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The use of ultraviolet radiation in the treatment of milk and other liquid foods is a very promising field of study since it reduces their bacterial load. It is rarely used to increase the vitamin D content and modify the protein and fatty acid composition of milk. The paper describes how different parameters of ultraviolet radiation influence such characteristics of raw and pasteurized milk as the mass fraction of total protein, nonprotein nitrogen content, active and titratable acidity, general bacterial load (QMA&OAMO), fatty acid composition, and vitamin D content. Low-pressure gas-discharge lamps were used to treat a 400 µm moving layer of milk with ultraviolet radiation. The radiation time, its doses, and the milk flow rate changed in the ranges of 5–25 min, 5.1–102 mJ/cm2, and 0.04453- 0.13359 m3/s, respectively. We identified optimal radiation ranges that lead to both a lower microorganism content and a higher vitamin D content. Our study also determined specific correlations in the mutual changes of the given parameters. The treatment ranges did not produce any significant changes in other physico-chemical properties of milk. We also found that vitamin D was synthesized in raw and pasteurized milk in a similar way. Moreover, there was an insignificant decrease in the vitamin D content in milk treated with ultraviolet radiation during storage for up to 48 hours. On the whole, the results indicate that the treatment of milk with ultraviolet radiation in the dosage range from 5.1 to 102 mJ/cm2 has a complex effect on the total bacterial load (QMA&OAMO) and vitamin D content, whereas it has almost no effect on the protein and fatty acid composition.

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