Structural and functional properties of starches from root tubers of white, yellow, and purple sweet potatoes

Food Hydrocolloids 89 (2019) 829–836

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Structural and functional properties of starches from root tubers of white, yellow, and purple sweet potatoes

T

Ke Guoa,b, Tianxiang Liua,b, Ahui Xua,b, Long Zhanga,b, Xiaofeng Bianc,∗∗, Cunxu Weia,b,∗ a

Key Laboratory of Crop Genetics and Physiology of Jiangsu Province / Key Laboratory of Plant Functional Genomics of the Ministry of Education, Yangzhou University, Yangzhou, 225009, China b Co-Innovation Center for Modern Production Technology of Grain Crops of Jiangsu Province / Joint International Research Laboratory of Agriculture & Agri-Product Safety of the Ministry of Education, Yangzhou University, Yangzhou, 225009, China c Institute of Food Crops, Jiangsu Academy of Agricultural Sciences, Nanjing, 210014, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Sweet potato Different colored root tubers Starch Structural properties Functional properties Cluster analysis

Sweet potato is an important starch resource and has different colored root tubers due to different genotype backgrounds. However, it is unclear whether starch properties are related to the color of root tuber. In this study, 3 white, 3 yellow, and 3 purple sweet potato varieties were planted in the same environment. The dry root tubers of white varieties had the highest starch ranging from 61.5 to 67.5% and the lowest soluble sugar ranging from 13.1 to 18.0%, and those of yellow varieties had the lowest starch ranging from 45.8 to 53.1% and the highest soluble sugar ranging from 28.9 to 31.5%. Starches from different varieties had different granule sizes (D[4,3] 12.3–18.1 μm) and amylose contents (24.1–27.2%), but all exhibited CA-type crystalline structure. The relative crystallinity ranged from 22.3 to 25.5%, and the IR absorbance ratios of 1045/1022 and 1022/995 cm−1 ranged from 0.665 to 0.775 and from 0.846 to 0.944, respectively, among 9 varieties. The starches from the same or different colored varieties had differences in swelling power, water solubility, gelatinization temperature, pasting viscosity, and digestion properties. The cluster analysis based on starch property parameters indicated that starch properties of sweet potato had no relationship with the color of root tuber but were determined by the genotype background of variety.

1. Introduction Sweet potato [Ipomoea batatas (L.) Lam.] is an important economic crop in Asia, Africa, and Latin America. Its root tuber is rich in starch, dietary fiber, vitamin C, provitamin A, iron, and minerals, and is usually used as an energy source in human diet and as an important starch resource in food and nonfood industries (Bovell-Benjamin, 2007; Zhu, Yang, Cai, Bertoft, & Corke, 2011). Sweet potato has white, yellow, orange, purple, and red varieties due to the difference in composition and content of phenolic compounds and pigments in root tuber (Wang et al., 2018). The yellow and purple of root tuber are due to the accumulation of lipid-soluble β-carotene and water-soluble anthocyanin, respectively (Odake, Terahara, Saito, Toki, & Honda, 1992). The yellow of root tuber can gradually become orange with the increase of β-carotene content (Tanaka, Sasaki, & Ohmiya, 2008). The anthocyanin is higher in red root tuber than in purple root tuber (Wang et al., 2018). Compared with yellow and purple root tubers, the white root

tuber contains very low phenolic compounds and β-carotene but has no anthocyanins (Kim et al., 2011; Teow et al., 2007). Starch is the main component of sweet potato root tuber. Its structural and functional properties determine its quality and applications in food and nonfood industries. Starch from sweet potato has widely been investigated in different colored root tubers (Lee & Lee, 2017; Osundahunsi, Fagbemi, Kesselman, & Shimoni, 2003; Sajeev, Sreekumar, Vimala, Moorthy, & Jyothi, 2012; Soison, Jangchud, Jangchud, Harnsilawat, & Piyachomkwan, 2015; Zhang et al., 2018). Osundahunsi et al. (2003) chose 2 different colored sweet potato cultivars representing the white and red root tubers to characterize their starches, and found that starches have similar physicochemical properties except that pasting properties are different between white and red cultivars. They concluded that the white cultivar is preferred when low retrogradation tendency is required. Soison et al. (2015) chose 4 different colored sweet potato varieties to represent the white, yellow, orange, and purple root tubers, and concluded that the physicochemical

∗

Corresponding author. Key Laboratory of Crop Genetics and Physiology of Jiangsu Province / Key Laboratory of Plant Functional Genomics of the Ministry of Education, Yangzhou University, Yangzhou, 225009, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Bian), [email protected] (C. Wei). https://doi.org/10.1016/j.foodhyd.2018.11.058 Received 17 August 2018; Received in revised form 31 October 2018; Accepted 28 November 2018 Available online 28 November 2018 0268-005X/ © 2018 Elsevier Ltd. All rights reserved.

