Submitted to: Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/15/2006
Publication Date: 2/24/2007
Citation: Teow, C.C., Truong, V., McFeeters, R.F., Thompson, R.L., Pecota, K.V., Yencho, C.G. 2007. Antioxidant activities, phenolic and ß-carotene contents of sweet potato genotypes with varying flesh colours. Food Chemistry. 103:829-838. Interpretive Summary: Fruits and vegetables are a rich source of phytochemicals, such as carotene, anthocyanins and other phenolic compounds. Studies have indicated that these phytochemicals have high antioxidant activity which helps to reduce the risk of chronic diseases. With flesh color varying from white, yellow and orange to purple, sweetpotatoes can be a rich source of these beneficial phytochemicals. However, limited information is available on the antioxidant activities of various types of sweetpotatoes. The objectives of this study were to determine the antioxidant activity of sweetpotatoes and calculate their correlations with total phenolic and beta-carotene contents. We found that the antioxidant activities varied widely among the sweetpotato genotypes. The purple color intensity of the sweetpotatoes tended to be associated with high antioxidant activity. Purple-fleshed sweetpotatoes would be a healthy food choice for consumers, as well as a potential source for natural food colorants. The antioxidant activities determined by radical quenching assays were highly correlated with the total phenolic contents, as measured by a simple spectrophotometric method. Therefore, the assay for total phenolics would be a useful technique for rapid evaluation of antioxidant activity in sweetpotatoes. This would be important for breeding programs where many samples must be evaluated, or for investigating the effect of processing and storage treatments on the retention of antioxidant activity.
Technical Abstract: Depending on flesh color (white, yellow, orange, purple), sweetpotatoes can be a rich source of carotene, anthocyanins, and other phenolic compounds, which are solubilized in lipophilic or hydrophilic solvents. Most of the reported antioxidant activities in sweetpotatoes have been based on the hydrophilic fraction. In order to obtain the total antioxidant activity (TAA), both hydrophilic and lipophilic fractions need to be considered. The objectives of this study were to determine the hydrophilic and lipophilic antioxidant activity of sweetpotatoes, and calculate their correlations with total phenolic and beta-carotene contents in different sweetpotato genotypes. Freeze-dried powders of 19 sweetpotato clones with varying flesh colors were extracted with hexane and acidified methanol. Antioxidant activity was measured using three methods: oxygen radical absorbance capacity (ORAC); 2,2 diphenyl-1-picrylhydrazyl (DPPH); and 2,2'-azinobis(3-ethyl-benzothiazoline-6-sulfonic acid (ABTS). Total phenolics were measured using the Folin-Ciocalteau method, total anthocyanins by the pH-differential method, and beta-carotene by HPLC. The TAA value (hydrophilic + lipophilic ORAC) was highest (27.2 mmol TE/g fw) for NC415 (purple-fleshed) and lowest (2.72 mmol TE/g fw) for Xushu 18 (white-fleshed). The hydrophilic-ORAC values were significantly correlated with the DPPH (R2 = 0.859) and ABTS (R2 = 0.761) values, indicating that these methods have similar predictive capacity for sweetpotato antioxidant activities. However, the lipophilic-ORAC values (2.7 to 52.9% of TAA) were poorly correlated with the beta-carotene contents (R2 = 0.480, p = 0.008). The total phenolic contents (0.011 to 0.949 mg chlorogenic acid equivalent/g fw) were highly correlated with the hydrophilic-ORAC (R2 = 0.937) and DPPH (R2 = 0.820) values. Therefore, the total phenolic content can serve as a useful indicator for the antioxidant activities of sweetpotatoes.