An on-line method for pressurized hot water extraction and enzymatic hydrolysis of quercetin glucosides from onions

• Total quercetin content was determined in different onion varieties.
• Hot water extraction was combined with hydrolysis using thermostable β-glucosidase.
• Enzyme activity was highest in water containing 5% ethanol at 84 °C and pH 5.5.
• The immobilized enzyme showed good stability over 72 h at the optimized conditions.
• The developed method gave comparable yield of quercetin compared to conventional methods.

A novel environmentally sound continuous-flow hot water extraction and enzymatic hydrolysis method for determination of quercetin in onion raw materials was successfully constructed using a stepwise optimization approach. In the first step, enzymatic hydrolysis of quercetin-3,4′-diglucoside to quercetin was optimized using a three level central composite design considering temperature (75–95 °C), pH (3–6) and volume concentration of ethanol (5–15%). The enzyme used was a thermostable β-glucosidase variant (termed TnBgl1A_N221S/P342L) covalently immobilized on either of two acrylic support-materials (Eupergit® C 250L or monolithic cryogel). Optimal reaction conditions were irrespective of support 84 °C, 5% ethanol and pH 5.5, and at these conditions, no significant loss of enzyme activity was observed during 72 h of use. In a second step, hot water extractions from chopped yellow onions, run at the optimal temperature for hydrolysis, were optimized in a two level design with respect to pH (2.6 and 5.5), ethanol concentration (0 and 5%) and flow rate (1 and 3 mL min−1) Obtained results showed that the total quercetin extraction yield was 1.7 times higher using a flow rate of 3 mL min−1 (extraction time 90 min), compared to a flow rate of 1 mL min−1 (extraction time 240 min). Presence of 5% ethanol was favorable for the extraction yield, while a further decrease in pH was not, not even for the extraction step alone. Finally, the complete continuous flow method (84 °C, 5% ethanol, pH 5.5, 3 mL min−1) was used to extract quercetin from yellow, red and shallot onions and resulted in higher or similar yield (e.g. 8.4 ± 0.7 μmol g−1 fresh weight yellow onion) compared to a conventional batch extraction method using methanol as extraction solvent.

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