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the homogenate was successively filtered through 100-, 200-, and 300mesh sieves, and settled overnight at 4 °C to obtain the starch precipitation. The starch precipitate was washed 5 times with 0.2% NaOH, 5 times with deionized water, and two times with anhydrous ethanol. Finally, the starch was dried at 40 °C, ground into powders and passed through a 100-mesh sieve, and used for measuring the structural and functional properties of starch.

properties of sweet potato starches are significantly affected by the color of root tuber. Lee and Lee (2017) chose 3 different colored sweet potato varieties to represent the white, orange, and purple root tubers, and found that their starches have similar physicochemical and structural properties and concluded that the colored sweet potato samples have potential as colored starchy food ingredients without structural or physical differences. However, starches from 7 purple sweet potatoes have different structural and functional properties due to their different genotype backgrounds (Zhang et al., 2018). Sajeev et al. (2012) used 3 white, 2 cream, and 2 orange sweet potato varieties as plant materials to investigate the textural and gelatinization characteristics of different colored sweet potatoes, and found that the textural, rheological, and gelatinization properties show significant differences among different varieties but have no relationship with the color of root tuber. Hundreds of sweet potato varieties with different colored root tubers have been cultivated. Variety selection and improvement have become of interest for the breeders of sweet potato (Zhang, Wang, Liu, & Wang, 2009). It is important to compare the difference of starch properties from different colored sweet potatoes. However, as the above reviews have illustrated, there are different opinions about whether the starch properties are related to the color of root tuber (Lee & Lee, 2017; Osundahunsi et al., 2003; Sajeev et al., 2012; Soison et al., 2015; Zhang et al., 2018). The disagreement is mainly caused by the reason that only one variety is chosen to represent the different colored root tubers. To our knowledge, it is unclear whether starch properties are related to the color of root tuber. In this study, 3 white, 3 yellow, and 3 purple sweet potato varieties were chosen to represent the different colored root tubers. They were planted in the same environment, and their starches were isolated from fresh root tubers. The structural and functional properties of starches were investigated and compared. The main purpose of this study was to reveal the relationship between starch properties and the color of root tuber, and provide some information for quality breeding and utilization of different colored sweet potato varieties.

2.4. Analysis of starch granule size The granule size of starch was measured using a laser diffraction particle size analyzer (Mastersizer 2000, Malvern, UK) as previously described by Cai et al. (2014). Briefly, starch-water slurry was analysed under the condition of 2000 rpm and the opacity between 10% and 11%. The surface- and volume-weighted diameters were chosen as granule size. 2.5. Measurement of apparent amylose content The starch-iodine absorption spectrum and apparent amylose content were measured following our previous method (Zhang et al., 2018). Briefly, 10 mg of starch was dissolved in 5 mL of dimethyl sulfoxide (DMSO) containing 10% of 6 M urea at 95 °C for 1 h. The 1 mL of starch-DMSO solution and 1 mL of iodine (0.2% I2, 2% KI, w/v) were mixed with water and made up to 50 mL in a volumetric flask, and stored in a dark environment for 20 min. The starch-iodine absorption spectrum was measured using a spectrophotometer (Ultrospec 6300 pro, Amersham Biosciences, Sweden). Apparent amylose content was evaluated from the absorbance at 620 nm. 2.6. Determination of protein content in starch

2. Materials and methods

The nitrogen content in isolated starch was determined using an element CHN-analyzer (Vario EL cube, Elementar Analysensysteme Gmbh, Germany), and then converted to protein content using a 6.25 conversion factor.

2.1. Plant materials

2.7. Analysis of crystalline structure

Three white sweet potato varieties, Su 24 (W1), Su 28 (W2), and Su 29 (W3), 3 yellow sweet potato varieties, Su 14 (Y1), Su 16 (Y2), and Su 25 (Y3), and 3 purple sweet potato varieties, Ningzi 1 (P1), Ningzi 2 (P2), and Ningzi 4 (P3) were planted under normal agronomic practices in the experimental field of Jiangsu Academy of Agricultural Sciences, Nanjing, China in 2017. Their fresh root tubers were used as plant materials in this study.

The crystalline structure of starch was analysed on an X-ray powder diffractometer (XRD) (D8, Bruker, Germany) following the method previously described by Wei et al. (2010). The starch was wetted in a closed container containing a saturated aqueous solution of NaCl for 2 weeks. The starch was scanned from 3° to 40° 2θ with a step size of 0.02° using the X-ray beam at 40 mA and 40 kV. 2.8. Analysis of short-range ordered structure

2.2. Measurement of soluble sugar and starch contents in dry root tuber Attenuated total reflectance-Fourier transforms infrared (ATR-FTIR) analysis of starch was carried out on a FTIR spectrometer (7000, Varian, USA) with a DTGS detector equipped with an ATR single-reflectance cell containing a germanium crystal (45° incidence angle) as previously described by Wei et al. (2010).

The fresh root tubers were washed cleanly and cut into small thin pieces. The samples were freezed-dried at −70 °C in a Freeze Dryer (FD-1A-50, Boyikang Corp., China) and ground extensively through a 100-mesh sieve to obtain the flour. The soluble sugar and starch contents in flour were measured following the method of Gao et al. (2014). Briefly, the soluble sugar was first extracted from flour with 80% (v/v) ethanol, and then the starch in flour was hydrolysed into soluble sugar with HClO4. The soluble sugar was finally determined using anthroneH2SO4 method, and then converted to soluble sugar and starch contents in flour by reference to a standard curve prepared with glucose (McCready, Guggolz, Silviera, & Owens, 1949).

2.9. Measurement of swelling power and water solubility The swelling power and water solubility of starch were measured following the method of Lin et al. (2016) with some modifications. Briefly, 30 mg of starch (W1) and 1.5 mL of water were suspended and heated at 95 °C for 30 min with continuous shaking (1000 rpm) in a ThermoMixer. The sample was cooled to room temperature and centrifuged at 5000 g for 10 min. The supernatant were transferred for quantifying the soluble carbohydrates (W2) using anthrone-H2SO4 method. The precipitated starch was weighed (W3). The swelling power and water solubility were calculated as W3/(W1-W2) and W2/ W1 × 100%, respectively.

