Abstract
An aqueous extract of polyphenols was prepared from China green tea under optimised conditions and then freeze-dried (green tea polyphenols extract, GTPE). This extract that contained 28% of total polyphenols had an antioxidant activity above 90% in solution when measured by the N,N,-diethyl-p-phenylenediamine (DPD) method. The GTPE was encapsulated either in the sodium-caseinate (Na-caseinate) or the calcium-caseinate (Ca-caseinate) beads in order to protect its antioxidant activity. The beads containing GTPE were stored at 21 °C and 44% relative humidity in darkness for 42 days. The antioxidant activity of the encapsulated GTPE was measured during storage using the DPD method. The antioxidant activity of the unencapsulated GTPE and the beads without GTPE was measured as controls under the same environmental conditions. Antioxidant activity of all beads was measured after dissolution in filtered and deionised water (80 °C) at pH 2.4 ± 0.1, before and after removing the hydrolysate caseinates that formed during beads' dissolution. The results showed that the antioxidant properties of unencapsulated tea polyphenols (GTPE) and of both the caseinate beads with GTPE were maintained almost stable over 42 days. No significant difference (P > 0.05) was observed between the antioxidant properties of the Na- or Ca-caseinate beads and unencapsulated GTPE. On the other hand, the presence of GTPE either in the Na- or in the Ca-caseinate beads did not change significantly (P > 0.05) the antioxidant properties of the protein beads. However, the antioxidant activities of the Na- or Ca-caseinate beads containing GTPE were significantly (P ≤ 0.05) higher than those of the same beads without GTPE after removing the caseinate hydrolysates present in the bead solutions (82% and 78% vs. 10% and 20%, respectively). These results showed that antioxidant activities of the beads without GTPE in solutions were principally caused by caseinate hydrolysates. Among the beads, Ca-caseinate beads with GTPE showed the best antioxidant properties. Antioxidant properties of caseinate beads were compared with those of Trolox. The results showed that a concentration of 5 mg*mL−1 of Na- or Ca-caseinate beads without GTPE had antioxidant activity equivalents to 192 and 205 μmol*L−1 of Trolox equivalent per mL of Trolox.
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