416 Modern Food Microbiology
growth and metabolic activities. The genera Bacillus, Clostridium, Alicyclobacillus, Geobacillus, and
Thermoanaerobacter contain the thermophiles of greatest importance in foods (see later section in
this chapter). Pasteurization (to destroy spoilage biota) of beers in the brewing industry is carried out
usually for 8–15 minutes at 60
◦ C.
Sterilization means the destruction of all viable organisms as may be measured by an appropriate
plating or enumerating technique. Canned foods are sometimes called “commercially sterile” to indicate that no viable organisms can be detected by the usual cultural methods employed or that the
number of survivors is so low as to be of no significance under the conditions of canning and storage.
Also, microorganisms may be present in canned foods that cannot grow in the product by reason of
undesirable pH, oxidation–reduction potential (Eh), or temperature of storage.
The processing of milk and milk products can be achieved by the use of ultrahigh temperatures
(UHT). Milk so produced is a product in its own right and is to be distinguished from pasteurized
milk. The primary features of the UHT treatment include its continuous nature, its occurrence outside
the package necessitating aseptic storage and aseptic handling of the product downstream from the
sterilizer, and the very high temperatures (in the range 140–150
◦ C) and the correspondingly short time
(a few seconds) necessary to achieve commercial sterility.
24
UHT-processed milks have higher consumer acceptability than the conventionally heated pasteurized products, and because they are commercially sterile, they may be stored at room temperatures
for up to 8 weeks without flavor changes.
FACTORS AFFECTING HEAT RESISTANCE OF MICROORGANISMS
Equal numbers of bacteria placed in physiologic saline or nutrient broth at the same pH are not
destroyed with the same ease by heat. Some 12 factors or parameters of microorganisms and their
environment have been studied for their effects on heat destruction, and they are presented below.
22
Water
The heat resistance of microbial cells increases with decreasing humidity, moisture, or water activity
(a w ), and this is illustrated in Table 17–1 for spores of Bacillus cereus. For example, at a w of 1.00 and
pH 6.5, D 95 was 2.386 minutes while at a w of 0.86, D 95 was 13.842 minutes.
18
Dried microbial cells placed in test tubes and then heated in a water bath are considerably more
heat resistant than moist cells of the same type. Because it is well established that protein denaturation
occurs at a faster rate when heated in water than in air, it is suggested that protein denaturation is either
the mechanism of death by heat or is closely associated with it (see a later section in this chapter).
The precise manner in which water facilitates heat denaturation of proteins is not entirely clear, but
it has been pointed out that the heating of wet proteins causes the formation of free–SH groups with
a consequent increase in the water-binding capacity of proteins. The presence of water allows for
thermal breaking of peptide bonds, a process that requires more energy in the absence of water and,
consequently, confers a greater refractivity to heat.
Fat
In the presence of fats, there is a general increase in the heat resistance of some microorganisms
(Table 17–2). This is sometimes referred to as fat protection and is presumed to increase heat resistance
growth and metabolic activities. The genera Bacillus, Clostridium, Alicyclobacillus, Geobacillus, and
Thermoanaerobacter contain the thermophiles of greatest importance in foods (see later section in
this chapter). Pasteurization (to destroy spoilage biota) of beers in the brewing industry is carried out
usually for 8–15 minutes at 60
◦ C.
Sterilization means the destruction of all viable organisms as may be measured by an appropriate
plating or enumerating technique. Canned foods are sometimes called “commercially sterile” to indicate that no viable organisms can be detected by the usual cultural methods employed or that the
number of survivors is so low as to be of no significance under the conditions of canning and storage.
Also, microorganisms may be present in canned foods that cannot grow in the product by reason of
undesirable pH, oxidation–reduction potential (Eh), or temperature of storage.
The processing of milk and milk products can be achieved by the use of ultrahigh temperatures
(UHT). Milk so produced is a product in its own right and is to be distinguished from pasteurized
milk. The primary features of the UHT treatment include its continuous nature, its occurrence outside
the package necessitating aseptic storage and aseptic handling of the product downstream from the
sterilizer, and the very high temperatures (in the range 140–150
◦ C) and the correspondingly short time
(a few seconds) necessary to achieve commercial sterility.
24
UHT-processed milks have higher consumer acceptability than the conventionally heated pasteurized products, and because they are commercially sterile, they may be stored at room temperatures
for up to 8 weeks without flavor changes.
FACTORS AFFECTING HEAT RESISTANCE OF MICROORGANISMS
Equal numbers of bacteria placed in physiologic saline or nutrient broth at the same pH are not
destroyed with the same ease by heat. Some 12 factors or parameters of microorganisms and their
environment have been studied for their effects on heat destruction, and they are presented below.
22
Water
The heat resistance of microbial cells increases with decreasing humidity, moisture, or water activity
(a w ), and this is illustrated in Table 17–1 for spores of Bacillus cereus. For example, at a w of 1.00 and
pH 6.5, D 95 was 2.386 minutes while at a w of 0.86, D 95 was 13.842 minutes.
18
Dried microbial cells placed in test tubes and then heated in a water bath are considerably more
heat resistant than moist cells of the same type. Because it is well established that protein denaturation
occurs at a faster rate when heated in water than in air, it is suggested that protein denaturation is either
the mechanism of death by heat or is closely associated with it (see a later section in this chapter).
The precise manner in which water facilitates heat denaturation of proteins is not entirely clear, but
it has been pointed out that the heating of wet proteins causes the formation of free–SH groups with
a consequent increase in the water-binding capacity of proteins. The presence of water allows for
thermal breaking of peptide bonds, a process that requires more energy in the absence of water and,
consequently, confers a greater refractivity to heat.
Fat
In the presence of fats, there is a general increase in the heat resistance of some microorganisms
(Table 17–2). This is sometimes referred to as fat protection and is presumed to increase heat resistance