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Chapter 33
Control by Heat
(thermal Processing)
introduction
The desirable effect of heat (from fire) on the taste of foods of animal and plant origin, especially
seeds, tubers, and roots, was probably accidentally discovered, following a natural forest fire, by
our ancestors long before civilization. They also possibly recognized that heated foods did not spoil
as rapidly as raw foods or cause health hazards. Since then, particularly following the invention of
pottery and ovens, heat has been used to roast, boil, bake, and concentrate foods to improve taste
and to enhance shelf life (and probably safety). However, it was not until approximately 1810 that
extended shelf life of perishable foods was achieved by a specific method, appertization. Nicolas
Appert, after whom the method was named, in France, reported that by filling a clean glass jar
with a desirable food, heating the contents in boiling water for six hours or more, and hermetically
sealing the container with a cork kept the food unspoiled and safe for a long time. 1 Some of his
products (meat stew) prepared in 1824 were found nontoxic when opened in 1938 but were found
to contain dormant spores. Heating of foods in cans was developed by Durand in England in
1810, and by 1820, the method was used in the United States. Methods to reduce the heating time
to approximately 30 minutes were also developed by adding different types of salts to the water to
raise the boiling point above 212°F (100°C). In 1870, the autoclave was invented, providing the
possibility of developing pressure vessels that could be used to heat canned food at a much higher
temperature for a relatively shorter period of time in order to retain the quality of foods as well as
to enhance shelf life.
All these developments occurred without clearly knowing why a perishable food, following
appertization or canning, did not spoil (or cause foodborne disease). The role of microorganisms
in the spoilage of wine and milk was discovered by Louis Pasteur in approximately 1870. He also
showed that mild heat treatment (pasteurization) killed these microorganisms, and in the absence
of recontamination, the products stayed unspoiled. Subsequently, the microbial role in foodborne
diseases was also recognized. Following these, studies were conducted to isolate the most heatresistant microorganisms (spores) that could survive heating in canned foods and cause spoilage
Chapter 33
Control by Heat
(thermal Processing)
introduction
The desirable effect of heat (from fire) on the taste of foods of animal and plant origin, especially
seeds, tubers, and roots, was probably accidentally discovered, following a natural forest fire, by
our ancestors long before civilization. They also possibly recognized that heated foods did not spoil
as rapidly as raw foods or cause health hazards. Since then, particularly following the invention of
pottery and ovens, heat has been used to roast, boil, bake, and concentrate foods to improve taste
and to enhance shelf life (and probably safety). However, it was not until approximately 1810 that
extended shelf life of perishable foods was achieved by a specific method, appertization. Nicolas
Appert, after whom the method was named, in France, reported that by filling a clean glass jar
with a desirable food, heating the contents in boiling water for six hours or more, and hermetically
sealing the container with a cork kept the food unspoiled and safe for a long time. 1 Some of his
products (meat stew) prepared in 1824 were found nontoxic when opened in 1938 but were found
to contain dormant spores. Heating of foods in cans was developed by Durand in England in
1810, and by 1820, the method was used in the United States. Methods to reduce the heating time
to approximately 30 minutes were also developed by adding different types of salts to the water to
raise the boiling point above 212°F (100°C). In 1870, the autoclave was invented, providing the
possibility of developing pressure vessels that could be used to heat canned food at a much higher
temperature for a relatively shorter period of time in order to retain the quality of foods as well as
to enhance shelf life.
All these developments occurred without clearly knowing why a perishable food, following
appertization or canning, did not spoil (or cause foodborne disease). The role of microorganisms
in the spoilage of wine and milk was discovered by Louis Pasteur in approximately 1870. He also
showed that mild heat treatment (pasteurization) killed these microorganisms, and in the absence
of recontamination, the products stayed unspoiled. Subsequently, the microbial role in foodborne
diseases was also recognized. Following these, studies were conducted to isolate the most heatresistant microorganisms (spores) that could survive heating in canned foods and cause spoilage