Download food-microbiology-preservation-by-radiations-2016

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PRESERVATION BY RADIATION
It is common to group the entire spectrum of radiation into two categories, one on each side of visible light.
Low-frequency, long-wavelength, low-quantum- energy radiation ranges from radio waves to infrared. The
effect of these radiations on microorganisms is related to their thermal agitation of the food. Conversely, the
high-frequency, shorter-wavelength radiations have high quantum energies and actually excite or destroy
organic compounds and microorganisms without heating the product. Microbial destruction without the
generation f high temperatures suggested the term “cold sterilization”
When applied to the food industry, shorter-wavelength radiation can be further divided into two groups.
Lower-frequency and lower-energy radiation, for example, the ultraviolet part of the spectrum, has sufficient
energy only to excite molecules. This area of the spectrum is employed in the food industry and s covered in
the section on ultraviolet irradiation. Radiations of higher frequencies have high energy contents and are
capable of actually breaking individual molecules into ions, hence the term ionizing irradiation
ULTRAVIOLET IRRADIATION
Of the various electromagnetic radiations, ultraviolet irradiation has been the most widely used in the food
industry. Radiation with wavelengths near 260 nm is absorbed strongly by purines and pyrirnidines and is
therefore the most germicidal. Ultraviolet radiation around 200 nm is strongly absorbed by oxygen, may
result in the production of ozone, and is ineffective against microorganisms.
Factors Influencing Effectiveness
It should be emphasized that only direct rays are effective unless they come from special reflectors, and even
then their effectiveness is reduced. The factors that influence the effectiveness of ultraviolet rays are as
follows:
1 Time. The longer the time of exposure to a given concentration, the more effective the treatment.
2 Intensity. The intensity of the rays reaching an object will depend on the power of the lamp, the distance
from the lamp to the object, and the kind and amount of interfering material in the path of the rays. Dust in
the air or on the lamp reduces the effectiveness, as does too much atmospheric humidity. \3 Penetration. The
nature of the object or material being irradiated has an important influence on the effectiveness of the
process. Penetration is reduced even by clear water, which also exerts a protective effect on microorganisms.
Dissolved mineral salts. especially of iron, and cloudiness greatly reduce the effectiveness of the rays. Even a
thin layer of fatty or greasy material cuts off the rays. There is no penetration through opaque material.
The successful use of these rays include treatment of water used for beverages; aging of meats; treatment of
knives for slicing bread; treatment of bread and cakes; packaging of sliced bacon; sanitizing of eating utensils;
prevention of growth of fiim yeast on pickle, vinegar, and sauerkraut vats; killing of spores on sugar crystals
and in syrups; storage and packaging of cheese; prevention of mold growth on walls and shelves; and
treatment of air used for, or in, storage and processing rooms.
Kinds of Ionizing Radiations
Radiation classified as ionizing includes x-rays or gamma rays, cathode or beta rays, protons, neutrons, and
alpha particles. Neutrons result in residual radioactivity in foods, and protons and alpha particles have little
penetration. Therefore, these rays are not practical for use in food preservation.
Definition of Terms
Before the utilization of ionizing radiations can be discussed, a few terms must be defined.
A roentgen (r) is the quantity of gamma or x-radiation which produces one electrostatic unit of electric charge
of either sign in a cubic centimeter of air under standard conditions.
The rad now is employed chiefly as the unit of radiation dosage, being equivalent to the absorption of 100
erg per gram of irradiated material.
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A megarad (Mrad) is 1 million rad, and a kilorad (Krad) is 1,000 rad.
An electronvolt (eV) is the energy gained by an electron in moving through a potential difference of 1 volt. A
meV is 1 million electronvolts.
A meV, then, is a measure of the intensity of the irradiation, and a rep is a measure of the absorbed energy
that is effective within the food.
A Gray (Gy) equals 100 rads and is being used as a term to replace rads in some references.
Radappertization is a term used to define “radiation sterilization” which would imply high dose
Radicidationtreatments, with the resulting product being shelf-stable. Radappertization is equivalent to
radiation sterilization or "commercial sterility," as it is understood in the canning industry. Typical levels of
irradiation are 3(MK) kGy.
Radurization refers to “radiation pasteurization” low-dose treatments, where the intent is to extend a
product’s shelf life. 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. Commondose levels are 0.75-2.5 kGy for fresh meats, poultry,
seafood, fruits, vegetables, and cereal grains.
Radididation also is a low-dose “radiation pasteurization” treatment, but with the specific intent being the
elimination of a particular pathogen. Radicidation is equivalent to pasteurization of milk, for example.
Specifically, it refers to the reduction of the number of viable specific nonspore- 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.
Picowaved is a term used to label foods treated with low-level ionizing radiation.
