Radiation Techniques for Preservation of Food

The following article highlights the four radiation techniques for preservation of food: The techniques are:- 1. Irradiation 2. Microwave Radiation 3. UV Radiation 4. Ionizing Radiation

Radiation Technique # 1. Irradiation:

Electromagnetic (e.m.) radiation is a way in which energy can be propagated through space. It is characterized in terms of its wavelength A, or its frequency v, and the product of these two properties gives the speed, c, at which it travels (3 x 10⁸ m sec ^_1 in a vacuum).

v = c (4.13)

The range of frequencies (or wavelengths) that e.m. radiation can have is known as the electromagnetic spectrum and is grouped into a number of regions, visible light being only one small region (Figure 4.8).

The energy carried by e.m. radiation is not continuous but is transmitted in discrete packets or quanta; the energy, E, contained in each quantum being given by the expression:

E = hv (4.14)

where h is a constant (6.6 x 10^-27 ergs sec –¹) known as Planck’s constant. Thus, the higher the frequency of the radiation the higher its quantum energy.

As far as food microbiology is concerned, only three areas of the e.m. spectrum concern us; microwaves, the UV region and gamma rays. We will now consider each of these in turn.

Radiation Technique # 2. Microwave Radiation:

The microwave region of the e.m. spectrum occupies frequencies between 10⁹ Hz up to 10¹² Hz and so has a relatively low quantum energy. For the two frequencies used in food processing, 2 450 MHz and 915 MHz, this is around 10^-18 ergs or 10^{– 6}eV.

Unlike the other forms of radiation we will discuss, microwaves act indirectly on micro-organisms through the generation of heat. When a food containing water is placed in a microwave field, the dipolar water molecules align themselves with the field. As the field reverses its polarity 2 or 5 x 10⁹ times each second, depending on the frequency used, the water molecules are continually oscillating.

This kinetic energy is transmitted to neighbouring molecules leading to a rapid rise in temperature throughout the product. In foods with a high salt content, surface heating due to ions acquiring kinetic energy from the microwave field can also contribute, but this is generally of minor importance.

Microwaves are generated using a magnetron, a device first developed in the UK during research into radar during the Second World War. Magnetrons are used both commercially and domestically, but their biggest impact has been in the domestic microwave oven and in catering where their speed and convenience have enormous advantages.

The principal problem associated with the domestic use of microwaves is non-uniform heating of foods, due to the presence of cold spots in the oven, and the non-uniform dielectric properties of the food.

These can lead to cold spots in some microwaved foods and concern over the risks associated with consumption of inadequately heated meals has led to more explicit instructions on microwaveable foods. These often specify a tempering period after heating to allow the temperature to equilibrate.

Microwaves have been slow to find industrial applications in food processing, although they are used in a number of areas. Microwaves have been used to defrost frozen blocks of meat prior to their processing into products such as burgers and pies thus reducing wear and tear on machinery.

There has also been a limited application of microwaves in the blanching of fruits and vegetables and in the pasteurization of soft bakery goods and moist (30% H₂O) pasta to destroy yeasts and moulds. In Japan, microwaves have been used to pasteurize high-acid foods, such as fruits in syrup, intended for distribution at ambient temperature.

These are packed before processing and have an indefinite microbiological shelf-life because of the heat process and their low pH. However the modest oxygen barrier properties of the pack has meant that their biochemical shelf-life is limited to a few months.

Radiation Technique # 3. UV Radiation:

UV radiation has wavelengths below 450 nm (v ⋍10¹⁵ Hz) and a quantum energy of 3-5 eV (10^-12 ergs). The quanta contain energy sufficient to excite electrons in molecules from their ground state into higher energy orbitals making the molecules more reactive. Chemical reactions thus induced in micro-organisms can cause the failure of critical metabolic processes leading to injury or death

Only quanta providing energy sufficient to induce these photochemical reactions will inhibit micro-organisms, so those wavelengths that are most effective give us an indication of the sensitive chemical targets within the cell. The greatest lethality is shown by wavelengths around 260 nm which correspond to a strong absorption by nucleic acid bases.

The pyrimidine bases appear particularly sensitive, and UV light at this wavelength will, among other things, induce the formation of covalently linked dimers between adjacent thymine bases in DNA (Figure 4.9). If left intact these will prevent transcription and DNA replication in affected cells.

The resistance of micro-organisms to UV is largely determined by their ability to repair such damage, although some organisms such as micrococci also synthesize protective pigments. Generally, the resistance to UV irradiation follows the pattern: Gram-negatives < Gram-positives ≈ yeasts < bacterial spores < moulds spores << viruses.

Death of a population of UV-irradiated cells demonstrates log-linear kinetics similar to thermal death and, in an analogous way, D values can be determined. These give the dose required to produce a tenfold reduction in surviving numbers where the dose, expressed in ergs or µW s, is the product of the intensity of the radiation and the time for which it is applied. Some published D values are presented in Table 4.7.

