Sweet potato starch

CHAPTER 3

Sweet potato starch Tai-Hua Mu and Miao Zhang Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China

Overview of starch Source of starch Starch is the most abundant storage reserve carbohydrate in plants. It is widely found in cereal grain seeds (e.g., corn, wheat, rice, and sorghum), tubers (e.g., potato), roots (e.g., sweet potato, cassava, and arrowroot), legume seeds (e.g., peas, beans, and lentils), fruits (e.g., green bananas, unripe apples, and green tomatoes), trunks (e.g., sago palm), and leaves (e.g., tobacco) (Chen et al., 2003). Generally root and tuber crops are rich sources of starch containing 70% 80% of water, 16% 24% of starch, and less than 4% of trace quantities of protein and lipids, beside other minerals and vitamins (Hoover, 2001). The major food consumed by human is starch, providing 75% 80% of the total caloric intake worldwide (Bemiller and Whistler, 1996). China is the second largest starch producing country in the world, after the United States, producing about 11.1 million metric tons of starch annually as of the year 2005 which principally comprises corn, cassava, potato, sweet potato, and wheat starch (Wang, 2005). Starch plays a vital role in developing food products either as a raw material or as a food additive, such as thickener, stabilizer, or texture enhancer (Aina et al., 2012). Starch is useful in maintaining the quality of stored food products; it improves moisture retention and consequently controls water mobility in food products. It could also be used as a delivery vehicle for substances of interest in the food and pharmaceutical industries such as antioxidants, colorants, flavors, and pharmaceutically active proteins (Guan et al., 2000). Starch is extracted from various starch-rich crops by wet separation techniques. The crops are harvested at full maturity, washed, and ground. The starch granules will settle in water due to their higher density. However, the sedimentation of starch granules in water is hindered by the presence of various nonstarch components like mucilage and latex, leading Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00003-X

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Sweet Potato

not only to the loss of starch, but also lowering of the quality of the extracted starch. In addition, the presence of nonstarch components and microorganisms affects the color of the starch, limiting its use in food and textile applications. Studies on the better recovery and economical extraction of starches with good functional properties indicated that lactic and citric acids improve the yield and color of starch from sweet potato tubers; on the other hand, an enzymatic method was developed for the enhancement of the recovery of starch from some roots and tubers (Balagopalan et al., 1996). In addition, sweet potato starch was also isolated by sour liquid processing and centrifugation (Deng et al., 2013).

Structural and physicochemical characteristics of starch Color and granule sizes of starch Native starch is whitish, odorless, bland, and insoluble in water. Color is an important criterion for starch quality, especially for use in the textile industries. Starch pastes should be clear and free from any off-color for better acceptability (Radley, 1976). Its granules exist in different ranges of size distribution, shapes, and dimensions (Table 3.1), which are greatly influenced by their botanical sources and growing and harvest conditions. Rice and oat starch have tiny granules ranging from 1.5 to 9 μm, whereas potato starch granules are as large as 100 μm (Chen et al., 2003). Suganuma and Kitahara (1997) reported the sizes of orange and purple Table 3.1 Characteristics of starch granules from different sources. Starch

Granule shape

Granule size range (µm)

Reference

Maize (waxy and normal)

Spherical, round, polygonal

2 30

Potato

Oval, spherical, lenticular Round, polygonal, oval, bell, round, Round, truncated, cylindrical, oval, spherical Spherical

5.5 72.2, 5 100

Chen et al. (2003); Tester and Karkalas (2002) Tester and Karkalas (2002) Chen et al. (2003); Moorthy (2002) Asaoka et al. (1992)

Sweet potato Cassava

Sorghum Mung bean Wheat

Round, oval Lenticular (A type), spherical (B type)

2 42, 3.4 27.5 4 35, 4 43, 5 45

5 20 1 45 15 35, 2 10

Tester and Karkalas (2002) Chen et al. (2003) Tester and Karkalas (2002)

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sweet potato starch to be in the range of 7 12 and 12 15 μm, respectively, with round, oval, and polygonal shapes. Granule size substantially affects the swelling power, solubility, and digestibility of starch. Proximate composition of starch Starch granules are composed of two major polymers: amylose and amylopectin, which are composed of glucose molecules linked together in linear and branched forms, respectively. Generally, depending on the plant, starch consists of 20% 25% of amylose and 75% 80% of amylopectin by weight. Waxy starches consist of less than 15% of amylose, and normal starches are 20% 35% amylose, while the high-amylose starches contain more than 40% amylose content (Tester et al., 2004). The structures and the relative amount of both polymers play an important role in determining starch properties. Amylose Amylose is a minor component of native starches; it forms a colloidal dispersion in hot water, whereas amylopectin is completely insoluble. The structure of amylose consists of long polymer chains of glucose units connected by α-acetal linkage. In an amylose chain all of the monomer units are α-D-glucose, and all the α-acetal links connect C-1 of one glucose molecule to C-4 of the next glucose molecule (Ophardt, 2003). The side chains range in length from 4 to over 100 (Hizukuri et al., 1981). Amylose content varies considerably among different starches, and genetic modifications have been carried out to obtain starch with amylose contents varying from 0% to .75%. The amylose fraction usually can be extracted by aqueous leaching procedures, dispersion, and precipitation (Adkins and Greenwood, 1969; Banks et al., 1971; Hizukuri, 1996). Considerable variations in the amylose content of sweet potato starches have been reported by various researchers (Chen et al., 2003; Noda et al., 1992; Zhu et al., 2011). Amylose content and degree of polymerization (DP) of amylose play a key role in influencing the physicochemical and technological properties of starch. Amylopectin Compared to amylose, the structure of amylopectin is more complex since 4% 5% of the total linkages form branches. Amylopectin molecules exist as heterogeneous mixtures and are thus usually characterized by the average values of DP and “chain length” (CL). CL is the total amount of

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carbohydrate divided by the number of nonreducing end groups (Chen et al., 2003; Hoover, 2001). The distribution of CL can be determined by size-exclusion chromatography and high-performance anion exchange chromatography with pulsed amperometric detection after debranching of amylopectin with isoamylase or pullulanase (Hizukuri, 1996). The α-acetal linkages connect C-1 of one glucose to C-4 of the next glucose molecule. The branches are formed by linking C-1 to C-6 through acetal linkages (Ophardt, 2003). Others The moisture content of air-equilibrated starch from cereals, roots, and tubers varies from about 6% to 18% (Moorthy, 2002; Tester et al., 2004). The moisture content prescribed for safe storage of dry starch in most of the starch producing countries is 13%. Higher levels of moisture can lead to microbial damage and subsequent deterioration in quality (Moorthy, 2002). The quantities of other components, such as lipids, protein, and minerals, are strongly influenced by the extraction methods and several other factors including the age of the crops and environmental conditions (Nkala et al., 1994). Lipids form another important component that has a strong effect on the starch properties (Sriroth et al., 1998). Generally, cereal starches contain higher lipid content (0.2% 0.8%) and protein (0.2% 0.5%) than root and tuber starches (Chen et al., 2003). Phosphorus content in sweet potato starch is similar to that in cassava starch (Asaoka et al., 1992), but much less than that in potato starch. Takeda et al. (1986) also found that in sweet potato, amylose contains less phosphorus (3 6 μg/g) than amylopectin (117 144 μg/g). Unlike other minerals phosphorus exerts significant effects on the functional properties of starches. High phosphorus content can impart high viscosity to starch and also improve its gel strength. High phosphorus starches can find use in food applications requiring high gel strength, such as jelly, etc. (Moorthy, 2002). Structural characteristics of starch Numerous investigations have been done to establish the level of intergranular organization within starch granules. The techniques used vary from X-ray diffraction (XRD) to atomic force microscopy and Fourier transform infrared spectroscopy, etc. In the native form of starch, amylose and amylopectin molecules are organized in granules as alternating semicrystalline and amorphous layers that form growth rings (Jobling, 2004).

