Quality evaluation of sweet potato products

CHAPTER 13

Quality evaluation of sweet potato products Yoshiyuki Nakamura Division of Field Crop Research, Institute of Crop Science, National Agriculture and Food Research Organization (NARO), Tsukuba, Japan

Sweet potato and its production and utilization in Japan Sweet potato is the 10th most important crop, the 7th most important crop for food, in terms of production in the world. The larger producing regions are now Asian and African countries, and China is the largest producing country with about 70% of the world’s annual production (FAOSTAT, 2016). Sweet potato is important not only as a food resource but also as industrial materials in these Asian and African countries (Woolfe, 1992). This crop was introduced into Japan from China about 400 years ago and has been now cultivated all over the country except in the north in Hokkaido. Its cultivation area is about 37,000 ha, and its production is about 860,000 tons, ranking it 13th in the world in recent years (FAOSTAT, 2016). Sweet potato was cultivated first in the Okinawa Islands and southern Kyushu areas, where people have been often struck by natural disasters, for example, typhoon and drought. Sweet potato is one of the most suitable crops in these regions since it is relatively tolerant to such disasters and can be grown in severe agricultural environments. Sweet potato was also sometimes utilized as an emergency crop to compensate for drastic decreases in rice production in the Edo era (1603 1867). In this era sweet potato production was propagated in the northeastern part of Japan (Kanto and Tohoku area) because the Japanese government recommended the production of sweet potato as an emergency crop in these areas where many people often suffered from famine (Kobayashi, 2010). The public sweet potato breeding system in Japan was established by the Japanese government in 1937, although the first cross-breeding of sweet potato was conducted in Okinawa prefecture in 1914 (Takahata, 2014).

Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00013-2

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At that time the Japanese government recommended the production of sweet potato for fuel materials as well as a staple food. According to the national statistical data, the cultivation area and the production of sweet potato in 1940 were about 270,000 ha and about 3.5 million tons, respectively. The cultivation area and production did not change much from 1920 to 1940. After World War II, the sweet potato production continued increasing together with the development of the starch industries and peaked at about 720 million tons in 1955. In this year about 30% of the production was taken up by the starch industries, which was the secondary to the largest consumption for food (about 38%). However, the consumption by the starch industries dramatically decreased when starch from the sugar industries could be inexpensively imported from foreign countries in 1960s. On the other hand, the consumption for food has not shown any drastic change for the last 70 years, and consequently it has made certain contribution to the sweet potato utilization in Japan. Although the larger part of the harvest was selfconsumed by producers in the 1950s and 1960s, the consumption through fresh markets gradually came to occupy a major part from the 1970s. At present, approximately 85% of the sweet potato production for food is consumed through fresh markets. Other uses for sweet potato in Japan are feed for livestock, materials for processed food, and materials for liquor (shochu) industries. The present percentages of the use for these in terms of the total consumption are around 0.3%, 6%, and 28%, respectively. Consequently, sweet potato is mostly consumed for food, including processed food (about 53%), alcohol (about 28%), and starch (about 15%) in Japan today (Katayama et al., 2017). As for food, Japanese people prefer to consume sweet potato after a simple cooking method, such as roasting, steaming, and frying. The roast sweet potato is called as “yaki-imo” and it is the most popular eating style of this crop. The utilization of sweet potato for processed food is performed mainly with two elements, namely its starch and its storage root. The starch isolated from sweet potato storage root was traditionally utilized as a material for confectioneries, cooking gel, and binding materials for mashed fish meat, and now it is mainly provided by the sugar industries. However, domestic sweet potato starch comprises only about 1.7% of the total consumption of starch in Japan. The processed foods made from sweet potato storage root include chips, steamed and cured sweet potato (called “hoshi-imo”), fried sweet potato coated with sugar syrup (called “daigaku-imo”), and soft cake made from sweet potato

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paste (called “imo-yokan”). In addition, sweet potato has been also available in the form of dehydrated powders and purees as the materials for processed food such as pigments and nutritional ingredients (Baba, 1990; Komaki and Yamakawa, 2006). The cultivars with orange and purple flesh are mostly suitable for such utilization because of their high content of physiologically functional components such as β-carotene and anthocyanins (Yamakawa and Yoshimoto, 2001).

Japanese sweet potato varieties for food and processed food Since sweet potato was first introduced into the southern Kyushu region via the Okinawa Islands about 400 years ago, many varieties of sweet potato have been produced by natural hybridization and mutation, or introduced from foreign countries. There were consequently more than 300 varieties in Japan at the beginning of the 1900s. For example, “Beniaka” was selected by farmers in Saitama prefecture from spontaneous mutants of “Yatsufusa” in 1898, and extended its production area to about 30,000 ha in the 1930s. “Genji” (“Genki”) was introduced into Hiroshima prefecture from Australia in 1895 and prevailed in 100,000 ha between 1940 and 1943. In addition to these two varieties, “Shichifuku,” “Taihaku,” and “Oiran” were also generally cultivated t that time. These five major native varieties were mainly used for food and their production area covered about 70% of the total production area in 1940s. “Shichifuku” was introduced from the United States in 1900 and cultivated mainly in western regions of Japan including Kyushu, Shikoku, and Setouchi areas. The production area of this variety was more than 25,000 ha in 1942. “Taihaku” was cultivated mainly in the Kanto area in eastern Japan. This variety was said to be domestically introduced from the Kyushu area—the place of origin—to the Kanto area in the 1910s, and became the leading variety in 1945 with 55,000 ha. This variety was also called “Yoshida,” and it played important roles as a parent in the breeding of early national varieties such as “Norin 2,” “Norin 3,” and “Norin 4.” “Oiran” originated in the Kyushu area, and was introduced into the Kanto area via Shizuoka prefecture at the beginning of the Meiji era in the 1870s. This variety has another name, “Iigoh,” and prevailed mainly in the Kanto area with 25,000 ha in 1943. “Oiran” (“Iigoh”) has a unique purple pattern in the center of the cross-section of its storage root, and was widely used as the material of “hoshi-imo” (local traditional

