International Journal of Biological Macromolecules 67 (2014) 79–84
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Effect of temperature-cycled retrogradation on in vitro digestibility and structural characteristics of waxy potato starch Yao-Yu Xie a , Xiao-Pei Hu a , Zheng-Yu Jin b , Xue-Ming Xu b , Han-Qing Chen a,∗ a
School of Biotechnology and Food Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei, Anhui 230009, PR China State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, 1800 Lihu Road, Wuxi, Jiangsu 214122, PR China b
a r t i c l e
i n f o
Article history: Received 10 February 2014 Received in revised form 3 March 2014 Accepted 8 March 2014 Available online 14 March 2014 Keywords: Waxy potato starch Slowly digestible starch Temperature-cycled retrogradation Structure Digestibility
a b s t r a c t The effects of temperature-cycled retrogradation treatment on the structural characteristics and in vitro digestibility of waxy potato starch were investigated in this study. The results showed that the maximum yield of slowly digestible starch (SDS) in waxy potato starch reached 38.63% by retrogradation treatment under temperature cycles of 4/25 ◦ C for 3 days with an interval of 24 h. The starch products prepared under the temperature cycles of 4/25 ◦ C exhibited a narrower melting temperature range (Tc − To ), a higher melting enthalpy (H) and a higher IR absorbance ratio (1047 cm−1 /1022 cm−1 ) than that prepared at a constant temperature of 25 ◦ C. Compared to native starch, X-ray diffraction pattern of treated starch was altered from B-type to C-type. Furthermore, the relative crystallinity of the starch products prepared under temperature-cycled retrogradation was the highest. This study suggests that more imperfect crystallites are formed in the crystalline matrix under temperature-cycled retrogradation, resulting in a high yield of SDS. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Starch is composed of essentially linear amylose and highly branched amylopectin with ␣-d-glucopyranose as the structure unit, and plays a very important role in supplying metabolic energy and nutrition for humans [1]. For specifying the nutritional fraction of starch or starch products, starch is generally classified into rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS) [2]. RDS is the starch fraction digested rapidly and completely in the small intestine, which is related to a series of health complications such as diabetes and cardiovascular diseases [3]. RS is not digested in the upper gastrointestinal tract that provides function control of glycemic index (GI) which is beneficial to the prevention of cardiovascular diseases [4]. SDS, as an intermediate starch fraction between RDS and RS, is digested slowly throughout the small intestine to provide a slow and prolonged release of glucose, and it has potential application in controlling and preventing hyperglycaemia-related diseases [5,6]. Therefore, SDS received more attention as a new functional food component or ingredient in novel food product development in recent years.
∗ Corresponding author. Tel.: +86 551 62901516; fax: +86 551 62901516. E-mail addresses: [email protected], [email protected] (H.-Q. Chen). http://dx.doi.org/10.1016/j.ijbiomac.2014.03.007 0141-8130/© 2014 Elsevier B.V. All rights reserved.
Recently, a number of techniques for preparing SDS products have been developed. These techniques include physical, chemical, enzymatic, and combined modification. Due to safety, cost-effectiveness, and more suitable for commercial use, therefore, physical method is widely used in starch modification. Physical method includes, but are not limited to, thermal treatment [7–11] and starch retrogradation [12–16]. Compared to thermal treatment, starch retrogradation is a relatively simple method, therefore, it receives more attention in recent years. Starch retrogradation, including short-term retrogradation and long-term retrogradation, is an unavoidable phenomenon and occurs readily during the storage of heat-processed starchy food. Through the process of retrogradation, the gelatinized starch is transformed from an amorphous state to a more ordered or crystalline state [17,18]. Its occurring could reduce the digestibility of starch by amylase and it was suitable for preparing RS and SDS. Park et al. [12] reported that temperature-cycled retrogradation could induce a great amount of RS from waxy maize starch gel. Zhou and Lim [16] indicated that the retrogradation increased SDS content in waxy and normal corn starch powders more effectively by the temperature cycling (4/30 ◦ C) than the isothermal storage (4 ◦ C). Zhang et al. [15] successfully prepared the SDS products with a high yield of 51.62% from waxy rice starch by temperature-cycled retrogradation. Moreover, Tian et al. [14] further confirmed that temperature-cycled retrogradation significantly increased the SDS
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yield and the slow digestibility of waxy rice starch, the maximum yield of SDS reached 54.5%. At present, hydrothermal treatment has been used to prepare slowly digestible starch from potato starch and waxy potato starch [8,9]. Waxy potato starch is a kind of newly developed starch source that lacks amylose, which is regarded as a good source of RS because of its B-type crystalline structure [9]. However, few studies reported the effects of temperature-cycled retrogradation treatment on the structural characteristics and digestibility of waxy potato starch. In order to explore its applications in food industry, we investigated the effects of temperature-cycled retrogradation on in vitro digestibility and structural characteristics of waxy potato starch.
