Determination of cis- and trans- α- and β-carotenoids in Taiwanese sweet potatoes (Ipomoea batatas (L.) Lam.) harvested at various times

Abstract

A HPLC method was improved to determine sweet potato carotenoids rapidly with good separation efficiency. A C30 column and a gradient solvent system consisting of methanol–acetonitrile–water (84/14/2, v/v/v, solvent A) and dichloromethane (solvent B) (a mixture of 80% A and 20% B was used initially, and then the mixing was programmed to 55% B within 15 min and kept to the end) were used for analysis. The flow rate was 1 ml/min and detection was at 450 nm. A total of 11 all-trans and cis forms of α- and β-carotene in Taiwanese sweet potato (Ipomoea batatas (L.) Lam.) could be resolved within 16 min. The orange-fleshed sweet potato (Tainung 66) had higher total carotenoid content than the yellow-fleshed one (Tainung 57) at the same harvest time. The total carotenoid levels in both crops harvested at various times were in the order: October > July > April > January.

Introduction

Sweet potato (Ipomoea batatas (L.) Lam.) (belongs to convoluvlaceae family) is an important and valuable staple crop worldwide (Food and Agriculture Organization, 1987). It is a nutritious and generous food source for humans and animals as well as a raw material for manufacturing starch, sugar, alcohol and so on (Kozai et al., 1997, Saiful Islam et al., 2002). The crop grows throughout the tropics and subtropics (Scott, 1992), especially in Asian and African countries where account for about 95% of the world’s sweet potato production (Mok, Zhang, & Carey, 1997). Sweet potato has high yield even though it was cultivated in adverse situations (Simonne, Kays, Koehler, & Eilenmiller, 1993). In Taiwan, the annual production of this crop is about 0.2 million metric tons (Yearly Report of Taiwan’s Agriculture, 2005). It can be harvested throughout the year.

Some reports (K’osambo et al., 1997, Van Jaarsveld et al., 2006) indicated that sweet potato contains abundant β-carotene particularly for that with orange flesh. Carotenoids are antioxidants that could suppress singlet oxygen (¹O₂) forming and lipid peroxidation (Burton and Ingold, 1984, Foote and Denny, 1968); moreover, they could reduce incidence of certain cancers and coronary heart disease in humans (Gester, 1993, Ziegler, 1989). However, high dose β-carotene supplements could not show protective effects against cancer and cardiovascular disease (Paiva & Russell, 1999). The agricultural research organizations of Taiwan have remarkable results in cultivation, harvesting technologies and post-harvest treatments of sweet potato. Huang, Chang, and Shao (2006) have evaluated the antioxidant activity of Taiwanese sweet potato by assessing contents of total phenols, flavonoids, anthocyanins and vitamin C. However, there are no thorough reports on the content of carotenoids in sweet potato cultivated in Taiwan.

The reversed-phase high performance liquid chromatography (RP-HPLC) has been used routinely to determine carotenoids because of its satisfactory separation efficiency (Chen et al., 2004, Inbaraj et al., 2006). Furthermore, some investigations (Chen et al., 2004, Lin and Chen, 2003, Sander et al., 2000) manifested that a C30 column had better resolution than a C18 column for separation of carotenoids and their geometric isomers.

In the present study, we improved a HPLC method with a C30 column for rapid and efficient analysis of carotenoids and their isomers in sweet potato. The contents and compositions of these compounds in Tainung 66 (with orange flesh) and Tainung 57 (with yellow flesh), the major and popular sweet potatoes in Taiwan, harvested in four different seasons of the year (2007) were also surveyed.