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processed food made from steamed sweet potato) because of its soft texture after steaming. Since a systematic cross-breeding of sweet potato started in Okinawa in 1914, several practical varieties were bred by the crossing of indigenous and/or introduced varieties. “Okinawa 100” was derived from a cross between “Shichifuku” and “Choshu,” and released in 1934 as a high yield variety. This variety was extensively cultivated during the postwar period as a food and energy supply, and its production area peaked at 81,000 ha (about 20% of the total production area of the crop in Japan) in 1946. “Gokoku-imo” was derived from a cross between “Genji” (“Genki”) and “Shichifuku,” and released in 1937. This variety was also called “Kokei 4,” and mostly cultivated in Japan in the latter half of the 1940s. Its maximum production area was recorded in 1949 at about 100,000 ha, which corresponded to about 25% of the total production area of sweet potatoes. In 1942, the first products of the Norin number varieties, “Norin 1” and “Norin 2,” were released. They occupied 25% 30% of the total sweet potato production area during the 1950s and 1960s, and their maximum production areas were recorded at about 100,000 ha in 1955 and 80,000 ha in 1962, respectively. These four varieties described earlier were used for food and starch materials. The most famous and important variety bred by crossing practiced in Okinawa is “Kokei 14,” which was derived from a crossing between “Nancy Hall” and “Siam” conducted in 1935 and released in 1945. This variety possessed a wide regional adaptability and thus produced many local derivative lines in many prefectures in Japan. The production area of the variety including its derivative lines was about 18,000 ha in 1973, which was equivalent to about 25% of the total area, and about 25,000 ha in 1985, equivalent to about 32% of the total area. Its production area was the largest for 20 years from 1973 to 1992, and was still the third largest in 2015. This variety can be adapted to the materials for various processed products, for example, “yaki-imo” (roast sweet potato), snacks, confectionaries, and paste because of the annual and local stability for the size and the starch content of the storage root. This variety is also used as the standard variety for quality evaluation in the current Japanese breeding system of sweet potatoes. After World War II, a new breeding system of sweet potato was started in 1947. In this system, a crossing was totally conducted in Kagoshima and then, a local selection was conducted in Chiba (mainly for the eastern region of the Japanese mainland), Kurashiki (mainly for the western region of the Japanese mainland), and Kumamoto

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(mainly for the southern region of Japan). This breeding system produced several major varieties, most of which are still available (Katayama et al., 2014). “Tamayutaka” was released in 1960 for starch production at first (Onoda et al., 1970), but today it has been utilized mainly as the material for “hoshi-imo” (Kuranouchi et al., 2006, 2010a,b, Nakamura et al., 2006, 2007). “Koganesengan” was also released initially for starch production in 1966 because of its high starch content (Sakai et al., 1967), and recently utilized mainly for the production of shochu, a liquor made from sweet potatoes. “Benikomachi,” derived from a cross between “Kokei 14” and “Koganesengan,” was released in 1975 (Sakai et al., 1978). This variety has nice shape, beautiful deep purple-red skincolor, and good taste, but was susceptible to some kinds of diseases such as soil rot and stem rot. In 1984 an improved variety, “Beniazuma,” was released (Shiga et al., 1985). This was also derived from a cross with “Koganesengan” with a father plant similar to “Benikomachi,” but is relatively resistant to soil rot and stem rot diseases. It also has beautiful red skin-color and good eating quality. The superiority of “Beniazuma,” that is, the practical resistance to diseases and good eating quality, accelerated the spread of this variety mainly in Kanto region, and in 1993 “Beniazuma” replaced “Kokei 14” as the number one variety in terms of production area in Japan. This variety had been the most popular sweet potato variety in Japan for about 30 years until 2010. Although the high sweetness is one of the great appeals for the eating quality of sweet potato, it may obstruct this crop in enlarging its utilization as a dietary food menu like as potato. Therefore sweet potato varieties with extremely low sweetness were developed from the end of the 20th century to the beginning of the 21st century. The varieties “Satsumahikari” released in 1987 (Kukimura, 1988; Kukimura et al., 1989) and “Okikogane” released in 2000 (Yoshinaga, 2010) produce almost no maltose during heat-cooking due to their lack or mere trace of β-amylase activity. In the 21st century other many new varieties of sweet potato having unique properties were also developed in Japan. “Quick Sweet,” released in 2002, was the first sweet potato variety containing starch with lower pasting temperature (Katayama et al., 2002). The temperature at which starch in sweet potato is gelatinized is ordinarily 70°C 75°C but the starch of this variety is able to become gelatinized at about 55°C. Thus the storage roots of the variety are able to produce maltose earlier during heat-cooking than other cultivars. In addition, the gelatinized gel made from such sweet potato starch with a lower pasting temperature was able

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to stay soft because it was hardly able to be retrogelatinized. Recently another variety containing a high content of such types of starch, “Konamizuki,” was developed (Katayama et al., 2012). Varieties with unique flesh colors have also been developed. The varieties with purple flesh color contain high anthocyanins in their flesh and are utilized mainly for food pigments. The varieties with orange flesh color contain high carotenoids and they are utilized for processed food, in particular, for “hoshi-imo” because of their soft texture. These varieties with purple and/ or orange flesh color have attracted the interest of health-conscious consumers in Japan (Tanaka et al., 2017). In 2007 a new variety “Beniharuka” with ordinary flesh color (yellow) was released. This variety is higher in sweetness and less mealy in texture compared to the current leading varieties such as “Kokei 14” and “Beniazuma” after cooking (Kai et al., 2017). During the last 10 years the production of this variety has been increasing in Japan due to the novelty of eating quality and the wide adaptability for utilization of the variety (Komaki and Yamakawa, 2006).

Free sugar components in relation to the sweetness of sweet potato products The sweetness is of great appeal in the eating quality of sweet potato, and is mainly due to the free sugars. The free sugars predominantly existing in the storage root of sweet potatoes are fructose, glucose, sucrose, and maltose (Picha, 1986a). Among them, maltose is hardly detected in raw fresh storage roots, while the other three sugars are present both in raw and in heat-cooked storage roots. It has been reported that these four free sugars were different for sweetness at the same temperature; the sweetness of fructose, glucose, and maltose are about 1.0, 0.55, and 0.35 times of that of sucrose at 40°C, respectively (Yoshizumi et al., 1986), and the total sweetness of the four free sugars, namely the summation of the products of the content and the relative sweetness of each free sugars were reported to be fairly correlated with sensory evaluation of the steamed sweet potato (Takahata et al., 1993a). Sweet potato cultivars and breeding lines are classified into three genotype groups based on the free sugar composition of their steamed storage roots (Takahata et al., 1992). Group 1 consists of cultivars and lines which contain large amounts of maltose in their steamed roots. Group 2 consists of cultivars and lines that generate small amounts of maltose by steaming. The cultivars and breeding lines which contain relatively large amounts of fructose and glucose are classified as group 3.

