Do Quercetin Glucosides Govern Skin Colour Changes in Brown Onion Bulbs during Curing?

Due to the maritime climate of the UK, onions are artificially cured in closed environments. Curing removes excess water from the outer skin and seals the neck to reduce infection and minimise weight loss from the flesh. Current curing practise involves holding onions at 28 ?C for three-six weeks which creates a golden brown colour that is appreciated by consumers. Cepaic acid, a possible oxidation product of quercetin, was recently identified as a novel brown pigment in dry onion skin. The aim of this work was to investigate further which compounds contribute to the colour change of brown onion skin during curing. Onion cv. Sherpa bulbs were cured at 20 or 28 ?C for 6 weeks (relative humidity 65 ? 75%). Replicated samples of onion skin were taken throughout the curing process. Objective colour, non-structural carbohydrates and flavonols were measured in the skin samples. Onions cured at 28 ?C were on average significantly more brown (according to hue angle (H?)) and darker (according to lightness (L*)) than those cured at 20 ?C. Quercetin glucoside concentrations were lower in the skins of onions cured at 28 ?C compared with those cured at 20 ?C. The relationship between colour and quercetin glucoside concentrations was confirmed using Partial Least Squares Discriminant Analysis (PLSDA) when nine physiological and biochemical variables (chroma (C*), dry weight, fructose, glucose, H?, L*, quercetin, quercetin 3,4-diglucoside and quercetin 4-glucoside) were considered. PLSDA clearly differentiated between onions cured at 20 and 28 ?C, with the variance mainly explained by differences in quercetin 3,4-diglucoside and quercetin 4-glucoside concentrations and H? and L* values. In summary and for the first time, the increase in brown pigmentation during curing at higher temperatures has been shown to be linked to the conversion of quercetin glucosides possibly into brown oxidative products. Source:Downes, K.; Chope, G. A.; Terry, L. A.Plant Science Laboratory, Cranfield University, Bedfordshire,MK43 0AL, United KingdomInternational Horticulture Congress, Lisboa 2010, ISHS ?