This document discusses physical methods of food preservation, focusing on the use of ionizing radiation and nonionizing radiation. It describes how food irradiation works as a "cold pasteurization" technique, exposing food to gamma rays, x-rays, or electron beams to kill microorganisms. Various factors affect the resistance of different types of microbes to radiation. Applications include inhibiting sprouting, decontamination, and delaying fruit maturation. Nonionizing microwave radiation generates heat to denature proteins and nucleic acids in microbes.

1. Basura J Jayasundara 1
PHYSICAL METHODS OF FOOD PRESERVATION
Use of Ionizing Radiation in Food Preservation
Food irradiation is a treatment of food with ionizing radiation belonging to the newest food
preservation techniques. Food irradiation is sometimes referred to as "cold pasteurization”
because the treatment by ionizing radiation can provide an effect similar to heat pasteurization,
without heating the food material as in as in heat pasteurization. The irradiation process involves
exposing the food, either prepackaged or in bulk, to a predetermined level of ionization radiation.
Three different types are used in the irradiation, gamma rays, x rays and electron beam. Among
the several methods available, gamma rays have a higher potential for effective and economical
use in food preservation. Several factors should be considered when the effects of radiation on
microorganisms are considered.
 Types of organisms: Gram positive bacteria are more resistant to irradiation than Gram
negative bacteria. In general, spore formers are more resistant than non-spore formers
 Numbers of Organisms: Larger the number, less effective
 Composition of suspending food: Proteins exert protective effect
 Presence or absence of Oxygen: Radiation resistance is greater in absence of oxygen
 Physical state of food: Dried cells are resistant than moist ones, frozen cells are resistant than
non-frozen cells
 Age of organisms: Bacteria resistant to radiation in lag phase, in log phase they are sensitive
The potential applications of irradiation are disinfestation, shelf life extension, decontamination
and product quality improvement. Some applications of irradiation in food industry are as
follows.
Table 2.1 Application of irradiation in food preservation
Application Commodity
Inhibition of sprouting Potatoes, yams, onions & garlic
Decontamination of food Spices, seafood, poultry & beef
Insect disinfestation Grains
Delay in fruit maturation Mangoes, papayas, strawberries
Inactivation of MO Poultry, eggs

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FDA requires that irradiated foods bear the international symbol for irradiation and the Radura
symbol along with the statement “Treated with radiation” or “Treated by irradiation” should be
mentioned on the food label.
Figure 2.1 International symbol for irradiation
Mode of action
Ionizing radiation can affect micro-organisms directly by interacting with key molecules within
the microbial cell, or indirectly through the inhibitory effects of free radicals produced by the
radiolysis of water. This method provides energy that destroys cell structures including DNA in
bacteria, parasites, insects and moulds. However some spores and viruses may be highly resistant
to ionizing energy. Main sites of damages are nucleic acid and the lipids of the cell membrane.
 Membrane lipid degradation
 Change the permeability of the cell membrane
 Leach out of cell components
 Inhibition of the DNA replication
Use of Nonionizing Radiation in Food Preservation
The microwave region of the electromagnetic spectrum occupies frequencies between 109
Hz up
to 1012
Hz and it has relatively low quantum energy. Two frequencies used in food processing
are 2450 MHz and 915 MHz. Domestic microwave ovens use 2450 MHz which is less
penetrating than the lower frequency.
Mode of action
Unlike the other forms of radiation, microwaves act indirectly on microorganisms through the
generation of heat. Destruction of microorganism is accomplished through the denaturation of
protein and nucleic acids.

3. Basura J Jayasundara 3
REFERENCES
 Adams, M.R., and Moss, M, O., 2008. Food Microbiology, third edition. RSC publishing,
UK.
 Ray, B., Fundamental Food Microbiology, third edition, 2004. CRC press.
 Jay, J.M., Loessner, M.J., and Golden, D.A., Modern Food Microbiology, seventh edition,
2005. Springer.
 Rahman, M.S., Handbook of Food Preservation, second edition, 2007. CRC press.
 www.fda.gov/Food/ResourcesForYou/Consumers/ucm261680.htm