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As more of the food that we consume is subject to some form of processing, a growing onus is being placed on the food industry to ensure that the food they produce is safe to eat.

Recent trends in certain sectors of the food industry directed at the centralization of production towards a smaller number of increasingly large food-processing facilities will have important implications for food safety. Such conditions enable a single incidence of food contamination at a facility from which food is distributed over a wide geographical area to potentially affect a large proportion of the population.

Microbiological food safety implies the inactivation or removal of pathogenic microorganisms associated with foods. This can, of course, be achieved in a number of ways but, increasingly, the use of chemical agents is becoming subject to ever tighter legislative control. This is in part a reflection of growing public anxieties about the possible harmful effects of such agents when ingested. Largely as a result of such concerns, interest is being shown in alternative, so-called “physical treatments.” The use of ultraviolet light (UV)falls within this category.

This article deals with microbiological food safety and its assurance through the use of UV light.

WHAT IS UV?
UV forms part of the electromagnetic spectrum and the UV wavelength range is from about 10 to 400 nm, placing it between X-rays and the visible part of the spectrum. Though frequently referred to as “non-ionizing radiation,” the shortest ultraviolet wavelengths do bring about some ionization. The UV portionof the spectrum has been sub-divided for convenience. The term “vacuumultraviolet” is reserved for wavelengths below 200 nm, because in thisregion, UV is strongly attenuated by air. It is usual to refer to the regionbetween 200 and 300 nm as “far ultraviolet” and that between 300and 400 nm as “near ultraviolet.” Alternative sub-divisions are oftenquoted in the scientific literature, thus, UV-C is used for wavelengths in therange 100 to 280 nm, UV-B for 280 to 315 nm, and UV-A for 315 to 400 nm.1 Itis only UV-C that is able to inactivate microorganisms directly. However, itisstill possible to employ the longer wavelengths to lethal effect in associationwith photocatalysts as will be explained below.

It is important to point out that UV is harmful to humans and, in any application, serious consideration must be given to protecting personnel from exposure to it. The eyes are particularly susceptible and the condition arising from exposure to UV, referred to as “welder’s eye,” is both painful and ultimately sight threatening. Exposure of skin to UV results in erythe-ma, or delayed reddening and, at sufficiently high doses, UV can have profound effects on the immune system that can lead to severe and potentially lethal consequences. However, all such harmful effects can be completely avoided by careful design of containment measures to eliminate stray UV through the use of shields and non-reflective surfaces.

MICROBIAL GROUP

D10 UV DOSE (mW-sec/cm2)

Bacteria (incl. spores)

0.4 to 30

Enteric viruses

5 to 30

Fungi

30 to 300

Protozoa

60 to 120

Algae

300 to 600

Table 1. D10 UV inactivation doses (in mW-sec/cm2) measured at 253.7 nm for various microbial groups.

HOW UV KILLS MICROORGANISMS
The lethal effects of UV towards microorganisms were discovered at the end of the nineteenth century and the first practical application of UV was in the disinfection of water. This remains the use to which UV is most commonly associated today and it is true to say that the technology for treating water can be thought of as relatively well accepted in the food and other industries. For this reason, UV treatment of water will not be touched upon further here and the interested reader is referred to one of the many handbooks on thesubject2 for further information.

Outside of the field of water treatment, UV is often referred to as a surface treatment. This view is only partially correct as I hope to show below, insofar as it describes only one particular aspect of UV treatment. Specifically with reference to surface treatment, it is important to realize that UV is strongly absorbed by most materials and cannot penetrate beyond the surface layers of solid objects. In such instances for both abiotic materials and many types of foods, it is only microorganisms that are present at the surface that one may ultimately hope to inactivate. For some types of food this may well be sufficient, for example, muscle flesh from a healthy animal immediately after slaughter is, for all intents, sterile. Where contamination does occur, it will be as a result of contact with contaminated surfaces or fluids and this will initially manifest itself at the surface.

The efficacy of UV surface treatment will be strongly influenced by surface topography. Crevices, and similar features, of dimensions comparable to the size of microorganisms (i.e., a few microns) may shield microorganisms from potentially lethal UV rays and enable them to survive. This was cited in recent work as the reason why the UV treatment of fish fillets from a smooth-fleshed species was more effective than that of a rough-fleshed one.