2.3. Starch isolation from root tubers The starch was isolated from fresh root tubers exactly following our previous method (Zhang et al., 2018). Briefly, the fresh root tubers were cut into small pieces and homogenized in deionized water. After that, 830

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that soluble sugar content ranges from 7.0 to 8.1% in white sweet potato, from 9.7 to 10.5% in purple sweet potato, and from 14.9 to 21.0% in orange sweet potato. Sajeev et al. (2012) reported that the soluble sugar content ranges from 11.5 to 14.9% in white sweet potato, from 15.8 to 16.9% in cream sweet potato, and from 18.4 to 23.1% in orange sweet potato. Though the soluble sugar content has some differences in different reports, the present study and previous reports all show that the soluble sugar contents in sweet potatoes are as follows: white/ cream-fleshed < purple-fleshed < yellow/orange-fleshed root tubers (Table 1, Grabowski et al., 2008; Kim et al., 2011; Sajeev et al., 2012). In addition, the soluble sugar content in root tuber of sweet potato is also influenced by cultivar, tuber maturity, and storage time (AduKwarteng et al., 2014; Zhang, Wheatley, & Corke, 2002).

2.10. Analysis of thermal properties The thermal properties of starch were analysed following the method of Cai et al. (2015). Briefly, 5 mg of starch and 15 μL of water were mixed and sealed in an aluminium pan. The sample was kept at 4 °C overnight. After equilibration at room temperature for 2 h, the sample was heated from 25 to 130 °C at 10 °C/min using a differential scanning calorimeter (DSC) (200-F3, Netzsch, Germany). 2.11. Analysis of pasting properties The pasting properties of starch were analysed following the method of Gao et al. (2014). Briefly, 2.5 g of starch and 25 mL of water were measured using a rapid visco analyzer (RVA-3D, Newport Scientific, Australia). The starch-water slurry was mixed by rotating the paddle at 960 rpm for the first 10 s and then at a constant speed of 160 rpm. The programmed temperature-time profile was: 50 °C for 1 min, heating to 95 °C at 12 °C/min, 95 °C for 2.5 min, cooling to 50 °C at 12 °C/min, and 50 °C for 1.4 min.

3.2. Granule size distribution of starch The 9 sweet potato starches all showed bimodal size distributions with small granules from 0.5 to 3 μm and large granules from 4 to 50 μm. The granule size of d(0.5), D[3,2], and D[4,3] ranged from 11.70 to 17.24 μm, from 5.90 to 8.51 μm, and from 12.33 to 18.09 μm, respectively, among 9 starches. The volume percentage of large-sized granules ranged from 91.11% to 93.30% (Table 1). The granule size in this study was comparable to the previous literature (Walter, Truong, Wiesenborn, & Carvajal, 2000; Zhang et al., 2018). It was noteworthy that the size distribution of starch granules was significantly different among different varieties but had no relationship with the color of root tuber. However, Lee and Lee (2017) investigated the different colored sweet potatoes with every colored root tuber having only one variety, and found that the purple root tuber has the smallest starch granules (D [4,3] 18.8 μm) and the white and orange root tubers have the similar starch granule size (D[4,3] approximate 22.8 μm). The starch granule size is affected by the variety, grown condition, and plant physiology (Abegunde, Mu, Chen, & Deng, 2013). In the present study, the 9 sweet potato varieties were planted in the same environment, indicating that the different size distributions of starch granules resulted from their different genotype backgrounds.

2.12. Analysis of digestion properties The native, gelatinized and retrograded starches were digested with both porcine pancreatic α-amylase (PPA, A3176, Sigma, USA) and Aspergillus niger amyloglucosidase (AAG, E-AMGDF, Megazyme, Ireland) following the method of Fan et al. (2016). Briefly, the 1% (w/ v) starch suspension was heated at 98 °C for 12 min to prepare the gelatinized starch. The gelatinized starch was stored at 4 °C for 36 h to obtain the retrograded starch. The starch was digested in enzyme solution (20 mM sodium phosphate buffer, pH 6.0, 6.7 mM NaCl, 0.01% NaN3, 2.5 mM CaCl2, 0.4 U/mg starch PPA, 0.4 U/mg starch AAG) at 37 °C with continuous shaking (1000 rpm) in a ThermoMixer. The released glucose was determined using a glucose assay kit (K-GLIC, Megazyme, Ireland), and then converted to the degraded starch. The rapidly digestible starch (RDS, degraded starch within 20 min), slowly digestible starch (SDS, degraded starch between 20 min and 2 h), and resistant starch (RS, undegraded starch after 2 h) were calculated according to the degraded rate of starch.

3.3. Iodine absorption spectrum and apparent amylose content The absorbance spectrum of starch-iodine complex is shown in Fig. 1A, and its derived parameters of maximum absorption wavelength (λmax), iodine blue value (BV) and the absorbance ratio of OD620 to OD550 (OD620/550) are presented in Table 2. The 9 sweet potato starches had some differences in their iodine absorption spectra. The apparent amylose content ranged from 24.1 to 27.2% among 9 sweet potato starches. The present result agreed with amylose contents ranging from 23.3 to 26.5% in 9 purple and 2 white sweet potato starches (Zhu, Cai, Yang, Ke, & Corke, 2010; Zhu et al., 2011) and from 24.6 to 31.0% in 7 purple sweet potato starches (Zhang et al., 2018). However, the apparent amylose contents ranging from 16.5 to 18.5% in white, yellow, orange, and purple sweet potatoes and from 33.8 to 35.5% in white, orange, and purple sweet potatoes are also reported (Lee & Lee, 2017; Soison et al., 2015). The amylose content is influenced by the genotype background, growing environment, and measuring method (Zhang et al., 2018). In the present study, the apparent amylose contents among 9 sweet potato starches had no relationship with the color of root tuber, and were influenced by their genotype backgrounds.

