Sweet potato polyphenols

CHAPTER 7

Sweet potato polyphenols Rie Kurata1, Hong-Nan Sun2, Tomoyuki Oki3, Shigenori Okuno4, Koji Ishiguro5 and Terumi Sugawara6 1

Division of Upland Farming Research, Kyusyu Okinawa Agricultural Research Center, NARO, Miyakonojo, Japan Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture, Beijing, People’s Republic of China 3 Graduate School of Health and Nutrition Sciences, Nakamura Gakuen University, Fukuoka, Japan 4 Department of Planning, Kyusyu Okinawa Agricultural Research Center, NARO, Kumamoto, Japan 5 Division of Field Crop Research and Development, Hokkaido Agricultural Research Center, NARO, Hokkaido, Japan 6 Crop Development and Agribusiness Research Division, Kyusyu Okinawa Agricultural Research Center, NARO, Kumamoto, Japan 2

Overview of sweet potato polyphenols A recent trend in the diet of most developed countries and some developing countries is the consumption of healthy food. To improve health the consumption of special dietary foods such as physiological functional foods, smoothies, and vegetable juices is increasing. The population in most developed countries and some developing countries is progressively aging and the desire to live as healthy for a long time has encouraged people to make efforts to maintain their health. Among the dietary strategies in some Asian countries, especially in China and Japan, sweet potatoes have recently become widely included in staple food, snack food, and vegetable juices. The Chinese Dietary Guidelines in 2016 recommend that 50
100 g sweet potato or potato should be consumed every day. With the implementation of the sweet potato and potato staple food strategy suggested by the Chinese Ministry of Agriculture, more and more staple food and snack food with high contents of sweet potato and potato are being researched and developed, and can be purchased in the market. Initially in Japan beverages popularly contained purple sweet potato pigment, but recently the whole purple sweet potato has been used to prepare vegetable juices containing anthocyanins, which are the purple pigment and the polyphenol content that are also available in the market. In addition, foods considered to have health functional effects because of their polyphenol content are also available as various products such as beverages, tablets, and jellies that are currently sold. The ingestion of Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00007-7

© 2019 Elsevier Inc. All rights reserved.

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polyphenols for health reasons is presently widespread in most developed countries, and the demand for polyphenols is increasing. Therefore more and more research institutes, especially the Potato and Sweet Potato Food Science Innovation Team, CAAS, China and Kyushu Okinawa Agricultural Research Center, NARO, Japan have been conducting research focusing on the high value of polyphenols in sweet potato. Polyphenolic compounds in sweet potato are separated into two main categories: flavonoids and phenolic acids. Flavonoids are mainly found in the tuberous root of sweet potato as pigments and can be classified by color. Purple, orange, and yellow sweet potatoes contain purple-colored anthocyanin, carotenoids (described in Chapter 8: Sweet potato carotenoids), and xanthophyll (as the main pigment), respectively. In addition, a patent reported that sweet potato tops contain quercetin glycosides, which are flavonoids (WO2006/014028). Phenolic acids in the sweet potato consist of a mixture of caffeic acid (CA) and caffeoylquinic acid (CQA) derivatives, and are commonly present in all parts of the leaves, petioles, stems, and tuberous roots. However, their content is most abundant in leaves, and the amount contained in the stems, petioles, and tubers is small. Therefore the main material source of polyphenols is the leaves. This chapter focuses on polyphenols, anthocyanins, and CQA derivatives, specifically their chemistry, functionality, and the processing required for their retention.

Chemistry Anthocyanins The anthocyanins (Greek anthos, flower, and kyanos, blue)—originally used to describe the blue pigment of the cornflower (Centaurea cyanus)— are the most important groups of water-soluble plant pigments visible to the human eye (Strack and Wray, 1989). They belong to the most widespread class of phenolic compounds collectively named flavonoids, and by the end of 1985, more than 4000 structures including 240 different naturally occurring anthocyanins had been found (Strack and Wray, 1989). They are present in a wide range of plant tissues, particularly in the flowers and fruits, but also in storage organs, roots, tubers, and stems. The basic anthocyanin structure consists of two or three chemical units, an aglycone base (also termed “anthocyanidin”), sugars, and organic acids in the case of acylated anthocyanins (Sui, 2017).

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In the last decade or two, sweet potato cultivars with purple flesh have been mainly grown in Japan, Korea, and New Zealand (Steed and Truong, 2008). Among them, the “Ayamurasaki” cultivar is the second generation of a local Japanese sweet potato variety “Yamagawamuarsaki” and accumulates high levels of anthocyanin pigments in the storage root, when cultivated in Japan. In “Yamagawamurasaki” and “Ayamuarsaki,” the major anthocyanins are cyanidin and peonidin 3-O-sophoroside-5-Oglucosides acylated with caffeic, ferulic, or p-hydroxybenzoic acids, abbreviated as YGM-1a, 1b, 2, 3, 4b, 5a, 5b, and 6 as listed in Table 7.1 (Odake et al., 1992, Goda et al., 1997, Terahara et al., 1999). China began to introduce purple sweet potato varieties in the 1980s, and then bred new varieties suitable for domestic cultivation, such as the purple sweet potato “Yan 176,” “135,” “Yan 337,” “Xushu 4,” “Jingshu 16,” “Yuzi 1,” and “Qunzi 1.” Currently, purple sweet potato are widely planted in northeastern China, including Shandong, Hebei, Guangxi, Jiangsu, and Guangdong provinces (Mu et al., 2017). With the increasing amount of purple sweet potato acreage in China, many researchers have studied the sweet potato anthocyanins structures because of their high photothermal stability. Yao (2009) found that the main anthocyanin of purple sweet potato (“Yan 176”) was 3-O-(6-Otranscaffeyl-2-O-β-glucopyranosyl-β-glucopyranoside)-5-O-β-glucoside peonidin. Qiu et al. (2009) found four anthocyanins from purple sweet potato, peonidin 3-O-(6-O-(E)-caffeoyl-2-O-β-D-glucopyranosyl-β-Dglucopyranoside)-5-O-β-D-glucoside, cyanidin 3-O-(6-O-p-coumaroyl)β-D-glucopyranoside, peonidin 3-O-(2-O-(6-O-(E)-caffeoyl-β-D-glucopyranosyl)-6-O-(E)-caffeoyl-β-D-glucopyranoside)-5-O-β-D-glucopyranoside, and peonidin 3-O-(2-O-(6-O-(E)-feruloyl-β-D-glucopyranosyl)6-O-(E)-caffeoyl-β-D-glucopyranoside)-5-O-β-D-glucopyranoside. Fig. 7.1 illustrates the chemical structure of anthocyanins isolated from purple-fleshed sweet potato. Other anthocyanins exist in the storage root, leaves, and cell lines of sweet potatoes as non-, mono, and diacylated forms with cyanidin, peonidin, or pelargonidin aglycone as listed in Table 7.1 (Islam et al., 2002a; Terahara et al., 2004; Tian et al., 2005; Truong et al., 2010; Kim et al., 2012). The anthocyanin composition affects the color of the paste, and those made from purple-fleshed sweet potatoes that are rich in peonidin and cyanidin anthocyanins exhibit a reddish- and bluish-purple color, respectively (Yoshinaga et al., 1999). Because commercial standards are not available, electrospray ionization mass

Table 7.1 Anthocyanins identified in storage root, leaf, and cell line of sweet potatoes. Anthocyanins

MW

Anthocyanins in Storage root

Leaf

3 3

3 3

3 3 3 3

3 3 3 3

3 3 3

3 3

Abbreviation Cell line

Cyanidin type

Cyanidin 3,5-diglucoside Cyanidin 3-sophoroside-5-glucoside Cyanidin 3-(60 -p-hydroxybenzoylsophoroside)-5-glucoside Cyanidin 3-(60 -p-coumarylsophoroside)-5-glucoside Cyanidin 3-(60 -caffeoylsophoroside)-5-glucoside Cyanidin 3-(6-caffeoylsophoroside)-5-glucoside Cyanidin 3-(60 -feruloylsophoroside)-5-glucoside Cyanidin 3-(6-caffeoyl-60 -p-hydroxybenzoylsophoroside)-5-glucoside Cyanidin 3-(6, 60 -dicoumarylsophoroside)-5-glucoside Cyanidin 3-(6-caffeoyl-60 -p-coumarylsophoroside)-5-glucoside Cyanidin 3-feruloyl-p-coumarylsophoroside-5-glucoside Cyanidin 3-(6, 60 -dicaffeoylsophoroside)-5-glucoside Cyanidin 3-(6-caffeoyl-60 -feruloylsophoroside)-5-glucoside

611 773 893 919 935 935 949 1055 1065 1081 1095 1097 1111

3 3 3 3 3 3 3 3 3 3 3 3

YGM-0a YGM-0c YGM-0d YGM-2 YGM-0g YGM-1a YGM-7a YGM-3' YGM-1b YGM-3

Peonidin type

Peonidin Peonidin Peonidin Peonidin Peonidin Peonidin Peonidin Peonidin Peonidin Peonidin Peonidin

3-sophoroside-5-glucoside 3-(60 -p-hydroxybenzoylsophoroside)-5-glucoside 3-(60 -p-coumarylsophoroside)-5-glucoside 3-(60 -caffeoylsophoroside)-5-glucoside 3-(6, 60 -caffeoylsophoroside)-5-glucoside 3-(60 -feruloylsophoroside)-5-glucoside 3-(6-caffeoyl-60 -p-hydroxybenzoylsophoroside)-5-glucoside 3-(6, 60 -dicoumarylsophoroside)-5-glucoside 3-feruloyl-p-coumarylsophoroside-5-glucoside 3-(6, 6'-dicaffeoylsophoroside)-5-glucoside 3-(6''-caffeoyl-6'''-feruloylsophoroside)-5-glucoside

787 907 933 949 949 963 1069 1079 1109 1111 1125

3 3

3

3 3 3 3

3 3 3

3 3 3

3

3 3 3 3 3 3 3 3 3

757 933

YGM-0f YGM-5b YGM-0i YGM-5a YGM-7b YGM-4b YGM-6

Pelargonidin type

Pelargonidin 3-sophoroside-5-glucoside Pelargonidin 3-feruloylsophoroside-5-glucoside

YGM-0b YGM-0e

3 3

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R1 3′ 2′ HO

6

10

1 + O 2

1′

3

5′ 6′

O

4 6

OH HO

5

9

O

OH O HO

8

7

OH 4′

O

1 O 1′

5 2 HO OR 2 4 O 5′ 3 6′ OR3 3′ HO 2′ HO OH OH 4′

Figure 7.1 Chemical structure of anthocyanins identified in purple-fleshed sweet potatoes: cyanidin, R1 5 OH; pelargonidin, R1 5 H; peonidin, R1 5 OCH3; R2, R3 5 H, caffeic acid; ferulic acid; p-hydroxybenzoic acid; and p-coumaric acid.

spectrometry and tandem MS have been used as powerful techniques for anthocyanin identification and characterization in purplefleshed sweet potatoes (Tian et al., 2005; Truong et al., 2010; Kim et al., 2012). The total anthocyanidin content was quantified using methods such as pH differential (Liu et al., 2013) and spectrometric (Oki et al., 2002b) methods. Individual anthocyanins were quantified using relatively stable standards such as cyanidin or peonidin 3,5-diglucoside (Kim et al., 2012; Lee et al., 2013) or using a relative response factor (Terahara et al., 2007; Oki et al., 2017a). Anthocyanin pigments in purple-fleshed sweet potatoes are highly stable against heat and ultraviolet irradiation owing to their acylated forms, which is an advantage when they are used in food additives as natural colorants (Hayashi et al., 1996). Anthocyanin colorants from purple-fleshed sweet potatoes are used as food additives such as E163 (E-number) and 73.260 (US CFR number). In addition to natural food colorants, purplefleshed sweet potato tubers are processed to produce juice concentrate, paste, and flour. Numerous kinds of processed foods such as noodles, bread, jams, chips, confections, beverages, and alcoholic beverages made from purple-fleshed sweet potato are currently available in Japanese stores (Suda et al., 2003). Major anthocyanins in purple-fleshed sweet potato remain intact in foods such as primary-processed foods, beverages, deepfried foods, and secondary-processed foods (Oki et al., 2010). In addition, with esthetic awareness increasing, many people, especially women, are

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paying more attention to skin care. The development of natural skin care products, characterized as safe, with an antiaging effect, has become an important research topic, and anthocyanins have these qualities, being classified as edible cosmetics and natural vitamins in Europe (Mu et al., 2017). They will have a vast market because of their applications in nontoxic makeup and their antioxidant capacity.