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The cultivars and lines belonging to group 1, which possess β-amylase activity, are the majority of currently grown sweet potato varieties, while group 2, which are completely or mostly lacking in this enzyme activity, contains only two varieties, “Satsumahikari” and “Okikogane,” that are currently available. The varieties belonging to group 3 that are used for food are relatively old ones, such as “Kokei 14” and “Tamayutaka.” The storage roots of ordinary sweet potato cultivars produce a large amount of maltose during heating due to the hydrolysis of their inner starch by β-amylase, and the maltose produced is largely responsible for the sweetness of heat-cooked sweet potato (Ito et al., 1968; Picha, 1986a; Takahata et al., 1992). Nakamura et al. (2014b, 2018) demonstrated that the sweetness of steamed storage roots, which is given in Brix% value of homogenate of the roots measured by a refractometer, increased linearly with maltose concentrations (wt.%) in the roots within 3 months after harvesting using the current Japanese sweet potato cultivars (Fig. 13.1). It was also reported that the maltose concentrations (wt.%) in the roots of the cultivars investigated ranged from approximately 0 wt.% to 15 wt.%. Older varieties such as “Kokei 14” and “Tamayutaka” contained maltose at concentrations less than 10 wt.% in their steamed storage roots, whereas recently developed varieties such as “Beniharuka” and “Himeayaka” contained maltose at concentrations higher than 12 wt.% (Nakamura et al., 2014b). Such new varieties 30 r = 0.855 ***

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Figure 13.1 Relationships between the maltose concentrations and the sweetness values in steamed storage roots of Japanese cultivars and breeding resources of sweet potato (Nakamura et al., 2014b) (n 5 221, 2012, and 2013). The experiments were conducted within 3 months of harvest.  Significant at P , .001.

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therefore exhibited higher sweetness in their steamed roots than the older varieties (Katayama et al., 2014) because they exhibited higher β-amylase activity in their fresh storage roots than older varieties (Nakamura et al., 2014b, 2017). In contrast, varieties with extremely low sweetness in their steamed roots have also been developed over the past 30 years, such as “Satsumahikari” and “Okikogane.” The sweetness of their steamed storage roots was very low (about 7 9 Brix%) as almost no maltose was produced during heating due to their extremely low β-amylase activity (Baba et al., 1987b; Kukimura, 1988; Kukimura et al., 1989; Kumagai et al., 1990). Kumagai et al. (1990) reported that a variant lacking or having only traces of β-amylase in sweet potato storage roots was controlled by a single recessive allele and inherited in a hexasomic or tetradisomic manner. They also described that a new type of sweet potato with or without extremely low β-amylase activity could easily be developed as the allele was frequently detected in cultivated germplasm of the genetic resources of sweet potato. Fig. 13.2 shows the effect of β-amylase activity in fresh storage root on the maltose concentration of its steamed storage root, which was 16

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Figure 13.2 Relationship between the β-amylase activities of fresh storage roots and the maltose concentrations in steamed storage roots of Japanese cultivars and breeding resources of sweet potato (Nakamura et al., 2014b) (n 5 221, 2012, and 2013).

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determined by quantification of the reducing sugar produced via starch hydrolysis by β-amylase isolated from fresh roots of the tested cultivars investigated. The maltose concentrations of the steamed storage roots increased with increasing β-amylase activity up to about 0.2 mmol maltose/(min mg) protein in enzyme solution, while the maltose concentrations did not clearly increase with increasing activity even if the activity increased over this level. The results suggested that maltose generation in sweet potato storage roots could be regulated not only by β-amylase but also by other factors regarding the roots, particularly those with higher [higher than 0.2 mmol maltose/(min mg) protein] β-amylase activity. Another factor could be starch gelatinization that is required prior to maltose generation by β-amylase in sweet potato, which is not able to digest raw starch (Kiribuchi and Kubota, 1976). The maltose concentration exhibited a negative correlation (r 5 20.53 , n 5 221) with the pasting temperature of starch isolated from the fresh storage roots of cultivars investigated (Fig. 13.3). This negative correlation between maltose concentration and starch pasting temperature was stronger (r 5 20.69 , n 5 111) for roots with higher β-amylase activity (Nakamura et al., 2014b). The starch gelatinization characteristics such as pasting temperature would be also important factors for maltose generation in sweet potato storage roots during heat-cooking as well as β-amylase activity of the root. The starch pasting temperature of sweet potato starch is closely related to the molecular structure of its amylopectin (Noda et al., 1998), and greatly affected by the soil temperature during the growth period of sweet potato (Noda et al., 2001). The storage roots of “Beniazuma” and “Beniharuka” cultivated in Hokkaido, the northernmost prefecture of Japan, generated higher amounts of maltose than those of the same varieties cultivated in Ibaraki located about 700 km south of Hokkaido, despite lower β-amylase activity in the roots cultivated in Hokkaido (Nakamura et al., 2014b). The pasting temperatures of starch isolated from the roots harvested in Hokkaido was 5°C 7°C lower than those harvested in Ibaraki, where the average temperature during the summer season was about 5°C higher than Hokkaido for both varieties. Thus the gelatinization of intracellular starch in the storage roots harvested in Hokkaido was practically recognized at a lower temperature than in those harvested in Ibaraki for variety “Beniazuma.” Sweet potato cultivars containing starch with a lower pasting temperature could have the potential for higher sweetness. In the 21st century some new varieties with lower starch pasting temperature have been developed for expanding the use of sweet potato

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16 r = – 0.53***

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Starch pasting temperature (°C) Beniazuma

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Figure 13.3 Relationship between the pasting temperatures of starch isolated from fresh storage roots and the maltose concentrations in steamed storage roots of Japanese cultivars and resources of sweet potato (Nakamura et al., 2014b) (n 5 221, 2012, and 2013). The starch pasting temperatures were estimated from the Rapid Visco Analyzer profiles of 7 wt.% suspensions of starch isolated from fresh storage roots of cultivars.  Significant at P , .001.

and its starch. “Quick Sweet” was the first of these new varieties (Katayama et al., 2002, 2004), and it possesses the lowest starch pasting temperature, at about 53°C determined by the Rapid Visco Analyzer, among the current cultivars (Katayama et al., 2015). This unique variety was able to produce maltose earlier during heat-cooking than other traditional popular varieties with higher pasting temperatures (70°C 75° C). The decrease in starch pasting temperature is effective for an increase in maltose generation because it will prolong the duration of maltose generation (Fig. 13.4). In addition, Nakamura et al. (2014a) reported that the activity of β-amylase isolated from “Quick Sweet” storage roots heated at 80°C almost maintained its original level in the fresh roots, whereas the

Quality evaluation of sweet potato products

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50 Time Starch gelatinization and maltose generation occur at the temperature from about 70°C – 80°C(white zone) for "Beniazuma" having ordinary starch pasting temperature. Slow heating prolongs the period( ) the sweet potato stays under the effective temperature zone, and thus increase maltose generation.

Time Starch gelatinization and maltose generation occur at the temperature from about 50°C – 80°C (White zone) for "Quick Sweet" due to its lower starch pasting temperature. The storage root of this variety stays under the effective temperature zone for long time even by rapid heating.

