Comparison of Oxidative Enzymes and Redox Active Compounds Between Fresh Squeezed and Processed Orange Juices

Commercially processed (CP) orange juice (OJ) and fresh squeezed juice with or without pasteurization (F and F+P, respectively) at 90 °C for 10 s, were compared for antioxidant compounds and activities of antioxidant-related enzymes in juice of Valencia oranges. Samples were examined at 0, 1, 2 and 4 days of 5 °C storage after juicing. Samples were extracted with a Tris buffer with protease inhibitors for measuring enzyme activities and with 6% trichloroacetic acid for redox active compounds. Total phenolic content (TPC) samples were extracted by boiling with 50% ethanol. All extractions were rapidly frozen in liquid nitrogen before storage at -80 °C. Enzyme activities measured were catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), superoxide dismutase (SOD), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR). Redox active compounds (RACs) measured were ascorbate (AsA), dehydroascorbate (DHA), hydrogen peroxide (H₂O₂), glutathione (GSH) and oxidized glutathione (GSSG). For a given enzyme, treatment effects gave converging activity levels over 4 days for CAT and DHAR, their activities were observed to be high in F and F+P juice extracts while APX activity was high in F+P, and MDHAR, SOD and GR activities were high in CP. Enzyme activities found to converge to a common level across 4 days of storage in F, F+P and CP juices were DHAR, GR and SOD. Enzyme activities diverging to different levels by treatment were POD and CAT (both with F high, CP low at 4 days). For RACs, only H₂O₂ was consistently high in F, and only DHA was consistently high in CP.  DHA was the only RAC to converge for all three treatments across 4 days, and AsA was the only RAC to diverge (F, F+P high, CP low). Relative to F and F+P juices, commercially processed juice had lower MDA levels, lower total antioxidant capacity, and lower antioxidant reductive capacity. TPC measured by  modified Folin-Ciocalteu showed commercial juice had higher TPC, consistent with HPLC-MS analysis of major phenolics in these juices. Headspace analysis for aroma volatiles showed terpenoid compounds higher in processed juice and esters higher in fresh juice. But, our fresh squeezed OJ had >8X higher levels of peel oil than processed juice, which may explain some of the observations above. Fines of solid orange constituents suspended in commercial OJ would not typically be seen in fresh squeezed product but may also influence the levels some of the above measures.