UVB treatments of packaged ready-to-eat salads: Induced enhancement of quercetin derivatives in baby-leaf lettuce (Lactuca sativa L.) and wild rocket (Diplotaxis tenuifolia L.)

Ready-To-Eat (RTE) salads conveniently pre-prepared in bags can promote the intake of natural bioactivecompounds, including antivirals such as quercetin. However, the content of these compounds in the species usedfor RTE salads is usually low due to limited solar UV exposure under tunnels and greenhouses in which they areusually cultivated. To address this, we treated commercial fresh-cut lettuce (Lactuca sativa L.) and wild rocket(Diplotaxis tenuifolia L.) leaves using a narrow-band UVB lamp directly through sealed polypropylene bags duringstorage (5-6 oC, 80% RH). The bagged leaf samples were kept under 20 ?mol m? 2 s? 1 of white light with a 12 hphotoperiod for 6 d. Half of the samples were additionally treated, 9 h d? 1, during the first 3 d by UVB narrowbandlamps delivering 2.8-3.6 ?mol m? 2s? 1 of UVB and 0.8-1.0 ?mol m? 2s? 1 of UVA radiation. The effects of theUVB treatments on epidermal phenolics, chlorophyll and photosynthetic parameters were monitored daily bynon-destructive fluorescence sensors over a 6-d storage period. At the end of the experiment, destructive HPLCDADanalysis of phenolics and photosynthetic pigments, antioxidant capacity assays and fresh weight loss determinationswere conducted. The UVB-treatment increased the epidermal phenolics (EPhen) Index with respectto unirradiated controls, while not affecting chlorophyll and carotenoids levels as well as photosynthetic efficiency.For both species, the EPhen Index changes were detected 15 h after the first UVB application. Then, wildrocket responded faster than baby-leaf lettuce and reached the maximal phenolic level with less than 1/3 of theenergy dose needed by lettuce. UVB-treated samples exhibited higher flavonoid concentrations (mainly quercetinderivatives) compared to controls (48-67% and 37-66% in lettuce and wild rocket, respectively). Leaf chlorophylland carotenoid contents were not affected by both UVB treatment and storage. We proved, for the firsttime, that it is possible to treat RTE salad leaves using through-packaging UVB radiation and enhance their totalphenolic and quercetin derivative contents. We also provided more insights concerning the dynamics of the UVBelicitationof phenolic compounds in postharvest leaves. Our results are propaedeutic for the optimization ofpotential UVB-treatments; selection of the most efficient wavelengths, intensity, single/multiple doses andproposals for application in the food industry.