Thesis (M.Sc.) - Cairo University - Faculty of Agriculture - Department of Food Science and Tencnology

The effect of the UV-C treatment on physico-chemical characteristics (pH, titratable acidity, TSS, color and ascorbic acid content), polyphenol oxidase activity (PPO), pectin methylesterase activity (PME) as well as microbial count of both apple and orange juices, compared to fresh juice was studied. The juice samples were irradiated using UV-C at 254 nm for different exposure times (15, 30, 45 and 60 min) and stored at 4±1 {u00BA}C for 30 days. UV-C treatment didn{u2019}t si gnificantly affect (p{u2264}0.05) pH values, titratable acidity and TSS (%) in both fresh and stored samples. A negative correlation between UV-C exposure time and L* values was noted for fresh and stored samples. On the other contrary, positive relationship was observed between UV-C exposure time and a* and b* values in apple juice. However, no significant differences were observed with L* and a* values in orange juice. Ascorbic acid content decreased as a result of increasing exposure time, where it was completely lost after 5 days in samples treated for 30, 45 and 60 min., and 10 days for samples treated for 15 min. compared with 15 days for control one. UV-C treatment didn{u2019}t significantly affect ascorbic acid content in both fresh and stored orange juice samples. PPO was partially deactivated (76.8 % loss of its initial activity after 60 min. of UV exposure) or during storage at a slower rate. On the contrary, PME didn{u2019}t affect after UV-C treatment. Total bacterial count as well as yeast and mold counts negatively affected by increasing the UV exposure time. The sensory characteristics, i.e. odor, color, taste, consistency and overall acceptability didn{u2019}t change as a result of UV-C treatment up to 30 min. and significantly decreased thereafter of apple juice. On the contrary, no changes in sensory characteristics were noticed apple juice treated with UV-C at any tested exposure times

Issued also as CD