Figure 13.4 Illustration of starch gelatinization and maltose generation during rapid or slow heating in the storage roots of sweet potato varieties containing starch with different pasting temperatures: “Beniazuma” and “Quick Sweet.”

activity of “Beniazuma” (with a starch pasting temperature of about 75° C) was severely inhibited at the same temperature. Takahata et al. (1994) indicated the importance of β-amylase stability during heat-cooking as well as starch gelatinization for maltose generation in sweet potato. “Quick Sweet” has an advantage for maltose generation during heating because β-amylase in its storage roots could maintain its activity at a higher temperature than the enzyme in “Beniazuma” storage roots. However, the activity of the enzyme isolated from the fresh roots of both varieties exhibited similar responses to temperature. This indicated that β-amylase in the heated storage roots of “Quick Sweet” remained stable due to starch gelatinization at lower temperatures before its inactivation during heating. Therefore maltose generation in “Quick Sweet” storage roots started at lower temperatures and continued at higher temperatures than that in “Beniazuma” during heating. In addition, such varieties as “Quick Sweet” and “Hoshikirari” that contain starch with lower pasting temperatures in their storage roots exhibited another useful function of being able to maintain a larger percentage of total ascorbic acid content in the fresh roots after heat-cooking than other older

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varieties (Nakamura et al., 2016). The earlier generation of maltose in heated storage roots of “Quick Sweet” and “Hoshikirari” containing starch with lower pasting temperature may also have a protective effect against the heat breakdown of ascorbic acid, a well-known antioxidant compound. Although maltose is one of the key components for the sweetness of steamed sweet potato storage roots as described earlier, it becomes less important for the sweetness of roots stored for longer periods (3 6 months) after harvesting. The long-term storage of sweet potato storage roots induces an increase in the content of sucrose together with a decrease in starch content (Picha, 1986b). Conversely, maltose concentration in steamed roots did not increase, but actually decreased during storage because of the decrease in β-amylase activity. The sweetness of sucrose is two to three times higher than that of maltose, and the sucrose concentration does not change significantly via heat-cooking. Thus sucrose instead of maltose played an important role in the sweetness of steamed sweet potato roots after long-term storage. Takahata et al. (1995) demonstrated that changes in sucrose-synthesizing enzymes, such as sucrose synthase (SUS) and sucrose-6-phosphate synthase (SPS), were possibly associated with sucrose accumulation in fresh sweet potato roots during storage. Masuda et al. (2007) reported that storage of sweet potato “Kokei 14” at 5°C or 10°C promoted sucrose accumulation due to the stimulation of SPS activity accompanying the suppression of β-amylase activity. Sucrose accumulation in sweet potato storage roots was also induced under stressful treatment, such as gamma irradiation (Hayashi et al., 1984). Although the enzymatic properties of SUS and SPS in higher plant species including sweet potato were extensively studied (Murata, 1971a,b; Ono and Ishimaru, 2006), the mechanism for sucrose accumulation in sweet potato storage roots still remains to be completely elucidated because the sucrose metabolisms are regulated by other metabolic enzymes such as acid invertases other than SUS and SPS. It had been reported that sucrose accumulation was induced in potato tubers stored at 5°C 7 °C due to the decrease in this enzyme activity together with the increase in SPS activity (Hironaka et al., 2004, 2005a,b). An acid invertase plays an important role in the conversion of sucrose into fructose and glucose, and a strong relationship between the hexose (fructose and glucose) content and the enzyme activity in various sweet potato cultivars (Takahata et al., 1996).

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Fructose and glucose are the predominant monosaccharides detected in sweet potato storage roots (Picha, 1986a). The concentrations of these monosaccharides in its fresh storage roots are up to about 1.5 wt.% in Japanese current commercial varieties. Among them, the older varieties with lower sucrose content such as “Kokei 14” and “Tamayutaka” contain the two kinds of monosaccharides at higher concentrations compared to current higher sucrose varieties such as “Beniazuma” and “Beniharuka” because these older varieties are higher in acid invertase activity among the current Japanese varieties. Although the concentrations of fructose and glucose in heat-cooked storage roots, which are little different from those in the fresh roots, are considerably lower than those of maltose and sucrose, the former two kinds of monosaccharides could play an important role for characterizing the sweetness of heat-cooked sweet potatoes. For example, fructose is the highest in sweetness among the four free sugars that exist in the sweet potato root, and its sweetness is about 1.2 times and three times that of sucrose and maltose at the same temperature, respectively. Thus fructose has a strong impact on the sweetness of heat-cooked sweet potato. On the other hand, glucose makes a soft impact on the sweetness due to its weak sweetness. Fructose and glucose therefore have complicating effects for the sweetness of sweet potato, and these effects still remain to be elucidated. In summary, the free sugars concerned with the sweetness of heat-cooked sweet potato are fructose, glucose, sucrose, and maltose. Among them, maltose generated by starch hydrolysis by β-amylase during heat-cooking is the most abundant constituent despite a gradual decrease in its content during storage even under normal conditions. The concentration of maltose in steamed sweet potato storage roots increases together with the enzyme activity up to about 0.2 mmol maltose/(min mg) protein of the enzyme solution but, however, does not increase in proportion to the activity when it increases beyond this level. The concentration of maltose exhibits a negative correlation with the pasting temperature of starch isolated from the fresh storage root. On the other hand, sucrose can be detected in fresh storage roots as well as heat-cooked ones at the same concentrations. Although its concentration is about one-third of the concentration of maltose, its sweetness is about 2.5 times that of maltose. Therefore sucrose contributes to the sweetness of heat-cooked sweet potato to a similar extent as maltose. Furthermore, sucrose plays a more important role for the sweetness of sweet potato roots instead of maltose

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when the root is stored for long period (more than 3 months) because its concentration practically increases after the long-term storage. Fructose and glucose are minor constituents of free sugars contained in sweet potato storage root, and are detected at lower concentrations in recently developed cultivars than in older cultivars in Japan.

