title: An on-line method for pressurized hot water extraction and enzymatic hydrolysis of quercetin glucosides from onions.
abstract: 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.
authors.affiliation: Lund University, Department of Chemistry, Centre for Analysis and Synthesis, Lund, Sweden.
authors.forename: Sofia
authors.initials: S
authors.lastname: Lindahl
authors.forename: Jiayin
authors.initials: J
authors.lastname: Liu
authors.forename: Samiullah
authors.initials: S
authors.lastname: Khan
authors.forename: Eva Nordberg
authors.initials: EN
authors.lastname: Karlsson
authors.forename: Charlotta
authors.initials: C
authors.lastname: Turner
mesh_terms: D000975:Antioxidants / Q000032:analysis / Q000302:isolation & purification; D055162:Biocatalysis; D002851:Chromatography, High Pressure Liquid; D000431:Ethanol / Q000737:chemistry; D005960:Glucosides / Q000032:analysis / Q000302:isolation & purification; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D019697:Onions / Q000737:chemistry*; D011312:Pressure; D011794:Quercetin / Q000032:analysis* / Q000302:isolation & purification; D013056:Spectrophotometry, Ultraviolet; D046949:Thermotoga neapolitana / Q000201:enzymology; D014867:Water / Q000737:chemistry*; D001617:beta-Glucosidase / Q000378:metabolism
chemical_list: D000975:Antioxidants; D005960:Glucosides; D014867:Water; D000431:Ethanol; D011794:Quercetin; D001617:beta-Glucosidase