Relationship between Colour and Biochemical Composition of Skin from 'Red Baron' Onion Bulbs Cured at Different Temperatures

Onions are cured to reduce incidence of neck rot and prevent water loss from the inner scales by forming a complete dry outer skin. Red onion skins contain anthocyanins, mainly comprising peonidin and cyanidin derivatives, which give them their characteristic colour. Additionally, it has been suggested that formation of brown colour in onion skin may be related to the conversion of quercetin glucosides to quercetin aglycon. This study aimed to determine the compounds responsible for colour change in the skins of 'Red Baron' red onions after curing at different temperatures. Onions 'Red Baron' were cured at 20, 24 and 28 degrees C (industry standard) for 6 weeks then transferred to cold storage (1 +/- 0.5 degrees C) for a further 7 months. Onion skin was sampled after curing and after seven months of cold storage. Objective colour, sugars, flavonols and individual anthocyanins were measured. Immediately after curing, fructose concentration in onion skins was significantly higher in those cured at 28 degrees C, although after 7 months of cold storage, those cured at 20 degrees C had a higher fructose concentration. There were no differences in quercetin aglycon or glucose concentrations over time or between different curing temperatures. After curing, onions cured at 20 degrees C had higher concentrations of quercetin 4'-glucoside, quercetin 3,4'-diglucoside, cyanidin and peonidin glucosides. However, after 7 months of cold storage, onions cured at 24 degrees C maintained higher concentrations of these compounds. Onion skin hue angle (H degrees) was higher in onions cured at 28 degrees C and H was negatively correlated with total anthocyanins (-0.704) and total flavonols (-0.639). The negative correlation between total flavonols/total anthocyanins with H suggests that differences in skin colour between onions cured at different temperatures may be due to a difference in flavonol and anthocyanin glucoside concentrations found in the skin tissue.