514 ◾ Fundamental Food Microbiology
Summary of Processing Methods
Some of the novel technologies currently being studied to process foods, to control microorganisms, as well as to produce new types of foods and their current status in the application of food
are briefly described. This includes processing under thermal and nonthermal conditions, and
the details are summarized in a recent document compiled by National Advisory Committee on
Microbiological Criteria for Foods (NACMCF). 5
Microwave and Radio-Frequency Processing 5–7
These methods involve exposing a food to electromagnetic waves at microwave and radio frequencies. Generally, microwaves at 915 MHz and 2450 MHz and radio frequencies at 13.6, 27.1,
and 40.7 MHz are used in food applications, but very little study has been conducted with radio
frequency. Both techniques generate internal heat by dielectric and ionic mechanisms; water molecules, because of their dipolar nature, are associated with dielectric heating. Heating of food is
very rapid but not uniform as compared with that in conventional heating. Destruction of microorganisms (cells and spores) is accomplished by the heat mainly through denaturation of proteins
and nucleic acids. Microwave ovens are widely used now to cook and reheat foods. This method
has the potential of producing commercially pasteurized and sterilized foods. However, because
of the possible presence of cold spots in solid and semisolid foods, the method cannot be used to
assure safety and is thus currently not regulated by the FDA (see Chapter 33). Many studies are
now being conducted to improve the uniform heating of the microwave oven.
Ohmic and Inductive Heating 5,8
In ohmic heating, electric currents are passed to heat food through an electrode that is in contact with the food. In contrast, in inductive heating, electric coils placed near a food generate an
oscillating electromagnetic field that sends electric currents to heat the food. Of the two, ohmic
heating has been studied well for its potential use in the production of commercially pasteurized
and sterilized food products. The destruction of microbial cells and spores is achieved primarily
by destabilizing proteins, nucleic acids, membrane, and other vital functional components by
heat. Food materials are heated rapidly and uniformly from the generation of internal energy. This
method also produces cold spots in foods containing high fats and solids. Techniques are being
studied to minimize their occurrence.
Infrared Heating 5,9
In infrared (IR) heating, also known as IR radiation, is an electromagnetic wave (wavelength 0.78
to 10,000 µm), which generates thermal energy that is absorbed by food materials to improve
product shelf life. The heating rate is fast and has a very high thermal efficiency and also provides
uniform heating, reduced quality losses, and significant energy saving. IR in combination with
microwave heating is also becoming an effective method for food processing.
Pulsed Electric Fields 10
Use of pulsed electric fields (PEF), a nonthermal method, involves application of high- intensity
electric pulses between two electrodes placed in a food for a very short time to inactivate
Summary of Processing Methods
Some of the novel technologies currently being studied to process foods, to control microorganisms, as well as to produce new types of foods and their current status in the application of food
are briefly described. This includes processing under thermal and nonthermal conditions, and
the details are summarized in a recent document compiled by National Advisory Committee on
Microbiological Criteria for Foods (NACMCF). 5
Microwave and Radio-Frequency Processing 5–7
These methods involve exposing a food to electromagnetic waves at microwave and radio frequencies. Generally, microwaves at 915 MHz and 2450 MHz and radio frequencies at 13.6, 27.1,
and 40.7 MHz are used in food applications, but very little study has been conducted with radio
frequency. Both techniques generate internal heat by dielectric and ionic mechanisms; water molecules, because of their dipolar nature, are associated with dielectric heating. Heating of food is
very rapid but not uniform as compared with that in conventional heating. Destruction of microorganisms (cells and spores) is accomplished by the heat mainly through denaturation of proteins
and nucleic acids. Microwave ovens are widely used now to cook and reheat foods. This method
has the potential of producing commercially pasteurized and sterilized foods. However, because
of the possible presence of cold spots in solid and semisolid foods, the method cannot be used to
assure safety and is thus currently not regulated by the FDA (see Chapter 33). Many studies are
now being conducted to improve the uniform heating of the microwave oven.
Ohmic and Inductive Heating 5,8
In ohmic heating, electric currents are passed to heat food through an electrode that is in contact with the food. In contrast, in inductive heating, electric coils placed near a food generate an
oscillating electromagnetic field that sends electric currents to heat the food. Of the two, ohmic
heating has been studied well for its potential use in the production of commercially pasteurized
and sterilized food products. The destruction of microbial cells and spores is achieved primarily
by destabilizing proteins, nucleic acids, membrane, and other vital functional components by
heat. Food materials are heated rapidly and uniformly from the generation of internal energy. This
method also produces cold spots in foods containing high fats and solids. Techniques are being
studied to minimize their occurrence.
Infrared Heating 5,9
In infrared (IR) heating, also known as IR radiation, is an electromagnetic wave (wavelength 0.78
to 10,000 µm), which generates thermal energy that is absorbed by food materials to improve
product shelf life. The heating rate is fast and has a very high thermal efficiency and also provides
uniform heating, reduced quality losses, and significant energy saving. IR in combination with
microwave heating is also becoming an effective method for food processing.
Pulsed Electric Fields 10
Use of pulsed electric fields (PEF), a nonthermal method, involves application of high- intensity
electric pulses between two electrodes placed in a food for a very short time to inactivate