The aim of this study was to evaluate the effectiveness of different UVC treatments, alone or in combination with ultra-high pressure homogenization (UHPH) on Bacillus subtilis spores in milk. Spores of B. subtilis (CECT4002) were inoculated in whole and skim milk to an initial concentration about 6 log CFU/mL. Milk was subjected to different ultraviolet radiation treatments at 254 nm (UVC) using a concentric tubular reactor in a dose ranging from 10 to 160 J/mL. Different number of passes were used to adjust the final dose received by the matrix. In general, increasing the number of passes (defined as number of entries to the tunnel-NET) increased the inactivation of spores of B. subtilis. The best lethality results (above 4 Log CFU/mL) were obtained by applying doses from 100 J/mL with several NET. When the same doses were achieved with a single pass lethality in most cases did not exceed 1 log CFU/mL. Increasing the NET also increased the likelihood for the spores to remain longer in the effective distance from the UVC source, estimated as 0.02 mm for whole milk and 0.06 mm for skim milk. Combination of UHPH and UVC did not clearly increase the efficiency of a single UVC treatment, and a lower lethality was even observed in some cases. UHPH treatments increased the turbidity and absorption coefficient (254 nm) of both whole and skim milk.
Liquid foods might present interferences in their optical properties that can reduce the effectiveness of short-wave ultraviolet radiation (UV-C) treatments used for sterilization purposes. The effect of turbidity as UV-C interference factor against the inactivation of bacterial spores was analysed by using phosphate-buffered saline solutions (PBS) of different turbidity values (2000, 2500, and 3000 NTU) which were adjusted with the addition of apple fibre. These suspensions were inoculated with spores of Bacillus subtilis and Alicyclobacillus acidoterrestris. While higher UV-C doses increased the inactivation rates of spores, these were reduced when turbidity values increased; a dose of 28.7 J/mL allowed inactivation rates of B. subtilis spores of 3.96 Log in a 2000-NTU suspension compared with 2.81 Log achieved in the 3000-NTU one. Spores of B. subtilis were more UV-C-resistant than A. acidoterrestris. Cloudy apple juice inoculated with A. acidoterrestris spores was processed by UV-C at different doses in a single pass and with recirculation of the matrix through the reactor. Inactivation increased significantly with recirculation, surpassing 5 Log after 125 J/mL compared with 0.13 Log inactivation after a single-pass treatment at the same UV-C dose. UV-C treatments with recirculation affected the optical properties (absorption coefficient at 254 nm and turbidity) of juice and increased browning as UV-C doses became higher.
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