Another important factor determining survival is the intrinsic resistance of the microorganism to the effects of UV. This will be influenced to some extent by the physiological state of the cell, and is therefore not a fixed quantity. Notwithstanding this important qualification, Table 1 shows the ranges of UV doses required to reduce populations of microbial groups by a single order of magnitude — a quantity referred to as the “D10 dose.” The range for bacteria excludes Deinococcus radiodurans, which is the most UV-resistant organism isolated to date. Fortunately, this bacterium is something of an oddity and highly unlikely to be found in normal food-processing operations.

Early planet Earth, lacking a protective ozone layer, was bathed in UV and while UV was an important agent of evolution in generating variation in early organisms, there ultimately was value to organisms in being able to protect themselves from its effects. Evolution appears to have conferred on microorganisms at least two independent strategies for specifically surviving UV exposure.

The first was to produce pigments that absorb UV strongly, and the protective effects of such pigments have been demonstrated by isolating non-pigmented mutants of the same species and comparing their UV resistance. The second has to do with the efficiency of DNA repair following UV exposure. While UV has the ability to chemically modify the structures of many of the chemical entities found in cells, it is its effects on DNA that will ultimately determine whether or not the organism will survive.3 UV is known to cause a number of different lesions in DNA but the most common is the dimerization of adjacent pyrimidine bases on the same strand of DNA. This effectively interferes with DNA replication and to counter this, enzyme-mediated repair processes have evolved that essentially restore the DNA to its original state. Given the fundamental importance of DNA replication, it is not surprising that all living organisms possess such repair mechanisms. However, their overall efficiency differs from species to species, and it turns out that the reason that D. radiodurans is so resistant to UV is because it possesses the most efficient DNA repair mechanism yet identified — essentially the repair mechanism is able to restore dimerized bases faster than the UV can generate them at all but the highest doses of UV. Some of these repair processes are activated by visible light, and it is in fact possible to reverse DNA damage completely by post-UV exposure to light of the correct wavelength. This is a factor that must be taken into account in commercial UV food treatment, that is, that the treated food is shielded from the relevant wavelengths for a sufficient period of time.

Fortuitously, UV generated using low-pressure mercury sources emits UV principally at a wavelength of 254 nm which is close to the peak absorptivity of DNA. This wavelength region is often referred to as “germicidal” because it may be thought of as highly biologically effective.

Yet another protective strategy that has been adopted by some microorganisms is growth in the form of “biofilms.” A biofilm may be thought of as a structured microbial community associated with solid surfaces. Attachment to surfaces occurs because certain members of the community are able to produce polysac-charides that serve as adhesives. Biofilms pose a very real threat in the food industry and contact of foods with biofilms invariably results in contamination as cells are shed from the biofilm to the food. While there is no evidence that growth in the form of biofilms arose specifically as a protection against environmental UV, organisms within biofilms are well protected from a variety of stresses, including UV. This is partly because the microorganisms within the biofilm are in a metabolic state that renders them less susceptible to environmental stresses, and partly because the polysaccharide matrix in which the cells are embedded offers a defense against both physical and chemical disinfectants. Many different approaches for neutralizing biofilms are being pursued but the best current advice appears to be to effect physical removal and then to thoroughly disinfect the underlying surface.

The effects described above in relation to DNA may be thought of as being “instantaneous” in that inactivation can only occur if the targeted microorganisms are actually undergoing exposure to UV. In other words, once removed from the UV field, the generation of harmful lesions ceases. However, a radically different form of UV treatment that can be applied to plant foods has been attracting much interest recently. It is based on a phenomenon known as “hormesis” that has been much championed in recent times by Edward J. Cal-abrese.4 The term hormesis is derived from Greek and has variously been interpreted as meaning “to excite,” but in more practical terms it may be taken as meaning “the stimulation by low doses of any potentially harmful agent.” The agents capable of bringing about these stimulatory effects may be either chemical or physical ones and included among the latter are various portions of the electromagnetic spectrum, including UV.

UV hormesis can be thought of as an induced effect that occurs over intervals of time measured in hours or days, in contrast to the virtually instantaneous effects of UV on DNA described above. It relies on eliciting a metabolic response by the plant tissue in countering what it perceives as an applied stress. The response is chemical; for certain types of fruit, the compounds produced as a result of low-dose UV treatment have been identified. These include a wide range of phy-toalexins and enzymes. The crucial factor is that these compounds confer resistance to attack by many different types of microorganisms, and molds in particular. What’s more, they are naturally occurring compounds and microbial inhibition can be achieved without the use of exogenous biocides. Many species of fruits and vegetables have been shown to respond to this form of treatment.1,5 In commercial terms, this stress response offers a way to extend the shelf life of fresh commodities. Hormetic treatments also have the potential to reduce waste through decreasing the rate of senescence. However, much work remains to be done in scaling up laboratory studies to enable hormet-ic treatments to be applied commercially.6 There is another benefit of hormetic treatment, and that is that many of the compounds produced by the plant in response to UV are actually beneficial to human health. The best studied example is resveratrol in grapes where recent work has shown that the levels of this cardio-protectant may be increased many fold by treating grapes with low doses of UV.

