UV processing holds potential to become a low-cost, nonthermal pasteurization technology for foods. It could also be used to pasteurize fresh juices, liquid egg and dairy products (such as milk, cheese milk, and whey-protein concentrates), and sweeteners. With the prevalent negative public reaction to chemical food additives, UV processing could reduce contamination levels because of its broad antimicrobial action, providing effective inactivation of viruses, vegetative bacteria, bacterial spores, yeast, conidia (fungal spores), and parasites. UV-light treatment of foods does not use chemicals or generate waste effluents, nor does it produce any byproducts, which makes it ecologically friendly. In addition, this approach can improve chemical and toxicological safety of a wide variety of liquid foods and beverages. Moreover, most nutritional components, which are sensitive to heat, are not destroyed by UV light or potentially suffer less destruction compared to heat treatment.1
The US Food and Drug Administration has approved UV light for use in processing of fresh juices to achieve a 5 log10 reduction of target pathogenic organisms, as required by regulations. Commercial technology already exists for juice processing using a monochromatic low-pressure mercury (LPM) lamp. In addition, use of pulsed UV (PUV) light has been approved for food production, processing, and handling.
Unfortunately, industrial applications of UV technology are still limited because UV-light transmittance through fluid foods and beverages is extremely low. The food industry is also dealing with different groups of micro-organisms, more chemical components, and more quality and nutrition issues compared to water treatment, where UV light has been used for many years. It is less effective in treating opaque and turbid liquids such as liquid eggs, apple cider, and orange juice, which strongly absorb scattered or reflected light. Consequently, effective UV treatment for liquid-food applications requires development of alternative approaches to those used for water.
We have been working at implementing an integrated approach that combines the advantages of available polychromatic UV sources as regards their propagation through specific liquids, impact on pathogenic and spoilage micro-organisms, overall quality, and nutritional compounds. Potentially, UV sources with polychromatic spectra or that are specifically shaped to match their wavelength to a liquid's transparency or opacity will allow improvements of inactivation performance. Knowledge of UV-light sensitivity of food compounds, in addition to their absorbing and scattering properties, is critical for correct UV-dose determination for the required safety and quality levels. Correct selection of the UV-light source and optimal design of the UV unit can also reduce the interference of high UV absorption and viscosity associated with fluid-food products, thus improving the inactivation efficiency. The flow pattern inside the UV reactor strongly influences the applied UV-dose delivery, since the position and residence time of the micro-organisms in certain regions of the irradiance field can vary significantly.
We used integrating-sphere spectroscopy to measure the total diffuse transmittance or reflectance of low-UV-transmittance fluids to compensate for any error of using traditional spectroscopy methods and approaches. We characterized the absorption behavior of different categories of food fluids, including apple and carrot juice, and raw, milk, and liquid eggs. Clear apple juice followed linear behavior according to the Beer-Lambert law, while milk, liquid eggs, and carrot juice showed nonlinear behavior. We measured the effect of vitamin C content on the absorption properties of juices.
We evaluated the performance of low- and medium-pressure mercury lamps, and of three pulsed sources with various energy levels per pulse of 31J (PUV-1), 344J (PUV-2), and 644J (PUV-3) by measuring their effects on pre-selected quality markers in apple juice, milk, and 30% fructose solution. We determined color, pH (a measure of acidity or basicity), soluble-solid content, viscosity, and absorption spectra for each product, and measured vitamin C and enzyme activity of polyphenol oxidase in apple juice and alkaline phosphatase in milk. Figure 1 shows examples of absorption spectra of apple juice and vitamin C and emission spectra of two UV sources.
Figure 1. Measured absorption spectra of apple juice and vitamin C, as well as emission spectra of low-pressure mercury (LPM) and high-intensity pulsed (HIP) UV sources.
The effects of the PUV-1 and 3 lamps on the beverage properties were comparable with those of the LPM lamp. For example, the pH of fructose decreased by 2.98% for the LPM lamp and by 3.18 and 4.51% for the PUV-1 and 3 lamps, respectively. Vitamin C content was reduced by 2.41% in apple juice and 35% in milk for LPM, while we observed reductions of 1.24 (PUV-1) and 2.01% (PUV-3) in apple juice, and 26 (PUV-1) and 24% (PUV-3) in milk, for the pulsed lamps. The PUV-2 lamp caused higher loss of vitamin C in both apple juice (10%) and milk (35%), and reduced the pH of fructose by 6.02%.
These results indicate that PUV lamps constitute a promising alternative for nonthermal treatment of beverages and liquid sweeteners, and they shorten processing time. Our next step will be to evaluate these novel PUV sources for their microbial efficacy and determine the most appropriate UV dose for the polychromatic spectrum.
Agriculture and Agri-Food Canada
Tatiana Koutchma, a research scientist in food-process engineering, focuses on emerging food technologies to improve the safety, quality, and consumer acceptance of foods. She also conducts research on applications of UV light in liquid foods.