Determination of UV D values is not usually a straightforward affair since the incident radiation can be absorbed by other medium components and has very low penetration. Passage through 5 cm of clear water will reduce the intensity of UV radiation by two-thirds.

This effect increases with the concentration of solutes and suspended material so that in milk 90% of the incident energy will be absorbed by a layer only 0.1 mm thick. This low penetrability limits application of UV radiation in the food industry to disinfection of air and surfaces.

Low-pressure mercury vapour discharge lamps are used: 80% of their UV emission is at a wavelength of 254 nm which has 85% of the biological activity of 260 nm.

Wavelengths below 200nm are screened out by surrounding the lamp with an absorbent glass since these wavelengths are absorbed by oxygen in the air producing ozone which is harmful. The output of these lamps falls off over time and they need to be monitored regularly.

Air disinfection is only useful when the organisms suspended in air can make a significant contribution to the product’s microflora and are likely to harm the product. For example, in the control of mould spores in bakeries.

UV lamps have also been mounted in the head space of tanks storing concentrates, the stability of which depends on their low a_w. Fluctuations in temperature can cause condensation to form inside the tank.

If this contacts the product, then areas of locally high a_w can form where previously dormant organisms can grow, spoiling the product. Process water can be disinfected by uv, this avoids the risk of tainting sometimes associated with chlorination, although the treated water will not have the residual antimicrobial properties of chlorinated water.

UV radiation is commonly used in the depuration of shellfish to disinfect the water re-circulated through the depuration tanks. Chlorina­tion would not be suitable in this situation since residual chlorine would cause the shellfish to stop feeding thus stopping the depuration process.

Surfaces can be disinfected by UV, although protection of micro-organisms by organic material such as fat can reduce its efficacy. Food containers are sometimes treated in this way and some meat chill store rooms have UV lamps to retard surface growth.

UV can however induce spoilage of products containing unsaturated fatty acids where it accelerates the development of rancidity. Process workers must also be protected from UV since the wavelengths used can cause burning of the skin and eye disorders.

Radiation Technique # 4. Ionizing Radiation:

Ionizing radiation has frequencies greater than 10¹⁸ Hz and carries sufficient energy to eject electrons from molecules it encounters. In practice three different types are used.

(1) High-energy electrons.in the form of β particles produced by radioactive decay or machine generated electrons. Strictly speaking they are particles rather than electromagnetic radiation, although in some of their behaviour they do exhibit the properties of waves.

Because of their mass and charge, electrons tend to be less penetrating than ionizing e.m. radiation; for example, 5 MeV β particles will normally penetrate food materials to a depth of about 2.5 cm.

(2) X-rays generated by impinging high energy electrons on a suitable target.

(3) Gamma γ rays produced by the decay of radioactive isotopes. The most commonly used isotope cobalt 60, ⁶⁰Co, is produced by bombarding non­radioactive cobalt, ⁵⁹Co, with neutrons in a nuclear reactor.

It emits high- energy γ –rays (1.1 MeV) which can penetrate food up to a depth of 20 cm (cf. β particles). An isotope of caesium, ¹³⁷Cs, which is extracted from spent nuclear fuel rods, has also been used but is less favoured for a number of reasons.

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 (Figure 4.10). These indirect effects play the more important role since in the absence of water doses 2-3 times higher are required to obtain the same lethality.

Removal of oxygen also increases microbial resistance 2-4 fold and it is thought that this may be due to the ability of oxygen to participate in free radical reactions and prevent the repair of radiation induced lesions.

As with UV irradiation, the main site of damage in cells is the chromosome. Hydroxvl radicals cause single- and double-strand breaks in the DNA molecule as a result of hydrogen abstraction from deoxyribose followed by elimination of phosphate which cleaves the molecule. They can also hydroxylate purine and pyrimidine bases.

Resistance to ionizing radiation depends on the ability of the organism to repair the damage caused. Inactivation kinetics are generally logarithmic, although survival curves often appear sigmoidal exhibiting a shoulder and a tail to the phase of log-linear death.

The shoulder is usually very slight but is more pronounced with bacteria which have more efficient repair mechanisms where substantially more damage can be accumulated before death ensues.

D values can be derived from the linear portion of these curves and Table 4.8 presents 6D values (the dose to produce a million fold reduction) reported for a number of foodborne organisms. These are expressed in terms of the absorbed dose of ionizing radiation which is measured in Grays (1 Gy = 1 joule kg^-1).

Resistance generally follows the sequence:

Food-associated organisms do not generally display exceptional resistance, although spores of some strains of Clostridium botulinum type A have the most radiation resistant spores.