Chemical factors in relation to the textural properties of steamed storage roots of Sweet potatoes The textural character of heat-cooked storage root is another key factor for the eating quality of heat-cooked sweet potato storage root, and it also plays an important role in the processing of sweet potato (Nara, 1951). In the current Japanese official breeding system of sweet potatoes for food, the breeding resources and lines were evaluated for texture as well as sweetness (Kitahara et al., 2017). The textural evaluation of breeding resources and lines tested is conducted by a comparison of their texture with the texture of the standard variety (“Kokei 14”) after eating their steamed storage roots together. The texture of the tested sample was sorted into five categories: mealy, slightly mealy, intermediate, slightly soggy, and soggy (Nakamura et al., 2010, 2015; Yoshinaga, 2014). The “intermediate” means that the sensory textural evaluation for the tested sample is almost equal to that of the standard variety. The “mealy” and “soggy” mean that the texture of the tested sample is mealy and soggy compared with the texture of the standard variety, respectively. However, such a sensory evaluation system has some problems, for example, the texture of the standard variety often differs between harvest locations or from year to year, and the tasting peculiarities of panel members are liable to influence the result of evaluation. Therefore it is necessary to determine the relationships between the texture of steamed sweet potato storage roots and physicochemical properties of their constituents using an invariant evaluating system of the textural properties of sweet potatoes. The most primary factor associated with the texture of sweet potato storage roots after cooking is starch content. Although the fresh storage roots with higher and lower starch contents tend to exhibit mealy and soggy texture in their steamed roots, respectively (Nara, 1957a), the textural differences in the steamed roots could not be definitely determined by starch content in the fresh roots. Nakamura et al. (2015) demonstrated that the groups of cultivars and the breeding resources having the five kinds of texture of their steamed storage roots were sorted into four

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Figure 13.5 The relationship between the starch content in fresh storage roots and the texture indices of steamed storage roots of Japanese sweet potato cultivars (Nakamura et al., 2015). Horizontal bars represent standard deviations in the starch contents of the root samples (n 5 60 65) for each texture index. Different letters indicate significant differences (P , .05) among the starch content by Tukey’s HSD test.

groups with significant differences (P , .05) based on the starch content in their fresh roots (Fig. 13.5). The starch content in the fresh storage root of sweet potato will change during heat-cooking via starch digestion by amylolytic enzymes, such as β-amylase, and therefore it is easily assumed that the content of starch remaining in the heat-cooked storage root must be much closely related to the texture of the cooked roots. However, there is little information about starch digestion regarding the texture of sweet potato storage roots (Nara, 1957b; Walter et al., 1975; Nakamura et al., 2017, 2018). The amount of starch digested by β-amylase during steaming was calculated from the maltose concentration in the steamed roots, and the starch contents of both in fresh and steamed roots were quantified after the complete digestion of starch into glucose (Nakamura et al., 2017). The differences in starch content between the fresh and the steamed roots were highly (r 5 0.94 , n 5 40) consistent with the amount of digested starch for the six varieties of sweet potato with three different levels of

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β-amylase. The starch digestion rate in steamed roots (i.e., ratio of starch content in steamed roots against that in fresh roots) was practically correlated to β-amylase activity in the fresh roots. The maltose concentration and the degradation rate of starch in steamed storage roots of three different groups for the levels of β-amylase activity in sweet potato varieties are shown in Table 13.1. The starch content in fresh storage roots of “Beniharuka” and “Himeayaka,” which have higher β-amylase activity, decreased remarkably to less than 50% during steaming. In contrast, older varieties such as “Kokei 14” and “Tamayutaka,” which have 50% 60% of the β-amylase activity in the former varieties, maintained approximately 70% of the starch content existing in their fresh roots after steaming due to lower starch digestion. Furthermore, in such varieties as “Okikogane” and “Satsumahikari” that lack or have extremely low levels of β-amylase activity, the starch content of their fresh roots hardly decreased during steaming. The steamed roots of these two varieties consequently contained the highest content of starch among the six varieties described earlier, and thus showed a mealy texture even though their fresh roots had the lowest starch content. The texture of the steamed roots of sweet potato could be closely correlated with the remaining content of starch after steaming rather than that prior to steaming. The starch content in steamed roots could be predicted by the starch content and β-amylase activity of fresh roots before steaming. Therefore β-amylase activity was thought to be an important factor in determining texture as well as the sweetness of steamed sweet potato. Table 13.1 β-Amylase activities, maltose concentrations, and starch digestion rates in three groups of sweet potato varieties with different levels of β-amylase activity (Nakamura et al., 2017). Activity level of sample

β-Amylase† activity [mmol maltose/(min mg)]

Maltose concentration† (wt.%)

Starch digestion rate , † (%)

High (n 5 12) Middle (n 5 11) Extremely low (n 5 11)

0.283 6 0.073a 0.139 6 0.037b 0.0126 6 0.022c

11.29 6 2.70a 7.25 6 2.20b 0.13 6 0.037c

51.19 6 8.45a 41.31 6 8.89b 0.92 6 2.43c

 High active varieties: “Beniharuka” (n 5 7), “Himeayaka” (n 5 5). Middle active varieties: “Kokei 14” (n 5 6), “Tamayutaka” (n 5 5). Extremely low active varieties: “Okikogane” (n 5 7), “Satsumahikari” (n 5 4). The number of samples examined is enclosed in parentheses.  Percentage of the amount of digested starch, which can be calculated as maltose content 3 0.95, against the starch content in fresh root. †Different alphabets in a column indicate significant differences at P , .05 as determined by Tukey’s HSD test among the three sample groups.

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The starch content of sweet potato storage roots also changed even in fresh roots during storage via starch digestion, and consequently textural change could occur after the heat-cooking of the root (Picha, 1986b; Walter, 1987). Although the starch degradation in sweet potato roots during storage is revealed to be concerned with α-amylase (Morrison et al., 1993; Takahata et al., 1995), there is little information about the enzymatic mechanism for starch degradation by α-amylase in sweet potato during the storage period (Hagenimana et al., 1994). Because sweet potato is normally high in β-amylase activity, the measurement of its α-amylase activity required the elimination of β-amylase activity. Baba et al. (1987a) measured the α-amylase activity from dehydrated sweet potato flour using β-limit dextrin, which is not susceptible to β-amylase digestion, as a substrate for the enzymatic reaction. It was reported that the activity of α-amylase in storage roots was extremely low (less than 0.1% of that of β-amylase) just after harvest, and still low (about 1% of β-amylase activity) even after storage for 8 months despite a prominent increase of the activity during storage (Baba, 1990). Although α-amylase showing very low activity compared to β-amylase, α-amylase could remarkably affect the texture of heat-processed sweet potato products, such as fried chips and granules, via the digestion of starch into free sugars. Although there have been some reports about the varietal difference in α-amylase activity of sweet potato cultivars in relation to textural differences (Baba, 1990; Morrison et al., 1993; Takahata et al., 1995), further investigation is still needed for the relationships between starch metabolism by α-amylase and the texture of heat-cooked sweet potato after storage under different conditions. It has been well known that the apparent amylose content in starch of rice grains shows a wide range of variation and significantly affects the texture of cooked rice. It was reported that the difference in amylose content in sweet potato starch had little effect on the texture of steamed storage roots (Nara, 1957c), because the diversity of the amylose content in the starch of Japanese sweet potato cultivars is relatively lower than that in rice starch (Kitahara et al., 1996; Tokimura et al., 2002; Katayama et al., 2004). Recently, amylose-free starch has been obtained by genetic transformation techniques (Kimura et al., 2001; Otani et al., 2007) in sweet potato as well as in other tuberous and root crops (Hovenkamp-Hermelink et al., 1987; Ceballos et al., 2007). However, the texture of the storage roots containing those starches has not been elucidated.