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The semicrystalline layer consists of ordered regions composed of double helices formed by short amylopectin branches, most of which are further ordered into crystalline structures known as the crystalline lamellae. The amorphous regions of the semicrystalline layers and the amorphous layers are composed of amylose and nonordered amylopectin branches. Starch crystallinity has been assigned to the well-ordered structure of the amylopectin molecules inside the granules (Moorthy, 2002). Starches present either A, B, or C crystalline types, and the C type is a mixture of the A and B type crystalline patterns. Most cereal starches display the A pattern, while tuber starches (e.g., potato, lily, canna, and tulip) exhibit the B pattern (Tester and Karkalas, 2002). Sweet potato starch was reported to possess the A and C patterns, or be intermediate between A and C (Moorthy, 2002). According to the report of Takeda et al. (1986), the A pattern was observed for two varieties, whereas another cultivar exhibited the C pattern. The distribution of crystallites in starch granules is an important factor controlling the rate of hydrolysis. Gérard et al. (2001) reported that B and C type crystalline patterns showed greater resistance to enzymatic hydrolysis compared to the A type pattern. Gelatinization and pasting properties of starch Gelatinization properties of starch are analyzed using differential scanning calorimetry. When starch is heated in the presence of abundant water, it results in the complete loss of crystallinity. This transformation process is referred to as “Gelatinization.” During gelatinization the structures of the starch granules are disrupted by extensive swelling of the granules. This is measured by the loss of birefringence and the point at which birefringence first disappears is regarded as the “gelatinization point” or “gelatinization temperature” (Whistler and Daniel, 1985). Gelatinization endotherms reflect the hydrogen-bond dissociation of double helices (Tester and Sommerville, 2003). Garcia and Walter (1998) investigated two Peruvian sweet potato varieties cultivated at different locations and found a range for the onset of gelatinization (To) to be between 58°C and 64°C, peak gelatinization (Tp) to be between 63°C and 74°C, and gelatinization conclusion (Tc) to be between 78°C and 83°C. The cultivation location was found to influence the thermal parameters. Various starch modification processes have also been reported to significantly affect the thermal properties of starches (Kawai et al., 2007; Umemoto et al., 2002).

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Starch paste is an interesting system for rheological studies because of its viscoelastic behavior. The utilization of starch in the textile, paper, adhesives, and food industries depends on the pasting properties, which include pasting temperature, peak viscosity (PV), breakdown viscosity (BDV), final viscosity (FV), and setback viscosity (SBV). Different varieties of sweet potato starches exhibit considerable variation in their pasting characteristics. However, sweet potato and cassava starches have some similarities in their pasting properties (Moorthy, 2002). The pasting temperature is the temperature at which a perceptible increase in viscosity occurs. This is always higher than the gelatinization temperature and is usually measured using a viscometer, such as a Brabender Viscograph or Rapid Visco Analyzer (RVA). The pasting temperature of sweet potato starch obtained using a Brabender Viscograph varied between 66.0°C and 86.3°C, while microscopic determination gave values between 57 70°C and 70 90°C (Moorthy, 2002). Based on previous investigations, sweet potato starch pastes possess a rigid viscous behavior and low gel strength (Jangchud et al., 2003). Collado et al. (1999) studied the pasting properties of 44 different sweet potato genotypes at 7% and 11% concentrations using RVA and worked out the correlations among the RVA parameters. They observed wide variation not only in the PV but also the broadness of peak, which has been attributed to another parameter, that is, time elapsed from the start of gelatinization to the time PV is reached. And a significant negative correlation was observed between PV and amylose content. Srichuwong et al. (2005) also reported that the PV of starch paste increased with low amylopectin content ratio, low amylose content, and large average granule size for 15 starches from different origins. Swelling power and solubility of starch Starch granule swelling ability is usually quantified by swelling power (the weight of sedimented swollen granules per gram of dry starch) or swelling volume (the volume of sedimented swollen granules per gram of dry starch) at the corresponding temperature (Konik et al., 2001). Swelling power provides evidence of noncovalent bonding between starch molecules. The degree of swelling and solubility in starches depends on the following factors: amylose amylopectin ratio, CL and molecular weight distribution, degree/length of branching, and conformation. Srichuwong et al. (2005) reported that the swelling power of starch granules increased with amylopectin unit chain ratio, and the swelling power and pasting

Sweet potato starch

33

properties were associated with the ratio of the relative molar distribution of amylopectin branch-chains in the starches from different botanical sources including sweet potato starch. The solubility of starch extracted from seven sweet potato collections from Peru increased to about 10% with an increase in temperature, while a higher degree of solubility (28%) was observed for the commercial starches. It was found that selection identity did not have noticeable effect, but location had significant influence at temperatures above 60°C (Garcia and Walter, 1998). Retrogradation of starch Sol stability or paste stability reflects the retrogradation tendency of starch pastes (Moorthy, 2002). Starch granules undergo irreversible swelling when heated in excess water above their gelatinization temperature, resulting in amylose leaching into the solution. In the presence of a sufficient starch concentration, this suspension will form an elastic gel on cooling. The molecular interactions that occur after cooling are mainly hydrogen bonding within the starch chains, known as retrogradation (Ratnayake et al., 2002). Amylose is considered to be primarily responsible for the short-term retrogradation process due to the fact that the dissolved amylose molecules reorient in a parallel alignment. The long-term retrogradation is represented by the slow recrystallization of the outer branches of amylopectin (Daniel and Weaver, 2000). The rate and the extent of retrogradation increase with the increase in the amount of amylose. In addition to the origin of the starch, retrogradation also depends on starch concentration, storage temperature, pH, temperature procedure, and the composition of the starch paste. Retrogradation is generally stimulated by a high starch concentration, low storage temperature, and pH values between 5 and 7 (Chen et al., 2003; Moorthy, 2002). Ishiguro et al. (2000) studied the retrogradation tendencies of starch isolated from 10 sweet potato cultivars with different amylose contents and CL distributions. Starches with fewer amylose and amylopectin molecules and higher contents of extrashort chains (around DP 10) retrograded more slowly compared to the others. Digestibility of starch The digestibility of starch by enzymes is important for evaluating their nutritive value as well as their use for various industrial purposes. A number of researchers have reported the effect of the action of amylases on the sweet potato starch granules (Noda et al., 1992; Rocha et al., 2010;

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Sweet Potato

Zhang and Oates, 1999; Zhu et al., 2011). These studies showed that starches varied in their susceptibility to enzymatic hydrolysis, and the variability in their susceptibilities to the interaction were attributed to various factors including the starch source, granule size, amylose lipid complex, type of enzymes, and hydrolysis conditions (Tester et al., 2006). Among noncereal starches, cassava starch had considerably higher susceptibility to enzymatic hydrolysis than other starches, such as potato and sweet potato starches (Zhang and Oates, 1999). The digestibility of raw starches from eight sweet potato varieties by glucoamylases was compared by Noda et al. (1992), who found that the digestibility was greater than 80% after 24 h of hydrolysis, and no significant correlation was observed between digestibility and amylose content of the starches.

Starch modification Starch modification involves the alteration of the physicochemical properties of native starch to improve its functional properties (Hermansson and Svegmark, 1996). The modification of native granular starches profoundly alters their gelatinization, pasting, and retrogradation properties (Choi and Kerr, 2003). Starch has been modified by various methods to achieve functionalities suitable for diverse industrial applications (Adebowale et al., 2006; Kawai et al., 2007; Lawal et al., 2005; Olayide, 2004). There are four broad kinds of modifications: chemical, physical, enzymatic, and genetic, and the main common modification methods are chemical, physical, and enzymatic. Different modification methods of starches Chemical modification of starches Chemical modification involves the introduction of functional groups into the starch molecules, resulting in markedly altered physicochemical properties. Such modification of native granular starches strongly changes the proximate compositions, gelatinization, retrogradation, and pasting characteristics. Modification is generally achieved through derivatization, for example, acetylation, cationization, oxidation, acid hydrolysis, and crosslinking. There has been a resistance toward the use of chemically modified starches in food applications since many chemicals are used for this kind of modification (Moorthy, 2002). Nevertheless, within the last decade researchers have developed an intense interest in developing novel methods of starch modification with greater emphasis on physical and enzymatic modifications.