2.13. Statistical analysis The data reported in all the tables were means ± standard deviations. The one-way analysis of variance with post hoc contrasts by Tukey's test was evaluated using the SPSS 19.0 Statistical Software Program. Hierarchical cluster analysis was employed using betweengroups linkage as the cluster method and Pearson correlation as the interval measure. 3. Results and discussion 3.1. Soluble sugar and starch contents of root tuber The contents of soluble sugar and starch in dry root tubers are presented in Table 1. Though soluble sugar and starch contents had differences among different varieties with the same colored root tubers, the white sweet potato varieties had the highest starch content ranging from 61.5 to 67.5% and the lowest soluble sugar content ranging from 13.1 to 18.0%, and the yellow sweet potato varieties had the lowest starch content ranging from 45.8 to 53.1% and the highest soluble sugar ranging from 28.9 to 31.5%. The 14 Virginia-grown sweet potato varieties including 6 orange, 4 white, 2 yellow, and 2 purple varieties have starch contents ranging from 42.3 to 64.1% with the white varieties having the highest starch content (Cartier et al., 2017), which is in agreement with the present results. For soluble sugar, Grabowski, Truong, and Daubert (2008) reported that an orange sweet potato variety puree contains 31.9% soluble sugar. Kim et al. (2011) reported

3.4. Protein content of starch The protein content in isolated starch is presented in Table 2, and ranged from 0.22 to 0.50 mg/g among 9 sweet potato varieties. Lee and Lee (2017) measured the protein contents of different colored sweet potato starches with one variety for every colored sweet potato, and found that the white-, orange-, and purple-fleshed sweet potato starch has 0.11, 0.09, and 0.10% protein content, respectively. However, Kim, 831

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Table 1 Starch and soluble sugar contents in dry root tuber and granule size of isolated starch.a Varieties

b

Starch content (%)

Soluble sugar content (%)

Granule size distribution d(0.5) (μm)

W1 W2 W3 Y1 Y2 Y3 P1 P2 P3

67.5 61.5 63.0 45.8 53.1 48.8 55.9 55.1 53.0

± ± ± ± ± ± ± ± ±

1.0f 1.3e 1.7e 1.1a 0.4c 0.7b 1.3d 1.1cd 0.3c

14.6 18.0 13.1 31.5 31.0 28.9 22.4 17.6 20.7

± ± ± ± ± ± ± ± ±

0.4b 0.1c 0.9a 0.2g 0.3g 0.6f 1.0e 0.2c 0.5d

14.75 17.24 16.81 13.71 15.53 11.70 17.10 12.97 11.95

± ± ± ± ± ± ± ± ±

c

D[3,2] (μm)

0.01e 0.01h 0.00g 0.20d 0.01f 0.01a 0.01h 0.01c 0.00b

7.48 8.51 8.12 7.03 7.63 5.90 8.47 6.56 6.17

± ± ± ± ± ± ± ± ±

c

0.00e 0.00h 0.00g 0.07d 0.08f 0.01a 0.00h 0.00c 0.00b

D[4,3] (μm) 15.22 18.09 17.72 14.32 16.43 12.33 17.75 13.51 12.57

± ± ± ± ± ± ± ± ±

c

0.01e 0.01h 0.00g 0.27d 0.01f 0.02a 0.01h 0.01c 0.01b

Small granules (%)

Large granules (%)

7.58 6.70 6.88 7.82 7.36 8.50 6.93 7.91 8.89

92.42 93.30 93.12 92.18 92.64 91.50 93.07 92.09 91.11

± ± ± ± ± ± ± ± ±

0.00e 0.00a 0.00b 0.00f 0.01d 0.01h 0.00g 0.01c 0.00a

± ± ± ± ± ± ± ± ±

0.00e 0.00i 0.00h 0.00d 0.01f 0.01b 0.00g 0.01c 0.00a

a

Data are means ± standard deviations, n = 3. Values in the same column with different letters are significantly different (p < 0.05). The W1, W2, and W3 represent the white sweet potato varieties, Su 24, Su 28, and Su 29, respectively. The Y1, Y2, and Y3 represent the yellow sweet potato varieties, Su 14, Su 16, and Su 25, respectively. The P1, P2, and P3 represent the purple sweet potato varieties, Ningzi 1, Ningzi 2, and Ningzi 4, respectively. c The d(0.5) is the granule size at which 50% of all the granules by volume are smaller. The D[3,2] and D[4,3] are the surface-weighted and volume-weighted mean diameter, respectively. b

granule can be determined by ATR-FTIR. The ordered degree and the proportion of amorphous to ordered carbohydrate structure can be measured through the absorbance ratios of 1045/1022 and 1022/ 995 cm−1, respectively (Sevenou, Hill, Farhat, & Mitchell, 2002). The ATR-FTIR spectra of 9 sweet potato starches are shown in Fig. 1C, and the absorbance ratios of 1045/1022 and 1022/995 cm−1 had some differences among 9 starches and ranged from 0.678 to 0.765 and from 0.852 to 0.943, respectively (Table 3). Similar ATR-FTIR spectrum and the absorbance ratios of 1045/1022 and 1022/995 cm−1 were also reported in 7 purple sweet potato starches (Zhang et al., 2018). Though the absorbance ratios of 1045/1022 and 1022/995 cm−1 were different among 9 sweet potato starches, they had no relationship with the color of root tuber.

