Hop leaf phenolics as a source of natural antioxidants

Hop leaves, a by-product of hop cone harvesting, are rich in quercetin and kaempferol 3-O-glycosides. This study characterised ethanolic hop leaf extracts and phenolic standards for their antioxidant activities (FRAP, DPPH, ORAC assays). Purified extracts were digested with snailase, a commercial enzyme blend, for glycoside hydrolysis. Digests were evaluated for antioxidant activities and phenolic content (LC-ESI-qTOF-MS/MS). Leaf flavonol glycosides had significantly lower antioxidant activities compared to their respective aglycones. Snailase treatment of leaf extracts achieved high hydrolysis rates (>99%) and significantly increased antioxidant activity (fold-change dependent on glycoside composition and antioxidant assay mechanism). Treatment was particularly effective for hop variety Cascade (4.5-fold increase in FRAP) due to high kaempferol 3-O-rutinoside content (400.56 mg/g). We demonstrate for the first time the preparation of antioxidant-rich hop leaf extracts using snailase and support findings with comprehensive underpinning analysis of the extracts themselves and the antioxidant activities of pure standards for the predominant compounds.