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Physical modification of starches Physical modification techniques can be safely used for food products as they do not involve chemicals. The use of high hydrostatic pressure (HHP) treatment for the physical modification produces starches with unique retrogradation and thermal properties. HHP-gelatinized starches show a lower quantity of released amylose (Stolt et al., 2001). HHP processing has been previously reported to result in the partial gelatinization of starch water mixtures at room temperature without altering their granular integrity (Blaszczak et al., 2005; Kawai et al., 2007). The extent of starch gelatinization during HHP treatment has been reported to be influenced by the starch concentration, holding time, temperature, and the starch origin (Bauer and Knorr, 2004; Kawai et al., 2007). And the degree of gelatinization (DG) starch suspensions increased with the increase in the pressure and decrease in starch content (Katopo et al., 2002). Enzymatic modification of starches Enzymatic modification mainly involves the use of hydrolyzing enzymes to modify the properties of starches (Kaur et al., 2012). Enzymatic hydrolysis of starches could be very effective in understanding the internal structure of starch granules. Amylolytic enzymes are the major type of enzymes involved in the breakdown of starch molecules. The amylases can be categorized into α-amylases, β-amylases, glucoamylases, and debranching enzymes depending on the configuration of the substrate involved or products formed (Goesaert et al., 2010). Furthermore, the amylases have two main classes, endo- and exo-acting enzymes, according to their type of action on substrates. A common type of α-amylase is the endo-acting enzymes that hydrolyze α-D-(1,4)-glycosidic linkages specifically depending on its origin, and internally yielding soluble products such as oligosaccharides and branched and low molecular weight α-limit dextrins; β-amylases are exo-acting enzymes, which hydrolyze the α-(1,4)linkages, beginning at the nonreducing ends of starch molecules, to form β-maltose and β-limit dextrins; while isoamylase and pullulanase are debranching enzymes which hydrolyze only the α-D-(1,6)-glycosidic bonds in an amylopectin chain branch which elevate linear short chains of glucan polymers and form the high-amylose starch (Butler et al., 2004). Value addition of starches by modification Starch modification is aimed at overcoming one or some of the shortcomings of native starches to enhance their versatility and to satisfy consumer

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demand, such as the loss of viscosity and thickening power, retrogradation characteristics, and syneresis (Tharanathan, 2005). In a way starch modification provides desirable functional attributes and economic alternative choices to other hydrocolloid ingredients, thus enhancing the versatility of starch and satisfying consumer demand. Nowadays, more concern has been shown to reducing the dietary calorie intake to avoid obesity complications by health-conscious people. Some of the starch derivatives are increasingly used as fat replacers or fat substitutes, and these derivatives are partially or totally undigested and contribute zero calories to the food consumption (Tharanathan, 2005). When hydrated starch-based fat replacers provide a slippery mouthfeel with various sensory perceptions depending on the different modification types, which work well in food systems with high moisture, such as meat emulsions, salad dressings, and some bakery products. Also resistant starch, an undigested starch fraction, has been found to have beneficial nutritional effects on humans. Resistant starch is a highly retrograded amylose fraction of starch, which can escape digestion in the small intestine but is fermented later in the colon to short-chain fatty acids (Weaver et al., 1992), and is considered to be a man-made dietary fiber with high physiological value and a potential ingredient of low calorie food. In addition, as a main source of starches, sweet potato has played an important role in the Chinese economy. Its characteristic high yield and wide adaptability once made great contributions to feeding the dramatically increasing Chinese population. The group members in the lab of the Potato and Sweet Potato Food Science Innovation Team, CAAS, China have investigated the structural and physicochemical characteristics of sweet potato starches and and/or their modified starches. The findings are summarized in the following sections.

Structural and physicochemical characteristics of sweet potato starch In China, the major commercial use of sweet potato is for its starch and the preparation of starchy food. As the physicochemical properties of starches dictate their functionality in various applications, this section therefore introduces the physicochemical properties of starches isolated from 11 sweet potato cultivars (Mixuan no.1, Chuangshu 217, Xichengshu 007, Xushu 28, Luoshu 10, Shangshu 19, Xushu 22, Xushu

Sweet potato starch

37

27, Chuanshu 34, Xushu 18, and Shi 5) popularly used for starch production in different regions of China.

Proximate composition The chemical composition of the different sweet potato starches is shown in Table 3.2. The purity of the starches was reasonably high ( . 91%). Moisture content (3.86% 6.52%) falls within the moisture level (,20%) recommended for commercial starches (Soni et al., 1993). It is also within the range (,13%) recommended for safe storage in most starch producing countries (ISI, 1970). The protein content of the starches varied between 0.28% and 0.75% with Mixuan no.1 having the highest protein content. A lower level of protein (0.23%) was previously reported for Xushu 18 by Chen et al. (2003), which could be ascribed to the extent of the removal of protein present in the starting material. The ash content varied significantly among the starches with values ranging from 0.10% to 0.47% (P , .05). This falls within the limit (#0.5%) recommended for grade A industrial starches (Radley, 1976). Many of the starches contained no lipid, except for starches of Shangshu 19 and Xushu 22 sweet potato cultivars. Amylose and amylopectin content varied significantly among the starches with values ranging from 13.33% to 26.83% and 73.17% to 86.67%, respectively. Among the starches, Chuanshu 217 showed the highest amylose content while the lowest was found in Chuanshu 34. Amylose and amylopectin content plays an important role in influencing the functional properties of starches. High-amylose starches are characterized by their high gelling strength which suggests their usefulness in the production of pasta, sweets, bread, and in coating fried products (Hung et al., 2005; Vignaux et al., 2005). Differences in the amylose content of sweet potato starches have been reported and ascribed to genotypic differences, environmental factors, and starch processing methods (Garcia and Walter, 1998; Oduro et al., 2000).

Thermal properties The thermal properties of the sweet potato starches are presented in Table 3.3. Gelatinization temperature is the temperature at which heated starch granules undergo the transition from a crystalline state to a gel. Starch gelatinization is an important parameter in starch characterization. The onset transition temperature (To) and peak temperature (Tp) of the starch ranged between 54.5 69.1°C and 62.5 75.9°C with mean values

Table 3.2 Chemical composition of sweet potato starches (w/w, %).a Cultivar

Origin

Moisture

Protein (dbb)

Ash (db)

Lipid (db)

Starch (db)

Amylose

Amylopectin

P3 (db)

Mixuan no.1 Chuanshu 217 Xichengshu 007 Xushu 28 Luoshu 10 Shangshu 19 Xushu 22 Xushu 27 Chuanshu 34 Xushu 18 Shi 5 Mean

Beijing

5.93 6 0.05cd

0.75 6 0.08a

0.15 6 0.01cd

0.00 6 0.00c

94.271.08abc

20.50 6 0.24f

79.5 6 0.24b

0.02 6 0.00

Sichuan

3.86 6 0.27h

0.33 6 0.01b

0.10 6 0.01d

0.00 6 0.00c

92.20 6 1.08dc

26.83 6 0.24a

73.17 6 0.24g

0.02 6 0.00

Sichuan

4.20 6 0.01g

0.34 6 0.09b

0.47 6 0.02a

0.00 6 0.00c

92.87 6 0.19bdc

24.17 6 0.24c

75.83 6 0.24e

0.02 6 0.00

Jiangsu Henan Henan

5.19 6 0.01f 5.77 6 0.03d 6.52 6 0.05a

0.31 6 0.06b 0.39 6 0.00b 0.39 6 0.01b

0.12 6 0.00d 0.31 6 0.11b 0.22 6 0.01c

0.00 6 0.00c 0.00 6 0.00c 0.05 6 0.01a

94.07 6 0.18abc 94.77 6 1.47ab 91.90 6 0.53d

22.00 6 0.47de 23.83 6 0.24c 21.50 6 0.24de

78.00 6 0.47dc 76.17 6 0.24e 78.50 6 0.24dc

0.02 6 0.01 0.02 6 0.00 0.02 6 0.00

Jiangsu Jiangsu Sichuan

6.12 6 0.00bc 5.79 6 0.03d 4.22 6 0.04g

0.32 6 0.10b 0.28 6 0.01b 0.29 6 0.02b

0.17 6 0.01cd 0.17 6 0.02cd 0.17 6 0.01cd

0.02 6 0.01b 0.00 6 0.00c 0.00 6 0.00c

93.17 6 1.49bdc 91.90 6 1.30d 93.13 6 0.58bdc

22.17 6 0.24d 21.33 6 0.47e 13.33 6 0.47g

77.83 6 0.24d 78.67 6 0.47c 86.67 6 0.47a

0.02 6 0.00 0.01 6 0.01 0.002 6 0.001

Jiangsu Sichuan

6.25 6 0.04b 5.53 6 0.04e 5.40

0.30 6 0.05b 0.30 6 0.05b 0.36

0.19 6 0.02cd 0.31 6 0.06b 0.214

0.00 6 0.00c 0.00 6 0.00c 0.01

95.60 6 1.10a 95.32 6 0.32a 93.56

23.50 6 0.24c 25.83 6 0.24b 22.27

76.50 6 0.24e 74.17 6 0.24f 77.73

0.01 6 0.00 0.01 6 0.00 0.02

P, phosphorus content. a Means in a column with the same letters are not significantly different at P , .05. b Dry basis.