Caffeoylquinic acids Chlorogenic acids belong to the ester compound family and are formed by the condensation of quinic acid and transcinnamic acids, which include CA, p-coumaric acid, and ferulic acid (Marques and Farah, 2009; Michael et al., 2006). Because of the different positions, varieties, and quantities of the condensation between quinic and transcinnamic acids, there are a variety of chlorogenic acids (molecular structures are shown in Fig. 7.2 and Table 7.2). 3-O-Caffeoylquinic acid (3-CQA) is the most common of the chlorogenic acids, with the molecular formula C16H18O9, and a molar mass of 354.30 g/mol. Additionally, the esterified chlorogenic acids (esterification reactions occur at R1 of the molecular structure of quinic acid in Fig. 7.2) are also chlorogenic acids, such as methyl chlorogenate and chlorogenic ethyl ester (Mu et al., 2017). CQAs in sweet potato roots have been investigated for many years, especially in the United States, Japan, and China. Investigations of sweet potato respiration in the United States during the first half of the 20th century isolated chlorogenic acid (5-O-caffeoylquinic acid, 5-CQA) from roots, but did not quantify it (Rudkin and Nelson, 1947). CA, OR3 6

4

2 5 1

HOOC HO

OR2

3

HO

OR1

Quinic acid, Q

H C 3

O

Ferulic acid, F

OH

Caffeic acid, C

O

HO

O

HO

HO

O OH

OH

Coumaric acid, p-Co

Figure 7.2 Composition and molecular structure of chlorogenic acids.

Table 7.2 Molecular structure and plant sources of chlorogenic acids. Name

R1

R2

R3

Plant sources

3-O-caffeoylquinic acid 4-O-caffeoylquinic acid 5-O-caffeoylquinic acid 3-O-coumaroyl guinic acid 4-O-coumaroyl guinic acid 5-O-coumaroyl guinic acid 3-O-feruloylquinic acid 4-O-feruloylquinic acid 5-O-feruloylquinic acid 4,5-di-O-caffeoylquinic acid 3,4-di-O-caffeoylquinic acid 3,5-di-O-caffeoylquinic acid 3,4-di-O-coumaroyl guinic acid 3,5-di-O-coumaroyl guinic acid 4,5-di-O-coumaroyl guinic acid 3-O-feruloyl,4-O-caffeoylquinic 3-O-caffeoyl,4-O-feruloylquinic 3-O-feruloyl,5-O-caffeoylquinic 3-O-caffeoyl,5-O-feruloylquinic 3-O-feruloyl,5-O-caffeoylquinic 3-O-caffeoyl,5-O-feruloylquinic 3,4,5-tri-O-caffeoylquinic acid

C H H p-Co H H F H H H C C p-Co p-Co H F C F C H H C

H C H H p-Co H H F H C C H p-Co H p-Co C F H H F C C

H H C H H p-Co H H F C H C H p-Co p-Co H H C F C F C

Existing widely Eucommia ulmoides Sweet potato leaves, coffee Hemerocallis fulva Coffee, Hemerocallis fulva Hemerocallis fulva Eucommia ulmoides, honeysuckle Eucommia ulmoides, coffee Tea, coffee Eucommia ulmoides, tea Honeysuckle, sweet potato Honeysuckle, Eucommia ulmoides Coffee Coffee Coffee Sweet potato leaves, coffee Sweet potato leaves, coffee Sweet potato leaves Sweet potato leaves Sweet potato leaves Sweet potato leaves Sweet potato leaves, coffee

acid acid acid acid acid acid

Note: R1, R2, and R3 represent the three different sites in the molecular structure of quinic acid in Fig. 7.2, respectively. H represents hydrogen. C, F, and p-Co represent caffeic acid, ferulic acid, and p-coumaric acid, respectively, in Fig. 7.2.

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isochlorogenic acid (di-O-caffeoylquinic acid; diCQA), 5-CQA, “Band 510” (4-O-caffeoylquinic acid; 4-CQA), and neochlorogenic acid (3CQA) were later isolated and quantified from different kinds of plant sources including sweet potato peelings by open column chromatography, and isochlorogenic acid was found to be the most abundant CQA in sweet potatoes (Sondheimer, 1958). 5-CQA, two isochlorogenic acids, and tentative 4-CQA in the roots of seven sweet potato cultivars harvested in the United States, including Jewel, Centennial, and Julian, were quantified by the use of high-performance liquid chromatography (HPLC) (Walter et al., 1979). This investigation showed that CQA contents varied greatly between cultivars, and 5-CQA and one of the isochlorogenic acids were the most abundant. A study using paper chromatography and thin-layer chromatography isolated and identified four CQAs, 4-CQA, 5-CQA, isochlorogenic acid, and 3-CQA, from 14 cultivars, including Centennial, Jasper, and Jewel, but did not quantify their contents (Thompson, 1981). HPLC analysis of the roots from the Jewel cultivar revealed the tissue location of three isochlorogenic acids to be in the order of outer. skin. inner (Walter and Shadel, 1981). A series of investigations into black rot of sweet potatoes in Japan during the mid-1950s isolated and identified several polyphenols, including 5-CQA, from the roots of the Norin No. 1 cultivar affected by the disease (Uritani and Muramatsu, 1953). Six polyphenols were found in roots (cultivar name not shown), one of the six was isolated and identified as 5CQA, and four of them were identified as three di-CQAs; the remaining polyphenol was tentatively identified as 4-CQA (Hayase and Kato, 1984). This investigation also quantified the contents of the four polyphenols in the roots of the Kintoki and Kokei No. 14 cultivars. Three CQAs, namely, 3,4-di-O-caffeoylquinic acid (3,4-diCQA), 3,5-di-O-caffeoylquinic acid (3,5-diCQA), and 4,5-di-O-caffeoylquinic acid (4,5-diCQA), were isolated from the roots of the Beniotome cultivar (Shimozono et al., 1996). CA and four CQAs, namely, 5-CQA, 3,4-diCQA, 3,5-diCQA, and 4,5-diCQA, were isolated from the raw roots of the Beniazuma cultivar and two CQAs, namely, 3-CQA and 4-CQA, were from its steamed roots (Takenaka et al., 2006). The roots of some sweet potato cultivars are known to have purple flesh and contain anthocyanins. The contents of CA and four CQAs, namely, 5-CQA, 3,4-diCQA, 3,5-diCQA, and 4,5-diCQA, were quantified in the roots of the Benimasari, Koganesengan, J-Red, and Murasakimasari cultivars (Ishiguro et al., 2007a). The Beniazuma, Benimasari, and Koganesengan cultivars have

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185

yellow-colored flesh. The J-Red cultivar has orange-colored flesh and is known to contain carotenoids (Ishiguro et al., 2010). The Murasakimasari cultivar has purple-colored flesh and is known to contain anthocyanins (Oki et al., 2002b). It is presumed that many types of CQAs are present in sweet potato roots, regardless of flesh color. The existence of an ester of CA and sugar in sweet potato roots has been reported. Beta-Dfructofuranosyl 6-O-caffeoyl-α-D-glucopyranoside (FCG), an ester of CA and sucrose, was isolated from the roots of the Beniazuma cultivar (Takenaka et al., 2006). FCG was also isolated from the roots of the JRed cultivar stored at 15°C (Ishiguro et al., 2007a). This investigation reported that FCG was undetectable in the nonstored roots of the J-red, Beniazuma, and Koganesengan cultivars, while it was present in those of the Murasakimasari cultivar, and the FCG content in these cultivars increased during storage in general. Sweet potato tops are also known to contain CQAs. Since the 2000s many studies on CQAs in sweet potato tops have been conducted in Japan, the United States, the United Kingdom, and China. The first isolation and identification of 3,4,5-tri-O-caffeoylquinic acid (3,4,5-triCQA) from sweet potato leaves and elucidation of their polyphenolic compositions (CA and five CQAs: 5-CQA, 3,4-diCQA, 3,5-diCQA, 4,5-diCQA, and 3,4,5-triCQA) was carried out in Japan (Islam et al., 2002b). This investigation of the leaves from 20 sweet potato genotypes by HPLC revealed that 3,5-diCQA was the most abundant among the six polyphenols in all genotypes tested. The polyphenol contents of sweet potato leaves reportedly vary depending on cultivation conditions, such as temperature and sunshine. In greenhouse experiments using the Simon No. 1, Kyushu No. 119, and Elegant Summer cultivars, the 3,5-diCQA content was higher after cultivation at 30°C than at 20°C or 25°C, and the contents of most CQAs (5-CQA, 3,4-diCQA, 3,5-diCQA, and 4,5diCQA) were decreased by shading (Islam et al., 2003a). Analyzing sweet potato leaf samples by HPLC using a conventional column with a length of 150 mm, an internal diameter of 4.6 mm, and a particle size of 5 μm and a methanol-based mobile phase proved very time consuming; the retention time for 3,4,5-triCQA was approximately 57 min (Islam et al., 2002b). HPLC analysis using short columns with a length of 75 mm and a particle size of 3 μm and an acetonitrile-based mobile phase shortened the retention time for 3,4,5-triCQA to approximately 15 min (Okuno et al., 2010). This investigation reported the contents of CA and five CQAs, namely, 5-CQA, 3,4-diCQA, 3,5-diCQA,

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4,5-diCQA, and 3,4,5-triCQA, in the leaves of 529 cultivars/lines as 0.08
3.09, 0.17
16.01, 0.50
34.08, 1.86
56.52, 0.69
20.73, and 0.25
13.81 mg/g of freeze-dried sample, respectively. The Suioh sweet potato cultivar was released for consumption of its leaves and petioles, and its leaves were shown to have a higher total polyphenol content and DPPH (1,1-dipheny-2-picrylhydrazyl) radical scavenging activity than were the leaves of other leafy vegetables (Ishiguro et al., 2004). The leaves of many cultivars have been shown to contain more CQAs than those of the Suioh cultivar (Okuno et al., 2010). However, the better taste of the leaves and petioles of Suioh than those of other cultivars is an important characteristic. Confirming the validity of polyphenol quantification methods will be increasingly important to consumers and manufacturers. In terms of CA and seven CQAs, namely, 3-CQA, 4-CQA, 5-CQA, 3,4-diCQA, 3,5-diCQA, 4,5-diCQA, and 3,4,5-triCQA, single-laboratory validation of quantification was conducted using Suioh leaves (Sasaki et al., 2014). In the validated quantification method, a column with a conventional size of 250 3 4.6 mm and a particle size of 3 μm and an acetonitrile-based mobile phase were used to separate the peaks of CA and CQAs from those of anthocyanins. The eight polyphenols mentioned above in the leaves of eight cultivars/lines (including Koganesengan, Suioh, Kokei No. 14, and others) were quantified by the validated method for two consecutive years (Sasaki et al., 2015b). Investigations of CQAs in sweet potato tops in the United States, the United Kingdom, and China were conducted as follows. The identification of CA, 5-CQA, 4,5-diCQA, 3,5-diCQA, and 3,4-diCQA in the leaves of the commercial cultivars Beauregard, Hernandez, and Covington in the United States was performed by HPLC with the aid of liquid chromatography-mass spectrometry (LC-MS) (Truong et al., 2007). This investigation showed that 3,5-diCQA and 4,5-diCQA were predominant in the leaves. Analysis by LC-MS3 identified phenolic compounds containing a feruloyl moiety in addition to CQAs in the stems of sweet potatoes (cultivar name not shown) cultivated in China (Zheng and Clifford, 2008). The compounds containing a feruloyl moiety were 3-Oferuloylquinic acid, 4-O-feruloylquinic acid, 5-O-feruloylquinic acid, 3-O-feruloyl-4-O-caffeoylquinic acid, 3-O-caffeoyl-4-O-feruloylquinic acid, 3-O-feruloyl-5-O-caffeoylquinic acid, and 3-O-caffeoyl-5-O-feruloylquinic acid, and the CQAs were 3-CQA, 4-CQA, 5-CQA, 3,5diCQA, and 4,5-diCQA. The four caffeoyl-feruloylquinic acids were presumed to be contained in smaller amounts than were the other

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compounds. LC-MS2 analysis identified 37 compounds, including 20 phenolic acids and 12 flavonoids, in sweet potato leaves (cultivar name not shown) cultivated in China (Zhang et al., 2015). The 20 phenolic acids included CQAs, such as 3-CQA, 3,4-diCQA, 3,5-diCQA, 4,5-diCQA, and triCQAs. In the lab of the Potato and Sweet Potato Food Science Innovation Team, CAAS, China, sweet potato leaves from 40 cultivars were collected (Sun et al., 2014a). After determination of the nutritional and functional composition, two sweet potato cultivars, Yuzi No. 7 and Simon No. 1, were selected. The polyphenols from these two cultivars were extracted, purified, and identified. Results (Xi et al., 2015) showed that polyphenols from sweet potato leaves consisted mainly of seven CQAs and a small amount of CA (Fig. 7.3). The 3,5-diCQA content was highest, while the 3CQA content was lowest among the CQAs (Table 7.3). (A) 5.0

6

Yuzi No.7

4.0

mAU

3.0 2.0

5

7

1.0 1 23

8

4

0 0

2

4

6

8

10

12

14

16

Time (min) (B) 3.0

Simon No.1

6

2.0

mAU

3 5

1.0

1 2

7 4

8

0 0

2

4

6

8

10

12

14

16

Time (min)

Figure 7.3 The HPLC of sweet potato leaf polyphenols. (A) Yuzi No. 7; (B) Simon No. 1. Peak 1: 5-CQA; peak 2: 3-CQA; peak 3: 4-CQA; peak 4: CA; peak 5: 4,5-diCQA; peak 6: 3,5-diCQA; peak 7: 3,4-diCQA; peak 8: 3,4,5-triCQA.