POTENTIATING THE EFFECTS OF UV
The lethal effects of UV may be increased by combining UV treatment with the use of powerful oxidants, such as hydrogen peroxide and/or ozone. Although both of these compounds are moderately germicidal in their own right, the added effect of UV is to bring about a synergistic inactivation through the enhanced generation of highly reactive free radical species. Ozone has been assigned the classification “generally recognized as safe” (GRAS), but hydrogen peroxide residuals can persist for considerable periods of time. This may prove problematic in the treatment of foods — as opposed to food processing equipment, or even the fabric of food processing facilities — andcare is needed to carefully control the peroxide concentration and the UV dose to ensure total photolysis of the peroxide.
Much interest has recently been shown in the ability of the anatase crystallineform of titanium dioxide (TiO2) to generate lethal free radicals when exposedto UV-A. TiO2 has been incorporated into a number of different materials, suchas ceramic tiles and other building materials. This results in the creation of “activesurfaces” that may, under the influence of UV-A illumination, be thoughtof as passively inactivating microorganisms.

CONCLUSIONS
Whatever measures are ultimately taken to ensure microbiological food safetywill require what is referred to in HACCP terminology as a “critical controlpoint” and in this context, this implies a killing stage. As yet, no meansof killing with kindness has yet been discovered and until such time, it mustbe recognized that whatever treatment is ultimately applied to a particular foodwill inevitably result in the compromise to some extent of the quality or wholesomenessof that foodstuff. The emphasis is on to some extent whether the changes thatare brought about are acceptable andwhether they offer improvements over current — essentially chemical — alternatives.In treating abiotic surfaces in food-processing facilities, such considerationsmatter far less. UV treatment in its many manifestations offers one possibility.However, it will prove necessary to assess carefully the effects of UV on thekey quality and nutritional attributes of a range of foods if it is to be morewidely used in the food industry.

Public concerns over what is being done to their food need to be acknowledged and squarely addressed. The use of certain terms will certainly not be helpful, and one that instantly springs to mind is the term “UV irradiation.” While this accurately describes any form of treatment with UV, it will automatically be linked in the public mind with ionizing radiation and ultimately with vague, but all too real, anxieties about rendering food radioactive. The solution to this is not simply one of slick marketing but of public education, and while this is a not inconsiderable task, it should not be shirked.

References

  1. Shama, G. 2006. Ultraviolet Light. In Handbook of Food Science, Technology, and Engineering, Hui, Y.H. (Ed.) CRC Press, Boca Raton, USA pp122-1 – 122-14.
  2. Schenk, G.O., 1987. Ultraviolet Sterilization. In: Lorch, W., (Ed.) Handbook of Water Purification, 2nd Edition, Ellis Horwood, Chichester.
  3. Harm, W., 1980. Biological Effects of Ultraviolet Radiation, Cambridge University Press, Cambridge.
  4. Calabrese E.J., 2005. Paradigm lost, paradigm found: The reemergence of hormesis as a fundamental dose response model in the toxicological sciences. Env. Pol. 138, 378-411.
  5. Shama, G., Alderson, P., 2005. UV hormesis in fruits: a concept ripe for commercialization. Trends Food Sci. Technol. 16, 128-136.
  6. Shama, G. 2006. Process Challenges in Applying Low Doses of Ultraviolet Light to Fresh Produce for Eliciting Beneficial Hormetic Responses. Postharvest Biology and Technology (in press).

 

Gilbert Shama B.Sc., M.Sc., Ph.D., DIC. is a Senior Lecturer in the Department of Chemical Engineering at Loughborough University, Loughborough, LEICS, LE11 3TU, UK.He has wide-ranging interests in the disinfection of foods and liquids by a variety of physical techniques that include UV treatment and the applicationof cold atmospheric plasmas.He can be reached at Tel.: +44 1509 222514;G.Shama@Lboro.ac.uk.

 

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