Table 3.3 Physicochemical and thermal properties of various sweet potato starches.a ΔHgel (J/g)

Cultivar

Syneresis (%)

Swelling power (g/ g)

Solubility (%)

To (°C)

Tp (°C)

Tc (°C)

Mixuan no.1 Chuanshu 217 Xichengshu 007 Xushu 28 Luoshu 10 Shangshu 19 Xushu 22 Xushu 27 Chuanshu 34 Xushu 18 Shi 5 Mean

37.23 6 1.50fe 44.68 6 1.50a 41.22 6 1.13bc

13.46 6 0.22f 17.08 6 0.48e 17.43 6 0.55e

8.56 6 0.44e 11.56 6 0.23de 12.82 6 0.53d

68.18 6 0.36a 69.11 6 0.28a 68.55 6 0.07a

75.41 6 0.14ab 74.53 6 0.00bc 75.17 6 0.00ab

81.60 6 0.96cde 82.18 6 0.27cd 81.88 6 0.95dc

6.98 6 0.57c 6.40 6 0.48c 9.32 6 0.07b

13.43 6 1.32f 13.08 6 0.01f 13.33 6 1.02f

43.88 6 0.38ab 39.63 6 1.88cde 38.30 6 1.50de 37.23 6 0.00fe 39.89 6 0.75cde 32.45 6 0.75g 34.57 6 2.56cde 40.16 6 1.13cd 39.02

22.20 6 0.22dc 21.37 6 1.93d 23.95 6 1.70abc 26.13 6 0.99a 24.48 6 1.51abc 23.33 6 1.65bcd 25.62 6 0.22ab 23.63 6 1.19abcd 21.70

12.92 6 2.12d 12.35 6 0.37d 18.77 6 1.27ab 13.15 6 1.82cd 12.12 6 1.98d 11.62 6 2.65de 19.97 6 1.42a 16.20 6 0.58bc 13.64

54.54 6 0.08f 58.79 6 1.01d 63.93 6 0.10c 59.97 6 0.11d 64.36 6 0.71c 55.11 6 0.05ef 66.92 6 1.2b 56.00 6 0.0f 62.31

63.81 6 0.00f 72.74 6 1.42d 73.90 6 0.00c 66.75 6 0.42e 75.90 6 0.19a 66.91 6 0.00e 75.84 6 0.57a 62.54 6 0.21g 71.23

78.88 6 0.74fg 82.75 6 0.54cd 84.69 6 0.34a 81.26 6 0.34de 84.46 6 0.12ab 80.16 6 0.54ef 83.03 6 1.08bc 78.59 6 0.88g 81.77

11.64 6 0.54a 10.84 6 1.53a 9.99 6 0.40b 10.08 6 0.39b 9.78 6 0.89b 11.89 6 0.26a 8.88 6 0.53b 8.95 6 0.46b 9.52

24.34 6 0.82ab 23.97 6 1.55ab 20.76 6 0.44dc 21.29 6 0.23dc 20.10 6 0.59d 25.06 6 0.59a 16.12 6 2.28e 22.59 6 0.91bc 19.46

To, Onset transition temperature; Tp, peak transition temperature; Tc, conclusion transition temperature; R, gelatinization range (Tc a Means in a column with the same letters not significantly different at P , .05.

To); ΔHgel, enthalpy of gelatinization.

R (°C)

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of 62.31°C and 71.23°C, respectively. Starches of Xushu 28, Xushu 22, Luoshu 10, Chuanshu 34, and Shi 5 showed significantly lower To, Tp, and conclusion transition temperature (Tc) compared to starches of other cultivars. Vasanthan et al. (1999) indicated starches with higher gelatinization transition temperatures (To and Tp) and enthalpy would require a higher heat of solubilization. However, sweet potato starches with higher transition temperatures, such as Mixuan no.1, Chuanshu 217, and Xichengshu 007, showed lower enthalpy of gelatinization. On the other hand, starches of Xushu 28, Luoshu 10, and Chuanshu 34 with lower transition temperatures showed higher enthalpy of gelatinization (WHgel) with values ranging from 11.64, 10.84, and 11.89 J/g, respectively. The gelatinization parameters (To, Tp, Tc, and WHgel) are strongly influenced by the molecular architecture of the crystalline region of starches (Noda et al., 1992). WHgel mainly reflects the loss of molecular order within the internal structure of starches (Cooke and Gidley, 1992). The gelatinization range (R) of the sweet potato starches varied significantly. Chuanshu 34 showed the highest gelatinization range, while the lowest was observed in Chuanshu 217. The gelatinization ranges showed that the numbers of double helices (in the amorphous and crystalline domains) that disentangled and melted during gelatinization were relatively similar in Mixuan no.1, Chuanshu 217, and Xichengshu 007 starches compared to the starches of other cultivars. It also showed that the degree of heterogeneity of the starch crystallites within Mixuan no.1, Chuanshu 217, and Xichengshu 007 starch granules was lower than those of other starches (Ratnayake et al., 2001). Furthermore, the variation in the gelatinization properties of the starches could be attributed to various factors including mineral composition, proportion of large and small granules, and the molecular architecture of the crystalline region of starches (Kaur et al., 2007).

Scanning electron micrograph The micrographs of the granules of the various sweet potato starches are shown on Fig. 3.1. The shapes of the various starch granules varied from polygonal, round, to cupuliform/bell shapes.

General physicochemical properties Color and granule sizes The color of the various sweet potato starches is presented in Table 3.4. There were significant differences in the color of the sweet potato starches

Sweet potato starch

41

Figure 3.1 Scanning electron micrographs of starch granules of various sweet potato cultivars, showing diversity in shapes and sizes. Values in parentheses denote the degree of magnification. (A) Mixuan no.1 ( 3 3500); (B) Xichengshu 007 ( 3 1000); (C) Xushu 28 ( 3 600); (D) Xushu 18 ( 3 3500); (E) Chuanshu 34 ( 3 3500); (F) Xushu 27 ( 3 1000); (G) Xushu 27( 3 600); (H) Shi 5 ( 3 3500).

Table 3.4 Color, granule size distribution, and shapes of various sweet potato starches.a Cultivar

Color 

L

Mixuan no.1 Chuanshu 217 Xichengshu 007 Xushu 28 Luoshu 10 Shangshu 19 Xushu 22 Xushu 27 Chuanshu 34 Xushu 18 Shi 5 Mean a

97.44 6 0.11fab 95.76 6 0.15g 96.75 6 0.15de 94.47 6 0.02h 97.19 6 0.32bc 97.61 6 0.16a 96.88 6 0.06de 96.44 6 0.06f 96.05 6 0.08g 96.65 6 0.08ef 96.98 6 0.09c 96.56

a



0.01 6 0.00cd 0.24 6 0.01b 0.02 6 0.01d 1.21 6 0.13e 0.03 6 0.01cd 0.03 6 0.01cd 0.22 6 0.02b 0.08 6 0.01cd 0.38 6 0.01a 0.00 6 0.00cd 0.09 6 0.01c 0.01



b

0.75 6 0.01de 0.61 6 0.03ef 0.68 6 0.01e 4.21 6 0.21a 0.89 6 0.00d 1.06 6 0.05c 0.51 6 0.06f 1.67 6 0.02b 0.63 6 0.02ef 1.62 6 0.05b 1.54 6 0.04b 1.29

Means in a column with the same letters are not significantly different at P , .05.

Diameter range (µm)

Average diameter (µm)

Shape

0.85 0.76 0.76 0.76 0.85 0.85 0.76 0.85 0.85 0.85 0.85

13.07 6 0.04a 8.83 6 0.23fg 9.43 6 0.24f 9.31 6 0.09f 8.67 6 0.00g 9.08 6 0.00c 8.10 6 0.03h 10.91 6 0.24d 12.37 6 0.12b 11.53 6 0.28cd 9.93 6 0.01e 10.31

Round, Round, Round, Round, Round, Round, Round, Round, Round, Round, Round,

44.69 26.17 29.12 29.12 26.17 36.08 23.51 36.08 40.15 40.15 32.41

cupuliform, cupuliform, cupuliform, cupuliform, cupuliform, cupuliform, cupuliform, cupuliform, cupuliform, cupuliform, cupuliform,

polygonal polygonal polygonal polygonal polygonal polygonal polygonal polygonal polygonal polygonal polygonal

