Control by Heat (Thermal Processing) ◾ 457
treatment is quite lethal to microorganisms, and the destruction is caused by the high temperature. At present, microwave-heated foods cannot be considered safe from pathogens. Generally,
when a food is heated in a microwave oven, it is not heated uniformly and some areas can remain
cold. If a food harbors pathogens, there are chances that they will survive in the cold spots. 7
Conclusion
Heating has been used in food preparation and preservation long before civilization. Following
the recognition of microbial roles in food spoilage and foodborne diseases, precise heating techniques have been developed to destroy vegetative cells of yeasts, molds, bacteria, viruses, and
spores of bacteria. These include pasteurization, commercial sterilization, and UHT. Destruction
of microorganisms by these methods is achieved by heat treatment of a food at a specific temperature for a specific time period. Microbial destruction at a high temperature results from structural
and functional destabilization of cells and spores. At a lower heating temperature, the cells and
spores can be sublethally injured and stressed. Because they are not dead, these two phenomena
have important implications in developing heating methods to destroy microorganisms in food.
Because extreme heat to kill all microorganisms cannot always be used, other methods are used
mainly to prevent growth of microorganisms as well as to maintain acceptance qualities of food.
One such method is low temperature and is discussed in Chapter 34.
QUESTIONS
1. Discuss the ideological differences in the use of heating in food preservation before and after
a.d. 1870.
2. List the microbiologically related objectives of heating a food.
3. Describe the mechanisms of sublethal and lethal heat injury in bacterial cells and spores.
How can stress proteins alter this effect in food processed slowly at low heat?
4. List three important factors that one should consider in designing thermal preservation of a
food. Discuss the implications of one of the factors.
5. Define D value, Z value, F value, and 12D value. Draw hypothetical plots to show how D
and Z values can indicate relative heat resistance of two microbial species.
6. List the objectives of pasteurization of food. How do these objectives differ from those used
in commercial sterilization of foods? Use a food system for each method of treatment to
justify your explanations.
7. How does microwave heating differ from conventional heating to ensure food safety?
8. The SPC/mL of raw milk from two suppliers is as follows: (A) SPC/mL 95,000 (with thermodurics 15,000/mL) and (B) SPC/mL 65,000 (with thermodurics 25,000/mL). Briefly discuss D value differences that a processor has to adopt to meet the legal bacterial counts for
pasteurized Grade A milk. Can both be sold as Grade A pasteurized milk? Explain.
References
1. Featherstone, S., A review of development in and challenges of thermal processing over the past 200
years: A tribute to Nicolas Appert, Food Res. Int., 47, 156–160, 2012.
2. Huang, K., Tian, H.P., Gai, L., and Wang, J.P., A review of kinetic models for inactivating microorganisms and enzymes by pulsed electric field processing, J. Food Eng., 111, 191–207, 2012.
treatment is quite lethal to microorganisms, and the destruction is caused by the high temperature. At present, microwave-heated foods cannot be considered safe from pathogens. Generally,
when a food is heated in a microwave oven, it is not heated uniformly and some areas can remain
cold. If a food harbors pathogens, there are chances that they will survive in the cold spots. 7
Conclusion
Heating has been used in food preparation and preservation long before civilization. Following
the recognition of microbial roles in food spoilage and foodborne diseases, precise heating techniques have been developed to destroy vegetative cells of yeasts, molds, bacteria, viruses, and
spores of bacteria. These include pasteurization, commercial sterilization, and UHT. Destruction
of microorganisms by these methods is achieved by heat treatment of a food at a specific temperature for a specific time period. Microbial destruction at a high temperature results from structural
and functional destabilization of cells and spores. At a lower heating temperature, the cells and
spores can be sublethally injured and stressed. Because they are not dead, these two phenomena
have important implications in developing heating methods to destroy microorganisms in food.
Because extreme heat to kill all microorganisms cannot always be used, other methods are used
mainly to prevent growth of microorganisms as well as to maintain acceptance qualities of food.
One such method is low temperature and is discussed in Chapter 34.
QUESTIONS
1. Discuss the ideological differences in the use of heating in food preservation before and after
a.d. 1870.
2. List the microbiologically related objectives of heating a food.
3. Describe the mechanisms of sublethal and lethal heat injury in bacterial cells and spores.
How can stress proteins alter this effect in food processed slowly at low heat?
4. List three important factors that one should consider in designing thermal preservation of a
food. Discuss the implications of one of the factors.
5. Define D value, Z value, F value, and 12D value. Draw hypothetical plots to show how D
and Z values can indicate relative heat resistance of two microbial species.
6. List the objectives of pasteurization of food. How do these objectives differ from those used
in commercial sterilization of foods? Use a food system for each method of treatment to
justify your explanations.
7. How does microwave heating differ from conventional heating to ensure food safety?
8. The SPC/mL of raw milk from two suppliers is as follows: (A) SPC/mL 95,000 (with thermodurics 15,000/mL) and (B) SPC/mL 65,000 (with thermodurics 25,000/mL). Briefly discuss D value differences that a processor has to adopt to meet the legal bacterial counts for
pasteurized Grade A milk. Can both be sold as Grade A pasteurized milk? Explain.
References
1. Featherstone, S., A review of development in and challenges of thermal processing over the past 200
years: A tribute to Nicolas Appert, Food Res. Int., 47, 156–160, 2012.
2. Huang, K., Tian, H.P., Gai, L., and Wang, J.P., A review of kinetic models for inactivating microorganisms and enzymes by pulsed electric field processing, J. Food Eng., 111, 191–207, 2012.