Ren, and Shin (2013) found that the protein content in isolated starch ranges from 0.01 to 0.28% in white sweet potato, from 0.02 to 0.23% in orange sweet potato, and from 0.05 to 0.07% in purple sweet potato. The protein content in starch depends on the species and variety of starch. Typical well washed cereal starches usually have approximate 0.3% protein, and root or tuber starches contain approximate 0.05% protein (Swinkels, 1985). The protein in starch granule contains granule surface and interior protein. The former is removed easily from starch granule without destroying the granule structure, but the latter for removal needs to disrupt granule structure (Baldwin, 2001). The isolation method of starch influences the protein content. The NaOH washing during starch isolation can remove the surface protein from starch granules (Xu et al., 2018). In the present study, the low protein content might be due to the species and isolation method of starch, and the variations among 9 starches might result from their different genotype backgrounds.

3.7. Swelling power and water solubility of starch The swelling powers and water solubilities of the sweet potato starches at 95 °C are presented in Table 4. Swelling powers ranged from 25.2 to 31.1 g/g, and water solubilities ranged from 11.7 to 16.6% among 9 sweet potato starches. Swelling powers and water solubilities at 92.5 °C range from 24.5 to 32.7 g/g and from 12.1 to 24.1%, respectively, among 44 sweet potato starches (Collado, Mabesa, & Corke, 1999). Swelling power and water solubility are affected by granule size, amylose content, amylopectin fine structure, crystalline structure, and protein and lipid contents (Kaur, Singh, McCarthy, & Singh, 2007; Qi, Tester, Snape, & Ansell, 2003; Srichuwong, Sunarti, Mishima, Isono, & Hisamatsu, 2005).

3.5. Crystalline structure of starch The XRD patterns of starches are shown in Fig. 1B. According to XRD patterns, starches are divided into A-, B-, and C-type. C-type starch consists of A- and B-type crystallinities and can be further classified to CA-, CC-, and CB-type according to the proportion of A- and B-type crystallinity from high to low (He & Wei, 2017). The CC-type starch has typical diffraction peaks at 5.6°, 15°, 17°, and 23° 2θ. Compared with CC-type starch, the CA-type starch has a shoulder peak at about 18° 2θ, and the CB-type starch has two shoulder peaks at about 22° and 24° 2θ (He & Wei, 2017). On the based of XRD patterns, the 9 sweet potato starches all showed CA-type XRD pattern (Fig. 1B) and had relative crystallinities ranging from 22.8 to 25.6% (Table 3). Lee and Lee (2017) reported that the orange, white, and purple sweet potatoes have CAtype starch, and Zhang et al. (2018) also reported that the purple sweet potatoes have CA-type starch. However, Kim et al. (2013) reported that some orange, white, and purple sweet potatoes have A-type starch and some white and purple sweet potatoes have CB-type starches. Genkina et al. (2003) reported that orange sweet potato has A-type starch when grown in 33 °C soil and CC-type starch when grown in 15 °C soil, indicating that the growing temperature has significant effects on crystalline structure. In the present study, the 9 sweet potato starches from 3 different colored root tubers grown at the same environment had the same crystalline structure, indicating that the crystalline structure had no relationship with the color of root tuber.

3.8. Thermal properties of starch The 9 sweet potato starches were analysed using DSC (Fig. 2), and their thermograms were significantly different. Some starches showed obvious two peak DSC curve, some starches had DSC curve with one major peak and one weak peak, and some starches had single and wide peak. Similar DSC thermograms have been reported in white, orange, yellow, and purple sweet potato starches (Genkina et al., 2003; Genkina, Wasserman, Noda, Tester, & Yuryev, 2004; Waramboi, Dennien, Gidley, & Sopade, 2011; Zhang et al., 2018). The two peaks of DSC thermogram in C-type starch result from the different gelatinization temperatures of A- and B-type crystallinities, and the first and the second peak responds to the B- and A-type crystallinity, respectively (Zhang et al., 2018). The thermal parameters are presented in Table 4. The gelatinization temperatures were different among different sweet potato starches. The gelatinization temperature ranges (ΔT) varied from 23.1 to 30.7 °C among 9 starches and were significantly wider than that of A- and B-type starch (Huang et al., 2015; Lin, Zhang, Zhang, & Wei,

