Radiation Protection of Foods

Radiation may be defined as the “emission and propagation of energy through

space or through a material medium”. Because they destroy microorganisms without

appreciably raising the temperature, the process is termed “cold sterilization”.

APPLICATION OF RADIATION The two most widely used techniques of

irradiating foods are: Gamma radiation (from either 60Co or 137Cs) use of electron beams from linear accelerators.

Gamma Radiation These are electromagnetic radiations emitted from the

excited nucleus of elements such as 60Co and 137Cs. This is the cheapest form of radiation for food

preservation, because the source elements are either byproducts of atomic fission or atomic waste products.

Gamma rays have excellent penetration power. 60Co has a half-life of about 5 years; and the half-life for

137Cs is about 30 years.

Electron Beams/Accelerated Electrons

The use of electron accelerators offers certain advantages over radioactive elements that make this form of radiation somewhat more attractive to potential commercial users.

X-ray is expensive and not usually used.

Some other sources of radiation: Microwaves When electrically neutral foods are placed in an

electromagnetic field, the charged asymmetric molecules are driven first one-way and then another.

As the molecules oscillate about their axes while attempting to go to the proper positive and negative poles, intermolecular friction is created and manifested as a heating effect.

Ultraviolet Light Ultraviolet (UV) light is a powerful bactericidal

agent, with the most effective wavelength being about 2,600 A˚ .

The mechanism of UV death in the bacterial cell is the production of lethal mutations as a result of action on cell nucleic acids.

The poor penetrative capacities of UV light limit its food use only to surface applications.

Also, it may catalyze oxidative changes that lead to rancidity and discolorations.

Small quantities of ozone may also be produced when UV light is used for the surface treatment of certain foods.

PRINCIPLES UNDERLYING THE DESTRUCTION OF MICROORGANISMS BY IRRADIATION

1-Types of Organisms Resistance to radiation (generally):

“Radio-resistance of microorganism generally parallels heat- resistance among bacteria”.

Most sensitive to radiations are: Pseudomonads Flavobacters

.

Among spore formers, Paenibacillus larvae seem to possess a higher degree of resistance than most other aerobic spore formers.

Spores of Clostridium botulinum type A appear to be the most resistant of all clostridial spores

The most radiation resistant spore-formers noted are:

Geobacillus stearothermophilus Clostridium sporogenes

The most resistant non-sporeformers are: Enterococcus faecium Alcaligenes spp., Moraxella-Acinetobacter group.

2- Numbers of Organisms The numbers of organisms have the same effect

on the efficacy of radiations as is the case for heat, chemical disinfection, and certain other phenomena:

“The larger the number of cells, the less effective is a given dose”

3- Composition of Suspending solvent (Food)

Microorganisms in general are more sensitive to radiation when suspended in buffer solutions than in protein-containing media.

The presence of nitrites tends to make bacterial endospores more sensitive to radiation.

4- Presence or Absence of Oxygen The radiation resistance of microorganisms is

greater in the absence of oxygen than in its presence.

5- Physical State of Food The radiation resistance of dried cells is, in

general, considerably higher than that for moist cells.

This is most likely a direct consequence of the radiolysis of water by ionizing radiations.

6- Age of Organisms Bacteria tend to be most resistant to radiation

in the lag phase just prior to active cell division. The cells become more radiation sensitive as

they enter and progress through the log phase and reach their minimum at the end of this phase.

PROCESSING OF FOODS FOR IRRADIATION Blanching or Heat Treatment Sterilizing doses of radiation are insufficient to

destroy the natural enzymes of foods. In order to avoid undesirable postirradiation

changes, it is necessary to destroy these enzymes.

The best method is a heat treatment—that is, the blanching of vegetables and mild heat treatment of meats prior to irradiation.

RADAPPERTIZATION, RADICIDATION, AND RADURIZATION OF FOODS

1- Radappertization: Radappertization is equivalent to radiation

sterilization or “commercial sterility,” as it is –understood in the canning industry.

Typical levels of irradiation are 30–40 kGy. The term was coined to honor N. Appert. The effect of this treatment on endospores and

exotoxins of C. botulinum (Type A, B, E) is of obvious interest.

Type E spores are the most radiation sensitive of these three types.

Type A spores are the most radiation resistant of these three types.

“Resistance increases at the colder temperatures and decreases at warmer temperatures”

The use of a radiation 12D process for C. botulinum in meat products would result in the survival of virus particles unless previously destroyed by other methods such as heating.

Enzymes are also highly resistant to radiation, and a dose of 20–60 kGy has been found to destroy only up to 75% of the proteolytic activity of ground beef.

2-Radicidation: Radicidation is equivalent to pasteurization—of

milk, for example. Specifically, it refers to the reduction of the

number of viable specific non-spore-forming pathogens, other than viruses, so that none is detectable by any standard method.

Typical levels to achieve this process are 2.5–10 kGy.