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In addition to the difference in starch properties, there are some other differences in cellular microstructure and water status between the steamed storage roots with different textures (Nakamura et al., 2010). In steamed roots having a mealy-type texture, the cells in the root tissues exist independently of each other for keeping their structures. On the other hand, in steamed roots having a soggy-type texture the cells in the root tissue melt together to lose their original structures (Fig. 13.6). The cells in the steamed roots with mealy-type texture are completely filled with starch gel and thus expand. On the other hand, the cells in the steamed roots with soggy-type texture shrink because of a decrease in expanding pressure due to a lower content of starch gel. The analysis of the molecular status of water using the magnetic resonance imaging technique revealed that the root tissue with mealy-type texture showed a smaller content of free water molecules with heterogeneous distribution compared to the root with soggy-type texture. The heterogeneous distribution of free water molecules was also concerned with some kind of deterioration in quality for “hoshi-imo” (Nakamura et al., 2007). “Hoshi-imo” is a local and traditional processed food made from steamed sweet potato roots and sometimes suffer from a deterioration in quality, known as “Shirota” (Nakamura et al., 2007; Kuranouchi et al., 2010a,b). Nakamura et al. (2007) observed that the portion of steamed storage roots showing a lower distribution of free water molecules will suffer from the “Shirota” damage after processing the storage roots into “hoshi-imo” product. Such a decrease in free water molecules in the “Shirota” part of the steamed storage roots could be caused by insufficient starch

Figure 13.6 The tissue microstructure of steamed storage root of sweet potato varieties showing mealy-type and soggy-type texture observed by scanning electron microscopy (magnification: 3 500). (A) Variety showing mealy-type texture; (B) variety showing soggy-type texture.

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gelatinization during steaming due to a rise in starch pasting temperature (Nakamura et al., 2006, 2007). The new varieties with lower starch pasting temperature, for example, “Quick Sweet” (Katayama et al., 2002) and “Hoshikirari” (Kuranouchi et al., 2012), are suitable materials for making “hoshi-imo” without the “Shirota” defect. In summary, the differences in cellular microstructure and molecular status of water in steamed storage root tissue seem to be concerned with the textural differences of steamed sweet potato storage roots. The tissue disintegration of storage roots after heat-cooking is also an important factor for eating quality and adaptability for cooking and manufacturing of sweet potato. The heat-cooked sweet potato storage roots exhibiting low tissue disintegration generally tend to have a mealy texture and are suitable for cooking and manufacturing. The degree of tissue disintegration of steamed storage roots of sweet potato can be evaluated by measuring the decreasing rate in their weights after shaking the root sections in water (Nakamura et al., 2015). The degree of disintegration was correlated with texture, and the textural differences could be accounted for by the differences in the degree of disintegration as well as by the differences in starch content. In particular, the textures of the roots with moderate starch content (16% 22%) in their fresh roots showed closer correlation to the degree of tissue disintegration because their textures were less affected by starch content compared to the roots with high and low starch content. The degree of tissue disintegration could be a different factor to the starch content affecting the texture of steamed sweet potato storage root. Matsuura-Endo et al. (2002a) observed the differences in the texture of steamed potato tubers with same starch content. They demonstrated that such differences in texture could be related to the degree of tissue disintegration caused by cell separation which was influenced by calcium and cell wall polysaccharides contents (Matsuura-Endo et al., 2002b). In sweet potato the disintegration rate of steamed storage root tissue exhibited a negative correlation (r 5 20.403) with the calcium content per fresh weight of the roots, which did not account for the textural differences, and also showed weak negative correlation (r 5 20.214) with the content of chelating agent soluble fraction in pectic substances extracted from starch residue (Nakamura et al., 2015). The chelating agent soluble ones play important roles together with divalent cations in cell adhesion in plant tissue (Grant et al., 1973). The treatment of storage root tissues of sweet potato with chelating agent decreases their firmness

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along with histological changes in the microstructure of cell wall (Fuchigami et al., 2002), and divalent cations such as calcium ions are reported to have a prevention effect against the decrease in firmness in the potato (Ross et al., 2011). Although the chemical compositions of other cell wall polysaccharides such as cellulose and hemicellulose in sweet potato storage roots are analyzed (Noda et al., 1994; Tsukui, 1988; Tsukui et al., 1994; Salvador et al., 2000), little information about their functions regarding the texture of cooked sweet potato roots is available (Nara 1958; Shen and Sterling, 1981) and further investigations are still needed. In summary, disintegration rates of steamed root tissues were associated with texture, which largely depended on starch content, and the rate related to calcium content and equally to starch content via interactions with chelating agent soluble pectic substances.

Carotenoids and anthocyanins—their association with discoloration in sweet potato storage root Another important factor for the quality of food is color. In the current Japanese breeding system of sweet potatoes for food, the colors of the skin and the flesh of the storage root of progeny strain are important items of investigation. The colors of the top parts, such as stem, petiole, and leaf blade, are also investigated mainly as markers of genetic characterization. The skin colors of sweet potato storage roots are caused by pigments accumulated in its periderm, and are categorized into eight groups: white, yellow, dark, orange, scarlet, red, purple (violet), and other. In Japan the cultivars with scarlet or red skin colors, for example, “Beniazuma” and “Beniharuka,” are thought to be suitable for fresh utilization, for example, for “yaki-imo.” On the other hand, the pale-colored cultivars, that is, with white and yellow skin colors, for example, “Konahomare” and “Kogenesengan,” are used for the production of starch and liquor (shochu) in order to avoid color-contamination from their skin. The major types of pigment associated with the colors of sweet potato storage root are carotenoids and anthocyanins (Woolfe, 1992), and these chemical components possess various health-promoting functions (Yoshimoto, 2010). In Japan many sweet potato varieties containing these components at high levels have been developed over the last 20 years (Takahata, 2014). The varieties containing high levels of carotenoids and anthocyanins in their storage roots have orange and purple flesh, respectively.