Sweet potato starch

43

with starch of Shangshu 19 being the whitest. The starch of Chuanshu 34 was found to be redder than other starches, while Xushu 28 showed a greater yellowness. Color is an important criterion in evaluating starch quality. Any form of pigmentation in starch will negatively affect its acceptability and that of its products (Galvez and Resurrection, 1992). A high value of lightness is desired for starches. In the case of starch particle size distribution (Table 3.4), Mixuan no.1 cultivar showed the highest mean granule size (13.07 μm) and the widest granule size range of 0.85 44.69 μm. On the other hand, starch of Xushu 22 showed the lowest mean granule size (8.10 μm) and the narrowest granule size range (0.76 29.12 μm). Hoover (2001) reported sweet potato starch granules as round, oval, and polygonal with sizes ranging from 2 to 42 μm. In comparison to granules of starches from other sources, higher (25.8 μm), lower (1.05 1.32 μm), and relatively similar values (7.3 9.7 μm) values of mean granules sizes were reported for potato, amaranth, and cassava starches, respectively (Chen et al., 2003; Kong et al., 2009). The differences in the granule sizes of the starches are presumably attributed to cultivar differences, growing conditions, and plant physiology. Moreover, starch granule size plays a significant role in influencing the pasting parameters of starches (Noda et al., 2004; Zaidul et al., 2007). Fine starch granules could be used as fat substitutes in high fat foods (Ma et al., 2006). However, starches with larger proportions of small starch granules, like Xushu 22, Luoshu 10, and Chuanshu 217, will find use in applications requiring relatively small starch granules. Digestibility, syneresis, swelling power, and solubility The enzyme digestibility of raw sweet potato starches as measured by pancreatin hydrolysis exhibited significant differences as shown in Fig. 3.2. Enzyme digestibility of raw starches is an important factor to be considered when evaluating their usefulness in diverse food applications. The digestibility of the starches showed variations from 10.35% in Xichengshu 007 to 15.15% in Xushu 18 cultivar with a mean value of 14.00%. The variability in the digestibility of the various starches might be due to environmental conditions associated with the crop growth location, such as temperature, precipitation, and soil. In addition, granule size and the structural characteristics of starches have been previously observed to exert substantial influence on in vitro digestibility of starches (Jayakody et al., 2005; Szylit et al., 1978). Also the interactions of various factors, including starch source, amylose, lipid complex, binding site, hydrolysis condition,

44

Sweet Potato

Figure 3.2 Enzyme digestibility of starches of various sweet potato cultivars. Error bars represent standard deviations. Columns with the same letters are not significantly different at P , .05.

and type of hydrolyzing enzyme, influence starch digestibility (Rocha et al., 2010). However, no significant correlation was found between the digestibility and the amylose content of starches obtained from eight sweet potato cultivars hydrolyzed by glucoamylases (Noda et al., 1992). Syneresis, an index for the degree of starch retrogradation at low temperatures is presented in Table 3.4. After 7 days of storage, the percentage of syneresis varied significantly between 32.45% and 44.68% with a mean value of 39.02%. Starch paste of Chuanshu 217 exhibited a higher retrogradation tendency due to the large volume of water expelled during the retrograding process compared with other starches regardless of the storage period, while Chuanshu 34 showed the lowest syneresis. The higher retrogradation tendency observed in the starch pastes from Chuanshu 217 might be due to its higher amylose content (Singh et al., 2003). Syneresis exhibited a significant positive correlation with amylose content (r 5 0.70, P # .05), while it showed a negative correlation with mean granule size (r 5 20.59, P # .05). The structural arrangement of the chains within the amorphous and crystalline regions of starches has been reported to strongly influence the interaction that occurs between these starch chains during gel storage (Singh et al., 2006).

Sweet potato starch

45

To understand the interactions between the water molecules and the starch chains in the crystalline and amorphous regions during heating, the swelling power and solubility of the starches are shown in Table 3.4. The various sweet potato starches exhibited different swelling power and solubility when heated in water at 90°C. The swelling power of the starches from the various sweet potato cultivars ranged from 13.46 to 26.13 g/g. Starch of Xushu 22 showed the highest swelling power, while the lowest was observed in Mixuan no.1 cultivar. The solubility of the starches ranged from 8.56% to 19.97%, with the lowest found in Mixuan no.1, whereas Xushu 18 showed the highest solubility. Differences in the swelling power and solubility of the starches could be attributed to the variations in the associative bonding forces within the starch granules. Previous studies attributed differences in the swelling and solubility patterns of starches to differences in amylose content, phosphorus, and starch granular properties (Kaur et al., 2007). However, no significant correlation was observed between the swelling power and amylose content in our studies but the swelling power and solubility of the starches positively correlated with each other (r 5 0.64, P # .05).

Pasting properties Sweet potato starches exhibited significant variations in their pasting behaviors (Table 3.5). The plant source, starch purity, and the interactions among starch components strongly influence the pasting properties of starches. The PV of the starches varied from 134 to 255 BU, being lowest for Shangshu 19 followed by Xushu 28 and being highest for Chuanshu 34 followed by Xichengshu 007, with an average value of 209.09 BU. Starch of Xushu 28 showed the lowest hot paste viscosity (HPV), while the highest was observed in Xushu 18. Aina et al. (2012) stated that starches with high HPV would be preferred in applications which require high starch consistency during prolonged cooking. However, amylose leaching, amylose lipid complex formation, friction between swollen granules, and granule swelling have been reported as the key influencing factors of HPV (Singh et al., 2006). BDV, a measure of the starch paste resistance to heat and shear, varied significantly between 91 and 162 BU in the various sweet potato starches, the lowest and highest values being observed for Shangshu 19 and Chuanshu 34 cultivars, respectively. The lower BDV observed in Shangshu 19 starch cultivar suggested its greater resistance to shear as

46

Sweet Potato

Table 3.5 Pasting properties of various sweet potato starches.a Cultivar

PV (BU)

HPV (BU)

BDV (BU)

CPV (BU)

SBV (BU)

PT (Min)

Ptemp (°C)

Mixuan no.1 Chuanshu 217 Xichengshu 007 Xushu 28 Luoshu 10 Shangshu 19 Xushu 22 Xushu 27 Chuanshu 34 Xushu 18 Shi 5 Mean

236d 226d 248b 138i 202f 134j 200g 237c 255a 236d 188h 209.09

81f 82e 91d 8k 75g 43j 71h 100b 93c 101a 58i 73

155c 144d 157b 130h 127j 91k 129i 137e 162a 135f 130g 136.09

167d 166d 201b 120g 134e 69i 130f 200b 192c 211a 94h 153.1

86e 84f 110b 112a 59g 26i 59g 100c 99d 110b 36h 80.09

6.90b 6.67f 6.73e 6.37h 7.03a 6.47g 6.30i 6.77d 6.00j 6.80c 5.97k 6.55

73.20a 72.50c 72.30d 69.70i 71.00g 71.10f 70.20h 72.60b 67.20k 72.10e 67.90j 70.89

PV, peak viscosity; HPV, hot paste viscosity; BDV, breakdown viscosity; CPV, cold paste viscosity; SBV, setback viscosity; PT, peak time; Ptemp, pasting temperature. a Means with the same letters in the same column are not significantly different at P , .05.

compared to the starches of other cultivars. SBV showed the tendency of starch pastes to retrograde. Starches of Xichengshu 007, Xushu 18, Xushu 27, and Xushu 28 showed higher retrogradation tendency due to their higher SBV. On the other hand, starch of Shangshu 19, Shi 5, Luoshu 10, and Xushu 22 showed lower SBV suggesting lower retrogradation tendency. The peak time (PT) of the sweet potato starches ranged from 5.97 to 7.03 min. The highest value was observed in Shi 5, while the lowest was observed in Luoshu 10. Low swelling starches are characterized by high PTs. The pasting temperature (Ptemp) of the starches varied significantly from 67.20°C to 73.20°C in starches of Chuanshu 34 and Mixuan no.1, respectively, with a mean value of 70.89°C. Pasting and thermal properties are the most important properties when considering starches for use as gelling and thickening agents. Starches with relatively high PV, high BDV, and low SBV like Luoshu 10, Xushu 22, and Shi 5 could be considered for use as thickening or gelling agents. However, low PV starches like Xushu 28 and Shangshu 19 would be suitable for the manufacture of weaning foods where low paste viscosity food ingredients are required. At present cereals used in weaning food applications need to be malted to reduce the viscosity of their pastes (Akingbala et al., 2002). Compositional and morphological properties of starch, such as amylose content, phosphorus content, and mean granule

Sweet potato starch

47

size, play crucial roles in influencing the pasting and rheological properties of starches (Liu et al., 2003; Singh et al., 2006; Zaidul et al., 2007). Overall the variability observed among the physicochemical properties of these various sweet potato starches further illustrated their useful potentials in various food and nonfood applications.

Structural and physicochemical properties of retrograded chemically modified sweet potato starch Sweet potato starches can result in a high glycemic index (GI) after cooking or gelatinization. However, a food product with low GI is preferable, not only in obese patients and individuals with diabetes, but also in healthy individuals (Björck and Asp, 1994). Thus it is necessary to improve the starch digestion resistibility and physicochemical properties of sweet potato starches to enhance their health value. This section therefore introduces the enhancement of the starch digestion resistibility of retrograded chemically modified sweet potato starches by retrogradation and further acetylation, as well as the changes of their physicochemical and morphological properties.

