Phenolic composition and antioxidant activity of sweetpotatoes [Ipomoea batatas (L.) Lam

Abstract

The influence of numerous factors on sweetpotato phenolic content and antioxidant activity was determined. Simplified, robust, and rapid methodologies were developed to quantify total phenolics and individual phenolic acids in sweetpotatoes. Quantification of total phenolic content using Folin-Denis reagent provided more reliable results than Folin-Ciocalteu reagent. Individual phenolic acids were quantified by reversed-phase high performance liquid chromatography (HPLC) and the best separation was achieved using a 5-µm, 4.6 × 250 mm column with a mobile phase of 1% (v/v) formic acid aqueous solution: acetonitrile: 2-propanol (70:22:8), pH 2.5. Methanol and ethanol provided higher phenolic extraction efficiency than acetone. In general, chlorogenic and 3,5-dicaffeoylquinic acid were the most prominent phenolics acids in sweetpotato root and leaf tissues. Immature roots and leaves at the initial stages of growth had the highest concentration of phenolics and antioxidant activity. In a comparison of plant parts, sweetpotato leaves had a significantly higher phenolic content and antioxidant activity than roots. Thermal processing of sweetpotato storage roots resulted in a significant loss of phenolic content and antioxidant activity. The outer skin tissue (raw or processed) contained the highest amounts of phenolic acids but also exhibited higher losses than cortex or pith tissue. After a 4 week exposure to 5 °C, the rate of increase in phenolic content and antioxidant activity in non-cured sweepotatoes was significantly higher than in cured roots. An ambient temperature exposure following low temperature storage accentuated the increase in phenolics and antioxidant activity. Minimally processed sweetpotatoes held at 5 °C accumulated more phenolic compounds and had a higher antioxidant activity than sweetpotatoes held at 0 °C. The increase in total phenolic content and antioxidant activity after 8 days was higher than 4 days. No fresh-cut tissue browning was observed after 8 days and the products were considered to be marketable. Sweetpotato genotypes differ in their phenolic content and antioxidant activity. A purple-fleshed breeding line was found to have higher total phenolic content and antioxidant activity than orange-fleshed and white-fleshed cultivars.