GAMMA RAYS AND CATHODE RAYS Since these two types of rays are equally effective in sterilization for
equal quantities of energy absorbed and apparently produce similar changes in the food being treated .
Sources
Chief sources of gamma rays are (1) radioactive fission products of uranium and cobalt, (2) the coolant
circulated in nuclear reactors, and (3) other fuel el use to operate a nuclear reactor.
Cathode rays usually are accelerated by special electrical devices. The greater this acceleration (i.e., the more
meV), the deeper the penetration into the food.
Penetration
Gamma rays have good penetration, but their effectiveness decreases experientially with depth. They have
been reported to be effective up to 20 cm in most foods.
Cathode rays, on the other hand, have poor penetration, being effective at only about 0.5 cm per meV when
“cross firing,” that is, irradiation from opposite sides, is employed.
Efficiency
Because cathode rays are directional, they can be made to hit the food and therefore are used with greater
efficiency than are gamma rays, which are constantly emitted in all directions from the radioactive sources.
Radioactive sources of gamma rays decay steadily and hence weaken with time.
Safety
The use of cathode rays presents fewer health problems than the use of gamma rays, since cathode rays are
directional and less penetrating, can be turned off for repair or maintenance work, and present no hazard of
radioactive materials after a fire, explosion, or other catastrophe. Gamma rays are emitted in all directions,
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are penetrating, are continuously emitted, and come from radioactive sources. Gamma rays require more
shielding to protect workers.
Effects on Microorganisms
The bactericidal efficacy of a given dose of irradiation depends on the following:
1 The kind and species of organism.
2 The numbers of organisms (or spores) originally present.
3 The composition of the food.
4 The presence or absence of oxygen.
5 The physical stale of the food during irradiation.
6 The condition of the organisms.
EFFECT OF IRRADIATION ON FOOD QUALITY
Radiation doses heavy enough to affect sterilization have been found to produce undesirable “side
reactions,” or secondary changes, in many kinds of foods, causing undesirable colors, odors, tastes, or
even physical properties.
Some of the changes produced in foods by sterilizing doses of radiation include
(I) in meat, a rise in pH, destruction of glutathione, and an increase in carbonyl compounds,
hydrogen sulfide, and methyl mercaptan,
(2) in fats and lipids, destruction of natural antioxidants, oxidation followed by partial
polymerization, and increase in carbonyl compounds, and
(3) in vitamins, reduction in most foods of levels of thiamine, pyridoxine, and vitamins B12, C, D, E,
and K; riboflavin and niacin are fairly stable.
The undesirable changes that occur in certain irradiated foods may be caused directly by irradiation or
indirectly as a result of post irradiation reactions.
Water undergoes radiolysis when irradiated in the following manner:
radiolysis
3H2O • H + OH + H2O2+ H2
4.In addition, free radicals are formed along the path of the primary electron and react with each other as
diffusion occurs.
5.By irradiating under anaerobic conditions, off-flavors and off odors are somewhat minimized due to the
lack of oxygen to form peroxides.
6.Irradiation of lipids and fats results in the production of carbonyls and other oxidation products such as
peroxides, especially if irradiation and/or subsequent storage takes place in the presence of oxygen.
7.High levels of irradiation lead to the production of "irradiation odors" in certain foods, especially meats.
8.Flavor and odor changes produced in certain foods by irradiation, certain detrimental effects have been
reported for irradiated fruits and vegetables. One of the most serious is the softening of these products
caused by the irradiation-degradation of pectin and cellulose, the structural polysaccharides of plants.
9.Ethylene synthesis in apples is affected by irradiation so that this product fails to mature as rapidly as
non irradiated controls.
10.In green lemons, however, ethylene synthesis is stimulated upon irradiation, resulting in a faster
ripening than in controls.
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Applications
Currently food irradiation has been approved only in a very limited way .
Low-level irradiation ([1 kiloGray) can be used on fresh fruits and vegetables to kill insects and to inhibit
spoilage.
Dry or dehydrated vegetables (herbs and spices) can be irradiated at up to 30 kiloGray to kill insects
MICROWAVE PROCESSING
Microwave heating and processing of foods is becoming increasingly popular, particularly at the consumer
level. Microwaves are electromagnetic waves between infrared and radio waves (Figure 10-1). Specific
frequencies are usually at either 915 megacycles or 2,450 megacycles. The energy or heat produced by
microwaves as they pass through a food is a result of the extremely rapid oscillation of the food molecules in
an attempt to align themselves with the electromagnetic field being produced. This rapid oscillation, or
intermolecular friction, generates heat. The preservative effect of microwaves or the bactericidal effect
produced is really a function of the heat that is generated. In other words, the microwaves themselves do not
result in any inactivation of foodborne microorganisms; rather, it is the heat produced by the excitation of
food molecules that actually results in microbial destruction.
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