Makoto Yoshimoto
Team of Crop Functionality and Utilization,
National Agricultural Research Center for Kyusyu Okinawa Region, Suya 2421, Koshi City, Kumamoto 861-1192, Japan
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BSTRACTSweet potato contains various kinds of physiologically functional components, polyphenolics, anthocyanins, fiber, and carotenoids in roots and leaves. Polyphenolic contents vary between the varieties and are relatively superior to other commercial vegetables. Polyphenolic components are mainly caffeoylquinic acid derivatives and the pigments of the purple fleshed sweet potato are acylated anthocyanins. The functional components show various kinds of physiological functions, anti-oxidation, anti-diabetes, anti-hypertension, and others in vitro or in vivo.
The component content can be increased by the harvest time, the controlled storage, and the use of moderate portion. The sweet potato roots or leaves with these functions are commercially used as not only fresh vegetables but also as a material of confectionery, noodles, alcohol drinks, and beverage. Further the waste of the shochu, a traditional Japanese liquor, is reused as raw material of vinegar-like beverage and breads with high content of polyphenolics. Sweet potato leaves also can be reused as functional feeds for egg-laying hens and beef cattle.
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BBREVIATIONSCA caffeic acid;
CQA caffeoylquinic acid;
ChA chlorogenic acid (3-O-caffeoylquinic acid);
Tel: 81-96-242-7873; Fax: 81-96-249-1002; E-mail:mak825@affrc.go.jp
4-O-CQA 4-O-caffeoylquinic acid;
3,4-diCQA 3,4-di-O-caffeoylquinic acid;
3,5-diCQA 3,5-di-O-caffeoylquinic acid;
4,5-diCQA 4,5-di-O-caffeoylquinic acid;
3,4,5-triCQA 3,4,5-tri-O-caffeoylquinic acid;
FA ferulic acid;
QA quinic acid;
Trp-P-1 3-amino-1,4-dimethyl-5H-pyrido-(4,3-b) indol
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NTRODUCTIONSweet potato represents the sixth most important food crop in the world and will be important in solving the global issues of food, energy, and natural resources and the environment in the 21st century (Kozai et al., 1996). Often called ―almost perfect nourishing food‖ sweet potato contains vitamins, minerals and many other nutrients in favorable ratios (Woolfe, 1992). Further, the sweet potato root has contributed to the development of the regional industries mainly as raw materials for starch and shochu (traditional alcohol drinks in Japan). However, demand for this crop is greatly declining due to diversified eating habits and deregulated import of farm produce, which is seriously affecting the local economy.
Therefore, the National Agricultural Research Center for Kyushu Okinawa Region (KONARC), Japan and other research institute are studying ways to enhance value addition of sweet potato by finding new uses.
Sweet potato leaves have largely been neglected except for their partial use as livestock feed in Japan and other Asian countries. The consumption of sweet potato leaves as a fresh vegetable is common in some parts of the world (Villareal et al., 1979). Food scientists are becoming increasingly interested in sweet potato leaves because it can contribute to alleviating food shortages and makes good use of natural resources. As sweet potato leaves can be harvested several times in a year, their yield is ultimately higher than many other leafy vegetables (Nwinyi, 1992). Furthermore, as one of the few vegetables that can be grown easily during the monsoon seasons of the tropics, sweet potato leaves are usually the only greens available in some countries after a flood or a typhoon. They are rich in vitamin, iron, calcium, zinc, and protein, and more tolerant of diseases, pests, and high moisture than many other leafy vegetables grown in the tropics (Pace et al., 1985; Woolfe, 1992; Yoshimoto, 2001). People have a great need to maintain and improve their health through the food they eat every day. Of three most important causes of death in Japan and other developed countries, cancer, heart diseases and cerebrovascular diseases, are said to be closely related to hypertension (Matsui, 2002). To prevent and alleviate these diseases, studies are exploring the health-promoting functions of various agricultural products. However, there are only a few studies concerning the sweet potato components that contribute to maintenance and improvement in health. In the recent decade, active research has been conducted regarding its health-promoting functions. These functions are important factors when developing new processing methods of sweet potato. Sweet potato is also an important crop as an industrial material for starch, sugar, and alcohol. Shochu is a traditional Japanese liquor made from rice, sweet potato, and barley. Recently sweet potato shochu becomes very popular alcohol drink
in Japan (Woolfe, 1992). Therefore, increases in shochu production have resulted in an enormous output of distillery by-products. Some of these by-products have been used as feedstuff (Mahfudz et al., 1996), but most have been discarded into the ocean. Dumping of these by-products into the ocean becomes problematic from the perspective of environmental protection. Optimal treatment of the distillery by-products is important for the success of commercial shochu production. Hence, developing a new use for the shochu-distillery by- products originating from the sweet potato is necessary from the standpoint both of the economy and of environmental conversion. The chapter describes the possible physiological functions, their related components, and utilizations of sweet potato roots, leaves and residues for food or non-food purposes (Table 1).
Table 1. Physiological functions and their related components of sweet potato
Function Related
components References Anti-oxidation Polyphenol,
anthocyanin
Cevallos-Casals and Cisneros-Zevallos (2003); Furuta et al., (1998); Hayase and Kato (1984); Islam et al., (2003a); Oki et al., (2002); Yoshimoto et al., (2004) Antimutagenicity Polyphenol,
anthocyanin
Yoshimoto et al., (1999a); Yoshimoto et al., (1999b); Yoshimoto et al., (2001);
Yoshimoto et al., (2002a) Anti-carcinogenesis Polyphenol,
anthocyanin Hou (2003); Hou et al., (2003); Kurata et al., (2007); Hagiwara et al., (2002) Anti-HIV 3,4,5-triCQA Mahmood et al., (1993); Tamura et al.,
(2006) Anti-Alzheimer‘s
disease diCQA Isoda et al., (2006)
Anti-melanogenesis Polyphenol Shimozono et al., (1996) Reduction of liver
injury Anthocyanin Suda et al., (1998)
Anti-hypertension Anthocyanin
polyphenol Yoshimoto et al., (1998): Mishima et al., (2005); Ishiguro et al. (2007a, 2007b) Antibacterial activity Fiber, pectin-like
polysaccharide Islam and Jalaluddin (2005); Yoshimoto et al., (2006)
Promotion of
Bifidobacterium growth Fiber Yoshimoto et al., (2005a, 2005b);
Takamine et al., (2005) Lowering cholesterol
level Fiber Lund (1984)
Anti-diabetes Anthocyanin, polyphenol
Matsui et al., (2002); Matsui et al., (2004a, 2004b); Terahara et al., (2003); Yoshimoto et al., (2006); Tsubata et al., (2004) Eye protection Lutein
β-carotene Ishiguro and Yoshimoto (2006); Okuno et al., (1998)
The physiological functions of the related components referred in the present chapter have been demonstrated by in vitro or in vivo study. It should be noted that many of the other physiological functions are proposed but are still undergoing research to explore these potential benefits.
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OMPONENTSAbout 400 years ago, Chinese medical report shows that peoples who do not eat rice, barley, and beans, but eat sweet potato roots have long life (Yamada, 1994). In Japan, sweet potato has been cultivated as an emergency crop in the time of famine and has prevented starvation several times (Duell, 1983). This means that sweet potato is not only an easily- cultivated crop, but has also sustained the communities at times of food scarcity. The nutrient composition of sweet potato roots is in details summarized by Woolfe (1992). Accordingly, the root nutrient components particularly not concerned with physiological functions are not described in the present chapter.