Content of resistant starch There are four kinds of resistant starch: Type 1 is physically inaccessible starch; Type 2 is nongelatinized starch; Type 3 is retrograded starch; and Type 4 is physically or chemically modified starch (Haralampu, 2000; Englyst et al., 1983, 1992). Retrogradation is the most common method to make resistant starch, because the processing method is very simple (Bao et al., 2007; Wu et al., 2009; Bravo et al., 1998). Acetylation is generally used to prepare Type 4 resistant starch. Recently, it was found that resistance to amyloglucosidase activity of acetylated retrograded (Type 3/ Type 4-retrograded chemically modified) potato starch was higher than that of retrograded and acetylated potato starch (Zie˛ba et al., 2011a,b, 2014). The retrograded sweet potato starch (RS) was obtained by suspending native sweet potato starch (NS) in distilled water, heating in boiling water to gelatinize the starch, cooling, freezing first and then melting, rinsing with distilled water, centrifuging, oven-drying, grinding, and sieving, while acetylated retrograded sweet potato starch (ARS) was prepared with acetic anhydride (Yu et al., 2015). The NS had high resistance to α-amylase without any processing, of which the resistant starch content

48

Sweet Potato

was 68.45%, and thus belonging to the goup of resistant starch granules (Type 2). The resistant starch content of RS was increased by retrogradation to 37.24% (Type 3). An acetyl residue can substitute the hydrogen of hydroxyl at the second and third carbon atoms that are neighboring the α-1,4 glycosidic bond, and thus it can inhibit the hydrolysis of amylase— enhance the hydrolysis resistance of starch (Sha et al., 2012). Therefore further acetylation dramatically increased the resistant starch content (Type 4, a kind of chemical modification starch due to joint chemical reagents) of ARS to 48.02%.

Thermal properties The gelatinization and retrogradation transition temperature and enthalpy of two times scanning were presented in Table 3.6. The retrogradation and further acetylation significantly influenced the To, Tp, and R1/2. The retrogradation decreased the WHgel of NS from 12.63 to 5.85 J/g, while further acetylation not only decreased the WHgel of RS from 5.85 to 2.64 J/g, but also significantly changed the To, Tp, and R1/2. After 2 days of retrogradation, the To, Tp, and WHgel of NS, RS, and ARS were significantly deceased; and the R1/2 of them became broader. The endothermic enthalpy and transition temperature of retrograded starch were usually lower than that for the native starch, and the temperature range of retrograded starch became broader (Singh et al., 2007). The retrogradation degree (RD) of NS was increased by retrogradation from 26.62% to 39.89%. Further acetylation treatment also increased the RD of RS from 39.89% to 52.30%, which was in agreement with the resistant starch content and digestion resistibility. Table 3.6 Gelatinization parameters of native, retrograded, and further acetylated sweet potato starch. Sample

To (°C)

NS

72.93 6 0.27 50.15 6 0.89b 52.98 6 0.24d 49.19 6 1.44c 53.60 6 0.25d 56.51 6 4.22a

RS ARS

1st 2nd 1st 2nd 1st 2nd

Tp (°C) a

WHgel (J/g)

R1/2 (°C)

79.93 6 0.18 61.75 6 0.56c 62.42 6 0.24e 61.69 6 0.48c 64.05 6 0.30d 64.40 6 0.50a b

7.29 6 0.18 12.23 6 0.87c 10.34 6 0.17c 13.97 6 1.42b 13.91 6 0.72b 12.65 6 1.18c d

RD (%)

12.63 6 0.31 3.36 6 0.59a 5.85 6 0.15b 2.33 6 0.28b 2.64 6 0.19c 1.39 6 0.24d a

26.62 6 4.62e 39.89 6 4.84c 52.30 6 5.62b

To, onset transition temperature; Tp, peak transition temperature; R1/2, half temperature range; WHgel, enthalpy of gelatinization; RD, retrogradation degree. Values followed by the different letter in the same column are significantly different (P , .05).

Sweet potato starch

49

Scanning electron micrograph Fig. 3.3 shows scanning electron micrograph (SEM) of both original and heated (boiling water for 20 min) NS, RS, and ARS. ARS is more compact than RS, because the acetylation will lead to the aggregation and fusion of starch, and acetylated granules also formed grooves on the surface of particles (Das et al., 2010). After heat treatment each starch sample changed to the porous structure, which gave them bigger surface area. However, with the retrogradation and further acetylation, the pores of heat-treated starches became bigger and the wall became more compact and thicker.

General physicochemical properties Digestion resistibility The resistance of enzyme digestibility of nonheat-treated and heat-treated NS, RS, and ARS are shown in Fig. 3.4. The digestion resistibility between nonheat-treated and heat-treated NS had more significant differences than RS and ARS, because the native starch in NS was totally gelatinized after heating in boiling water for 20 min since it was easily hydrolyzed by enzyme. The resistibility of RS also showed a significant decrease between the nonheat-treated and the heat-treated forms, but the resistibility of ARS showed the least difference. Therefore ARS were more resistant to heat treatment than NS and RS, and thus further acetylation can significantly increase the digestion resistibility and thermal stability of starch. Zie˛ba et al. (2011b, 2014) found that acetylated retrograded starch was more resistant to the hydrolysis of amyloglucosidase than native and retrograded starch. Water-soluble index, water absorption index, and swelling capacity The water-soluble index (WSI), water absorption index (WAI), and swelling capacity (SWC) of NS, RS, and ARS are shown in Table 3.7. The RS showed the lowest WSI (4.29/100 g dry solids) compared to NS and ARS (P , .05). It was reported that the solubility of retrograded starch was lower than that of native starch (Zie˛ba et al., 2011a). The WAI and SWC of starches were decreased with the retrogradation and acetylation treatment (Table 3.7). As commonly observed, the retrogradation of starch would make the gelatinized starch recrystallize to form a more compact and dense structure. Compared to the NS, the water binding capacity of acetylated starch was slightly higher (Das et al., 2010), which

50

Sweet Potato

Figure 3.3 Resistance of the activity of pancreatin of NS, RS, and ARS. Nonheattreated, before pancreatin treatment without any pretreatment; heat-treated, heated in boiling water for 20 min before pancreatin treatment. Values followed by different letters in the same composition and sample are significantly different (P , .05).

Sweet potato starch

51

Figure 3.4 Scanning electron micrographs of the NS, RS, ARS, and heat-treated sample, respectively. HNS, HRS, and HARS are heat-treated NS, RS, and ARS, respectively. Table 3.7 WSI (water-soluble index), WAI (water absorption index), and SWC (swelling capacity) of native, retrograded, and further acetylated sweet potato starch. Sample

WSI (g/100 g dry solids)

WAI (g/g dry solids)

SWC (g/g dry solids)

NS RS ARS

6.08 6 0.33e 4.29 6 0.18f 13.77 6 0.28c

16.49 6 0.55a 14.91 6 0.94b 13.18 6 0.29c

17.55 6 0.45a 15.58 6 0.38b 15.29 6 0.53b

Values followed by the different letters in the same column are significantly different (P , .05).

was due to the fact that the water could easily bind in the looser area in amylose and amylopectin (Singh et al., 2004). Particle size distribution The particle size distribution parameters are shown in Table 3.8. The starch particle size became bigger and the span (range) became broader after retrogradation and further acetylation. From the ratio of particle size distribution, the main particle sizes of NS, RS, and ARS were around 13, 20, and 30 μm, respectively. Acetylation could cause a slight aggregation or a cluster of starch particles (Das et al., 2010).

Pasting property Among starch samples, ARS showed the lowest viscosity, with PV of 24 mPa s, trough viscosity (TV) of 16 mPa s, and FV of 21 mPa s

52

Sweet Potato

Table 3.8 Particle size distribution parameters of native, retrograded, and further acetylated sweet potato starch (μm). Sample

NS RS ARS

Span (range)a

D50b

D[4,3]c

35.37 6 1.09 (0.69 36.09)e 39.39 6 1.25 (0.76 40.15)e 116.08 6 2.10 (1.05 117.13)c

9.83 6 0.39e 16.11 6 0.10d 20.27 6 0.26c

10.65 6 0.39e 18.09 6 0.14d 27.16 6 0.28c

Values followed by the different letter in the same column are significantly different (P , .05). a Range representing the particle size distribution range of sample and Span was the wide of the range. b D50 representing 50% of total particle size distribution. c D[4,3] representing particle size derived from the volume distribution.

Table 3.9 The viscosity parameters of native, retrograded, and further acetylated sweet potato starch. Sample

PV (mPa s)

TV (mPa s)

NS RS ARS

1909 6 33 1323 6 28c 24 6 5e a

1213 6 35 1298 6 43a 16 6 2e

b

BD (mPa s)

FV (mPa s)

SB (mPa s)

PT (min)

696 6 24 25 6 3cd 8 6 3cd

1784 6 34 1970 6 37e 21 6 6e

571 6 10 672 6 23a 5 6 3e

4.6 6 0.1 7 6 0.1a 2.1 6 1.4c

b

ab

b

Ptemp (°C) b

80 6 0.0c 70.2 6 0.3d ND

PV, peak viscosity; TV, trough viscosity; BD, breakdown viscosity; FV, final viscosity; SB, setback; Pt, peaking time; Ptemp, pasting temperature. Values followed by the different letter in the same column are significantly different (P , .05).

