Ultraviolet light stimulates flavonol accumulation in peeled onions and controls microorganisms on their surface

The effects of ultraviolet (UV) light on flavonol content in peeled onions (Allium cepa L.) and on microbial survival on their surface were investigated. The content of phenolic compounds showed a gradient within the onion bulb, with the highest level in the external dry "skin" (tunic) and the lowest level in the center. Peeled bulbs were treated with UV light comprising the bands of UV-C (more than half of the total UV output), UV-A, and UV-B. The response to UV depended upon the position of the scales within the bulb. In the outer fleshy scales, the UV doses of 1.2-6 kJ m(-2) approximately doubled the accumulation of flavonols and the total antioxidant capacity. When mid-depth (5th from the outside) scales were exposed to UV, the lowest dose tested (1.2 kJ m(-2)) had no significant effect on flavonols accumulation, whereas the higher doses decreased their levels. The low-dose UV treatment reduced the count of Escherichia coli on artificially contaminated peeled onions by 1.5-3 logs and alleviated the decay of Penicillium-inoculated bulbs. The present study has demonstrated a potential of UV light for simultaneous decontamination of peeled onions and their enrichment in health-enhancing phytonutrients.