3.6. Short-range ordered structure of starch The short-range ordered structure in the external region of starch 832

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3.9. Pasting properties of starch Pasting properties determine the quality and utilization of starch (Abegunde et al., 2013). The pasting properties of 9 sweet potato starches are presented in Table 5. Peak, hot, and final viscosities among 9 sweet potato starches ranged from 4773 to 5959 mPa s, from 2202 to 2929 mPa s, and from 3101 to 3699 mPa s, respectively. The breakdown and setback viscosity ranged from 2216 to 3222 mPa s and from 630 to 899 mPa s, respectively. Breakdown viscosity reflects the starch paste resistance to heat and shear, and starch with high breakdown means the lower resistance to heat (Abegunde et al., 2013). Setback viscosity reflects the gelling ability or retrogradation tendency of starch (Simi & Abraham, 2008). Final viscosity is formed at the end of cooling at 50 °C, and indicates the ability of starch to form a paste or gel after cooling (Shimelis, Meaza, & Rakshit, 2006). The granule morphology and size, amylose content, crystalline structure, and starch purity affect the pasting properties (Abegunde et al., 2013; Singh, Kaur, Ezekiel, & Guraya, 2005). In the present study, pasting properties of starches were significantly different among different sweet potato varieties but had no relationship with the color of root tuber. 3.10. Digestion properties of starch Digestion properties of native, gelatinized, and retrograded starches are presented in Table 6. For native starches, the RDS, SDS, and RS among 9 sweet potato varieties ranged from 3.4 to 5.5%, from 8.9 to 19.5%, and from 75.0 to 87.2%, respectively. The digestion of native starch is affected mainly by granule morphology and size, starch component, and crystalline structure (Lin et al., 2018; Wang & Copeland, 2013). In the present study, starches from different sweet potato varieties had different granule sizes, amylose contents, relative crystallinities, and ordered degrees, which might lead to the different digestion properties of native starches. For gelatinized starches, the RDS, SDS, and RS among 9 sweet potato varieties ranged from 80.4 to 85.0%, from 4.0 to 9.2%, and from 7.4 to 14.5%, respectively. For retrograded starches, the RDS, SDS, and RS among 9 sweet potato varieties ranged from 78.8 to 84.1%, from 1.8 to 3.6%, and from 12.7 to 17.6%, respectively. The inter- and intra-molecular hydrogen bonds between starch chains are disrupted during starch gelatinization, leading to that the gelatinized starch is more rapidly degraded than the native starch. The amylose chains of gelatinized starch again associate to form the double helices and the amylopectins recrystallize to form the crystallites during gelatinized starch retrogradation, leading to that the retrograded starch has higher resistance to digestive enzymes than the gelatinized starch (Chung, Lim, & Lim, 2006). Due to the destruction of granule and crystalline structures by heating, the gelatinized and retrograded starches are mainly influenced by amylose content, amyloselipid complex, and amylopectin structure (Cai et al., 2015; Lin et al., 2017; Wang & Copeland, 2013). In the present study, starches from different sweet potato varieties had different digestion properties, but the digestion properties had no relationship with the color of root tuber. Trung, Ngoc, Hoa, Tien, and Hung (2017) also reported that the RDS, SDS, RS of native starch are similar among white, yellow, and purple sweet potatoes.

Fig. 1. Iodine absorbance spectrum (A), XRD pattern (B), and ATR-FTIR spectrum (C) of starch. The abbreviations of sweet potato varieties are explained in Table 1.

2017). The wide gelatinization temperature range is also reported in sweet potato starches from white (Lee & Lee, 2017), yellow (Kim et al., 2013), and purple root tubers (Zhang et al., 2018), which is due to that the B- and A-type crystallinity in C-type starch has low and high gelatinization temperature, respectively (Bogracheva, Morris, Ring, & Hedley, 1998). The thermal properties of starch are affected by many factors, such as granule size, amylose content, and crystalline structure (Kim et al., 2013; Lindeboom, Chang, & Tyler, 2004; Osundahunsi et al., 2003). Though the 9 sweet potato varieties had CA-type starch, the different DSC thermograms among different varieties might result from their different granule size, amylose content, relative crystallinity, and ordered degree.

3.11. Cluster analysis of starch In order to compare the relationships of different colored sweet potatoes, the hierarchical cluster was performed based on granule size D[4,3], apparent amylose content, relative crystallinity, IR absorbance ratios of 1045/1022 and 1022/995 cm−1, swelling power, water solubility, thermal parameters (Table 4), pasting properties (Table 5), and digestion properties (Table 6). The dendrogram consisted of two major clusters (Fig. 3). On the basis of similarities and differences in starch property parameters, the white sweet potato varieties, Su 28 and Su 29, and yellow sweet potato variety, Su16, were separated from the other 833

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Table 2 Iodine absorption spectrum parameters, apparent amylose content, and protein content of starch.a Varieties

b

W1 W2 W3 Y1 Y2 Y3 P1 P2 P3 a b

λmax (nm)

Iodine blue value

OD620/OD550

Apparent amylose content (%)

Protein content (mg/g)

598.2 598.0 598.3 600.3 603.5 602.5 602.3 605.2 603.5

0.330 0.315 0.311 0.311 0.343 0.340 0.314 0.305 0.336

1.114 1.119 1.124 1.147 1.156 1.158 1.132 1.157 1.166

27.1 25.5 25.2 24.7 27.2 27.0 25.1 24.1 26.5

0.22 0.31 0.22 0.50 0.31 0.31 0.47 0.44 0.31

± ± ± ± ± ± ± ± ±

0.6a 1.3a 1.3a 0.3ab 2.2bc 0.5bc 1.0bc 1.8c 0.0c

± ± ± ± ± ± ± ± ±

0.002c 0.002b 0.003ab 0.002ab 0.005e 0.003de 0.002b 0.003a 0.004cd

± ± ± ± ± ± ± ± ±

0.009a 0.009a 0.009a 0.003bc 0.005c 0.005c 0.004ab 0.016c 0.013c

± ± ± ± ± ± ± ± ±

0.2de 0.2c 0.2bc 0.2b 0.4e 0.4de 0.3bc 0.3a 0.2d

± ± ± ± ± ± ± ± ±

0.04a 0.01ab 0.04a 0.01c 0.01ab 0.09ab 0.04c 0.01bc 0.01ab

Data are means ± standard deviations, n = 3. Values in the same column with different letters are significantly different (p < 0.05). The abbreviations of sweet potato varieties are explained in Table 1.