(Table 3.9). Thus further acetylation can significantly decrease the viscosity of RS. Colussi et al. (2014) found that the viscosity of acetylated potato starch was lower than that of native potato starch. Zie˛ba et al. (2014) also found that the viscosity of native potato starch was higher than that of retrograded starch and retrograded acetylated starch. Within the pasting property parameter, the PV represents the swelling ability of the samples, TV and BDV represent the stability and shear resistance of gelatinized sample, while FV and SBV represent the retrogradation properties of the sample and the ability to increase the consistency of the food system (Deng et al., 2013). The PV of NS was higher than RS and ARS, but RS showed the higher TV, SBV, and FV, which suggested that the RS had the higher retrogradation tendency. The RS showed lower amylographic viscosity parameters compared to NS. This attribute could be a result of the destruction of granules subjected to hydrolysis during heating, which improved the levels of linear short-chain molecules. Moreover, the molecular motion decreased by the retrogradation and the microcrystalline beam were formed by hydrogen bonding between amylose and amylopectin at the same time. These two reasons might lead to the reduction of the viscoelasticity properties.

Sweet potato starch

53

Structural and physicochemical properties of physically modified sweet potato starch HHP is a nonthermal food processing technique, which has interesting functional effects in foods (Kim et al., 2012). To produce high-quality HHP products, it is important to know the effects of different HHP treatment conditions on different food components, including starch. When scattered in aqueous solutions, starch is gelatinized by HHP treatment, resulting in structural and physicochemical changes (Kim et al., 2012; Vallons et al., 2014). Starch is present in complex food matrices, so it is necessary to evaluate the effects of other food components, especially of salts or ions (e.g., sodium, calcium, and chloride ions) on HHP-induced starch gelatinization, which is a hydration and hydrogen-bond disrupting process (Buckow et al., 2007; Rumpold and Knorr, 2005). This section therefore introduces the effects of different inorganic salts on the structural and physicochemical properties of HHP-gelatinized sweet potato starch.

Thermal properties The thermal properties of native and HHP-treated sweet potato starches are shown in Table 3.10. Compared with the native starch, HHP-treated sweet potato starches with and without inorganic salts had higher To and Tp, but lower R, WHgel, and peak height index (PHI) values. Due to the interference or strengthening of the water molecule hydrogen-bond network, the dissociative ions had an effect on the water absorption of sweet potato starch. It led to a greater requirement for calories in the melting process, and prevented the damage of the crystal structure. It was noteworthy that the starch suspensions with calcium chloride or sodium chloride had significantly higher To and Tp values than those suspensions without salts. Among the HHP-treated sweet potato starches, the highest To and Tp values were obtained with 0.01 M of calcium and 0.01 M sodium chloride (73.10°C and 72.68°C, respectively). It was shown that To and Tp values at 0.1 M (in both salts) were lower than those at 1 M. It might be caused by the relatively high concentrations of salt osmotic pressure, and the interaction between starch and salt would also reduce the liquidity of starch molecular chain segments. On the other hand, because of the combined impact of starch molecule and ions, particles would be stretched to a certain extent (Eleni et al., 2001). With increasing salt concentrations, the R of HHP-treated sweet potato starches decreased first and subsequently increased. The starch suspension with 0.001 M salt had

Table 3.10 The thermal property parameters of native and HHP-treated sweet potato starch. Pressure (MPa)

Salt (M)

To (°C)

Tp (°C)

R (°C)

WHgel (J/g)

PHI

0.1 600

0 (Native) 0 CaCl2 0.001 0.01 0.1 1 NaCl 0.001 0.01 0.1 1

65.02 6 0.21d 69.41 6 0.07b

74.90 6 0.3bc 75.47 6 0.28bc

19.75 6 0.86ab 12.12 6 0.43de

12.03 6 0.49a 0.64 6 0.13f

1.22 6 0.06a 0.11 6 0.02ef

70.23 6 1.36b 73.10 6 1.25a 67.53 6 0.32c 71.15 6 1.39b

77.91 6 0.11a 77.73 6 0.78ab 75.25 6 0.08bc 77.69 6 0.23ab

15.36 6 2.85cd 9.25 6 0.98e 15.45 6 0.49cd 13.07 6 2.32d

1.77 6 0.20e 2.43 6 0.15d 6.92 6 0.14c 9.37 6 0.16b

0.23 6 0.02ef 0.53 6 0.06d 0.90 6 0.05c 1.43 6 0.22b

68.53 6 0.71c 72.68 6 0.12a 70.82 6 0.92b 71.09 6 2.69b

77.73 6 0.04ab 78.34 6 0.85a 77.66 6 0.03ab 76.00 6 0.41b

18.41 6 1.34abc 11.33 6 1.47de 13.67 6 1.85cde 10.82 6 6.18de

1.21 6 0.14e 2.13 6 0.07d 6.03 6 0.22c 9.22 6 0.10b

0.13 6 0.02ef 0.38 6 0.05de 0.89 6 0.13c 0.89 6 0.22c

600 600 600 600 600 600 600 600

Values followed by the different letter in the same column are significantly different (P , .05).

Sweet potato starch

55

the highest R-value, while that with 0.01 M salt had the lowest R-value (Table 3.10). The addition of salts increased WHgel and PHI of HHPtreated sweet potato starch in a dose-dependent manner. The WHgel and PHI values of HHP-treated sweet potato starch with calcium chloride were higher than those with sodium chloride at similar concentrations, but with no significant difference (Table 3.10). HHP treatment induced gelatinization of starch when dispersed in excess water. Therefore the crystalline structure of starch granules changed from an ordered to a disordered conformation (Blaszczak et al., 2005, 2010). The DG of HHP-treated starches had the same tendency as WHgel and PHI. The HHP-treated sweet potato starch with no salt had the highest DG value (94.7%), while starch with 1 M calcium and sodium chloride had the lowest DG values (22.11% and 23.36%, respectively). A satisfactory correlation index (r2 5 0.96) obtained between DG and salt concentration with a mathematical model was y 5 223.214x 1 119.39. During the HHP-induced gelatinization process, the amorphous regions of starch granules were hydrated and formed lamellae among crystalline regions. Subsequently, granules swelled and gelatinized. In starch granules, sodium, calcium, and chloride ions had higher polarity than glucose molecules (Rumpold and Knorr, 2005). Therefore these ions had more strength to combine with water molecules and prevent the hydration of amorphous regions. Calcium ions had higher polarity than sodium and chloride ions, which had a higher capacity to prevent HHP-induced gelatinization or the loss of crystallinity of sweet potato starches. As we all know, partially gelatinized starch granules are more difficult to digest in the human body due to the integrity of the spherical structure compared to the gelatinized starches. Moreover, the partially gelatinized starch would slowly release glucose to provide energy continuously. So this could avoid the possibility of blood sugar rapidly reaching a high level after the intake of starchy foods. In other words, it could regulate the blood sugar metabolism. Therefore this kind of starch is expected to be applied as an auxiliary food for the improvement of hyperglycemia.

Scanning electron micrograph The SEM micrographs ( 3 3000) of native and HHP-treated sweet potato starch granules are shown in Fig. 3.5. HHP-treated starch granules without salts lost their characteristic shape and had irregular sections with smooth surfaces (Fig. 3.5). For 0.001 M calcium chloride and 0.001 and

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Figure 3.5 SEM micrographs of native and HHP-treated sweet potato starch samples.

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57

0.01 M sodium chloride, almost all starch granules disintegrated into small fragments or aggregated to larger sections with rough surfaces. With increasing salt concentrations, more granules remained intact. Compared with starch granules in sodium chloride, starch granules in calcium chloride had higher structural integrity, because calcium chloride could prevent HHP-induced starch gelatinization. Calcium ions as a kind of divalent cation showed a more significant cross-linking function and hydration ability in the starch system. Salt contended water molecules with starch granules, which led to more complicated expansion of starch granules (Cai et al., 2012). HHP-induced starch gelatinization has two stages. In the first stage, changes in starch granule morphology could not be observed; when pressure reached a certain value, the starch granules began to disintegrate (Wuzburge and Whistler, 1964). However, the starch granules almost remained intact at 0.1 and 1 M calcium chloride and 1 M sodium chloride (Fig. 3.5). Therefore the addition of inorganic salts increased the pressure of HHP-induced sweet potato starch gelatinization.

