ABSTRACTRepresentative samples of fresh orange juice (OJ) with various flavor scores were assayed for peroxidase activity. In thk assays, pphenyl‐enediamiue (PPDA), the hydrogen donor, was oxidized by H2O2after a short time lag (about 1 min) caused by interaction of ascorbic acid in the juice with a PPDA‐oxidation intermediate. A search was made for compounds that are native substrates of OJ peroxidase. Thus, the peroxidase activity of a protein fraction precipitated from neutralized OJ with ammonium sulfate and dialyzed free of ascorbic acid was tested with H2O2and a number of hydrogen donor compounds that are normal juice constituents. Ascorbic acid was very reactive, as were the phenolic acids (caffeic, gentisic and coumaric). The flavonoids, criodic‐tyol, hesperidin and naringin were unreactive. Reduced nicotinamide adenine dinucleotide (NADH) was also reactive in the presence of hydroquinone and other compounds that mediate electron transfer through intermediate states. The dialyzed protein fraction also catalyzed the oxidation of pyridoxal‐PO4, indoleacetic acid. dihvdroxv‐maleic acid, and NADH+p‐cresol by O2plus Mn++. Although OJ appears to contain many compounds that are reactive with peroxidase, their reactivities in OJ are apparently very slow due to the level of H2O2. Concentrations of ascorbic and caffeic acids did not change in OJ incubated at 30°C for 4 hr. Processing conditions that increased peroxidase activity and pulp content in OJ decreased quality
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