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The main carotenoid in these orange-fleshed varieties is β-carotene, which is the dominant component (more than 80% of total carotenoids). On the other hand, the carotenoids mainly detected in yellow-fleshed varieties are not β-carotene but β-carotene diepoxide and β-cryptoxanthin epoxide, which are produced from β-carotene via metabolic pathways (Maoka et al., 2007). The content of β-carotene in the storage roots have been extensively investigated for various sweet potato cultivars with yellow and orange flesh (Takahata et al., 1993b; Kimura et al., 2007). The carotenoid contents in the orange-fleshed cultivars were about 10 times that of the ordinary yellow-fleshed cultivars (Ishiguro et al., 2010) ranging from 13.5 to 39.9 mg/100 g DW. The high β-carotene varieties such as “Hitachi Red” (Tarumoto et al., 1995) and “J-Red” (Yamakawa et al., 1998) are suitable for processed food, for example, “hoshi-imo” and juices, because they have soft and moist textures due to their lower starch content. The deep and bright yellow-fleshed varieties are preferable for food such as “yaki-imo” and “hoshi-imo.” The deep yellow-fleshed variety “Beniazuma” replaced “Kokei 14” with pale yellow flesh as a leading variety for “yaki-imo,” and “Beniharuka,” a bright yellow-fleshed variety, became a leading variety for “hoshi-imo” (steamed and cured sweet potato) instead of “Tamayutaka” with its white yellow flesh color. Over the last 20 years various sweet potato varieties with purple flesh were also developed. They contain anthocyanins at high levels in their storage roots, and most of them are utilized as food colorants in powder or puree forms. Eight major peaks of anthocyanins were detected by investigations using HPLC for the crude pigments extracted from the storage root of “Yamagawamurasaki,” a local cultivar in the southern Kyushu area, and were abbreviated as YGM-1a, -1b, -2, -3, -4b, -5a, -5b, and -6 after the name of cultivar “Yamagawamurasaki” (Goda et al., 1997). The molecular identification of the eight peaks demonstrated that the basic molecular structures of the former four major anthocyanins (YGM-1a, -1b, -2, and -3) are acylated forms of cyanidin, and those of the later four anthocyanins (YGM-4b, -5a, -5b, and -6) are acylated forms of peonidin (Terahara et al., 1999). The sweet potato cultivars containing cyanidin-type anthocyanins predominantly have a bluish purple color, and those containing peonidin-type anthocyanins predominantly have reddish purple color in the flesh of their storage roots, respectively (Yoshinaga et al., 1999). In addition, the cyanidin-type anthocyanins in purplefleshed sweet potatoes were revealed to have higher antioxidative activity

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estimated by radical-scavenging activity of DPPH (Ishiguro et al., 2007). Takahata et al. (2011) reported that cyanidin-based anthocyanins are closely related to DPPH radical-scavenging activity in sweet potato storage roots. Besides the antioxidant activity, various physiological functionalities of anthocyanins isolated from purple-fleshed sweet potatoes have been extensively investigated in the recent decades in Japan (Suda et al., 2003, 2008; Yoshimoto, 2010). The anthocyanins contained in sweet potato storage roots have high potential as natural colorants for processed foods (Oki et al., 2010) because of their advantages such as high yield of pigment (Yamakawa et al., 1997), color stability against heating and light irradiation, and bright tone (Hayashi et al., 1996; Tsukui et al., 1999). In Japan most of the sweet potato varieties with purple flesh were first utilized as the source of food colorants, while a few official varieties became available for table use. “Purple Sweet Lord” (Tamiya et al., 2003) was the first national variety for table use and “Kyushu No. 137” (Yoshinaga et al., 2006) was the second variety. These two varieties have lower anthocyanin content (about 20% 35%) and better eating quality (higher sweetness and softer texture), compared to the special varieties for food colorants such as “Murasakimasari” and “Akemurasaki.” “Kyushu No. 137” also possesses a high suitability for “hoshi-imo” manufacturing because this variety has a soggy texture and a light color tone in its steamed storage root. Soggy texture in the steamed storage root is highly required for “hoshi-imo” manufacturing, and light color due to its lower content of phenolic compounds including anthocyanins is effective in preventing the product’s color from becoming dark. The “hoshi-imo” made from “Kyushu No. 137” is excellent in color quality because of its lower content of phenolic compounds. The darkening discoloration of storage roots during processing is also one of the serious problems for the quality of sweet potato products. The darkening of the flesh color of sweet potato storage roots is caused through enzymatic or nonenzymatic mechanisms. The enzymatic darkening occurs in the outer portion of fresh or heat-cooked storage roots at a not sufficiently high temperature in the presence of oxygen, and is caused by oxidation of phenolic compounds via polyphenol oxidase (Walter and Schadel, 1981; Ma et al., 1992). The degree of this enzymatic discoloration is closely correlated to the content of phenolic compounds (Walter and Purcell, 1980) and the activity of polyphenol oxidase (Ma et al., 1992).

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On the other hand, the nonenzymatic darkening of the flesh color can occur even in the storage root after being cooked at a temperature high enough to inhibit polyphenol oxidase. It is caused by the binding of ferric ions, which are transformed from ferrous ions by oxidation during heating, to phenolic compounds, which are predominately chlorogenic acids and their isomers in sweet potato (Nakabayashi, 1970). Reduction of the contents of ferric ions or chlorogenic acids, which are active factors for the formation of the nonenzymatic darkening in heat-cooked sweet potato storage roots, was necessary to prevent the nonenzymatic darkening in heat-cooked sweet potato roots. Shimozono et al. (2000) reported that a treatment of sweet potato paste prepared from the varieties with high chlorogenic acid contents with chlorogenic acid esterase decomposed the isomers of chlorogenic acid presented in the paste into low active isomers such as caffeic and quinic acid, and thus effectively suppressed the darkening of the paste. Baba (1992) reported that reducing or chelating the ferric ions by some kind of chemical agent, for example, ascorbic acid and calcium chloride, was also effective for the prevention of the darkening in peeled sweet potato storage roots. The content of chlorogenic acids and the activity of polyphenol oxidase in sweet potato storage roots are the important factors for the darkening discoloration of sweet potato storage roots, whether it is caused by enzymatic mechanisms or not, and therefore a cultivar with lower chlorogenic acids content and polyphenol oxidase activity has a great potential for the development of sweet potato products with excellent colors. It has been reported that there are cultivar differences for the contents and the enzyme activity, and hence the contents of phenolic compounds and polyphenol oxidase activity for breeding materials are roughly checked in the present sweet potato breeding system in Japan (Komaki et al., 1992). “Aikomachi,” a recently developed variety in which hardly darkening discoloration in its flesh color occurs, was suitable for paste and confectioneries due to its light yellow flesh color (Ohara-Takada et al., 2016).