Confocal laser scanning microscopy and polarized light microscopy The confocal laser scanning microscopy (CLSM) (green) and polarized light microscopy (PLM) (black and white) micrographs of native and HHP-treated sweet potato starches are presented in Fig. 3.6. The growth ring in the CLSM optical slice represents the internal structure of the starch granules, while the Maltese cross of the starch granules under polarized light indicates the degree of molecular order and crystallinity (Suda et al., 2003). NS had a typical round growth ring and Maltese cross. At 600 MPa with no salt addition, only a small section of the starch fractions and granules was visible in CLSM due to starch gelatinization (Karim et al., 2000), while all the Maltese crosses disappeared. Almost all starch granules and Maltese crosses were detectable with 0.1 and 1 M calcium chloride and 1 M sodium chloride. Both the CLSM optical slice and Maltese cross represent the degree of molecular order and crystallinity (Suda et al., 2003). With increasing starch granule DG (or decreasing ion strength), fewer starch granules were visible in CLSM and more Maltese crosses disappeared from the inner section, suggesting that sweet potato starch granules were gradually gelatinized from the inner to the outer section, and the inner section was more sensitive to the HHP treatment. This explained why the starch granules

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Figure 3.6 CLSM and PLM micrographs of native and HHP-treated sweet potato starch samples.

retained their original shape in the first stage of the HHP-induced starch gelatinization process (Vallons and Arendt, 2009).

X-ray diffraction The XRD profiles and relative degrees of crystallinity of native and HHP-treated sweet potato starches are shown in Fig. 3.7. The XRD

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59

Figure 3.7 XRD curve and relative crystallinity of native and HHP-treated sweet potato starch samples.

pattern of sweet potato starch depended on the cultivar, which could be A, C, or a combination of A and C types (Schirmer et al., 2013). NS had peaks at 15.2°, 17.2°, 18.1°, and 22.86° with a characteristic A type XRD pattern (Fig. 3.6). Compared with the A type, the C type XRD pattern had an extra and weak peak around 5.6°, which might disappear after drying (Ann-Charlotte, 2009). In Fig. 3.6, no obvious diffraction peak change was observed at about 5.6°. Therefore seeking other changes was also important. A single diffraction peak at about 17.2° could act as forceful evidence to prove the pattern change (Ann-Charlotte, 2009). Following HHP treatment, the peak at 18.1° disappeared, which indicated that HHP treatment converted the XRD pattern of sweet potato starch from an A to a C type. The addition of salts did not affect the XRD pattern of HHP-treated sweet potato starch. The peak intensities in the XRD profile were indicative of the degree of crystallinity of the starch granule (Takeda et al., 1986). To assess the change in the degree of crystallinity after HHP treatment, the relative degree of crystallinity of each sample was estimated. Native starch showed the highest degree of crystallinity (30.53%), while the HHP-treated starch with no salt had the lowest

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degree of crystallinity (15.66%). With increasing salt concentration, the peaks in the XRD profiles became stronger, and the degree of crystallinity significantly increased. At similar concentrations, starches in calcium chloride (22.11% 28.64%) had stronger peaks and higher degrees of crystallinity than those in sodium chloride (16.07% 27.29%). However, the changes in the relative degrees of crystallinity did not correspond to changes in DG (Fig. 3.5), because rapid retrogradation occurred inside the starch granules following HHP treatment (Kawai et al., 2007).

Pasting properties Pasting property parameters of native and HHP-treated sweet potato starch samples are shown in Table 3.11. Compared with NS, the viscosity of HHP-treated sweet potato starch with no salt increased significantly (highest viscosity: 7357.33 cP). The addition of salts decreased viscosity and increased PT and pasting temperature of sweet potato starch pastes (Table 3.11). Salt could dissolve into anions and cations as one kind of electrolyte in water. Hydrogen bonds in starch molecules were damaged by these ions, as well as those between starch particles and water molecules. This led to more complications in terms of starch gelatinization and a significant reduction in the PV (Meera et al., 2008). With increasing salt concentrations the breakdown and pasting temperatures of starch pastes increased, while peak and hold viscosity of starch pastes from calcium chloride starch suspensions increased. It might be due to the increasing number of charged ions, which enhanced the adsorption capacity of starch. Final viscosity, setback, and PT of starch pastes reduced with sodium chloride starch suspensions (Table 3.11). The viscosity of HHPtreated starch in calcium chloride was lower than in sodium chloride at similar concentrations (Table 3.11), because calcium chloride has a higher ion strength than sodium chloride (Liu et al., 2010).

Swelling power and solubility The swelling power and solubility of native and HHP-treated sweet potato starch are shown in Fig. 3.8A and B. Compared with native starch, HHP-treated starch with no salt had lower swelling power and solubility (Fig. 3.8). However, the addition of salts increased the swelling power and solubility of HHP-treated sweet potato starch (Fig. 3.8). This change indicated that the cluster structure of amylopectin had been opened due to the influence of electronics carried by the ions, which enabled the

Table 3.11 The pasting property parameters of native and HHP-treated sweet potato starch. Pressure (MPa)

Salt (M)

Peak viscosity (cP)

Hold viscosity (cP)

Breakdown (cP)

Final viscosity (cP)

Setback (cP)

Peak time (min)

Pasting temp. (°C)

0.1 600

0 (Native) 0 CaCl2 0.001 0.01 0.1 1 NaCl 0.001 0.01 0.1 1

6024.67 6 102.77b 7357.33 6 0.58a

3526.67 6 12.70cd 5077.33 6 2.31a

2498.00 6 90.07a 2280.00 6 1.73ab

4480.33 6 15.01de 6822.67 6 4.62b

953.67 6 2.31d 1745.33 6 2.31c

4.42 6 0.03bcd 4.76 6 0.08bc

76.78 6 0.14ab 76.48 6 0.38ab

3661.33 6 69.14e 4273.67 6 96.50d 5687.67 6 74.27c 6175.00 6 1.73b

2751.67 6 60.34e 3546.33 6 14.15cd 3753.67 6 9.02c 3907.33 6 10.97c

909.67 6 13.80d 1127.33 6 84.81cd 1934.00 6 74.67b 2267.67 6 9.24ab

4460.67 6 89.03de 5298.33 6 5.51d 5234.33 6 32.62d 4883.00 6 19.05d

1709.00 6 28.69c 2152.00 6 18.03b 1480.67 6 40.50c 975.67 6 30.02d

4.84 6 0.08bc 5.00 6 0.12ab 4.49 6 0.04bcd 4.79 6 0.02bc

77.57 6 0.45ab 76.02 6 0.33ab 76.85 6 0.62ab 78.65 6 0.26a

6164.25 6 45.14b 4718.32 6 15.48cd 6086.09 6 29.22b 6179.20 6 3.45b

5153.51 6 12.55a 3802.15 6 25.35c 4690.00 6 31.26b 3742.55 6 19.87c

1011.54 6 49.55d 916.16 6 55.27d 1396.54 6 29.58c 2428.59 6 50.23a

7941.19 6 80.14a 6044.38 6 29.59c 7079.48 6 38.56b 4787.55 6 45.68d

2788.59 6 45.58a 2242.68 6 29.63b 2388.98 6 16.59ab 1047.79 6 41.56d

5.47 6 0.00ab 5.21 6 0.09ab 4.80 6 0.05bc 4.53 6 0.05bcd

70.71 6 0.55b 75.5 6 0.29ab 74.7 6 0.47ab 76.65 6 0.19ab

600 600 600 600 600 600 600 600

Values followed by the different letters in the same column are significantly different (P , .05).

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Figure 3.8 Swelling power (A) and solubility (B) of native and HHP-treated sweet potato starch samples. Values followed by the different letters are significantly different (P , .05).

release of more amylase (Tester and Morrison, 1990). With increasing salt concentrations, the swelling power of HHP-treated starch in sodium chloride increased (Fig. 3.8A), while the solubility of HHP-treated starch in calcium chloride decreased (Fig. 3.8B). The addition of calcium and

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sodium chloride to sweet potato starch suspensions not only inhibited the hydration of amorphous regions but might also limit the formation of amylose lipid complexes in starch granules during the HHP-induced starch gelatinization (Oh et al., 2008).

Research and development trend of sweet potato starch Research on starches from different food resources has attracted widespread attention around the world. In some areas or aspects, further studies are necessary as follows: (1) carry out pilot and industrial production demonstrations of processing technologies of modified sweet potato starches, to expand their application in the food system; (2) develop the application of sweet potato starches and/or their modified starches in snack foods, to make full use of their physicochemical properties; and (3) develop the application of sweet potato starches and/or their modified starches on staple foods, to take full advantage of their structural and physicochemical properties. In any case, the sweet potato is a potential source of high-quality starches and/or modified starches, and it is necessary to conduct continuous and in-depth studies on them.

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