisms. In contrast to most other psychrotrophs, Micrococcus cryophilus undergoes
chain shortening in response to low temperatures, which apparently decreased the melting point of its
membrane lipids.
59
The widespread occurrence of low-temperature-induced changes in fatty acid composition suggests
that they are associated with physiological mechanisms of the cell. It is known that an increase in the
degree of unsaturation of fatty acids in lipids leads to a decrease in the lipid melting point. It has been
suggested that increased synthesis of unsaturated fatty acids at low temperatures has the function of
maintaining the lipid in a liquid and mobile state, thereby allowing membrane activity to continue to
function. This concept, referred to as the lipid solidification theory, was first proposed by Gaughran
20
and Allen.
3 It has been shown by Byrne and Chapman
10 that the melting point of fatty-acid side chains
in lipids is more important than the entire lipid structure.
When L. monocytogenes was cultured in the growth range of 12–13
◦ C, there were significant
deviations in the percentages of both i15:O (i = iso) and a15:O (a = anteiso) fatty acids together
with a suggested deviation in a17:O, which resulted in a significant change in the total branched-chain
fatty acids.
54 The cytoplasmic membrane of L. monocyogenes cultured at low temperatures contains
a large proportion of shorter-chain fatty acids where branching switches from iso to anteiso (see
reference 38).
Psychrotrophs synthesize high levels of polysaccharides. Well-known examples of this effect include
the production of ropy milk and ropy bread dough, both of which are favored by low temperatures. The
production of extracellular dextrans by Leuconostoc and Pediococcus spp. are known to be favored at
temperatures below the growth optima of these organisms. The greater production of dextran at lower
temperatures is due apparently to the fact that dextransucrase is very rapidly inactivated at temperatures
in excess of 30
◦ C.
53 A temperature-sensitive dextransucrase synthesizing system has also been shown
for a Lactobacillus sp.
12
From a practical standpoint, increased polysaccharide synthesis at low temperatures manifests itself
in the characteristic appearance of low-temperature spoiled meats. Slime formation is characteristic
of the bacterial spoilage of frankfurters, fresh poultry, and ground beef. The coalescence of surface
colonies leads to the sliminess of such meats, and no doubt contributes to the increased hydration
capacity that accompanies low-temperature meat spoilage. This extra polymeric material undoubtedly
plays a role in biofilm formation (see Chapter 22).
Pigment production is favored. This effect appears to be confined to those organisms that synthesize phenazine and carotenoid pigments. The best-documented example of this phenomenon involves
pigment production by Serratia marcescens. The organism produces an abnormally heat-sensitive
enzyme that catalyzes the coupling of a monopyrrole and bipyrrole precursor to give prodigiosin (the
red pigment).
72 The increased production of pigments at suboptimum temperatures is also reported
by others.
65,72 It is interesting that a very large number of marine psychrotrophs (and perhaps psychrophiles) are pigmented. This is true for bacteria as well as yeasts. On the other hand, none of the
more commonly studied thermophiles are pigmented.
Some strains display differential substrate utilization. It has been reported that sugar fermentation at
temperatures below 30
◦ C gives rise to both acid and gas, whereas above 30
◦ C, only acid is produced.
23
Similarly, others have found psychrotrophs that fermented glucose and other sugars with the formation
of acid and gas at 20
◦ C and lower, but produced only acid at higher temperatures.
67 The latter was
ascribed to a temperature-sensitive formic hydrogenase system. These investigators studied a similar
405
in total phospholipids of vibrios as growth temperatures were lowered from 15 to −5
◦ C but not among
the pseudomonads over their range of growth.
6,7,27 A change from saturated to unsaturated lipids
would not be expected to occur in pseudomonads as growth temperatures are lowered because the
psychrotrophic strains contain between 59% and 72% unsaturated lipids, making them more versatile
than many other organisms. In contrast to most other psychrotrophs, Micrococcus cryophilus undergoes
chain shortening in response to low temperatures, which apparently decreased the melting point of its
membrane lipids.
59
The widespread occurrence of low-temperature-induced changes in fatty acid composition suggests
that they are associated with physiological mechanisms of the cell. It is known that an increase in the
degree of unsaturation of fatty acids in lipids leads to a decrease in the lipid melting point. It has been
suggested that increased synthesis of unsaturated fatty acids at low temperatures has the function of
maintaining the lipid in a liquid and mobile state, thereby allowing membrane activity to continue to
function. This concept, referred to as the lipid solidification theory, was first proposed by Gaughran
20
and Allen.
3 It has been shown by Byrne and Chapman
10 that the melting point of fatty-acid side chains
in lipids is more important than the entire lipid structure.
When L. monocytogenes was cultured in the growth range of 12–13
◦ C, there were significant
deviations in the percentages of both i15:O (i = iso) and a15:O (a = anteiso) fatty acids together
with a suggested deviation in a17:O, which resulted in a significant change in the total branched-chain
fatty acids.
54 The cytoplasmic membrane of L. monocyogenes cultured at low temperatures contains
a large proportion of shorter-chain fatty acids where branching switches from iso to anteiso (see
reference 38).
Psychrotrophs synthesize high levels of polysaccharides. Well-known examples of this effect include
the production of ropy milk and ropy bread dough, both of which are favored by low temperatures. The
production of extracellular dextrans by Leuconostoc and Pediococcus spp. are known to be favored at
temperatures below the growth optima of these organisms. The greater production of dextran at lower
temperatures is due apparently to the fact that dextransucrase is very rapidly inactivated at temperatures
in excess of 30
◦ C.
53 A temperature-sensitive dextransucrase synthesizing system has also been shown
for a Lactobacillus sp.
12
From a practical standpoint, increased polysaccharide synthesis at low temperatures manifests itself
in the characteristic appearance of low-temperature spoiled meats. Slime formation is characteristic
of the bacterial spoilage of frankfurters, fresh poultry, and ground beef. The coalescence of surface
colonies leads to the sliminess of such meats, and no doubt contributes to the increased hydration
capacity that accompanies low-temperature meat spoilage. This extra polymeric material undoubtedly
plays a role in biofilm formation (see Chapter 22).
Pigment production is favored. This effect appears to be confined to those organisms that synthesize phenazine and carotenoid pigments. The best-documented example of this phenomenon involves
pigment production by Serratia marcescens. The organism produces an abnormally heat-sensitive
enzyme that catalyzes the coupling of a monopyrrole and bipyrrole precursor to give prodigiosin (the
red pigment).
72 The increased production of pigments at suboptimum temperatures is also reported
by others.
65,72 It is interesting that a very large number of marine psychrotrophs (and perhaps psychrophiles) are pigmented. This is true for bacteria as well as yeasts. On the other hand, none of the
more commonly studied thermophiles are pigmented.
Some strains display differential substrate utilization. It has been reported that sugar fermentation at
temperatures below 30
◦ C gives rise to both acid and gas, whereas above 30
◦ C, only acid is produced.
23
Similarly, others have found psychrotrophs that fermented glucose and other sugars with the formation
of acid and gas at 20
◦ C and lower, but produced only acid at higher temperatures.
67 The latter was
ascribed to a temperature-sensitive formic hydrogenase system. These investigators studied a similar