The effect of UV radiation on the microbiological population within different liquids has received much attention in research, as the primary goal is to achieve germicidal efficacy upon the treatment of liquids with UV radiation. However, limited research has been conducted on the effect of UV treatment on the chemical and biochemical composition of liquids. In the instance that some studies have investigated such impact, the studies were mainly limited to water, fruit juices, sugar concentrates and goat’s milk (as summarized in Table 2.4).
Total Solids (Soluble and Insoluble)
The dissolved organic solutes in liquids will have a strong UV attenuating effect (Koutchma, 2009). Suspended solids will not only attenuate the UV dose, but could also provide a ‘shielding’ effect towards microorganisms, as the microorganism could aggregate on the surfaces of suspended particles that could absorb, reflect, refract and scatter the UV light (Christenen & Linden, 2001). Although limited research has been conducted on the effect of suspended solids, Koutchma (2009) states that the UV dose delivered may be underestimated as a result of fluids with a high concentration of suspended solids thus negatively impacting the UV dose response through lower UV absorbance to the targeted microorganisms. From this it was concluded that a thorough investigation and analysis of the liquid should be concluded in order to equate UV absorbance needed for an optimal UV dose response.
Macronutrients
Foods vary enormously in their sensitivity to UV light. Degradation of macronutrients in food following exposure to sunlight or to UV radiation has been investigated since the early 1950’s. Spikes (1981) reported that unsaturated fatty acid residues in oils, solid fats, and phospholipids are usually regarded as light “light sensitive”. Degradation of especially fat and protein in foodstuffs due to light exposure occurs in the UV and visible light region of the spectrum (between 280 nm & 780 nm), but the blue to green band of the visible region (430-460 nm) is considered the main band responsible for light-induced oxidation (Borle et al., 2001). Koutchma (2009) indicates nucleic acids are the strongest light absorbers at the germicidal wavelength of 254.7 nm, and states that unsaturated organic molecules usually absorb at wavelengths greater than 220 nm. Carbohydrates were also indicated not to be especially sensitive to light degradation (Koutchma, 2009), however, Grandison (2006) reports that UV radiation of carbohydrate rich substances results in loss of texture or viscosity due to the breakdown of the glycosidic bond in starch, pectin or cellulose to give smaller carbohydrates.
Some defects that can occur as a result of photooxidation of fat, protein and carbohydrates include altering the nutritional profile of the food being irradiated, the development of off-flavours and aromas and changes to the rheological properties of
the food (texture and appearance). A more detailed review is given in section 2.5. with specific emphasis on the effect of UV radiation on milk, as a model liquid.
Redox potential
The UV dose response can also be subject to the redox potential of the liquid being treated and in some instances the germicidal efficacy can be enhanced when using UV in combination with other oxidizing agents such hydrogen peroxide (H2O2) and ozone
(Blank & Cumming, 2001). The use H2O2 of in combination with UV is well-known
in the aseptic packaging and wastewater industries, as the free radical formation of H2O2 and resulting germicidal efficacy is improved with UV radiation (Warriner et
al., 2000; Thiruvenkatachari et al., 2006).
pH
Various authors reported that the pH of the medium treated does not influence the killing efficacy of the target organism (Ngadi et al., 2003; Koutchma et al., 2004; Murakami et al., 2006; Hijnen et al., 2006). However, the physical properties of the liquids in question as per their reports were not altered and in more complex liquid systems, such as milk, a change in pH could induce structural changes in organic compounds, such as protein. These changes could therefore alter the physical and biochemical properties of the liquid and could impact on the UV dose response due to a change in UV absorbance needed to have the desired effect. Therefore the effect of UV on the product characteristics of more complex biological liquids, such as milk, remains of interest.
Vitamins
Bekb let (1990) stated that off-flavours and odours introduced following photooxidation are directly related to the source of light, the radiation dosage, the type of liquid being treated, the temperature of the liquid, the presence of oxygen and wavelength; however other constituents responsible for photochemical changes include proteins, amino acids, riboflavin, and vitamins. According to Grandison (2006), the destruction of vitamins is dependent on the chemical structure of the vitamin itself.
Various researchers have shown that exposure to visible light between 365 and 500 nm causes a significant increase in light oxidation in milk (Bosset et al., 1995; Hansen & Skibsted, 2000; Lennersten & Lingnert, 2000; Van Aardt et al., 2001). Riboflavin (Vitamin B2) is implicated in this oxidation because it acts as a photosensitizer when exposed to specific wavelengths (400, 446, and 570 nm) within this range (Lee, 2002; Bekb let, 1990). Other water-soluble vitamins susceptible to photooxidation include Thiamine (vitamin B1) and Ascorbic acid (vitamin C). Wilson & Droby (2001) reported that ascorbic acid in fruit juice was converted to dehydro-ascorbic acid, the oxidized form, which is biologically active and converted back to the reduced form during storage. The order of sensitivity of the water-soluble vitamins to photooxidation is as follows: thiamin > ascorbic acid > pyridoxine > riboflavin > folic acid > cobalamin > nicotinic acid (Fellows, 2000). The fat-soluble vitamins vary in their sensitivity to radiation, with Vitamins D and K largely unaffected while vitamins A and E are sensitive to photooxidation. The order of sensitivity of the fat-soluble group of vitamins is: vitamin E > vitamin A > vitamin K > vitamin D (Fellows, 2000). To minimize the loss of vitamins, liquids should be packaged in the absence of oxygen and irradiated at low temperatures with further losses that can be prevented by storing the irradiated products at low temperatures (Wilson & Droby, 2001).
Furan
It is well-known that furan is formed in food and drinks during thermal processing, and is present in foods such as coffee and processed baby foods (Moro et al., 2012). Furan derivatives, particularly furfural and hydroxymethylfurfural, are major volatile constituents of heated milk (Burton, 1988). Furans are not found in raw milk, but appear in milk heated above 90°C due to the Maillard reaction (Ferretti et al., 1971; Shibamoto, 1980). Furan has been reported as a possible human carcinogen, and therefore is undesirable in any consumed food or drinks (Bakhiya & Appel, 2010). In a study by Fan & Geveke (2007) they showed that UV-C treatment of apple cider caused the formation of furan at high doses of radiation. It was concluded that fructose was probably the main source of furan following UV radiation, however, it was also shown that very low amounts of furan were formed at the UV-C dose that was required for the inactivation of E. coli K12.
Cholesterol and cholesterol oxides
Cholesterol is the main sterol in animal-derived products, such as milk. It is well- known that cholesterol, like other sterols, are susceptible to oxidation by reactive oxygen species, light, UV light, ionizing radiation, chemical catalysts, lipid hydroxyperoxides, and enzymatic reactions, leading to the formation of sterol oxidation products (SOPs) (Sieber, 2005). Furthermore, human studies reveal that cholesterol oxidation products (COPs) could be absorbed from the diet (Linseisen & Wolfram, 1998).