Table 7.3 The constituents of polyphenols purified from two sweet potato leaf cultivars (Yuzi No. 7 and Simon No. 1). Peak No.

Retention time (min)

Identification

1 2 3 4 5 6 7 8

1.47 1.91 2.10 2.92 4.16 4.54 4.88 6.87

5-CQA 3-CQA 4-CQA CA 4,5-diCQA 3,5-diCQA 3,4-diCQA 3,4,5triCQA

Standard curve

y 5 11.372x 2 0.4279 y 5 9.9086x 1 0.2857 y 5 25.894x 2 17.128 y 5 28.183x 2 1.2114 y 5 9.2077x 2 7.244 y 5 18.056x 2 18.405 y 5 15.353x 2 12.021 y 5 6.2184x 2 5.1579

R2

0.9962 1.0000 0.9988 1.0000 0.9987 0.9981 0.9987 0.9949

Yuzi No. 7

Simon No. 1

Peak area

Content (%, DW)

Peak area

Content (%, DW)

55.55 6 0.92 17.45 6 0.07 35.90 6 0.57 3.80 6 0.14 378.7 6 57.00 1115.10 6 13.29 388.30 6 4.67 27.70 6 0.57

2.46 6 0.03 0.87 6 0.00 1.02 6 0.01 0.36 6 0.01 20.96 6 0.27 31.39 6 0.26 13.04 6 0.11 2.64 6 0.03

130.85 6 7.85 34.65 6 0.64 277.35 6 0.92 384.90 6 0.99 683.15 6 3.13 132.80 6 0.14 31.95 6 0.21 1.95 6 0.07

5.77 6 0.25 1.73 6 0.02 5.69 6 0.01 0.22 6 0.01 21.29 6 0.04 19.45 6 0.06 4.72 6 0.00 2.98 6 0.01

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Physiological function Anthocyanins Purple-fleshed sweet potatoes exhibit a brilliant reddish-purple color with high levels of anthocyanins, total phenolics, and antioxidant activities (Yoshinaga et al., 1999; Oki et al., 2003; Steed and Truong, 2008). Recently sweet potato cultivars with deep purple flesh have been developed in China, Japan, Korea, New Zealand, and other countries to meet the growing demand for healthy food (Mu et al., 2017; Steed and Truong, 2008). The metabolism of ethanol induces reactive oxygen species (ROS) generation and depletion of the cell antioxidant activity, which leads to development of alcohol-related pathologies. The group members in the lab of the Potato and Sweet Potato Food Science Innovation Team, CAAS, China investigated the dealcoholic effect and preventive effect of purple sweet potato anthocyanins (PSPAs) (cultivar: Yan 176) on acute and subacute alcoholic liver damage (ALD) (Sun et al., 2014b). Sevenweek-old male inbred mice were grouped into five groups: control group (without PSPAs and ethanol treatments), model group (with ethanol treatment only), low-dose group (50 mg PSPAs/kg body weight), middle-dose group (125 mg PSPAs/kg body weight) and high-dose group (375 mg PSPAs/kg body weight), and the mice in all groups were administered intragastrically. Biochemical parameters of serum and liver were determined, and a histopathological analysis of liver tissue was also achieved. Results showed amelioration of all tested parameters following administration of PSPAs (Figs. 7.4
7.9). The abovementioned result suggested that PSPAs have a preventive effect on acute and subacute ALD, and could be used as a complementary reagent during prophylactic and therapeutic managements of ALD. In Japan the most notable example is the Ayamurasaki cultivar, which is the second generation of a local variety, Yamagawamuarsaki. The extracted anthocyanins from the Ayamurasaki storage root exhibited multiple physiological functions such as radical scavenging activity (RSA) (Furuta et al., 1998; Oki et al., 2002b; Kano et al., 2005), oxygen radical absorbance capacity (Oki et al., 2009), angiotensin I-converting enzyme inhibition (Suda et al., 2003), α-glucosidase inhibition (Matsui et al., 2001a, b), and antimutagenic activity (Yoshimoto et al., 1999; 2001). A cell line derived from the Ayamurasaki storage root exhibited potent antimutagenic activity against 3-amino-1,4-dimethyl-5H-pyrido[4,3-b] indole

Sweet Potato

190

(B)

200 180 160 140 120

Control Model Low-dose group Middel-dose group High-dose group

** ##

## ##

100 80 60 40

** ## ## ##

Activity of LDH (U/L)

Activity of ALT and AST (U/L)

(A)

ALT

##

350 300 250 200 150 100

Control

AST

(C)

(D)

450 400

160

Model

140

Low-dose group Middel-dose group High-dose group

100 80 60 #

40

Model

Low-dose Middel-dose High-dose group group group

**

Control

Activity of LDH (U/L)

Activity of ALT and AST (U/L)

**

0

0

120

**

400

50

20

180

**

450

# ##

350

##

300 250 200 150 100 50

20

0

0

ALT

AST

Control

Model

Low-dose Middel-dose High-dose group group group

Figure 7.4 Effect of purple sweet potato anthocyanins (PSPAs) on the activity of serum ALT, AST, and LDH of ALD mice. (A) Effect of PSPAs on the activity of serum ALT and AST of the acute ALD mice. (B) Effect of PSPAs on the activity of serum LDH of the acute ALD mice. (C) Effect of PSPAs on the activity of serum ALT and AST of the subacute ALD mice. (D) Effect of PSPAs on the activity of serum LDH of the subacute ALD mice. Compared with control,  P , .05,  P , .01; compared with model, # P , .05, ##P , .01. ALT, alanine aminotransferase; AST, aspartate aminotransferase; LDH, lactate dehydrogenase; TCH, total cholesterol; MDA, malondialdehyde; SOD, superoxide dismutase; GST, glutathione S-transferase

(Trp-P-1) and inhibition of HL-60 cell proliferation (Konczak-Islam et al., 2003). Despite the complex chemical structure of the anthocyanins in purple-fleshed sweet potatoes, studies have confirmed that following administration of an anthocyanin-rich extract of Ayamurasaki storage root to rats (Suda et al., 2002; Harada et al., 2004) or human consumption of a beverage prepared from purple-fleshed sweet potato (Harada et al., 2004; Kano et al., 2005; Oki et al., 2006), two anthocyanin components, YGM-2 and YGM-5b, were rapidly absorbed into the body (detectable in the blood). Furthermore, they were rapidly excreted

Sweet potato polyphenols

(A)

(B) 3

0.3

2.5

*#

2

Control Model Low-dose group Middle-dose group High-dose group

1.5

# # 1 0.5

Serum LDL_C levels (mmol/L)

Serum TCH and TG levels (mmol/L)

191

0.25

** **# **##

0.2

##

0.15 0.1 0.05 0 Control

0

TCH

Model

TG

Low-dose Middle-dose High-dose group group group

(D) 0.3

(C)

3

#

2.5

#

Control Model Low-dose group Middle-dose group High-dose group

2

* 1.5

# 1

## ##

Serum LDL_C levels (mmol/L)

Serum TCH and TG levels (mmol/L)

3.5 0.25 0.2 0.15 0.1 0.05

0.5 0 0

Control

TCH

TG

Model

Low-dose Middle-dose High-dose group group group

Figure 7.5 Effect of purple sweet potato anthocyanins (PSPAs) on the serum TCH, triglyceride (TG), and LDL-C levels of ALD mice. (A) Effect of PSPAs on the serum TCH and TG levels of the acute ALD mice. (B) Effect of PSPAs on the serum LDL-C level of the acute ALD mice. (C) Effect of PSPAs on the serum TG and TCH levels of the subacute ALD mice. (D) Effect of PSPAs on the serum LDL-C level of the subacute ALD mice. Compared with control,  P , .05,  P , .01; compared with model, #P , .05, ## P , .01.

together with nonacylated anthocyanins in the urine. In addition, it has been reported that the purple-fleshed sweet potato or derived beverages containing anthocyanins show pharmacological effects such as antihyperglycemic effects mediated by α-glucosidase inhibition (Matsui et al., 2002) and antiatherosclerotic effects (Miyazaki et al., 2008) in animal models. Moreover, beneficial effects on hypertension and hepatitis have been observed in both animal models (Suda et al., 2003; Kobayashi et al., 2005) and clinical trials (Suda et al., 2003, 2007; Oki et al., 2016, 2017b). Other pharmacological effects of anthocyanins in other purple-fleshed cultivars include the attenuation of oxidative stress and inflammatory responses induced by D-galactose in mouse liver (Zhang et al., 2009),

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*

Hepatic MDA level (mg/g liver)

(A) 30

#

25

## 20

##

15 10 5 0 Control

Model

Low-dose Middle-dose High-dose group group group

(B) Hepatic MDA level (mg/g liver)

30 25 20

## 15

##

##

10 5 0 Control

Model

Low-dose Middle-dose High-dose group group group

Figure 7.6 Effect of purple sweet potato anthocyanins (PSPAs) on the hepatic MDA level of the ALD mice. (A) Effect of PSPAs on the hepatic MDA level of the acute ALD mice. (B) Effect of PSPAs on the hepatic MDA level of the subacute ALD mice. Compared with control,  P , .05,  P , .01; compared with model, #P , .05, ##P , .01.

protection of mouse liver against D-galactose-induced apoptosis (Zhang et al., 2010), attenuation of hepatic lipid accumulation in high-fat diet-fed mice (Hwang et al., 2011a), attenuation of dimethylnitrosamine-induced liver injury in rats (Hwang et al., 2011b), attenuation of domoic acid
induced cognitive deficit in mice (Lu et al., 2012), improvement of fasting blood glucose level, glucose, and insulin tolerance in high-fat diet-treated mice (Zhang et al., 2013), beneficial effects on high-fat diet-induced kidney dysfunction and damage in high-fat diet mice (Shan et al., 2014), alleviation of high-fat diet-induced obesity in rats (Zhang et al., 2015), protective effects against high-fat diet-induced hepatic inflammation in

Sweet potato polyphenols

(B)

2 # 1.5 * 1

0.5

0 Control

Model

240 # *##

200 # 160 120 80 40 0

Low-dose Middle-dose High-dose group group group

(C)

Control

Model

Low-dose Middle-dose High-dose group group group

(D) 250

3.5

**##

**##

3

**##

2.5 2

*

1.5 1 0.5 0

Hepatic GST level (U mg/g protein)

Hepatic SOD level (U mg/g protein)

Hepatic GST level (U mg/g protein)

Hepatic SOD level (U mg/g protein)

(A)

193

200

*##

150

*##

**

100

50

0 Control

Model

Low-dose Middle-dose High-dose group group group

Control

Model

Low-dose Middle- High-dose group dose group group

Figure 7.7 Effect of purple sweet potato anthocyanins (PSPAs) on the hepatic SOD and GST levels of the ALD mice. (A) Effect of PSPAs on the hepatic SOD level of the acute ALD mice. (B) Effect of PSPAs on the hepatic GST level of the acute ALD mice. (C) Effect of PSPAs on the hepatic SOD level of the subacute ALD mice. (D) Effect of PSPAs on the hepatic GST level of the subacute ALD mice. Compared with control,  P , .05,  P , .01; compared with model, #P , .05, ##P , .01.

rats (Wang et al., 2017), protection of PC-12 cells against β-amyloidinduced injury (Ye et al., 2010), and beneficial effects on the growth of human retinal pigment epithelial cells (Sun et al., 2015).

Caffeoylquinic acids Currently quality of life and healthy life expectancy are considered important worldwide, and attention has been focused on strategies for living long and healthily. In working generations being overweight causes lifestyle diseases and increases medical expenses, and so health management with daily healthy meals is recommended. Sweet potato tops have a high content of polyphenolics that consist of CA and CQAs as described previously. This content is very high compared with that found in other

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Hepatic ADH level (U mg/g protein)

(A) 50

40

30 ** 20

* **##

**

10

0 Control

Model

Low-dose Middle-dose High-dose group group group

Hepatic ADH level (U mg/g protein)

(B) 45 40 35 30

**##

**#

25 20

**

15

**

10 5 0 Control

Model

Low-dose Middle-dose High-dose group group group

Figure 7.8 Effect of purple sweet potato anthocyanins (PSPAs) on the hepatic ADH level of the ALD mice. (A) Effect of PSPAs on the hepatic ADH level of the acute ALD mice. (B) Effect of PSPAs on the hepatic ADH level of the subacute ALD mice. Compared with control,  P , .05,  P , .01; compared with model, #P , .05, ##P , .01.

vegetables (Ishiguro et al., 2004). In addition, the 2,2-diphenyl-1-picrylhydrazyl (DPPH) RSA and total polyphenolic content are very highly correlated, and most antioxidants contained in sweet potato tops are presumed to be CQA polyphenols (Islam et al., 2003b). Therefore the investigation of the functionality of CQAs was first considered based on its high associated antioxidant capacity. Antioxidant capacity and related effects ROS are a series of metabolic by-products involved in degenerative and pathological processes in the human body (Li et al., 2016).