Table 3 Relative crystallinity and IR absorbance ratio of starch.a Varieties

b

W1 W2 W3 Y1 Y2 Y3 P1 P2 P3

Relative crystallinity (%)

22.8 24.5 23.5 23.7 23.1 23.3 24.6 25.6 24.0

± ± ± ± ± ± ± ± ±

0.7a 1.2a 1.2a 0.8a 0.3a 0.5a 0.2a 0.2a 0.7a

IR absorbance ratio 1045/1022 (cm−1)

1022/995 (cm−1)

0.726 0.745 0.691 0.716 0.746 0.720 0.765 0.721 0.678

0.875 0.853 0.906 0.876 0.852 0.880 0.858 0.866 0.943

± ± ± ± ± ± ± ± ±

0.011ab 0.013b 0.013a 0.014ab 0.013b 0.013ab 0.014b 0.015ab 0.019a

± ± ± ± ± ± ± ± ±

0.009ab 0.006a 0.007c 0.004ab 0.008a 0.007b 0.008ab 0.007ab 0.002d

a Data are means ± standard deviations, n = 2. Values in the same column with different letters are significantly different (p < 0.05). b The abbreviations of sweet potato varieties are explained in Table 1.

six varieties at the linkage distance of 25. As for the remained six varieties, there were two groups at the distance of approximate 8. One group contained white sweet potato variety, Su 24, and purple sweet potato varieties, Ningzi 1 and Ningzi 4. The another group had purple sweet potato variety, Ningzi 2, and yellow sweet potato varieties, Su 14 and Su 25. The Su 29 could be further separated from Su 28 and Su 16 at the distance of 6, the Ningzi 2 did from Su 14 and Su 25 at the distance of 4, and the Su 24 did from Ningzi 1 and Ningzi 4 at the distance of 2. The cluster result indicated that starches from different varieties with the same colored root tuber had different physicochemical properties, and the structural and functional properties of starches had no relationship with the color of root tuber and were determined by genotype background of variety. Sajeev et al. (2012) investigated the relationship of 3 white, 2 cream, and 2 orange sweet potato varieties

Fig. 2. DSC thermogram of starch. The abbreviations of sweet potato varieties are explained in Table 1.

using clustering and principal component analysis based on textural properties of root tuber and pasting and gelatinization properties of flour. Their results also showed that there is no similarity in starch properties among the varieties with the same colored root tuber.

4. Conclusion Nine sweet potato varieties with 3 white, 3 yellow, and 3 purple root tubers were investigated in this study. The white root tuber contained the highest starch and the lowest soluble sugar, and the yellow

Table 4 Swelling power, water solubility, and thermal parameters of starch.a Varieties

b

Swelling power (g/g)

Water solubility (%)

Thermal parameters To (°C)

W1 W2 W3 Y1 Y2 Y3 P1 P2 P3

31.1 29.2 28.2 25.2 27.5 30.8 30.0 30.6 25.5

± ± ± ± ± ± ± ± ±

1.0d 0.4bcd 0.7bc 1.0a 1.5b 0.8d 0.2cd 0.6d 0.8a

13.0 13.7 16.0 14.0 13.2 12.7 11.7 12.3 16.6

± ± ± ± ± ± ± ± ±

0.8ab 0.0b 0.6c 0.9b 0.8ab 0.2ab 0.1a 0.9ab 0.9c

58.0 52.1 55.1 60.5 55.6 56.4 58.9 64.1 56.1

± ± ± ± ± ± ± ± ±

c

Tp (°C)

0.1d 0.6a 0.9b 0.2e 0.1bc 0.6c 0.3d 0.7f 0.2bc

a

70.2 74.5 70.1 73.1 64.7 70.5 79.9 78.9 70.0

± ± ± ± ± ± ± ± ±

c

0.1b 0.3d 0.5b 0.5c 0.1a 0.9b 0.1e 0.2e 0.8b

Tc (°C) 84.6 83.7 83.2 86.3 86.1 87.1 86.6 87.2 84.3

± ± ± ± ± ± ± ± ±

c

0.7b 0.7ab 0.4a 0.3c 0.3c 0.1c 0.9c 0.4c 0.5ab

ΔT (°C)

c

ΔH (J/g)

26.6 31.6 28.1 25.8 30.5 30.7 27.7 23.1 28.2

0.7bc 0.6d 1.3c 0.5b 0.3d 0.5d 0.6c 0.8a 0.4c

10.3 ± 0.6ab 10.0 ± 0.8ab 10.1 ± 1.1ab 10.9 ± 0.1ab 10.9 ± 0.7ab 10.5 ± 0.1ab 11.6 ± 0.2b 10.9 ± 0.6ab 9.5 ± 0.5a

± ± ± ± ± ± ± ± ±

c

Data are means ± standard deviations, n = 3. Values in the same column with different letters are significantly different (p < 0.05). The abbreviations of sweet potato varieties are explained in Table 1. c To, gelatinization onset temperature; Tp, gelatinization peak temperature; Tc, gelatinization conclusion temperature; ΔT, gelatinization temperature range (Tc – To); ΔH, gelatinization enthalpy. b

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Table 5 Pasting properties of starch.a Varieties

b

PV (mPa s)

W1 W2 W3 Y1 Y2 Y3 P1 P2 P3

c

HV (mPa s)

5959 ± 29g 5586 ± 16e 5424 ± 20c 5347 ± 10b 5525 ± 9d 5661 ± 29f 5326 ± 41b 4773 ± 13a 5922 ± 29g

c

BV (mPa s)

