The lability of anthocyanins and flavonols extracted from grape (Vitis vinifera L.) skin with commonly used extraction solvents was examined. Results indicate that both anthocyanins and flavonol glycosides are unstable in even mildly acidic extraction solvents and highly labile in 1% hydrochloric acid in methanol, the most commonly used extraction solvent. While lability of anthocyanins has previously been reported, this is the first report of flavonol glycoside stability in these solvents. As a result of these observations, an extraction protocol was developed that maximized anthocyanin (malvidin-3-O-glucoside, malvidin-3-O-acetylglucoside, and malvidin-3-O-p-coumaroylglucoside) and flavonol (quercetin-3-O-glucoside and quercetin-3-O-glucuronide) extraction while minimizing degradation in the time between extraction and HPLC analysis. A range of acidified methanol solvents including hydrochloric, formic, acetic, citric, and maleic acids were tested together with acidulated ethanol and several methanol water mixtures ranging from 0% to 100% methanol. The most efficacious solvent system was 50% aqueous methanol, which had the advantage of incorporating both high anthocyanin and flavonol extractability and subsequent stability prior to HPLC analysis.
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