Sweet potato polyphenols

195

Figure 7.9 Effect of purple sweet potato anthocyanins (PSPAs) on the histopathology of acute and subacute ALD mice. (A) The histopathology of mice in control group. (B) The histopathology of acute ALD mice in model group. (C) Low-dose PSPAs group. (D) Middle-dose PSPAs group. (E) High-dose PSPAs group. (F) The histopathology of subacute ALD mice in model group. (G) Low-dose PSPAs group. (H) Middledose PSPAs group. (I) High-dose PSPAs group.

Overproduction of ROS could disturb cellular redox balance, resulting in cell injury or apoptosis (Zhang et al., 2017), further triggering oxidative damage of tissues and organs, which accelerates the development of various diseases, such as cancer, atherosclerosis, diabetes, chronic inflammatory disease, cardiovascular disease, and Alzheimer’s disease (Babbar et al., 2011; Fiuza et al., 2004; Shoham et al., 2008; Valko et al., 2007). Although humans and other organisms have endogenous antioxidant defenses against ROS, these systems may sometimes not be sufficient to prevent the occurrence of cell damage (Rechner et al., 2002). In the lab of the Potato and Sweet Potato Food Science Innovation Team, CAAS, China, the antioxidant activity and inhibition of intracellular ROS of the total and individual phenolic compounds from Yuzi No. 7 sweet potato leaves were investigated. Results showed that sweet potato leaf polyphenols possessed significantly higher antioxidant activity than ascorbic acid,

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tea polyphenols, and grape seed polyphenols (Table 7.4). Among the individual phenolic compounds CA showed the highest antioxidant activity, followed by mono-CQAs and di-CQAs, while 3,4,5-triCQA showed the lowest value (Table 7.5). Sweet potato leaf polyphenols could significantly decrease the level of intracellular ROS in a dose-dependent manner (Fig. 7.10). Among the individual phenolic compounds, CA and 3-CQA showed higher value than other individual phenolic compounds (Fig. 7.11) (Sun et al., 2018). Numerous reports have indicated the antimutagenic and anticancer effects of substances with high antioxidant capacity. Additionally, the inhibitory effect of CQAs was confirmed using the Ames method (Yoshimoto et al., 2002) which is used to measure antimutagenic activity and growth suppression of cultured human cancer cells (the stomach, large intestine, and leukemia) (Kurata et al., 2007). Furthermore, administering sweet potato green extract to a nude rat cancer model forcibly induced by the transplantation of cancer cells, suppressed the proliferation of cancer cells (Karna et al., 2011). Since both antimutagenic and growth inhibitory effects against cancer cells in vitro and in vivo were observed, it is likely that carcinogenesis-related processes leading to proliferation were suppressed. However, because there are no clear epidemiological results of the effect of dietary CQA intake on cancer, only possible cancer suppression has been shown. Therefore determining the further functionality of CQAs has mainly focused on those contained in sweet potato tops, which would be effective following consumption. Lifestyle-related disease prevention effect Lifestyle-related diseases include diabetes, hypertension, dyslipidemia, and hyperuricemia. In addition, obesity in association with one or more of these conditions is called metabolic syndrome. Dietary therapy has been the basis for the amelioration of obesity, and polyphenols also contribute to improving lifestyle diseases and metabolic syndrome. Therefore research on the effect of CQAs on lifestyle-related diseases targeted at improving these conditions by the dietary intake of sweet potato stems and leaves have been conducted in recent years. The white-skinned sweet potato appears to have been used in folk remedies for its diabetes improving effect (Kusano and Abe, 2000; Ludvik et al., 2004). Insulin secretion was also confirmed to be promoted in rat pancreatic cells (RIN-5F) treated with sweet potato foliage extract containing CQAs (Yoshimoto et al., 2006). Furthermore, STZ-induced

Table 7.4 Antioxidant activity of Yuzi No. 7 sweet potato leaf polyphenols, tea polyphenols, and grape seed polyphenols. Samples

Sample concentration (μg/mL) 5 UO2 2

SPLP TPP GPP

10

20

scavenging activity (μg ACE/mL)

14.57 6 0.31a 3.60 6 0.28b 3.02 6 0.11c

30.56 6 2.59a 7.29 6 0.31b 3.18 6 0.42c

5

10

20

Oxygen radical absorbance capacity (μg TE/mL)

62.71 6 2.99a 10.62 6 0.45b 6.73 6 0.12c

22.35 6 1.59a 16.67 6 2.98b 13.75 6 0.62b

SPLP, total polyphenols from sweet potato leaves; TPP, total polyphenols from tea; GPP total polyphenols from grape seeds. a
c Data in the same column that are significantly different are represented by different letter (P , .05).

33.72 6 2.61a 32.23 6 1.22a 29.21 6 1.68b

55.68 6 1.45a 43.53 6 0.59b 43.54 6 0.77b

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Sweet Potato

Table 7.5 Antioxidant activity of individual phenolic compounds from sweet potato leaves. Samples

UO2 2 scavenging activity (μg ACE/mL)

Oxygen radical absorbance capacity (μg TE/mL)

SPLP CA 3-CQA 4-CQA 5-CQA 3,4-diCQA 3,5-diCQA 4,5-diCQA 3,4,5-triCQA

30.56 6 2.59b 51.12 6 5.35a 22.97 6 2.81c 19.36 6 1.45c 20.12 6 2.79c 20.68 6 1.55c 21.69 6 1.42c 22.14 6 2.15c 15.03 6 1.12d

33.72 6 2.61c 56.78 6 4.12a 41.23 6 1.06b 39.15 6 1.58bc 42.58 6 3.66b 39.91 6 8.37bc 35.21 6 2.11bc 42.16 6 3.89b 32.21 6 1.62c



The concentration of all tested samples was 10 μg/mL. SPLP, total polyphenols from sweet potato leaves; CA, caffeic acid; 3-CQA, 3-O-caffeoylquinic acid; 4-CQA, 4-O-caffeoylquinic acid; 5-CQA, 5-O-caffeoylquinic acid; 3,4-diCQA, 3,4-di-Ocaffeoylquinic 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. a
d Data in the same column that are significantly different are represented by different letter (P , .05).

insulin-deficient diabetic rats fed foliar powder exhibited reduced blood glucose levels (Yoshimoto et al., 2006). Moreover, α-glucosidase and aldose reductase inhibitors used as oral antidiabetic agents were found in CQAs (Matsui et al., 2004; Kurata et al., 2011). It is interesting that both enzymes show strong inhibitory effects in the following order of increasing magnitude: mono , di , tri caffeoyl groups. In addition, the leaf and stem extracts of sweet potatoes containing CQAs have been reported to lower the blood glucose level of type 2 diabetes model AA-Ky mice after 4 weeks administration (Nagamine et al., 2014). The suppressive mechanisms of CQAs involve promoting the secretion of glucagon-like peptide-1 (GLP-1), which could be a type 2 diabetes remedy. As described previously, CQAs in sweet potato tops have been reported to have various effects in improving diabetes. Currently, reports of the effects on humans are pending. In addition, the inhibitory effect of CQAs on the angiotensin I-converting enzyme (ACE) and the subsequent amelioration of a hypertensive animal model were observed following the administration of foliar powder (Ishiguro et al., 2007b). Studies of obese rats administered foliage powder in combination with a high-fat diet revealed the reduction of body fat and adipose tissue, as well as serum triglyceride, total cholesterol, and liver total cholesterol levels. This result shows that sweet potato leaf powder improved obesity and dyslipidemia (Kurata et al, 2017). As a

Sweet potato polyphenols

199

Figure 7.10 Protective effect of total polyphenols from sweet potato leaves on human hepatocytes LO2 oxidative stress. (A) The effect of total polyphenols from sweet potato leaves on the cell viability of oxidative stress LO2 cells. (B) The effect of total polyphenols from sweet potato leaves on the level of intracellular reactive oxygen species (ROS). Note: Control was LO2 cells without H2O2 and antioxidants treatment; H2O2 control was LO2 oxidative stress model group which was treated by H2O2 of 100 μM; Trolox, ascorbic acid, TPP, and GPP were LO2 cells pretreated by 100 μg/mL Trolox, ascorbic acid, tea polyphenols, and grape seed polyphenols, respectively, and then treated by 100 μM H2O2; SPLP1
SPLP6 were LO2 cells pretreated by sweet potato leaf polyphenols of 25, 50, 100, 200, 400, and 800 μg/mL, respectively, and then treated by 100 μM H2O2; dotted line represents the values of blank control group, while solid line represents the values of LO2 oxidative stress model group. Values are means 6 SD of five determinations. Different letters above different bars mean the cell viability or the level of intracellular reactive oxygen species (ROS) are significantly different (P , .05).

200

Sweet Potato

Figure 7.11 Protective effect of individual phenolic compounds from sweet potato leaves on human hepatocytes LO2 oxidative stress. (A) The effect of individual phenolic compounds from sweet potato leaves on the cell viability of oxidative stress LO2 cells. (B) The effect of individual phenolic compounds from sweet potato leaves on the level of intracellular reactive oxygen species (ROS). Note: Control was LO2 cells without H2O2 and antioxidants treatment; H2O2 control was LO2 oxidative stress model group which was treated by H2O2 of 100 μM; SPLP, CA, 3-CQA, 4-CQA, 5-CQA, 3,4-diCQA, 3,5-diCQA, 4,5-diCQA, and 3,4,5-triCQA were LO2 cells pretreated by 100 μg/mL sweet potato leaf polyphenols, caffeic acid, 3-Ocaffeoylquinic acid, 4-O-caffeoylquinic acid, 5-O-caffeoylquinic acid, 3,4-di-O-caffeoylquinic acid, 3,5-di-O-caffeoylquinic acid, 4,5-di-O-caffeoylquinic acid, 3,4,5-tri-O-caffeoylquinic acid, respectively, and then treated by 100 μM H2O2; dotted line represents the values of blank control group, while solid line represents the values of LO2 oxidative stress model group. Values are means 6 SD of five determinations. Different letters above different bars mean the cell viability or the level of intracellular reactive oxygen species (ROS) are significantly different (P , .05).

Sweet potato polyphenols

201

related function, sweet potato foliage containing CQAs was reported to inhibit the oxidation of low-density lipoprotein (LDL), which causes arteriosclerosis (Nagai et al., 2011; Taira et al., 2013). Other functionality Several other effects that could be expected to be prevented by daily dietary intake of functional foods have also been reported. The antiosteoporotic effect of CQAs was revealed by the inhibition of osteoclast formation and suppression of bone destruction in adjuvant-induced arthritic rats (Tang et al., 2006). Moreover, anti-Alzheimer's disease effects were observed in human neuroblastoma clonal cells (SH-SY5Y) and senescence-accelerated-prone 8 mice, which exhibit age-related deterioration of memory and learning (Han et al., 2010; Sasaki et al., 2013). In conducting these tests, it was very interesting that CQAs promoted ATP production by SH-SY5Y cells (Miyamae et al., 2011), which is suggested to be attributed to the polyphenols. Some CQAs have been reported to possess properties such as antibacterial activity not suitable for consumption as food. In addition, only the characteristic polyphenol component 3,4,5-triCQA, which is the most functionally effective component in the leaves, has been reported to have anti-human immunodeficiency virus (HIV) effects (Tamura et al., 2006). No plants containing higher 3,4,5-triCQA levels than sweet potato leaves have been found to date. Therefore possible novel functional and medicinal effects of 3,4,5-triCQA are expected to be discovered. In Japan, there is a company that manufactures the dry powder of sweet potato foliage, which has been added to the class of processed foods. Furthermore, companies that extract polyphenols from the foliage of sweet potato and sell them as food materials have recently emerged. Therefore it appears that the variety of available sweet potato
based processed foods will expand further. Functional studies of sweet potato foliage are increasing in number, and its further development and widening application are expected.