2927 ± 8f 2456 ± 37b 2202 ± 38a 2730 ± 13e 2474 ± 44bc 2865 ± 7f 2639 ± 27d 2557 ± 11cd 2905 ± 77f

c

FV (mPa s)

3032 ± 25d 3130 ± 32e 3222 ± 19f 2617 ± 6b 3050 ± 42de 2796 ± 35c 2686 ± 60b 2216 ± 7a 3017 ± 56d

c

SV (mPa s)

3557 ± 3c 3253 ± 13b 3101 ± 6a 3568 ± 36c 3316 ± 28b 3699 ± 42d 3327 ± 14b 3293 ± 24b 3690 ± 59d

c

PTime (min)

630 ± 7a 797 ± 50cd 899 ± 33e 838 ± 22de 842 ± 26de 834 ± 49de 687 ± 24ab 736 ± 14bc 784 ± 56cd

4.8 4.7 4.6 4.8 4.7 4.8 5.1 5.1 4.6

± ± ± ± ± ± ± ± ±

c

PTemp (°C)

0.1cd 0.0bc 0.0a 0.0d 0.0bc 0.0cd 0.0e 0.0e 0.0ab

74.2 73.3 72.5 75.3 73.3 74.5 76.1 78.4 74.1

± ± ± ± ± ± ± ± ±

c

0.1c 0.1b 0.0a 0.5d 0.1b 0.5c 0.5e 0.4f 0.1bc

a

Data are means ± standard deviations, n = 3. Values in the same column with different letters are significantly different (p < 0.05). The abbreviations of sweet potato varieties are explained in Table 1. c PV, peak viscosity; HV, hot viscosity; BV, breakdown viscosity (PV – HV); FV, final viscosity; SV, setback viscosity (FV – HV); PTime, peak time; PTemp, pasting temperature. b

Table 6 Digestion properties of native, gelatinized, and retrograded starches.a Varieties

b

Native starch RDS (%)

W1 W2 W3 Y1 Y2 Y3 P1 P2 P3 a b c

3.4 5.1 5.0 4.4 4.8 5.0 3.9 3.8 5.5

± ± ± ± ± ± ± ± ±

c

0.1a 1.0bc 0.3bc 1.0abc 0.0abc 0.2abc 0.8ab 0.1ab 0.2c

Gelatinized starch SDS (%)

c

11.9 ± 1.2b 14.5 ± 1.5bcd 16.9 ± 1.0cde 15.0 ± 0.6cd 17.2 ± 1.3de 16.1 ± 0.8cd 8.9 ± 1.3a 14.0 ± 0.9bc 19.5 ± 1.4e

RS (%) 84.7 80.4 78.1 80.6 78.0 79.0 87.2 82.1 75.0

± ± ± ± ± ± ± ± ±

c

1.2d 0.8bc 1.1b 0.9bc 1.3b 0.8b 0.6e 0.8c 1.5a

Retrograded starch

RDS (%)

SDS (%)

RS (%)

RDS (%)

SDS (%)

RS (%)

83.6 83.0 82.4 80.6 80.8 80.4 84.5 85.0 84.2

8.1 9.2 7.0 6.7 6.9 5.1 4.0 7.5 6.1

8.3 ± 1.3ab 7.8 ± 0.9a 10.6 ± 0.6abc 12.8 ± 1.8cd 12.3 ± 1.0cd 14.5 ± 1.5d 11.5 ± 1.1bcd 7.4 ± 1.6a 9.7 ± 0.9abc

83.5 83.6 81.9 80.2 78.8 79.5 82.1 84.1 82.8

2.7 2.7 3.0 3.6 3.6 3.5 1.8 3.2 2.6

13.8 13.7 15.1 16.1 17.6 17.0 16.1 12.7 14.5

± ± ± ± ± ± ± ± ±

0.4bc 0.5bc 1.0ab 0.2a 1.0a 0.6a 0.7c 0.3c 1.4bc

± ± ± ± ± ± ± ± ±

1.2ab 1.4b 1.1ab 1.9ab 1.3ab 1.6ab 1.5a 1.5ab 1.6ab

± ± ± ± ± ± ± ± ±

0.2c 2.4c 0.7abc 1.4abc 0.6a 2.0ab 0.9abc 1.0c 1.8bc

± ± ± ± ± ± ± ± ±

1.1a 0.6a 1.1a 0.8a 1.5a 0.3a 0.8a 1.6a 0.3a

± ± ± ± ± ± ± ± ±

1.0ab 1.8ab 1.4abc 1.3abc 1.2c 1.8bc 0.2abc 1.0a 1.5abc

Data are means ± standard deviations, n = 3. Values in the same column with different letters are significantly different (p < 0.05). The abbreviations of sweet potato varieties are explained in Table 1. RDS, rapidly digestible starch; SDS, slowly digestible starch; RS, resistant starch.

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Fig. 3. Dendrogram generated by hierarchical cluster analysis based on starch property parameters of different colored sweet potato varieties.

root tuber had the lowest starch and the highest soluble sugar. Starches from different varieties had different granule sizes and amylose contents, but all exhibited CA-type crystalline structure. The swelling power, water solubility, thermal parameters, pasting properties, and digestion properties were different among starches from different varieties. The differences of structural and functional properties of starches had no relationship with the color of root tuber and were determined by their genotype backgrounds. This study could provide important information for quality breeding and utilization of different colored sweet potato varieties. Acknowledgements This study was financially supported by grants from the National 835

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