Antioxidant Activity and Bioactive Compounds of Sweetpotatoes

Abstract

Sweetpotatoes are rich in dietary fiber, minerals, vitamins and bioactive compounds such as beta-carotene, phenolic acids and anthocyanins which provide sweetpotato clones with distinctive flesh colors (cream, yellow, orange and purple). Limited studies have been conducted on the total antioxidant capacity of various sweetpotato types in relation to their phytochemical content and composition. This study aimed to determine the free radical scavenging activity of sweetpotatoes using several common methods, to assess the correlation among the antioxidant assays, and to determine the relationship between the antioxidant activity and the phenolic, anthocyanin and β-carotene content in breeding lines. Oxygen radical absorbance capacity (ORAC), 2,2-diphynyl-1-piicrylhydrazyl (DPPH) and 2,2'-azinobis(3-ethyl-benzothiazoline-6-sulfonic acid (ABTS) methods were used to evaluate antioxidant activity . Total phenolics were measured using the Folin-Ciocalteau method, total anthocyanins by the pH-differential method, and β-carotene by HPLC. Results indicated that dark purple-fleshed sweetpotatoes had high antioxidant activity, total phenolic and anthocyanin contents. The total phenolic and β-carotene contents were highly correlated with the antioxidant activity of the hydrophilic and lipophilic extracts, respectively. There were good correlations between ORAC and ABTS, and ORAC and DPPH suggesting that these methods have similar predictive capacity for sweetpotato antioxidant capacity.

With high antioxidant capacity, the purple-fleshed sweetpotatoes have good potential for health food markets. Characterization of the pigments in a genotypically diverse group of purple-fleshed clones for physiological functionality and polyphenolic compounds associated with the color of the derived progeny is important for breeding program as well as suitability for processing. The results showed that the anthocyanin and phenolic contents as well as antioxidant activities greatly varied among the cultivars. Heat treatment resulted in significant increase in the polyphenolic content and composition of the extracts. Based on visual color measurement, the clones were categorized as a red-dominant group (peonidin-rich). However, HPLC analysis revealed that the ratios of peonidin and cyanidin varied widely among the samples with several genotypes in a cyanidin-rich group. The genotypic diversity of purple-fleshed sweetpotatoes may make it possible to utilize them in a variety of food and nutraceutical applications.