Processing and utilization Sweet potato storage root Sweet potato is often used to produce processed foods after storage. The optimal storage temperature and relative humidity for sweet potato are 13°C
16°C and 80%
85%, respectively (Woolfe, 1992). Furthermore, prolonged exposure to lower temperatures induces irreversible

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deterioration, and the product is more easily infected by nonpathogenic fungi (Uritani, 1999). Because sweet potato is a tropical root crop, it is susceptible to physiological damage during low-temperature storage. In addition, some biochemical changes, including in sugars and starch, occur in the roots during storage, especially at low temperature (Nakatani and Komeichi, 1991; Picha, 1987). However, sweet potato is stored at low temperature to enhance the sweetness before the preparation of “Hoshiimo,” which is made from sweet potato by steaming and then drying, in Japan. Ishiguro et al. (2007a) investigated changes in the polyphenolic content and RSA of four cultivars (Benimasari, Koganesengan, J-Red, and Murasakimasari) during storage at optimal and low temperatures. Storage time had a significant effect on all cultivars tested while temperature did not in the case of Koganesengan and Murasakimasari. Significant interactions were observed between storage time and temperature for Benimasari and J-Red using a two-way repeated measure analysis of variance (ANOVA). Levels of polyphenolics increased during storage except with Murasakimasari. The largest increase was observed in Benimasari after 13 days storage at 5°C, which was significantly greater than that observed at 15°C. After 37 days storage at 5°C, the polyphenolic content in Benimasari increased 3.7-fold relative to the initial sample. Lieberman et al. (1959) previously reported that 5-CQA increased more in sweet potato roots during storage at 7.5°C than at 15°C. The analysis of Benimasari showed consistent results with those of Lieberman et al., but with a cultivar-dependent pattern. Exposure to nonfreezing temperatures has been shown to stimulate an increase in unique phenolics in various fruits and vegetables (Blankenship and Richardson, 1985; Lattanzio and van Sumere, 1987; Lattanzio et al., 2001). Lattanzio et al. (1994) reported that there is a low, critical temperature below which an increase in phenylpropanoid metabolism, including that of phenylalanine ammonia lyase, is stimulated during the storage of plant tissues and this temperature varies between commodities. It has been suggested that the temperaturesensitivity of sweet potato roots at 5°C differs as a function of genotype. The RSA was found to increase during storage and was highly correlated with the polyphenolic content. Low-temperature storage may induce active oxygen species, radicals, or superoxide formation as a consequence of an imbalance in oxidative and reductive processes (Lattanzio et al., 1994). Benimasari and Murasakimasari, which have higher RSA because of inducible or preformed polyphenolics during storage, showed excellent

Sweet potato polyphenols

203

stability to low-temperature storage. This suggests that the cold resistance of sweet potato cultivars may, at least in part, be mediated by the RSA of polyphenolics. The main CQAs in all the cultivars at the beginning of storage were 5-CQA and 3,5-diCQA, which increased extensively during storage except in Murasakimasari (Fig. 7.12). The pattern of increase in 5-CQA (A)

(B) 10

10

BM

BM CA ChA

6

3,4-diCQA

4

3,5-diCQA

8

μmol/g DW

μmol/g DW

8

3,4-diCQA

4

4,5-diCQA

2

CA ChA

6

3,5-diCQA 4,5-diCQA

2 0

0 0

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0

10

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6

KS CA

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ChA 3,4-diCQA 3,5-diCQA

2

μmol/g DW

KS μmol/g DW

20

Storage (days)

Storage (days)

CA

4

ChA 3,4-diCQA 3,5-diCQA

2

4,5-diCQA

4,5-diCQA

0

0 0

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0

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Storage (days) 10

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JR

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JR

8 CA ChA

6

3,4-diCQA

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3,5-diCQA 4,5-diCQA

2

μmol/g DW

μmol/g DW

8

CA ChA

6

3,4-diCQA

4

3,5-diCQA 4,5-diCQA

2 0

0 0

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Storage (days)

30

40

16

MM

MM

14

12

CA

10

ChA

8

3,4-diCQA

6

3,5-diCQA

4

4,5-diCQA

2

μmol/g DW

14

20

Storage (days)

16

μmol/g DW

20

Storage (days)

12

CA

10

ChA

8

3,4-diCQA

6

3,5-diCQA

4

4,5-diCQA

2

0

0 0

10

20

Storage (days)

30

40

0

10

20

30

40

Storage (days)

Figure 7.12 Changes in caffeoylquinic acids during storage 15°C (A) or 5°C (B). Data are represented as means 6 SD of five different roots. BM, Benimasari; KS, Koganesengan; JR, J-Red; MM, Murasakimasari.

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and 3,5-diCQA levels differed as a function of storage temperature. The increase in 5-CQA was significantly greater with storage at 5°C than at 15°C in Benimasari and Koganesengan after 13 and 26 days of storage, respectively. Whereas, the 3,5-diCQA content was greater at 15°C than it was at 5°C in Koganesengan and J-Red after 13 days of storage. Kojima and Kondo (1985) reported that 5-CQA was enzymatically converted to 3,5-diCQA by esterification of CA with 5-CQA. Lower enzyme activity could explain the lower amounts of 3,5-diCQA generated at 5°C. More 3,5-diCQA might be produced from 5-CQA by the converting enzyme with higher activity at 15°C. The CQA content in Murasakimasari, except for CA, was higher than in other cultivars and the level and composition remained nearly constant. A component other than the five aforementioned CQAs was identified during storage (Fig. 7.13A) and was purified using successive chromatographic steps. This structure was identified as gluco-6-O-caffeoyl sucrose (CSu, 6-O-caffeoyl-[β-D-fructofuranosyl-{2-1}]-α-D-glucopyranoside)

Figure 7.13 HPLC of polyphenolics in “J-Red” sweet potato roots stored for 37 days at 15°C (A) and the structure of caffeoyl sucrose (CSu) (B).

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based on nuclear magnetic resonance, infrared, and fast atom bombardment mass spectrometry (Fig. 7.13B) analyses. Generally, the CSu content significantly increased during storage, and the increase was greater at 15°C than it was at 5°C. This result is consistent with the higher levels of CSu found in sweet potato stored over the long term than in fresh sweet potato (Takenaka et al., 2006). The change was the greatest in J-Red stored at 15°C and after 37 days of storage, where the content increased to 0.646 μmol/g DW. CA might react with the sucrose generated during storage at the optimal storage temperature of 15°C. A variety of plant polyphenolics are currently the focus of considerable scientific attention because of their perceived beneficial pharmacological effects. Postharvest storage at a controlled temperature and storage duration is one possible approach for increasing the functional value of sweet potato roots. The sweet potato storage root is consumed after processing and cooking. Takenaka et al. (2006) monitored phenolic compounds in sweet potato during several model cooking and processing treatments. When blocks prepared from fresh sweet potatoes were boiled in water, CQAs, especially 5-CQA and 3,5-diCQA, decreased considerably. A part of these compounds was eluted into the boiling water as the boiling time increased. Furthermore, 3-CQA, 4-CQA, and 4,5-diCQA levels increased slightly as the boiling time increased, but the CA content hardly changed. Since their total amounts in boiling water and boiled sweet potato decreased, 5-CQA and 3,5-diCQA appeared to decompose during heating in boiling water. This decomposition was possibly mediated by the enzyme polyphenol oxidase (PPO) because purified 5-CQA and 3,5diCQA are stable under heating conditions. This possibility was supported by the observation that the decomposition was prevented in the presence of ascorbic acid, which is known to inhibit PPO. The findings are summarized as follows: when sweet potato is cooked whole or in larger pieces, heat is gradually transmitted from the surface to the interior, and PPO reacts with the phenolic compounds. The enzyme reaction progresses gradually, and the phenolic compounds are affected until the enzyme is deactivated at 60°C
80°C.

Sweet potato tops Sweet potato tops (mainly the leaves and petioles) are used as green leafy vegetables in tropical and subtropical regions such as Asia and Africa. In

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Japan they are cultivated and used in some areas such as Okinawa. Since it has become clear that sweet potato leaves contain abundant nutrients such as vitamins and minerals, as well as functional ingredients such as polyphenols, the use of the tops has been examined, and some studies on the use of sweet potato will be introduced. Effect of cooking on polyphenols As mentioned previously, sweet potato tops are a good source of polyphenols, and they are usually cooked before consumption similar to other green vegetables. Cooking causes a number of changes in the physical characteristics and chemical composition of polyphenols. Therefore the effects of cooking methods on polyphenol concentration in sweet potato tops have been investigated. It is also important to estimate the appropriate amount of an ingredient that can be safely consumed. In the lab of the Potato and Sweet Potato Food Science Innovation Team, CAAS, China, the effects of boiling, steaming, microwaving, baking, and frying on proximate composition, total and individual polyphenol content, and antioxidant activity of sweet potato leaves (cultivar: Pushu 53) were investigated (Sun et al., 2014c). An increase of 9.44% in total polyphenol content was observed after steaming, whereas decreases of 30.51%, 25.70%, and 15.73% were noticed after boiling, microwaving, and frying, respectively (Fig. 7.14A). Decrease of 63.82% and 32.35% in antioxidant activity were observed after boiling and microwaving, respectively, whereas increases of 81.40%, 30.09%, and 85.82% in antioxidant

Figure 7.14 Effect of different domestic cooking methods on total polyphenol content (A) and antioxidant activity (B) of sweet potato leaves. Values are means 6 SD of three determinations. Cooking methods that were not significantly different were represented by same letter (P..05).

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activity were observed after steaming, baking, and frying, respectively (Fig. 7.14B). The phenolic compounds in the sweet potato leaf extracts cooked by different methods were determined by the HPLC method. Eight phenolic compounds, 5-CQA, 3-CQA, 4-CQA, CA, 4,5-diCQA, 3,5-diCQA, 3,4-diCQA, and 3,4,5-triCQA, were identified (Fig. 7.15). In a comparison of the amounts of the eight phenolic compounds present in raw sweet potato leaves, the order was 4,5-diCQA. 3,5-diCQA. 3,4-diCQA. 5

5 6

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Figure 7.15 Chromatograms of phenolic standards and sweet potato leaf extracts at 326 nm in HPLC.

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4-CQA. 3,4,5-triCQA. CA. 3-CQA. 5-CQA (Table 7.6) (Sun et al., 2014c). For 5-CQA, boiling, steaming, baking, and frying decreased its content significantly, while microwaving did not cause significant change. For 3-CQA, boiling, steaming, microwaving, and frying induced significant loss, while baking did not change its content significantly. 4-CQA was the most abundant mono-CQA in sweet potato leaves, and among the used cooking methods, only boiling decreased its content significantly. For CA, baking increased its content significantly, while the other cooking methods caused significant loss. 4,5-diCQA was the most abundant di-caffeoylquinic acid in sweet potato leaves, and only boiling induced significant loss of 4,5-diCQA. For 3,5-diCQA, only boiling decreased its content significantly. For 3,4-diCQA, steaming and frying increased its content significantly, while boiling decreased its content significantly. For 3,4,5-triCQA, steaming, baking, and frying induced significant increase, while there were no significant differences between raw and other treated sweet potato leaves (Table 7.6). The abovementioned results indicate that most of the individual phenolic compounds measured by HPLC in sweet potato leaves show the same general trend on their content, that is, a decrease after boiling and a better retention after other cooking methods. In particular, there is a common point for steamed and fried sweet potato leaves that the contents of both 3,4-diCQA and 3,4,5-triCQA increased significantly compared with raw samples, which is in accordance with the trend of antioxidant activity. This result suggests that steaming would be a preferred method for maintaining the polyphenols and antioxidant activity of sweet potato leaves (Sun et al., 2014c). Sugawara et al. (2011) examined the content of functional components such as lutein, total CA and CQA derivatives, and total polyphenols in cooked leaves of the Suioh cultivar (Sugawara et al., 2011). The leaf blades of Suioh were cooked by steaming, simmering, boiling, and stirfrying. The total CA and CQA derivative content was 244.7
428.2 mg/ 100 g in cooked products. The total polyphenol content of the leaf blades remained at a high value when steamed or simmered. Since the content of polyphenols in the leaves does not decrease significantly in usual cooking, it was evaluated that a considerable amount can be ingested from the cooked products. Sasaki et al. (2015a) also examined individual components such as CA and seven kinds of CQA derivatives in Suioh leaves before and after boiling treatment in detail. The result showed that the Suioh leaf’s contents

Table 7.6 Contents of individual phenolic compounds in sweet potato leaves treated by different domestic cooking methods (mg/g of DW). Peak No.

1 2 3 4 5 6 7 8

Identity

Cooking methods Raw

5-O-caffeoylquinic acid 3-O-caffeoylquinic acid 4-O-caffeoylquinic acid Caffeic acid 4,5-Di-O-caffeoylquinic acid 3,5-Di-O-caffeoylquinic acid 3,4-Di-O-caffeoylquinic acid 3,4,5-Tri-Ocaffeoylquinic acid

Boiling

Steaming

Microwave

Baking

Frying

2.58 6 0.16 3.06 6 0.31a 13.55 6 1.36ab 4.62 6 0.81b 27.23 6 2.28ab

1.55 6 0.11 1.16 6 0.35c 11.20 6 0.00c 0.62 6 0.00c 19.60 6 0.13c

1.93 6 0.16 1.63 6 0.25b 14.67 6 0.33a 0.75 6 0.04c 29.84 6 2.88a

2.63 6 0.22 1.14 6 0.12c 13.17 6 0.45b 0.74 6 0.02c 24.28 6 1.47b

2.25 6 0.05 3.06 6 0.06a 13.53 6 0.52b 5.80 6 0.59a 30.10 6 3.81a

1.76 6 0.00cd 1.32 6 0.06bc 12.86 6 0.19b 0.75 6 0.02c 30.04 6 0.54a

25.02 6 1.26ab

18.09 6 0.07c

26.96 6 2.53a

23.14 6 1.09b

26.01 6 2.80ab

25.90 6 0.38ab

14.18 6 0.44b

12.90 6 0.04c

17.71 6 1.16a

14.91 6 0.60b

14.38 6 1.04b

18.32 6 0.08a

10.68 6 0.00c

10.84 6 0.00bc

11.08 6 0.17a

10.81 6 0.03c

11.07 6 0.28ab

11.11 6 0.10a

a

d

c

a

b

Note: Values are means 6 SD of three determinations. Cooking methods that were not significantly different are represented by same letter (P..05). a-dValues within same lines with different letters are significantly different (p , 0.05).

