456 ◾ Fundamental Food Microbiology
microorganisms to be destroyed. Most products are given a commercially sterile treatment to
destroy microorganisms growing in a product under normal storage conditions. Low-acid or
high-pH (pH > 4.6) products are given 12D treatment to destroy Clo. botulinum Type A and B
spores (the most resistant spores of a pathogen). However, the products can have viable spores of
thermophilic spoilage bacteria (e.g., Bacillus stearothermophilus, Bac. coagulans, Clo. thermosaccharolyticum, and Desulfotomaculum nigrificans; see Chapter 20). As long as the products are stored
at or below 30°C (86°F), these spores will not germinate. If the products are temperature abused
to 40°C (104°F) and above even for a short time, the spores will germinate. Subsequent storage
at or below 30°C will not prevent outgrowth and multiplication of these thermophiles to cause
food spoilage. The time and temperature required for commercial sterility of a particular food
are determined by actual pack inoculation studies. Generally, Clo. sporogenes PA 3679 is used
to simulate Clo. botulinum because this is a nonpathogenic strain, but the spores have the same
heat resistance as Clo. botulinum Type A or B (both proteolytic and nonproteolytic). For spoilage
control studies, spores of Bac. stearothermophilus are used because spores of this species are the
most heat resistant.
For high-acid or low-pH (pH ≤ 4.6) products, such as tomato products, fruit products, and
acidified foods, a much lower heat treatment is used. Because Clo. botulinum spores cannot germinate or grow at this low pH, their presence is of little health hazard significance. The spore formers
that can germinate and grow in low-pH products (e.g., Bac. coagulans) and the aciduric non-sporeforming bacteria (e.g., Lactobacillus and Leuconostoc spp.), yeasts, and molds that can grow at
low pH are relatively heat sensitive. These products are generally heated to approximately 100°C
(212°F) for a desired period of time. High-heat-treated products are either first packed in containers and then heated or heated first and then packed in sterile containers while still hot (hot pack).
Commercial sterility is also obtained by heating a food at very high temperatures for a short
time. This process is designated as ultrahigh temperature (UHT) processing. Milk heated to
approximately 150°C (302°F) for two to three seconds can be stored at room temperatures (≤30°C),
and the products generally have a three-month shelf life. However, if microbial heat- stable proteinases or lipases are present in the raw milk, the product can show spoilage. In the UHT process,
the milk is heated by injecting steam at high pressure for a rapid temperature increase. Following
heat treatment in bulk, the milk is packed in small serving containers. Microbial heat-sensitive
toxins will be destroyed, but heat-stable toxins may remain active even after heating for commercial sterility.
Under special circumstances, foods are heated to destroy all microorganisms (cells and spores)
and to achieve sterility. Sterile foods are necessary for immunosuppressed individuals in order to
avoid any complications from the microorganisms that are normally present in heated but nonsterile foods.
Microwave Heating
Heating or cooking foods in the microwave at home is quite common in both developed and
developing countries. 7,8 Frozen foods can be thawed and heated very rapidly, in a few minutes,
depending on the size of the product. However, the method has not been well accepted as a source
of rapidly generated high heat for commercial operations.
In a microwave oven, the waves change their polarity very quickly. Oppositely charged water
molecules in a food rapidly move to align along the waves. The movement of the water molecules
generates frictional heat, causing the temperature of the food to rise very rapidly. Depending
on the exposure time and intensity of the wave, the temperature can be very high. Microwave
microorganisms to be destroyed. Most products are given a commercially sterile treatment to
destroy microorganisms growing in a product under normal storage conditions. Low-acid or
high-pH (pH > 4.6) products are given 12D treatment to destroy Clo. botulinum Type A and B
spores (the most resistant spores of a pathogen). However, the products can have viable spores of
thermophilic spoilage bacteria (e.g., Bacillus stearothermophilus, Bac. coagulans, Clo. thermosaccharolyticum, and Desulfotomaculum nigrificans; see Chapter 20). As long as the products are stored
at or below 30°C (86°F), these spores will not germinate. If the products are temperature abused
to 40°C (104°F) and above even for a short time, the spores will germinate. Subsequent storage
at or below 30°C will not prevent outgrowth and multiplication of these thermophiles to cause
food spoilage. The time and temperature required for commercial sterility of a particular food
are determined by actual pack inoculation studies. Generally, Clo. sporogenes PA 3679 is used
to simulate Clo. botulinum because this is a nonpathogenic strain, but the spores have the same
heat resistance as Clo. botulinum Type A or B (both proteolytic and nonproteolytic). For spoilage
control studies, spores of Bac. stearothermophilus are used because spores of this species are the
most heat resistant.
For high-acid or low-pH (pH ≤ 4.6) products, such as tomato products, fruit products, and
acidified foods, a much lower heat treatment is used. Because Clo. botulinum spores cannot germinate or grow at this low pH, their presence is of little health hazard significance. The spore formers
that can germinate and grow in low-pH products (e.g., Bac. coagulans) and the aciduric non-sporeforming bacteria (e.g., Lactobacillus and Leuconostoc spp.), yeasts, and molds that can grow at
low pH are relatively heat sensitive. These products are generally heated to approximately 100°C
(212°F) for a desired period of time. High-heat-treated products are either first packed in containers and then heated or heated first and then packed in sterile containers while still hot (hot pack).
Commercial sterility is also obtained by heating a food at very high temperatures for a short
time. This process is designated as ultrahigh temperature (UHT) processing. Milk heated to
approximately 150°C (302°F) for two to three seconds can be stored at room temperatures (≤30°C),
and the products generally have a three-month shelf life. However, if microbial heat- stable proteinases or lipases are present in the raw milk, the product can show spoilage. In the UHT process,
the milk is heated by injecting steam at high pressure for a rapid temperature increase. Following
heat treatment in bulk, the milk is packed in small serving containers. Microbial heat-sensitive
toxins will be destroyed, but heat-stable toxins may remain active even after heating for commercial sterility.
Under special circumstances, foods are heated to destroy all microorganisms (cells and spores)
and to achieve sterility. Sterile foods are necessary for immunosuppressed individuals in order to
avoid any complications from the microorganisms that are normally present in heated but nonsterile foods.
Microwave Heating
Heating or cooking foods in the microwave at home is quite common in both developed and
developing countries. 7,8 Frozen foods can be thawed and heated very rapidly, in a few minutes,
depending on the size of the product. However, the method has not been well accepted as a source
of rapidly generated high heat for commercial operations.
In a microwave oven, the waves change their polarity very quickly. Oppositely charged water
molecules in a food rapidly move to align along the waves. The movement of the water molecules
generates frictional heat, causing the temperature of the food to rise very rapidly. Depending
on the exposure time and intensity of the wave, the temperature can be very high. Microwave