Conclusion The sweetness and texture of steamed sweet potato storage roots, which were thought to be major factors regarding the eating quality and the suitability for processing of sweet potatoes, were investigated for various varieties in Japan. The sweetness of steamed storage roots stored for not

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longer than 3 months after harvesting were correlated with the concentration of maltose in the roots. The maltose concentration increased to a peak of approximately 10 wt.%, with β-amylase activity in the fresh roots increasing up to about 0.2 mmol maltose/(min mg) protein of the enzyme solution. The maltose concentration, however, did not increase along with increased β-amylase activity even if the activity exceeded these levels. The maltose concentration also exhibited a negative correlation with the pasting temperature of starch isolated from the fresh roots. The recently developed varieties with high sweetness, such as “Quick Sweet” and “Beniharuka,” produced a larger amount of maltose due to their higher β-amylase activity and/or lower starch pasting temperature than older varieties. The textures of steamed sweet potato storage roots were correlated to the contents of starch remaining in the steamed roots after digestion into maltose by β-amylase during steaming. The digestion rates of starch were significantly correlated to the activities of β-amylase in the fresh roots. The disintegration of steamed storage root tissues is also an important factor for eating quality and suitability for processing of sweet potato. In general, the steamed storage roots with a lower degree of tissue disintegration showed a mealy texture regardless of their starch content. The texture of steamed roots is highly correlated to the degree of tissue disintegration, particularly in the roots with moderate starch contents (16% 22%). The disintegration rate of steamed root tissue showed negative correlations with the contents of calcium (r 5 20.403) and chelating agent soluble pectin fraction from starch residue (r 5 20.214) per the fresh weight of the root. From these results the texture of steamed sweet potato storage root is largely affected by the content of starch remaining after the digestion by β-amylase during steaming and the degree of tissue disintegration, which is correlated with the contents of calcium and pectin probably existing in cell wall of the root tissue. The color of the skin and the flesh of the storage root are another important factors for the quality of sweet potato products, such as foods and food materials. In Japan there are many varieties of sweet potato with different colored skin and flesh. Among them purple- and orange- fleshed varieties have high contents of anthocyanins and β-carotene, respectively, and the functionality of these compounds has been extensively investigated in recent decades. Such sweet potato varieties therefore have been of great interest to health-conscious people, thus resulting in the promotion of the production of sweet potato products. In contrast, a negative

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aspect of sweet potato flesh color is the darkening discoloration caused by the chemical reactions of phenolic compounds that are plentiful in sweet potato storage roots. The discoloration appearing in flesh storage roots is primarily induced by enzymatic oxidation of phenolic compounds, and when it occurs after heat-cooking and processing it is caused by nonenzymatic binding of phenolic compounds to ferric ions transformed from ferrous ions by heating. The darkening discoloration clearly decreases the quality of sweet potato food products, including food materials such as starch and powder, and thus the degree of the darkening discoloration is an important characteristic to check when breeding sweet potatoes as a food crop.

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Hironaka, K., Ishibashi, K., Koaze, H., Umezaki, K., Mori, M., Tsuda, S., et al., 2004. Relationship between invertase activity and reducing sugar content of cold-stored Japanese processing potatoes. Food Preserv. Sci. 30, 295 299. Hironaka, K., Ishibashi, K., Koaze, H., Kobayashi, S., Mori, M., Tsuda, S., et al., 2005a. Changes in invertase, sucrose-6-phosphate synthase and UDP-glucose pyrophosphorylase activities, and their relations to reducing sugar content in Japanese processing potato varieties stored at low temperature. Food Preserv. Sci. 31, 9 14. Hironaka, K., Ishibashi, K., Koaze, H., Miyashita, H., Mori, M., Tsuda, S., et al., 2005b. Effect of storage temperature on invertase, sucrose-6-phophate- synthase and UDP-glucose pyrophosphorylase activities of Japanese processing potatoes. Food Preserv. Sci. 31, 67 74. Hovenkamp-Hermelink, J.H.M., Jacobsen, E., Ponstein, A.S., Visser, R.G.F., Vos-Scheperkeuter, G.H., Bijmolt, E.W., et al., 1987. Isolation of an amylose-free starch mutant of the potato (Solanum tuberosum L.). Theor. Appl. Genet. 75, 217 221. Ishiguro, K., Yahara, S., Yoshimoto, M., 2007. Changes in polyphenolic content and radical scavenging activity of sweet potato (Ipomoea batatas L.) during storage at optimal and low temperatures. J. Agric. Food Chem. 55, 10773 10778. Ishiguro, K., Yoshinaga, M., Kai, Y., Maoka, T., Yoshimoto, M., 2010. Composition, content and antioxidative activity of the carotenoids in yellow-fleshed sweetpotato (Ipomaea batatas L.). Breed. Sci. 60, 324 329. Ito, T., Ando, T., Ichikawa, K., 1968. Effects of cooking processes on the saccharification of sweet potato (part 1): the relation between heating temperature and increase of sugar contents. J. Home Econ. Jpn. 19, 170 173 [in Japanese]. Kai, Y., Sakai, T., Katayama, K., Kumagai, T., Ishiguro, K., Nakazawa, Y., et al., 2017. “Beniharuka”: a new sweetpotato cultivar for table use. Bull. NARO Kyushu Okinawa Agric. Res. Cent. 66, 87 119 [in Japanese with English summary]. Katayama, K., Komae, K., Kohyama, K., Kato, T., Tamiya, S., Komaki, K., 2002. New sweet potato line having low gelatinization temperature and altered starch structure. Starch/Stärke 54, 51 57. Katayama, K., Tamiya, S., Ishiguro, K., 2004. Starch properties of new sweet potato lines having low pasting temperature. Starch/Stärke 56, 563 569. Katayama, K., Sakai, T., Kai, Y., Nakazawa, Y., Yoshinaga, M., 2012. “Konamizuki”: a new sweetpotato cultivar. Bull. NARO Kyushu Okinawa Agric. Res. Cent. 58, 15 36 [in Japanese with English summary]. Katayama, K., Ohara-Takada, A., Kuranouchi, T., Nakamura, Y., Kai, Y., Kumagai, T., et al., 2014. New sweetpotato cultivars bred for food recently in Japan. In: Takahata, Y. (Ed.), New Era of Sweetpotato Research in East Asia: Proceedings of 6th Japan China Korea Sweet Potato Workshop, Kagoshima, pp. 14 15. Katayama, K., Tamiya, S., Sakai, T., Kai, Y., Ohara-Takada, A., Kuranouchi, T., et al., 2015. Inheritance of low pasting temperature in sweetpotato starch and the dosage effect of wild-type alleles. Breed. Sci. 65, 352 356. Katayama, K., Kobayashi, A., Sakai, T., Kuranouchi, T., Kai, Y., 2017. Recent progress in sweetpotato breeding and cultivars for diverse applications in Japan. Breed. Sci. 67, 3 14. Kimura, T., Otani, M., Noda, T., Ideta, O., Shimada, T., Saito, A., 2001. Absence of amylose in sweet potato (Ipomoea batatas (L.) Lam.) following the introduction of granule-bound starch synthase I cDNA. Plant Cell Rep. 20, 663 666. Kimura, M., Kobori, C.N., Rodoriguez-Amaya, D.B., Nestel, P., 2007. Screening and HPLC methods for carotenoids in sweetpotato, cassava and maize for plant breeding trials. Food Chem. 100, 1734 1746.

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