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of CA, 3-CQA, 5-CQA, 3,5-diCQA, and 3,4,5-triCQA were significantly decreased, which was inferred to be attributable to the formation of structural isomers of mono-CQAs or di-CQAs, in addition to the outflow of CA and CQAs in the leaves. These results indicate that sweet potato leaves cooked using common household methods possibly produce products with a certain level of consumable polyphenols. Furthermore, the results showed that steaming rather than boiling would minimize the loss of polyphenols. In addition, it has been reported that polyphenols are generally sensitive to adverse environmental conditions, including unfavorable temperatures, light, pH, etc., and are therefore susceptible to degradative reactions during product processing and storage (Fang and Bhandari, 2011). In order to preliminarily clear the stability of sweet potato leaf polyphenols, and provide a theoretical basis for the application of sweet potato leaf polyphenols in food, medicine, and other fields, the group members in the lab of the Potato and Sweet Potato Food Science Innovation Team, CAAS, China investigated the effects of different pH values (3.0, 5.0, 7.0, and 8.0), temperatures (55°C, 65°C, 80°C, and 100°C), and light treatments on the total polyphenol content and antioxidant activity of polyphenols extracted from the leaves of two sweet potato cultivars, Jishu No. 04150 and Shangshu No. 19 (Sun et al., 2017). Results showed that sweet potato leaf polyphenols have higher stability in neutral and weakly acidic solutions (pH 5
7). There is no significant effect of light and lowtemperature heat treatment (50°C and 65°C) on total polyphenol content and antioxidant activity. High temperature heat treatment (80°C and 100°C) will cause the significant decrease of the antioxidant activity of sweet potato leaf polyphenols (Figs. 7.16
7.18). Application to food processing Oki et al. (2002a) reported the RSA and polyphenol contents of the hot water extraction of leaves of the sweet potato cultivar Simon-1 to investigate the use of the leaves as an ingredient for a tea-like beverage. The activity of hot water extraction on the freeze-dried powder at various temperatures (60°C, 80°C, and 100°C) was measured. The results indicated that higher extraction temperatures induced higher RSA in the extract. Furthermore, the hot water extract was confirmed to contain polyphenols such as CA and 5-CQA. Ishida et al. (2003) investigated the palatability and preservability of cookies containing sweet potato leaf powder by preparing samples in

Sweet potato polyphenols

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Shangshu No. 19

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Jishu No. 04150

Shangshu No. 19

Antioxidant activity (mgTE/mL)

3.5

a 3 2.5

bc

b c

2 1.5

a

a ab b

1

3

5

7

8

pH values

Figure 7.16 Effect of different pH value on the total polyphenol content (A) and antioxidant activity (B) of sweet potato leaf polyphenols from Jishu No. 04150 and Shangshu No. 19. Values are means 6 SD of three determinations. Data on the same broken line that are not significantly different are represented by the same letter (P..05).

which 2% of the wheat flour was replaced with sweet potato leaf powder. The results showed that the sweet potato-containing cookies were more palatable and preferred to cookies prepared without the sweet potato, and had a similar texture (Ishida et al. 2003). Although some green discoloration was observed during storage, there was no significant decrease in quality. Suzuno et al. (2004) also examined the effects of adding sweet potato leaf powder to bread. Compared to the unsupplemented bread samples, the addition of sweet potato did not affect the specific volume, moisture content, and degree of gelatinization of the starch and bread hardness.

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Figure 7.17 Effect of different heat treatment on the retention rates (%) of total polyphenol content and antioxidant activity of sweet potato leaf polyphenols from Jishu No. 04150 at 50°C (A), 65°C (B), 80°C (C), and 100°C (D), and that of Shangshu No. 19 at 50°C (E), 65°C (F), 80°C (G), and 100°C (H). Values are means 6 SD of three determinations. Data on the same broken line that are not significantly different are represented by the same letter (P..05).

Figure 7.18 Effect of different light treatment on the retention rates (%) of total polyphenol content of sweet potato leaf polyphenols from Jishu No. 04150 (A) and Shangshu No. 19 (B), and effect of different light treatment on the retention rates (%) of antioxidant activity of sweet potato leaf polyphenols from Jishu No. 04150 (C) and Shangshu No. 19 (D). Values are means 6 SD of three determinations. Data on the same broken line that were not significantly different are represented by same letter (P..05).

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Specifically, 100 g of bread containing 2% sweet potato leaf powder had 2.8 g dietary fiber, 49 μg carotenes, 87 mg calcium, and 11 mg polyphenols, indicating that the bread functionality was improved. Dishes and products using new cultivar Suioh The new sweet potato cultivar Suioh was developed for the use of its tops by NARO, Japan, and its tops and leaves were shown to have higher total polyphenols content and DPPH RSA than those of other leafy vegetables (Ishiguro et al., 2004). The leaves are tasty and rich in nutrients (e.g., vitamins and minerals), and studies on cooking and processing methods were conducted to investigate possible additional uses. Because little is known about methods for preparing Suioh for dietary consumption, several recipes have been developed for home cooking. These sweet potato leaf recipes include “Ohitashi” (boiled greens seasoned with soy sauce), “Tempura” (deep fried after coated with batter mix), and stir-fried with garlic, as well as noodles, soups, and confections. In addition, processed items such as dietary supplements have been developed so that consumers can use the product more easily. Some Suioh products are commercially available in Japan, such as a green juice (called “Aojiru” in Japan) used as a nutritional food. It is commonly used as a dried powder. Kale and young barley leaves are mainly used as raw materials, and Suioh is used in some products. In addition to preparing this product as a beverage consumed after dissolving it in water, there are various other methods such as dissolving the powder in milk or soup or mixing it into yogurt, cookies, and pancake. The production process of green juice powder consists of washing, cutting, drying, and coarsely crushing, followed by finely grinding the material to obtain the final product.

Research and development trend of sweet potato polyphenols Research on polyphenols has increased all over the world, and with its increase applications are becoming more extensive. However, there is still room for improvement in some areas, such as (1) developing new raw materials for polyphenols: polyphenols could be extracted from agricultural by-products and food processing wastewater and residues. This comprehensive utilization of waste materials is of great environmental significance; (2) in-depth studies on the separation, purification, and the structure
activity relationships of polyphenols: polyphenols are composed

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of a variety of components. At present, the separation and purification processes of individual components are limited, and the differences in activity levels among different components needs further study. Additionally, the mechanisms responsible for polyphenol activities, especially the in vivo mechanisms are still not clear, and as a result there is no reliable theoretical support for the utilization of polyphenol activities; and (3) expanding the application range, and improving the application depth: polyphenols have a variety of biological activities. At present the applications of their activities in food, medicine, and daily chemical fields are mainly focused on free radical scavenging, antisepsis, and antiinflammation. The development and application of activities, such as anticancer, cardiovascular protection, and diabetes prevention, need to be expanded. Thus there is still a lot of work to be done in the study of polyphenols.

References Babbar, N., Oberoi, H.S., Uppal, D.S., Patil, R.T., 2011. Total phenolic content and antioxidant capacity of extracts obtained from six important fruit residues. Food Res. Int. 44, 391
396. Blankenship, S.M., Richardson, D.G., 1985. Changes in phenolic acids and internal ethylene during long term cold storage of pear. J. Am. Soc. Hortic. Sci. 110, 336
339. Fang, Z.X., Bhandari, B., 2011. Effect of spray drying and storage on the stability of bayberry polyphenols. Food Chem. 129, 1139
1147. Fiuza, S.M., Gomes, C., Teixeira, L.J., Girão, M.T., Cordeiro, M.N., Milhazes, N., et al., 2004. Phenolic acid derivatives with potential anticancer properties—a structure
activity relationship study. Part 1: methyl, propyl and octyl esters of caffeic and gallic acids. Bioorgan. Med. Chem. 12, 3581
3589. Furuta, S., Suda, I., Nishiba, Y., Yamakawa, O., 1998. High tert-butylperoxyl radical scavenging activities of sweet potato cultivars with purple flesh. Food Sci. Technol. Int. Tokyo 4, 33
35. Goda, Y., Shimizu, T., Kato, Y., Nakamura, M., Maitani, T., Yamada, T., et al., 1997. Two acylated anthocyanins from purple sweet potato. Phytochemistry 44, 183
186. Han, J., Miyake, Y., Shigemori, H., Isoda, H., 2010. Neuroprotective effect of 3,5-di-Ocaffeoylquinic acid on SH-SY5Y cells and senescence-accelerated-prone mice 8 through the up-regulation of phosphoglycerate kinase-1. Neuroscience 169, 1039
1045. Harada, K., Kano, M., Takayanagi, T., Yamakawa, O., Ishikawa, F., 2004. Absorption of acylated anthocyanins in rats and humans after ingesting an extract of Ipomoea batatas purple sweet potato tuber. Biosci. Biotechnol. Biochem. 68, 1500
1507. Hayase, F., Kato, H., 1984. Antioxidative components of sweet potatoes. J. Nutr. Sci. Vitaminol. 30, 37
46. Hayashi, K., Ohara, N., Tsukui, A., 1996. Stability of anthocyanins in various vegetables and fruits. Food Sci. Technol. Int. Tokyo 2, 30
33. Hwang, Y.P., Choi, J.H., Han, E.H., Kim, H.G., Wee, J.-H., Jung, K.O., et al., 2011a. Purple sweet potato anthocyanins attenuate hepatic lipid accumulation through activating adenosine monophosphate-activated protein kinase in human HepG2 cells and obese mice. Nutr. Res. 31, 896
906.

216

Sweet Potato

Hwang, Y.P., Choi, J.H., Yun, H.J., Han, E.H., Kim, H.G., Kim, J.Y., et al., 2011b. Anthocyanins from purple sweet potato attenuate dimethylnitrosamine-induced liver injury in rats by inducing Nrf2-mediated antioxidant enzymes and reducing COX-2 and iNOS expression. Food Chem. Toxicol. 49, 93
99. Ishida, H., Suzuno, H., Innami, S., Maekawa, A., Tadokoro, T., 2003. Quality, preference characteristic and preservation stability of cookies added with sweet potato leaf powder. Food Preserv. Sci. 29, 75
81. Ishiguro, K., Toyama, J., Islam, M.S., Yoshimoto, M., Kumagai, T., Kai, Y., et al., 2004. Suioh, a new sweet potato cultivar for utilization in vegetable greens. Acta Hortic. 637, 339
345. Ishiguro, K., Yahara, S., Yoshimoto, M., 2007a. Changes in polyphenolic content and radical-scavenging activity of sweet potato (Ipomoea batatas L.) during storage at optimal and low temperature. J. Agric. Food Chem. 55, 10773
10778. Ishiguro, K., Yoshimoto, M., Tsubata, M., Takagaki, K., 2007b. Hypertensive effect of sweet potato tops. Nippon Shokuhin Kagaku Kogaku Kaishi 54, 45
49. Ishiguro, K., Yoshinaga, M., Kai, Y., Maoka, T., Yoshimoto, M., 2010. Composition, content and antioxidative activity of the carotenoids in yellow-fleshed sweet potato (Ipomoea batatas L.). Breed Sci. 60, 324
329. Islam, M.S., Yoshimoto, M., Terahara, N., Yamakawa, O., 2002a. Anthocyanin compositions in sweet potato (Ipomoea batatas L.) leaves. Biosci. Biotechnol. Biochem. 66, 2483
2486. Islam, M.S., Yoshimoto, M., Yahara, S., Okuno, S., Ishiguro, K., Yamakawa, O., 2002b. Identification and characterization of foliar polyphenolic composition in sweet potato (Ipomoea batatas L.) genotypes. J. Agric. Food Chem. 50, 3718
3722. Islam, M.S., Yoshimoto, M., Ishiguro, K., Okuno, S., Yamakawa, O., 2003a. Effect of artificial shading and temperature on radical scavenging activity and polyphenolic composition in sweet potato (Ipomoea batatas L.) leaves. J. Am. Soc. Hortic. Sci. 128, 182
187. Islam, M.S., Yoshimoto, M., Yamakawa, O., 2003b. Distribution and physiological functions of caffeoylquinic acid derivatives in leaves of sweet potato genotypes. J. Food Sci. 68, 111
116. Kano, M., Takayanagi, T., Harada, K., Makino, K., Ishikawa, F., 2005. Antioxidative activity of anthocyanins from purple sweet potato, Ipomoera batatas cultivar Ayamurasaki. Biosci. Biotechnol. Biochem. 69, 979
988. Karna, P., Gundala, S.R., Guptal, M.V., Shamsi, S.A., Pace, R.D., Yates, C., et al., 2011. Polyphenol-rich sweet potato greens extract inhibits proliferation and induces apoptosis in prostate cancer cells in vitro and in vivo. Carcinogenesis 32, 1872
1880. Kim, H.W., Kim, J.B., Cho, S.M., Chung, M.N., Lee, Y.M., Chu, S.M., et al., 2012. Anthocyanin changes in the Korean purple-fleshed sweet potato, Shinzami, as affected by steaming and baking. Food Chem. 130, 966
972. Kobayashi, M., Oki, T., Masuda, M., Nagai, S., Fukui, K., Matsugano, K., et al., 2005. Hypotensive effect of anthocyanin-rich extract from purple-fleshed sweet potato cultivar “Ayamurasaki” in spontaneously hypertensive rats. Nippon Shokuhin Kagaku Kogaku Kaishi 52, 41
44. Kojima, M., Kondo, T., 1985. An enzyme in sweet potato root which catalyzes the conversion of chlorogenic acid, 3-caffeoylquinic acid, to isochlorogenic acid, 3,5-dicaffeoylquinic acid. Agric. Biol. Chem. 49, 2467
2469. Konczak-Islam, I., Yoshimoto, M., Hou, D.X., Terahara, N., Yamakawa, O., 2003. Potential chemopreventive properties of anthocyanin-rich aqueous extracts from in vitro produced tissue of sweet potato (Ipomoea batatas L.). J. Agric. Food Chem. 51, 5916
5922.

