Food Protection with High Temperatures
435
Genetics
A significant discovery toward an understanding of the genetic bases of thermophilism was made
by McDonald and Matney.
36 These investigators effected the transformation of thermophilism in B.
subtilis by growing cells of a strain that could not grow above 50
◦ C in the presence of DNA extracted
from one that could grow at 55
◦ C. The more heat-sensitive strain was transformed at a frequency of
10
−4 . These authors noted that only 10–20% of the transformants retained the high-level streptomycin
resistance of the recipient, which indicated that the genetic loci for streptomycin resistance and that
for growth at 55
◦ C were closely linked.
Although much has been learned about the basic mechanisms of thermophilism in microorganisms, the precise mechanisms underlying this high-temperature phenomenon remain a mystery. The
facultative thermophiles such as some B. coagulans strains present a picture as puzzling as the obligate thermophiles. The facultative thermophiles display both mesophilic and thermophilic types of
metabolism. In their studies of these types from the genus Bacillus, which grew well at both 37
◦ C
and 55
◦ C, Bausum and Matney
6 reported that the organisms appear to shift from mesophilism to
thermophilism between 44
◦ C and 52
◦ C.
CANNED FOOD SPOILAGE
Although the objective in the thermal canning of foods is the destruction of microorganisms, these
products nevertheless undergo microbial spoilage under certain conditions. The main reasons for this
are underprocessing, inadequate cooling, contamination of the can resulting from leakage through
seams, and preprocess spoilage. Because some canned foods receive low-heat treatments, it is to be
expected that a rather large number of different types of microorganisms may be found upon examining
such foods.
As a guide to the type of spoilage that canned foods undergo, the following classification of canned
foods based on acidity is helpful.
Low Acid (pH > 4.6)
This category includes meat and marine products, milk, some vegetables (corn, lima beans), meat and
vegetable mixtures, and so on. These foods are spoiled by the thermophilic flat-sour group (Geobacillus stearothermophilus, B. coagulans), sulfide spoilers (Clostridium nigrificans, C. bifermentans),
and/or gaseous spoilers (Thermoanaerobacterium thermosaccharolyticum). Mesophilic spoilers include putrefactive anaerobes (especially PA 3679 types). Spoilage and toxin production by proteolytic
C. botulinum strains may occur if they are present. Medium-acid foods are those with a pH range of
5.3–4.6, whereas low-acid foods are those with pH ≥ 5.4.
Acid (pH 3.7–4.0 to 4.6)
In this category are fruits such as tomatoes, pears, and figs. Thermophilic spoilers include B.
coagulans types. Mesophiles include P. polymyxa, P. macerans (B. betanigrificans), C. pasteurianum,
C. butyricum, Thermoanaerobacterium thermosaccharolyticum, lactobacilli, and others.
435
Genetics
A significant discovery toward an understanding of the genetic bases of thermophilism was made
by McDonald and Matney.
36 These investigators effected the transformation of thermophilism in B.
subtilis by growing cells of a strain that could not grow above 50
◦ C in the presence of DNA extracted
from one that could grow at 55
◦ C. The more heat-sensitive strain was transformed at a frequency of
10
−4 . These authors noted that only 10–20% of the transformants retained the high-level streptomycin
resistance of the recipient, which indicated that the genetic loci for streptomycin resistance and that
for growth at 55
◦ C were closely linked.
Although much has been learned about the basic mechanisms of thermophilism in microorganisms, the precise mechanisms underlying this high-temperature phenomenon remain a mystery. The
facultative thermophiles such as some B. coagulans strains present a picture as puzzling as the obligate thermophiles. The facultative thermophiles display both mesophilic and thermophilic types of
metabolism. In their studies of these types from the genus Bacillus, which grew well at both 37
◦ C
and 55
◦ C, Bausum and Matney
6 reported that the organisms appear to shift from mesophilism to
thermophilism between 44
◦ C and 52
◦ C.
CANNED FOOD SPOILAGE
Although the objective in the thermal canning of foods is the destruction of microorganisms, these
products nevertheless undergo microbial spoilage under certain conditions. The main reasons for this
are underprocessing, inadequate cooling, contamination of the can resulting from leakage through
seams, and preprocess spoilage. Because some canned foods receive low-heat treatments, it is to be
expected that a rather large number of different types of microorganisms may be found upon examining
such foods.
As a guide to the type of spoilage that canned foods undergo, the following classification of canned
foods based on acidity is helpful.
Low Acid (pH > 4.6)
This category includes meat and marine products, milk, some vegetables (corn, lima beans), meat and
vegetable mixtures, and so on. These foods are spoiled by the thermophilic flat-sour group (Geobacillus stearothermophilus, B. coagulans), sulfide spoilers (Clostridium nigrificans, C. bifermentans),
and/or gaseous spoilers (Thermoanaerobacterium thermosaccharolyticum). Mesophilic spoilers include putrefactive anaerobes (especially PA 3679 types). Spoilage and toxin production by proteolytic
C. botulinum strains may occur if they are present. Medium-acid foods are those with a pH range of
5.3–4.6, whereas low-acid foods are those with pH ≥ 5.4.
Acid (pH 3.7–4.0 to 4.6)
In this category are fruits such as tomatoes, pears, and figs. Thermophilic spoilers include B.
coagulans types. Mesophiles include P. polymyxa, P. macerans (B. betanigrificans), C. pasteurianum,
C. butyricum, Thermoanaerobacterium thermosaccharolyticum, lactobacilli, and others.