A radiation dosage up to 7 kGy has been approved by the World Health Organization (WHO) as being “unconditionally safe for human consumption”

3- Radurization: Radurization may be considered equivalent to

pasteurization. It refers to the enhancement of the keeping

quality of a food by causing substantial reduction in the numbers of viable specific spoilage microbes by radiation.

Common dose levels are 0.75–2.5 kGy for fresh meats, poultry, seafood, fruits, vegetables, and cereal grains.

The ultimate spoilage of radurized, low-temperature-stored foods is invariably caused by one or more of the Acinetobacter-Moraxella or lactic acid types noted above.

In general, shelf-life extension is not as great for radurized fruits as for meats and seafood because molds are generally more resistant to irradiation than the Gram-negative bacteria that cause spoilage of the latter products.

Insect eggs and larvae can be destroyed by 1kGy.

LEGAL STATUS OF FOOD IRRADIATION In 1983, the FDA permitted spices and vegetable

seasonings to be irradiated up to 10 kGy In 1981, a joint Food and Agriculture Organization (FAO)/International Atomic Energy Agency (IAEA)/WHO Expert Committee on food irradiation found that foods given an overall average of up to 10.0 kGy were unconditionally safe.

It is considered an additive rather than a process, which it is. This means that irradiated foods must be labeled as such.

Another area of concern is the fate of C. botulinum spores, and yet another is the concern that nonpathogens may become pathogens or that the virulence of pathogens may be increased after exposure to subradappertization doses.

However, There is no evidence that the latter occurs.

The U.S. Centers for Disease Control and Prevention has estimated that if only one-half of the ground beef, pork, poultry, and processed luncheon meats in the U.S. were irradiated, there would be over 880,000 fewer cases of foodborne illness (Food Protection Trends, July 2003).

When low-acid foods are irradiated at doses that do not effect the destruction of C. botulinum spores, legitimate questions about the safety of such foods are raised, especially when they are held under conditions that allow for growth and toxin production.

Because these organisms would be destroyed by radappertization, only products subjected to radicidation and radurization are of concern here.

EFFECT OF IRRADIATION ON FOOD QUALITY The undesirable changes that occur in certain

irradiated foods may be caused directly by irradiation or indirectly as a result of post irradiation reactions.

1- Water : Water undergoes radiolysis when irradiated in

the following manner:

In addition, free radicals are formed along the path of the primary electron and react with each other as diffusion occurs.

By irradiating under anaerobic conditions, off-flavors and off-odors are minimized due to the lack of oxygen to form peroxides.

One of the best ways to minimize off-flavors is to irradiate at subfreezing temperatures.

The effect of subfreezing temperatures is to reduce radiolysis and its consequent reactants.

2- proteins and other nitrogenous compounds :

Other than water, proteins and other nitrogenous compounds appear to be the most sensitive to irradiation effects in foods.

Among the most sensitive to irradiation are: Methionine Cysteine Histidine Arginine Tyrosine

3- Lipids: Irradiation of lipids and fats results in the

production of oxidation products (such as carbonyls and peroxides), especially if irradiation and/or subsequent storage take(s) place in the presence of oxygen.

The most noticeable organoleptic effect of lipid irradiation in air is the development of rancidity.

Fruits and vegetables: One of the most serious detrimental effects is

the softening of these products caused by the irradiation–degradation of pectin and cellulose, the structural polysaccharide of plants.

STORAGE STABILITY OF IRRADIATED FOODS Foods subjected to doses of ionizing radiation

may be expected to be as shelf stable as commercially heat-sterilized foods. There are, however, two differences between

foods processed by radappertization and heatappertization that affect storage stability:

1) Radappertization does not destroy enzymes

2) Some postirradiation changes may be expected to occur.

NATURE OF RADIATION RESISTANCE OF MICROORGANISMS

The most resistant of all known non–sporeforming bacteria consist of:

four species of the genus Deinococcus (isolated from meat, the hides of animals, water and feces)

one specie of Deinobacter (isolated from animal feces and freshwater fish)

one specie of Rubrobacter (isolated from a radioactive hot spring in Japan)

one specie Acinetobacter (isolated from cotton and soils)

Apparent Mechanisms of Resistance 1-The extreme resistance of deinococci to desiccation

has been observed and presumed to be related in some way to radioresistance.

2- All are highly pigmented and contain various carotenoids, a fact that suggests some relationship to radiation resistance.

3-DNA repair process and abundance of DNA. 4-The deinococci do not contain phosphatidylgylcerol

or diphosphatidylglycerol in their phospholipids but contain, instead, phosphoglycolipids as the major component.

5- possession of an outer membrane (deinococci), unlike other Gram-positive bacteria.

6-Among other unusual features of deinococci is their possession of palmitoleate (16:1), which makes up about 60% of the fatty acids in their envelope and about 25% of the total cellular fatty acid.