Sweet potato polyphenols

217

Kurata, R., Adachi, M., Yamakawa, O., Yoshimoto, M., 2007. Growth suppression of human cancer cells by polyphenolics from sweet potato (Ipomoea batatas L.) leaves. J. Agric. Food Chem. 55, 185
190. Kurata, R., Yahara, S., Yamakawa, O., Yoshimoto, M., 2011. Simple high-yield purification of 3,4,5-tri-O-caffeoylquinic acid from sweet potato (Ipomoea batatas L.) leaf and its inhibitory effects on aldose reductase. Food Sci. Technol. Res. 17, 87
92. Kurata, R., Kobayashi, T., Ishii, T., Niimi, H., Niisaka, S., Kubo, M., et al., 2017. Influence of sweet potato (Ipomoea batatas L.) leaf consumption on rat lipid metabolism. Food Sci. Technol. Res. 23, 57
62. Kusano, S., Abe, H., 2000. Antidiabetic activity of white skinned sweet potato (Ipomoea batatas L.) in obese zucker fatty rats. Biol. Pharm. Bull. 23, 23
26. Lattanzio, V., van Sumere, C.F., 1987. Changes in phenolic compounds during the development and cold storage of artichoke (Cynara scolymus L.) heads. Food Chem. 24, 37
50. Lattanzio, V., Cardinali, A., Palmieri, S., 1994. The role of phenolics in the postharvest physiology of fruits and vegetables: browning reactions and fungal diseases. Ital. J. Food Sci. 1, 3
22. Lattanzio, V., Di Venere, D., Linsalata, V., Bertolini, P., Ippolito, A., Salerno, M., 2001. Low temperature metabolism of apple phenolics and quiescence of Phlyctaena vagabunda. J. Agric. Food Chem. 49, 5817
5821. Lee, M.J., Park, J.S., Choi, D.S., Jung, M.Y., 2013. Characterization and quantitation of anthocyanins in purple-fleshed sweet potatoes cultivated in Korea by HPLC-DAD and HPLC-ESI-QTOF-MS/MS. J. Agric. Food Chem. 61, 3148
3158. Li, F., Zhang, X., Zheng, S., Lu, K., Zhao, G., Ming, J., 2016. The composition, antioxidant and antiproliferative capacities of phenolic compounds extracted from tartary buckwheat bran [Fagopyrumtartaricum (L.) Gaerth]. J. Funct. Foods 22, 145
155. Lieberman, M., Craft, C., Wilcox, M.S., 1959. Effect of chilling on the chlorogenic acid and ascorbic acid content of Port Rico sweet potatoes. J. Am. Soc. Hortic. Sci. 74, 642
648. Liu, X., Mu, T., Sun, H., Zhang, M., Chen, J., 2013. Optimisation of aqueous two-phase extraction of anthocyanins from purple sweet potatoes by response surface methodology. Food Chem. 141, 3034
3041. Lu, J., Wu, D., Zheng, Y., Hu, B., Cheng, W., Zhang, Z., 2012. Purple sweet potato color attenuates domoic acid-induced cognitive deficits by promoting estrogen receptor-α-mediated mitochondrial biogenesis signaling in mice. Free Radic. Biol. Med. 52, 646
659. Ludvik, B., Neuffer, B., Pacini, G., 2004. Efficacy of Ipomoea batatas (Caiapo) on diabetes control in type 2 diabetic subjects treated with diet. Diabetes Care 27, 436
440. Marques, V., Farah, A., 2009. Chlorogenic acids and related compounds in medicinal plants and infusions. Food Chem. 113, 1370
1376. Matsui, T., Ueda, T., Oki, T., Sugita, K., Terahara, N., Matsumoto, K., 2001a. α-Glucosidase inhibitory action of natural acylated anthocyanins. 1. Survey of natural pigments with potent inhibitory activity. J. Agric. Food Chem. 49, 1948
1951. Matsui, T., Ueda, T., Oki, T., Sugita, K., Terahara, N., Matsumoto, K., 2001b. α-Glucosidase inhibitory action of natural acylated anthocyanins. 2. α-Glucosidase inhibition by isolated acylated anthocyanins. J. Agric. Food Chem. 49, 1952
1956. Matsui, T., Ebuchi, S., Kobayashi, M., Fukui, K., Sugita, K., Terahara, N., et al., 2002. Anti-hyperglycemic effect of diacylated anthocyanin derived from Ipomoea batatas cultivar Ayamurasaki can be achieved through the α-glucosidase inhibitory action. J. Agric. Food Chem. 50, 7244
7248. Matsui, T., Ebuchi, S., Fujise, T., Abesundara, K.J.M., Doi, S., Yamada, H., et al., 2004. Strong antihyperglycemic effects of water-soluble fraction of Brazilian propolis and its

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bioactive constituent, 3,4,5-tri-O-caffeoylquinic acid. Biol. Pharm. Bull. 27, 1797
1803. Michael, N.C., Weiguo, W., Nikolai, K., 2006. The chlorogenic acids of Hemerocallis. Food Chem. 95, 574
578. Miyamae, Y., Kurisu, M., Han, J., Isoda, H., Shigemori, H., 2011. Structure
activity relationship of caffeoylquinic acids on the accelerating activity on ATP production. Chem. Pharm. Bull. 59, 502
507. Miyazaki, K., Makino, K., Iwadate, E., Deguchi, Y., Ishikawa, F., 2008. Anthocyanins from purple sweet potato Ipomoea batatas cultivar Ayamurasaki suppress the development of atherosclerotic lesions and both enhancements of oxidative stress and soluble vascular cell adhesion molecule-1 in apolipoprotein E-deficient mice. J. Agric. Food Chem. 56, 11485
11492. Mu, T.H., Sun, H.N., Zhang, M., Wang, C., 2017. Sweet Potato Processing Technology, first ed Elsevier, London, pp. 279
400. Nagai, M., Tani, M., Kishimoto, Y., Iizuka, M., Saita, E., Toyozaki, M., et al., 2011. Sweet potato (Ipomoea batatas L.) leaves suppressed oxidation of low density lipoprotein (LDL) in vitro and in human subjects. J. Clin. Biochem. Nutr. 48, 203
208. Nagamine, R., Ueno, S., Tsubata, M., Yamaguchi, K., Takagaki, K., Hira, T., et al., 2014. Dietary sweet potato (Ipomoea batatas L.) leaf extract attenuates hyperglycaemia by enhancing the secretion of glucagon-like peptide-1 (GLP-1). Food Funct. 5, 2309
2316. Nakatani, M., Komeichi, M., 1991. Effect of low temperature treatment during storage on carbohydrate composition of sweet potato tubers. Newsl. Natl. Agric. Res. Ctr 8, 13
19. Odake, K., Terahara, N., Saito, N., Toki, K., Honda, T., 1992. Chemical structures of two anthocyanins sweet potato, Ipomoea Batatas. Phytochemistry 31, 2127
2130. Oki, T., Masuda, M., Osame, M., Kobayashi, M., Furuta, S., Nishiba, Y., et al., 2002a. Radical-scavenging activity of hot water extract from leaves of sweet potato cultivar“Simon-1”. Nippon Shokuhin Kagaku Kogaku Kaishi 49, 683
687. Oki, T., Masuda, M., Furuta, S., Nishiba, Y., Terahara, N., Suda, I., 2002b. Involvement of anthocyanins and other phenolic compounds in radical-scavenging activity of purple-fleshed sweet potato cultivars. J. Food Sci. 67, 1752
1756. Oki, T., Osame, M., Masuda, M., Kobayashi, M., Furuta, S., Nishiba, Y., et al., 2003. Simple and rapid spectrophotometric method for selecting purple-fleshed sweet potato cultivars with a high radical-scavenging activity. Breed. Sci. 53, 101
107. Oki, T., Suda, I., Terahara, N., Sato, M., Hatakeyama, M., 2006. Determination of acylated anthocyanin in human urine after ingesting a purple-fleshed sweet potato beverage with various contents of anthocyanin by LC-ESI-MS/MS. Biosci. Biotechnol. Biochem. 70, 2540
2543. Oki, T., Sato, M., Yoshinaga, M., Sakai, T., Sugawara, T., Terahara, N., et al., 2009. 1,1Diphenyl-2-picrylhydrazyl radical-scavenging capacity and oxygen radical absorbance capacity of sweet potato cultivars with various flesh colors. Nippon Shokuhin Kagaku Kogaku Kaishi 56, 655
659. Oki, T., Miyoshi, A., Goto, K., Sato, M., Shiratsuchi, H., Terahara, N., et al., 2010. Determination of major anthocyanins in processed foods made from purple-fleshed sweet potato. Nippon Shokuhin Kagaku Kogaku Kaishi 57, 128
133. Oki, T., Kano, M., Watanabe, O., Goto, K., Boelsma, E., Ishikawa, F., et al., 2016. Effect of consuming a purple-fleshed sweet potato beverage on health-related biomarkers and safety parameters in Caucasian subjects with elevated levels of blood pressure and liver function biomarkers: a 4-week, open-label, non-comparative trial. Biosci. Microbiota Food Health 35, 129
136.

Sweet potato polyphenols

219

Oki, T., Sato, M., Terahara, N., 2017a. A modified method for the determination of acylated anthocyanins in purple-fleshed sweet potato (Ipomoea batatas (L).) tubers by high-performance liquid chromatography with visible absorption. Food Sci. Technol. Res. 23, 855
862. Oki, T., Kano, M., Ishikawa, F., Goto, K., Watanabe, O., Suda, I., 2017b. Double-blind, placebo-controlled pilot trial of anthocyanin-rich purple sweet potato beverage on serum hepatic biomarker levels in healthy Caucasians with borderline hepatitis. Eur. J. Clin. Nutr. 71, 290
292. Okuno, S., Ishiguro, K., Yoshinaga, M., Yoshimoto, M., 2010. Analysis of six caffeic acid derivatives in sweet potato leaves by high-performance liquid chromatography using a short column. Jpn. Agric. Q. 44, 415
420. Picha, D.H., 1987. Chilling injury, respiration, and sugar changes in sweet potatoes stored at low temperature. J. Am. Soc. Hortic. Sci. 112, 497
502. Qiu, F., Luo, J., Yao, S., Ma, L., Kong, L., 2009. Preparative isolation and purification of anthocyanins from purple sweet potato by high-speed counter-current chromatography. J. Sep. Sci. 32, 2146
2151. Rechner, A.R., Kuhnle, P., Bremner, G.P., Hubbard, K.P., Moore, G.C.A., Rice-Evans, C.A., 2002. The metabolic fate of dietary polyphenols in humans. Free Radic. Biol. Med. 33, 220
235. Rudkin, G.O., Nelson, J.M., 1947. Chlorogenic acid and respiration of sweet potatoes. J. Am. Chem. Soc. 69, 1470
1475. Sasaki, K., Han, J., Shimozono, H., Villareal, M.O., Isoda, H., 2013. Caffeoylquinic acidrich purple sweet potato extract, with or without anthocyanin, imparts neuroprotection and contributes to the improvement of spatial learning and memory of SAMP8 mouse. J. Agric. Food Chem. 61, 5037
5045. Sasaki, K., Oki, T., Kobayashi, T., Kai, Y., Okuno, S., 2014. Single-laboratory validation for the determination of caffeic acid and seven caffeoylquinic acids in sweet potato leaves. Biosci. Biotechnol. Biochem. 78, 2073
2080. Sasaki, K., Oki, T., Kai, Y., Okuno, S., 2015a. Changes in contents of caffeic acid and seven species of caffeoylquinic acids in sweet potato cultivar “Suioh” leaves during boiling treatment. Nippon Shokuhin Kagaku Kogaku Kaishi 62, 470
476. Sasaki, K., Oki, T., Kai, Y., Nishiba, Y., Okuno, S., 2015b. Effect of repeated harvesting on the content of caffeic acid and seven species of caffeoylquinic acids in sweet potato leaves. Biosci. Biotechnol. Biochem. 79, 1308
1314. Shan, Q., Zheng, Y., Lu, J., Zhang, Z., Wu, D., Fan, S., et al., 2014. Purple sweet potato color ameliorates kidney damage via inhibiting oxidative stress mediated NLRP3 inflammasome activation in high fat diet mice. Food Chem. Toxicol. 69, 339
346. Shimozono, H., Kobori, M., Shinmoto, H., Tsushida, T., 1996. Suppression of the melanogenesis of mouse melanoma B 16 cells by sweet potato extract. Nippon Shokuhin Kagaku Kogaku Kaishi (in Japanese with English abstract) 43, 313
317. Shoham, A., Hadziahmetovic, M., Dunaief, J.L., Mydlarski, M.B., Schipper, H.M., 2008. Oxidative stress in diseases of the human cornea. Free Radic. Biol. Med. 45, 1047
1055. Sondheimer, E., 1958. On the distribution of caffeic acid and the chlorogenic acid isomers in plants. Arch. Biochem. Biophys. 74, 131
138. Steed, L.E., Truong, V.D., 2008. Anthocyanin content, antioxidant activity, and selected physical properties of flowable purple-fleshed sweet potato purees. J. Food Sci. 73, S215
S221. Strack, D., Wray, V., 1989. Anthocyanins. In: Dey, P.M. (Ed.), Methods in Plant Biochemistry, vol. 1. Academic Press, London, pp. 325
356.

