Effects of UV Irradiation on the Reduction of Bacterial Pathogens and Chemical Indicators of Milk

Consumer demand for fresher and minimally processed foods has brought about a movement to find effective, non-thermal processing technologies for the treatment of milk. The influence of temperature on bacterial reduction in UV irradiated milk was tested. Commercially processed skim, reduced fat (2%), and whole milk samples were inoculated with a naladixic acid resistant E. coli O157:H7 surrogate (ATCC 25922), maintained at or brought to 4oC and 20oC, respectively, and then exposed to a UV light dose between 5.3-6.3 mJ/cm2 for approximately 1.5 sec using the CiderSure 3500 apparatus (FPE Inc., Macedon, NY). Bacterial concentrations before and after UV exposure were enumerated and the results indicated that processing temperature was not significantly related to bacterial reduction (p > 0.05). The results did indicate that skim milk samples had a greater bacterial reduction, regardless of processing temperature compared to reduced fat milk and whole milk samples (p < 0.05). Solids such as milk fat, protein, lactose and minerals, in the milk have a greater effect over bacterial reductions than processing temperatures.

Traditional goat cheeses are produced using unpasteurized milk, which increases the food safety concerns for these types of products. Fresh goat's milk was inoculated to 107 cfu/ml with Listeria monocytogenes (L-2289) and exposed to UV light using the CiderSure 3500 apparatus. Inoculated milk was exposed to an ultraviolet dose range between 0 and 20 mJ/cm2 to determine the optimal UV dose. A greater than 5-log reduction was achieved (p < 0.0001) when the milk was processed 12 times for a cumulative exposure time of roughly 18 sec and a cumulative UV dose of 15.8 +/- 1.6 mJ/cm2. The results of this study indicate that UV irradiation could be used for the reduction of L. monocytogenes in goat's milk.

Organoleptic consequences of goat's milk treated with UV technology were assessed. Olfactory studies were conducted and a highly significant difference was determined between the odor of fresh goat's milk and UV processed milk (p < 0.05). The extent of lipid oxidation and hydrolytic rancidity was measured by thiobarbituric acid reactive substances (TBARS) and acid degree values (ADVs). Results indicated that as the UV dose increased, there was a significant increase in TBARS values and ADVs of the milk samples (p < 0.05). Milk samples were processed using the UV processor under the same conditions as previously described without exposure to the UV source to determine if the agitation from pumping was causing off-flavors by way of hydrolytic rancidity. The ADVs from these samples increased at the same rate as the UV irradiated samples; however, sensory studies indicated that the increase of free fatty acids (FFA) was not enough to cause detectable off-odors in the milk. Solid phase microextraction and gas chromatography (SPME-GC) was utilized to quantify the production of volatile compounds that were formed due to UV processing. The formation of pentanal, hexanal and heptanal was identified after as little as 1.3 mJ/cm2 UV dose. Peak areas were measured and analyzed after 7.8 mJ/cm2 and 15.6 mJ/cm2 and were determined to increase significantly as UV dose increased (p < 0.05). The chemical analyses supported the findings from the olfactory studies. The outcome of this research showed that UV irradiation at the wavelength 254 nm, was detrimental to certain chemical properties of fluid milk. The properties that were perceived as negative in fluid milk may be considered an attribute in certain types of cheese and future studies in the cheese production sector should be considered. Other applications for this technology could be for use in developing countries where milk is not typically processed because of the high costs of thermal pasteurization. On-farm applications for the treatment of replacement milk should also be considered.