Since studies on food irradiation started, a number of bacteria which are highly resistant to radiation have been isolated and described. Although one of these, Deinococcusradiodurans, was first isolated from meat, their role in foods is not significant in the normal course of events.

Although patents describing the use of ionizing radiation in the treatment of food appeared soon after the discovery of radioactivity at the turn of the Century, it was not until after the Second World War that food irradiation assumed commercial potential.

This was largely due to technological advances during the development of nuclear weapons, but also to a strong desire to demonstrate that nuclear technology could offer the human race something other than mass destruction.

In particular, food irradiation has the advantage of being a much more precisely controlled process than heating, since penetration is deep, instantaneous and uniform. It also retains the fresh character of the product as low level irradiation produces no detectable sensory change in most products.

This failure of low doses of radiation to produce appreciable chemical change in the product has been an obstacle to the development of simple tests to determine whether a food has been irradiated.

Although availability of such a test is not essential for the control of irradiation, it is generally accepted that it would facilitate international trade in irradiated food, enhance consumer confidence and help enforce labelling regulations.

A number of promising methods have been identified including thermoluminescence, electron spin resonance, high-field nuclear magnetic resonance and chemical analysis for specific markers such as hydroxylated tyrosine in high protein foodstuffs.

One proposed test is based on differences between irradiated and non-irradiated foods in the ratio between the viable plate count and the quantity of Gram-negative endotoxin detected using the Limulus amoebocyte lysate test.

Food irradiation is not without its disadvantages, but a lot of the concerns originally voiced have proved to be unfounded. In 1981 an expert international committee of the FAO/WHO and the International Atomic Energy Authority recommended general acceptance of food irradiation up to a level of 10 kGy.

They held the view that it ‘constitutes no toxicological risk. Further toxicological examinations of such treated foods are therefore not required’.

It had been thought that irradiation could lead to pathogens becoming more virulent but, apart from one or two exceptions, it has been found that where virulence is affected it is diminished. In the exceptions noted, the effect was slight and not sufficient to compensate for the overall reduction in viable numbers.

No example has been found where a non-pathogenic organism has been converted to a pathogen as a result of irradiation. Although it has been reported that spores of some mycotoxigenicmoulds which survive irradiation may yield cultures with increased mycotoxin production.

Morphological, biochemical and other changes which may impede isolation and identification and increased radiation resistance have been noted as a result of repeated cyclic irradiation.

However, these experiments were performed under the most favourable conditions and for this to occur in practice would require extensive microbial regrowth after each irradiation; a condition that is readily preventable by good hygienic practices and is most unlikely to be met.

The levels of radiation proposed for foods are not sufficient to induce radioactivity in the product and there is no evidence that consumption of irradiated foods is harmful. Food irradiation facilities do require stringent safety standards to protect workers but that is already in place for the irradiation of other materials such as the sterilization of medical supplies and disposables.

By far the greatest obstacle to the more widespread use of food irradiation is not technical but sociological in the form of extensive consumer resistance and distrust.

Much of this is based on inadequate information and false propaganda and parallels very closely earlier arguments over the merits of milk pasteurization. Among the same objections raised then were that pasteurization would be used to mask poor quality milk and would promote poor practices in food preparation.

While it has to be agreed that those who take the most cynical view of human nature are often proved correct, this did not prove to be the case with milk pasteurization where the production standards and microbiological quality of raw milk are now higher than they have ever been.

Depending on the lethality required, food irradiation can be applied at two different levels. At high levels it can be used to produce a safe shelf stable product in a treatment known as radappertization.

Though this has been investigated in the context of military rations, it is unlikely to be a commercial reality in the foreseeable future. C. botulinum spores are the most radiation resistant known, so very high doses are required to achieve the minimum standard of a 12D reduction (≈ 45 kGy) for low-acid foods.

In the event of a process failure, the growth of more resistant, non­pathogenic Clostridia would not act as a warning as it can in thermal processing. High radiation doses are also more likely to produce unacceptable sensory changes and the product has to be irradiated in the frozen state to minimize migration of the radio lytic species that cause such changes.

These considerations would not apply when the food was inhibitory to the growth of C. botulinum as a result of low pH or the presence of agents such as curing salts.

Two terms are used to distinguish different types of radiation pasteurization. Radicidation is used to describe processes where the objective is the elimination of a pathogen, as, for example, in the removal of Salmonella from meat and poultry.

Radurization applies to processes aiming to prolong shelf-life. This distinction may be thought a little over elaborate since, as with thermal pasteurization, irradiation treatments are relatively non-discriminating and will invariably improve both safety and shelf-life.

A number of potential applications have been identified (Table 4.9) and food irradiation for specific applications is now permitted in more than 35 countries. In the UK, the applications described in Table 4.10 have been allowed since 1991, although the requirement for labelling irradiated products and consumer resistance has limited the adoption of these so far.