C: Food Chemistry Antioxidant Capacity and Antioxidant Content in Roots of 4 Sweetpotato Varieties Fanie Rautenbach, Mieke Faber, Sunette Laurie, and Robert Laurie Abstract: The antioxidant contents (β -carotene, chlorogenic acid, and vitamin C) as well as the antioxidant capacity (ORAC, FRAP, and ABTS) of 4 sweetpotato varieties were measured in this study. The sweetpotato varieties were cultivated under different water regimes and also subjected to thermal processing. The results show that the 2 orange- ﬂeshed varieties (Resisto and W-119) contain signiﬁcant more β -carotene, chlorogenic acid, and vitamin C than the 2 cream-ﬂeshed varieties (Bosbok and Ndou). Thermal processing decreased the carotenoid and vitamin C content of all the varieties but increased the chlorogenic acid content and antioxidant capacity. Drought stress appears to increase the β -carotene, vitamin C, and chlorogenic acid contents as well as the antioxidant capacity of some of the sweetpotato varieties, especially W-119. Keywords: antioxidant capacity, antioxidant content, drought stress, sweetpotato, thermal processing Introduction Antioxidants such as ﬂavonoids and carotenoids, present in fruits and vegetables, have been positively correlated to the reduced in- cidence of heart disease, some cancers, and age-related degener- ative diseases. The beneﬁcial properties of ﬂavonoids and other polyphenols have been linked to their ability to scavenge free rad- icals, while carotenoids, such as β -carotene, can also scavenge free radicals and it is a vitamin A precursor (Kaur and Kapoor 2001). There is a large focus on the potential role that β -carotene-rich sweetpotato, particularly those with a dark yellow or orange ﬂesh can play in strategies contributing toward the combating of vi- tamin A deﬁciency. A recent clinical trial done in South Africa showed that consumption of boiled and mashed β -carotene-rich orange-ﬂeshed sweetpotatoes did improve the vitamin A status of children aged between 5 and 10 y (Van Jaarsveld and others 2005). Globally, an estimated 190 million children younger than 5 y are vitamin A deﬁcient (WHO 2009), and in 2004, 6% (0.6 million) of under-5 deaths were attributed to vitamin A deﬁciency (Black and others 2008). Improving the vitamin A status of children between 6 mo and 5 y reduces the all-cause mortality by 23% in areas with high vitamin A deﬁciency (Beaton and others 1993). According to a national survey done in 2005, approximately 2 in 3 children (aged between 1 and 9 y) in South Africa are vitamin A deﬁcient (Labadarios 2007). Vitamin A deﬁciency was also ranked 14th as a risk factor cause of the burden of disease in South Africa accounting for 0.4% of disability-adjusted life year (MRC 2008). β -carotene is the predominant carotenoid found in orange- ﬂeshed sweetpotato and a β -carotene content as high as 22.6 mg/ 100 g fresh weight was measured in an orange-ﬂeshed sweet- MS 20091008 Submitted 10/12/2009, Accepted 3/8/2010. Author Rautenbach is with Antioxidant Research Laboratory, Faculty of Applied Sciences, Cape Peninsula Univ. of Technology, P.O. Box 652, Cape Town, South Africa. Author Faber is with Nutritional Intervention Research Unit, Medical Research Council, P.O. Box 19070, Tygerberg 7505, South Africa. Authors Laurie and Laurie are with Agricultural Research Council—Roodeplaat Vegetable and Ornamental Plant Inst., Private Bag X293, Pretoria 0001, South Africa. Direct inquiries to author Rautenbach (E-mail: rautenbachf@cput.ac.za). potato variety (Teow and others 2007). Apart from the carotenoid content of sweetpotatoes, other phytochemicals with antioxidant capabilities have also been measured in sweetpotatoes. Individual polyphenols that have been identiﬁed and quantiﬁed in sweet- potato varieties are mainly phenolic acids such as chlorogenic-, caffeic-, and dicaffeoylquinic acid (Padda and Picha 2007). Sweet- potatoes also contain vitamin C and vitamin E (Woolfe 1992). The antioxidant capacity of purple-, white-, yellow-, and orange-ﬂeshed sweetpotatoes was determined by measuring the antioxidant activity (ferric thiocyanate assay), radical scavenging capability (DPPH (2,2-diphenyl-1-picrylhydrazyl) assay), reduc- ing power and the iron-chelating capacity by Rumbaoa and others (2009). Teow and others (2007) employed a battery of assays (ORAC [oxygen radical absorbance capacity]), ABTS (2,2 ′ - azinobis(3-ethyl-benzothiazoline-6-sulfonic acid)) and DPPH) on sweetpotatoes with varying ﬂesh colors to measure the hydrophilic (H) antioxidant capacity of an acidiﬁed methanol extract and the lipophilic (L) ORAC of a hexane extract (dried and redis- solved in 50% acetone containing RMCD [randomly methylated β -cyclodextrin]). Published results indicated that environmental factors, genetic factors, and cultural management strategies can signiﬁcantly inﬂu- ence the level of β -carotene in sweetpotato varieties. The speciﬁc factors reported to inﬂuence β -carotene content are air and soil temperature, radiation, location, variety, fertilization, and maturity of roots (K’osambo and others 1998; Bartoli and others 1999). Drought has become a major constraint for good production especially in areas where cultivation depends on rain-fed condi- tions (Anselmo and others 1998). Drought causes stomata closure and as a result it reduces carbon dioxide uptake for photosyn- thesis which in turn affects the yield and growth of the plant. Consequently, different water regimes are expected to inﬂuence nutrient content of crops. In wheat leaves, for example, a 2.6-fold increase in β -carotene content was detected after drought stress (Bartoli and others 1999). Sweetpotato is regarded as a moderately drought tolerant crop that is very sensitive to water stress during development and root initiation (Indira and Kabeerathuma 1988). Van Jaarsveld and others (2006) studied the effect of thermal processing on β -carotene in an orange-ﬂeshed sweetpotato and C  2010 Institute of Food Technologists R  C400 Journal of Food Science  Vol. 75, Nr. 5, 2010 doi: 10.1111/j.1750-3841.2010.01631.x Further reproduction without permission is prohibited