Effect of thermal processing on the phenolic antioxidants of colored potatoes

Foods with antioxidant capacity contribute health benefits and provide protection against certain cancers, Alzheimer's dementia, and cardio-vascular diseases caused by oxidative damage. Colored potatoes are a significant source of antioxidants from polyphenols, carotenoids, and ascorbic acid. In this research, retention of total phenolics and antioxidant activity were studied in fresh colored potatoes, and processed flakes were prepared as potential ingredients for snack foods using freeze-drying, drum-drying and refractance window-drying. Extruded products prepared from purple colored potatoes ('Purple Majesty' cv) and yellow peas using a twin-screw extruder were analyzed for the effect of extrusion cooking on their antioxidant activity and anthocyanin level. Bioavailability of the phenolic antioxidants of extruded products was analyzed using HepG2 liver cancer cells in a cellular antioxidant assay and compared with unprocessed samples. Antioxidant potential of degradation products from purple potato anthocyanins during high temperature processing and contribution to the total antioxidant activity were measured using chemical assays (DPPH and ABTS). Thermal degradation kinetics of anthocyanins prepared from 'Purple Majesty' potatoes was conducted over a temperature range of 100 -- 150 °C. Dehydrated potato flakes showed no significant losses (P > 0.05) in total antioxidant capacity (TAC) and total phenolics (TP) content, whereas 23 - 45% losses in total anthocyanins (TA) were observed during dehydration of potatoes. The quantity of TAC was unchanged and the TP was largely retained (73 -- 83%) in the extruded products prepared from colored potatoes and yellow pea flours. Severe losses in TA (60 -- 70%) of extruded products were observed due to high temperature during extrusion cooking. The large bioavailability of the extruded products was well supported by the ORAC antioxidant activity, total phenolics (including free and bound fractions) and total flavonoids content irrespective of the degradation of anthocyanins. The bioavailability in the extrudates could be attributed to the breakdown of the conjugated phenolic antioxidants to their free forms, and formation of Maillard reaction products with potential antioxidant activity. Thermal degradation in prepared anthocyanins followed a first order reaction, but the degradation compounds had antioxidant potency contributing to the TAC in the processed foods.