220

Sweet Potato

Suda, I., Oki, T., Masuda, M., Nishiba, Y., Furuta, S., Matsugano, K., et al., 2002. Direct absorption of acylated anthocyanin in purple-fleshed sweet potato into rats. J. Agric. Food Chem. 50, 1672
1676. Suda, I., Oki, T., Masuda, M., Kobayashi, M., Nishiba, Y., Furuta, S., 2003. Physiological functionality of purple-fleshed sweet potatoes containing anthocyanins and their utilization in foods. Jpn. Agric. Res. Q. 37, 167
173. Suda, I., Ishikawa, F., Hatakeyama, M., Miyawaki, M., Kudo, T., Hirano, K., et al., 2007. Intake of purple sweet potato beverage affects on serum hepatic biomarker levels of healthy adult men with borderline hepatitis. Eur. J. Clin. Nutr. 62, 60
67. Sugawara, T., Negishi, Y., Kai, Y., Ishiguro, K., Oki, T., Suda, I., 2011. Effect of cooking method on the lutein and polyphenol contents in edible leaves of the Suioh sweet potato cultivar. J. Jpn. Soc. Cook. Sci. 44, 291
298. Sui, X., 2017. Literature review. In: Sui, X. (Ed.), Impact of Food Processing on Anthocyanins. Springer, Singapore, pp. 5
14. Sun, H.N., Mu, T.H., Xi, L.S., Zhang, M., Chen, J.W., 2014a. Sweet potato (Ipomoea batatas L.) leaves as nutritional and functional foods. Food Chem. 156, 380
389. Sun, H.N., Mu, T.H., Liu, X.L., Zhang, M., Chen, J.W., 2014b. Purple sweet potato (Ipomoea batatas L.) anthocyanins: preventive effect on acute and sub-acute alcoholic liver damage and dealcoholic effect. J. Agric. Food Chem. 62, 2364
2373. Sun, H.N., Mu, T.H., Xi, L.S., Song, Z., 2014c. Effects of domestic cooking methods on polyphenols and antioxidant activity of sweet potato leaves. J. Agric. Food Chem. 62, 8982
8989. Sun, M., Lu, X., Hao, L., Wu, T., Zhao, H., Wang, C., 2015. The influences of purple sweet potato anthocyanin on the growth characteristics of human retinal pigment epithelial cells. Food Nutr. Res. 59, 27830. Sun, H.N., Mu, T.H., Xi, L.S., 2017. Effect of pH, heat and light treatments on the antioxidant activity of sweet potato leaf polyphenols. Int. J. Food Prop. 20, 318
332. Sun, H.N., Mu, B.N., Song, Z., Ma, Z.M., Mu, T.H., 2018. The in vitro antioxidant activity and inhibition of intracellular reactive oxygen species of sweet potato leaf polyphenols. Oxid. Med. Cell Longev. 4, 1
11. Suzuno, H., Ishida, H., Innami, S., Maekawa, A., Tadokoro, T., 2004. Bread-making properties and quality of bread contains a freeze-drying powder of sweet potato leaves. J. Integr. Study Diet. Habits 15, 29
34. Taira, J., Taira, K., Ohmine, W., Nagata, J., 2013. Mineral determination and anti-LDL oxidation activity of sweet potato (Ipomoea batatas L.) leaves. J. Food Compos. Anal. 29, 117
125. Takenaka, M., Nanayama, K., Isobe, S., Murata, M., 2006. Changes in caffeic acid derivatives in sweet potato (Ipomoea batatas L.) during cooking and processing. Biosci. Biotechnol. Biochem. 70, 172
177. Tamura, H., Akioka, T., Ueno, K., Chujyo, T., Okazaki, K., King, P.J., et al., 2006. Anti-human immunodeficiency virus activity of 3,4,5-tricaffeoylquinic acid in cultured cells of lettuce leaves. Mol. Nutr. Food Res. 50, 396
400. Tang, Q.Y., Kukita, T., Ushijima, Y., Kukita, A., Nagata, K., Sandra, F., et al., 2006. Regulation of osteoclastogenesis by Simon extracts composed of caffeic acid and related compounds: successful suppression of bone destruction accompanied with adjuvant-induced arthritis in rats. Histochem. Cell Biol. 125, 215
225. Terahara, N., Shimizu, T., Kato, Y., Nakamura, M., Maitani, T., Yamaguchi, M., et al., 1999. Six diacylated anthocyanins from the storage roots of purple sweet potato, Ipomoea batatas. Biosci. Biotechnol. Biochem. 63, 1420
1424. Terahara, N., Konczak, I., Ono, H., Yoshimoto, M., Yamakawa, O., 2004. Characterization of acylated anthocyanins in callus induced from storage root of

Sweet potato polyphenols

221

purple-fleshed sweet potato, Ipomoea batatas L. J. Biomed. Biotechnol. 2004, 279
286. Terahara, N., Oki, T., Matsui, T., Matsumoto, K., Fukui, K., Sugita, K., et al., 2007. Simultaneous determination of major anthocyanins in purple sweet potato. Nippon Shokuhin Kagaku Kogaku Kaishi 54, 33
38. Thompson, D.P., 1981. Chlorogenic acid and other phenolic compounds in fourteen sweet potato cultivars. J Food Sci 46, 738
740. Tian, Q., Konczak, I., Schwartz, S.J., 2005. Probing anthocyanin profiles in purple sweet potato cell line (Ipomoea batatas L. Cv. Ayamurasaki) by high-performance liquid chromatography and electrospray ionization tandem mass spectrometry. J. Agric. Food Chem. 53, 6503
6509. Truong, V.D., McFeeters, R.F., Thompson, R.T., Dean, L.L., Shofran, B., 2007. Phenolic acid content and composition in leaves and roots of common commercial sweet potato (Ipomea batatas L.) cultivars in the United States. J. Food Sci. 72, C343
C349. Truong, V.D., Nigel, D., Thompson, R.T., Mcfeeters, R.F., Dean, L.O., Pecota, K.V., et al., 2010. Characterization of anthocyanins and anthocyanidins in purple-fleshed sweet potatoes by HPLC-DAD/ESI-MS/MS. J. Agric. Food Chem. 58, 404
410. Uritani, I., 1999. Biochemistry on postharvest metabolism and deterioration of some tropical tuberous crops. Bot. Bull. Acad. Sin. 40, 177
183. Uritani, I., Muramatsu, K., 1953. Phytopathological chemistry of black-rotted sweet potato. Part 4. Isolation and identification of polyphenols from the injured sweet potato (I). Nippon Nogeikagaku Kaishi (in Japanese with English abstract) 27, 29
33. Valko, M., Leibfritz, D., Moncol, J., Cronin, M.T.D., Mazur, M., Tesler, J., 2007. Free radicals and antioxidants in normal physiological functions and human disease. Int. J. Biochem. Cell B 39, 44
84. Walter Jr, W., Shadel, W.E., 1981. Distribution of phenols in ‘‘Jewel’’ sweet potato [Ipomoea batatas (L.) Lam.] roots. J. Agric. Food Chem. 29, 904
906. Walter Jr, W.M., Purcell, A.E., McCollum, G.K., 1979. Use of high-pressure liquid chromatography for analysis of sweet potato phenolics. J. Agric. Food Chem. 27, 938
941. Wang, X., Zhang, Z.F., Zheng, G.H., Wang, A.M., Sun, C.H., Qin, S.P., et al., 2017. The inhibitory effects of purple sweet potato color on hepatic inflammation is associated with restoration of NAD1 levels and attenuation of NLRP3 inflammasome activation in high-fat-diet-treated mice. Molecule 22, 1315
1330. Woolfe, J.A., 1992. Sweet potato-past and present. Sweet Potato: An Untapped Food Resource. Cambridge University Press, Cambridge, pp. 15
40. Xi, L.S., Mu, T.H., Sun, H.N., 2015. Preparative purification of polyphenols from sweet potato (Ipomoea batatas L.) leaves by AB-8 macroporous resins. Food Chem. 172, 166
174. Yao, Y.R., 2009. The Purification, Stability, and Antioxidant of Sweet Potato. Hebei Agricultural University, Hebei (in Chinese). Ye, J., Meng, X., Yan, C., Wang, C., 2010. Effect of purple sweet potato anthocyanins on β-amyloid-mediated PC-12 cells death by inhibition of oxidative stress. Neurochem. Res. 35, 357
365. Yoshimoto, M., Okuno, S., Yoshinaga, M., Yamakawa, O., Yamaguchi, M., Yamada, J., 1999. Antimutagenicity of sweet potato (Ipomoea batatas) roots. Biosci. Biotechnol. Biochem. 63, 537
541. Yoshimoto, M., Okuno, S., Yamaguchi, M., Yamakawa, O., 2001. Antimutagenicity of deacylated anthocyanins in purple-fleshed sweet potato. Biosci. Biotechnol. Biochem. 65, 1652
1655.

222

Sweet Potato

Yoshimoto, M., Yahara, S., Okuno, S., Islam, M.S., Ishiguro, K., Yamakawa, O., 2002. Antimutagenicity of mono-, di-, and tricaffeoylquinic acid derivatives isolated from sweet potato (Ipomoea batatas L.) leaf. Biosci. Biotechnol. Biochem. 66, 2336
2441. Yoshimoto, M., Kurata, R., Okuno, S., Ishiguro, K., Yamakawa, O., Tsubata, M., et al., 2006. Nutritional value and physiological functions of sweet potato leaves. Acta Hortic. 703, 107
115. Yoshinaga, M., Yamakawa, O., Nakatani, M., 1999. Genotypic diversity of anthocyanin content and composition in purple-fleshed sweet potato (Ipomoea batatas (L.) Lam). Breed Sci. 49, 43
47. Zhang, Z.F., Fan, S.H., Zheng, Y.L., Lu, J., Wu, D.M., Shan, Q., et al., 2009. Purple sweet potato color attenuates oxidative stress and inflammatory response induced by D-galactose in mouse liver. Food Chem. Toxicol. 47, 496
501. Zhang, Z.F., Lu, J., Zheng, Y.L., Hu, B., Fan, S.H., Wu, D.M., et al., 2010. Purple sweet potato color protects mouse liver against d-galactose-induced apoptosis via inhibiting caspase-3 activation and enhancing PI3K/Akt pathway. Food Chem. Toxicol. 48, 2500
2507. Zhang, Z.F., Lu, J., Zheng, Y.L., Wu, D.M., Hu, B., Shan, Q., et al., 2013. Purple sweet potato color attenuates hepatic insulin resistance via blocking oxidative stress and endoplasmic reticulum stress in high-fat-diet-treated mice. J. Nutr. Biochem. 24, 1008
1018. Zhang, Y., Niu, F., Sun, J., Xu, F., Yue, R., 2015. Purple sweet potato (Ipomoea batatas L.) color alleviates high-fat-diet-induced obesity in SD rat by mediating leptin’s effect and attenuating oxidative stress. Food Sci. Biotechnol. 24, 1523
1532. Zhang, L., Xu, Y., Li, Y., Bao, T., Gowd, V., Chen, W., 2017. Protective property of mulberry digest against oxidative stress—a potential approach to ameliorate dietary acrylamide-induced cytotoxicity. Food Chem. 230, 306
315. Zheng, W., Clifford, M.N., 2008. Profiling the chlorogenic acids of sweet potato (Ipomoea batatas) from China. Food Chem. 106, 147
152.