Stability of antioxidants in an apple polyphenol-milk model system

The stability of antioxidants in an apple polyphenol-milk model system was examined. The model system consisted of skim milk fortified with pH-neutralised apple polyphenols (AP, 0-200 mg per 100 ml milk), with or without ascorbic acid (100 mg per 100 ml milk). Physical and chemical changes were evaluated after thermal treatment (120 degrees C, 5 min) and oxidative storage (20 degrees C and 38 degrees C, up to 12 weeks). Antioxidant capacity was determined using both oxygen radical absorbance capacity (ORAC) assay and ferric reducing antioxidant power (FRAP) assay. Significant antioxidant capacity was detected in the presence of milk. Antioxidant capacity was retained during thermal treatment but decreased slowly during storage. The concentration of ascorbic acid decreased rapidly, and was close to zero after 2-week storage at 38 degrees C or 10-week storage at 20 degrees C. The browness of the polyphenol-milk system increased over storage duration of 0-12 weeks; this effect was retarded by the addition of ascorbic acid. This high polyphenol-milk has demonstrated good physical stability. (C) 2008 Elsevier Ltd. All rights reserved.