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Abstract
Seven proanthocyanidin fractions (PAs, P0–P6) were obtained from China bitter hop Tsingtao (Humulus lupulus) by organic solvent extraction, macroporous resin AB-8 adsorption, and the classification of oligomers/polymers. The average degrees of polymerization (ADP) for fractions P0–P6 were 2.22, 2.59, 3.54, 3.88, 2.63, 3.02, and 5.57, respectively. Antioxidative activities including the reducing power (RP), inhibition capability of lipid peroxidation (ICLP), antioxidant in linoleic acid system (ALAS), ability of scavenging hydroxyl free radical [HO·] (ASHFR), and thiobarbituric acid (TBA) value (carbonyl compounds in beer could react with TBA and produce a typical color at 530 nm) were observed in this study. The antioxidation of PAs was related to its structure and the reaction system and there was no obvious correlation between ICLP (ALAS/ASHFR) and the ADP of PAs. The RP (the reducing power of antioxidants by converting Fe3+ in potassium ferricyanide into Fe2+) of PA fractions from strong to weak were Pn fractions (n = 0–5) > (+)-catechin > Vc (the reference substance of antioxidants, ascorbic acid) > P6 fractions (ADP = 5.57). A considerable concentration effect was observed in the inhibition of both the liposome peroxidation process and the linoleic system. However, the ICLP was above 20% at 234 nm and 532 nm when the addition of P3 or P4 fraction was 30 μg/mL, and the inhibition effect of P2 fraction was the strongest in the linoleic acid system. The distinct inhibition difference of PAs in autoxidation of linoleic acid system existed when the addition was 10 μg/mL, and Pn fractions (n = 3–6) showed strong antioxidation compared to Vc in this situation. The ASHFR gradually strengthened with concentration. When the concentration reached 50 μg/mL, the ASHFR were all above 5% and increased with the ADP of all PA fractions, and the ASHFR of P6 fraction (ADP = 5.57) exceeded 20%. During wort boiling process, P3 and P4 fractions had powerful antioxidative ability, which could significantly inhibit the formation of aging precursors.