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Know your onions....
Curing onions reduces water loss and suppresses incidence of disease, and can promote a darker skin finish. The curing involves heating at 28ºC for six weeks (65-75% RH); reducing curing temperatures may reduce energy usage, but does this affect flavonol concentration in the skin of brown onions or flavonol and anthocyanin concentration in the skin of red onions? A Cranfield University team looked at this and reported their studies in Post Harvest Biology and Biotechnology.
Brown skinned varieties Sherpa and Wellington, and red onion Red Baron, were cured at 20, 24 and 28°C for six weeks. Samples were analysed immediately after curing and after seven months cold storage at 1 ± 0.5°C. Objective colour measurement showed that skin of Sherpa and Wellington was darker immediately after being cured at 28°C compared to 20°C. No significant correlations were found between colour and sugar concentrations. Flavonols were measured in the skin of all varieties and anthocyanins in the skin of Red Baron immediately after curing were higher in those cured at 20°C. Anthocyanins and flavonols may play a major role in varying the skin colour of red onions, but the difference between curing temperatures may not have been sufficient to give a correlation between darkening of Sherpa and Wellington and flavonol concentration.
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