Chemicals and reference standards were obtained from Sigma-Aldrich (Steinheim, Germany): caffeic acid (CAA), (+)-catechin (CAT), 3,4-dihydroxybenzoic acid (DBA), (−)-epicatechin (EPC), ferulic acid (FEA), gallic acid (GAA), 4-hydroxybenzoic acid (HBA), hesperetin (HES), kaempferol (KAE), morin (MOR), naringenin (NAN), p-coumaric acid (PCA), phloridzin (PHD), phloretin (PHT), quercetin-3-D-galactoside (QGA3), quercetin-3-D-glucoside (QGU3), quercetin-7-D-glucoside (QGU7), quercetin (QUR), sinapic acid (SIA), siringic acid (SRA), taxifolin (TAF) and FC reagent. The standard reference narirutin (NAR) was obtained from K&J Scientific (Marbach am Neckar, Germany) and isorhamnetin (IRT) and naringin (NAG) from Carl Roth (Karlsruhe, Germany). Stock solutions were prepared by dissolving the reference standards in analytical grade absolute ethanol and diluting each of them in seven steps for the measurements.
The reducing capacity was determined by spectrophotometry as orignally described [28 ] with slight modifications. Briefly, 20 μL of each sample (reference standard dilution or ethanol) was mixed with 100 μL demineralized water (H O ) and 100 μL FC reagent and were incubated for 3 in the dark. We then added 1580 μL of H O and 200 μL 7.5% (w/v) sodium carbonate and incubated for another 30 in the dark. The absorption of the samples was measured at 765 using a Specord 210 plus spectrophotometer (Analytik Jena, Jena, Germany). The slope was determined dy linear regression and presented as the reducing capacity.
For statistical analysis, Sigma Plot (Systat Software, San Jose, CA, USA) was used for one-way analysis of variance (ANOVA) corresponding to an unpaired t-test. If there was a significant difference, an additional pairwise test was carried out using the Holm–Šidák method. The significance level for both tests was 0.05. Statistical analysis was always carried out with all significant decimal places.
The reducing capacity was determined by spectrophotometry as orignally described [28 ] with slight modifications. Briefly, 20 μL of each sample (reference standard dilution or ethanol) was mixed with 100 μL demineralized water (H O ) and 100 μL FC reagent and were incubated for 3 in the dark. We then added 1580 μL of H O and 200 μL 7.5% (w/v) sodium carbonate and incubated for another 30 in the dark. The absorption of the samples was measured at 765 using a Specord 210 plus spectrophotometer (Analytik Jena, Jena, Germany). The slope was determined dy linear regression and presented as the reducing capacity.
For statistical analysis, Sigma Plot (Systat Software, San Jose, CA, USA) was used for one-way analysis of variance (ANOVA) corresponding to an unpaired t-test. If there was a significant difference, an additional pairwise test was carried out using the Holm–Šidák method. The significance level for both tests was 0.05. Statistical analysis was always carried out with all significant decimal places.
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