Changes in antioxidant flavonoids during freeze-drying of red onions and subsequent storage

The aim of this study was to elucidate the effect of the freeze-drying process and storage on onion flavonoids content. Major flavonols of a mature red onion bulb were confirmed as quercetin derivatives (3,4′-O-diglucoside (Qdg) and 4′-O-monoglucoside (Qmg)) using a combination of chromatographic comparisons, UV–visible spectra and scientific literature data. These two components account for about a 93% of the total flavonols (334 ± 60 mg Q kg−1, fresh weight), and are the main responsible for the IC50 antioxidant activity of 15.84 ± 3.73 g/kg. The remaining flavonol fraction (approx. 7%) comprises up to 8 different components of which quercetin-3-O-glucoside and isorhamnetin-4-glucoside are prominent members although each contribute less than 3% of the total flavonol fraction; instead, quercetin represents less than 1% of total flavonols. In this red onion, eight anthocyanins (which represent less than 1% of total flavonols) were also found at a total level of 2.1 ± 0.05 mg C3g kg−1 (fresh weight). Anyway, 4 of them (cyanidin 3-glucoside (C3g) > cyanidin 3-(6′′-malonylglucoside) (C3,6mg) > cyanidin 3-(6′′- malonyl-laminaribioside) (C3,6mlmb) > cyanidin 3-laminaribioside (C3lmb)) account for the 95% of total anthocyanins. Freeze-drying process concludes in an increase in the extraction of flavonoids up to a 32% for flavonols and 25% for anthocyanins. Freeze-dried onion powder storage at room temperature, in dark, in air- and water-tight glass bottles for up to 6 months was keeping rather stable all flavonoids.