percentages of RDS, SDS, and RS in the product were calculated by following equations: RDS (%) = SDS (%) = RS (%) =
(G − FG) × 0.9 20
(G
120
TS
× 100
− G20 ) × 0.9 × 100 TS
(TS − RDS − SDS) TS
(1)
× 100
(2) (3)
where G20 and G120 are glucose content released after 20 and 120 min, respectively; FG is the free glucose; TS is the weight of total starch sample used for each test. 2.4. Differential scanning calorimetry (DSC)
2. Materials and methods 2.1. Materials Waxy potato starch was obtained from National Starch Specialties (Shanghai) Ltd. (Shanghai, China). ␣-amylase type VI-B from porcine pancreas (EC 3.2.1.1, A3176) and amyloglucosidase (EC 3.2.1.3) were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA) and Shanghai Yuanye Bio-Technology Co., Ltd. (Shanghai, China), respectively. All other chemicals (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
2.2. Preparation of slowly digestible starch products Waxy potato starch (5.0 g) and distilled water (10 mL) were placed in a 150 mL glass container, sealed, and heated in boiling water for 30 min. The resultant gels were cooled to room temperature, then hermetically sealed and stored for 1, 3, 5, 7 days under three different temperature conditions: temperature cycles of 4 and 25 ◦ C with an interval of 24 h, or constant temperatures of 4 and 25 ◦ C. After treatment, each of resulting gels was cut into pieces (less than 5 cm in length and thickness), then freeze-dried. All samples were milled (Model LG-04, Baixin Yaoji Corporation, Ruian, China) to pass through a 100-mesh sieve to obtain the starch products.
2.3. Measurement of in vitro digestibility of starch products The in vitro digestibility of the starch products were analyzed by previously described methods [19,20] with some modification. Briefly, starch sample (200 mg, dry solids) was hydrolyzed by a mixed enzyme solution of porcine pancreatic ␣-amylase (290 U/mL) and amyloglucosidase (15 U/mL), where 1 U is defined as the amount of enzyme that liberates 1.0 mg glucose from starch in 1 min at pH 5.2 and 37 ◦ C. Phosphate buffer (15 mL, 0.2 mol/L, pH 5.2) and five glass balls (10 mm in diameter) were added to each of the conical tubes containing starch samples (200 mg, dry base). After equilibration at 37 ◦ C for 5 min, the enzyme solution (5 mL) was added to the sample tube, followed by incubation in a water bath at 37 ◦ C with shaking (170 r/min). Aliquots (0.5 mL) were taken at intervals of 20 and 120 min and mixed with 4 mL of 80% ethanol to deactivate the enzymes. The mixed solution was centrifuged at 2000 r/min for 10 min, and the glucose content in the supernatant was measured using the 3,5dinitrosalicylic acid (DNS) assay [21]. Percentage of hydrolyzed starch was calculated by multiplying a factor of 0.9 with the glucose content [20]. Each sample was analyzed in triplicate. The
The thermal properties of each starch sample were determined by a TA Q200 (TA Instruments, New Castle, DE, USA). Approximately, 3 mg anhydrous starch sample was mixed with 6 L deionized water and hermetically sealed in an aluminum pan [15]. Then the pan was equilibrated at 4 ◦ C for 24 h. After equilibration, the pan was heated from 20 to 85 ◦ C at a rate of 8 ◦ C/min. An empty pan was used as a reference. Onset temperature (To ), peak temperature (Tp ), conclusion temperature (Tc ) and melting enthalpy (H) were calculated using DSC software (TA Instruments, New Castle, DE, USA). Experiments were conducted in triplicate. 2.5. Fourier transform-infrared spectroscopy (FT-IR) The FT-IR spectra were obtained on a Nicolet 6700 spectrometer (Thermo Electric Corporation, Waltham, MA, USA). A spectral resolution of 4 cm−1 was employed and 64 scans were acquired for each spectrum. Spectra were baseline-corrected and deconvoluted by drawing a straight line at 1200 and 800 cm−1 . The absorbance ratio of 1047 cm−1 /1022 cm−1 was obtained from the deconvoluted spectra using Omnic version 8.0 software (Thermo Fisher Scientific Inc. Waltham, MA, USA). The ratio of 1047 cm−1 /1022 cm−1 from deconvoluted FT-IR spectrum has been used to express the amount of ordered crystalline to amorphous domains in starches [27]. 2.6. X-ray diffraction X-ray diffraction analysis was performed with an X-ray diffractometer (D/MAX 2500 V, Rigaku Corporation, Japan), operating at ˚ The starch 40 kV and 40 mA with Cu K␣ radiation (k = 1.5406 A). sample was scanned at a rate of 2/min from 4◦ to 35◦ at room temperature. The degree of relative crystallinity was calculated according to the method of Nara and Komiya [22] using MDI-Jade 6.0 software (Material Date, Inc. Livermore, California, USA). 2.7. Statistical analysis Results are expressed as the mean ± standard deviation of the triplicate experiments. Data were analyzed by one-way analysis of variance (ANOVA), followed by Duncan’s multiple range test using SPSS 17.0 Statistical Software Program (SPSS Incorporated, Chicago). A value of P < 0.05 was considered statistically significant. 3. Results and discussion 3.1. In vitro digestibility of starch products The in vitro digestibility of waxy potato starch products is significantly affected by the storage conditions. The yield of SDS in waxy potato starch increased as the retrogradation time increased, reaching the maximum content (38.63%) after retrogradation at
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Fig. 2. Melting thermograms of waxy potato starch products stored at a constant temperature of 4 ◦ C, 25 ◦ C or cycles of 4/25 ◦ C (24 h each) for 3 days. (a) Native waxy potato starch (18.37% RDS, 8.64% SDS, 72.99% RS), (b) waxy potato starch product stored at 4 ◦ C for 3 days (32.89% RDS, 31.57% SDS, 35.54% RS), (c) waxy potato starch product stored at 25 ◦ C for 3 days (29.09% RDS, 32.65% SDS, 38.26% RS), (d) waxy potato starch product stored at cycles of 4/25 ◦ C(24 h each) for 3 days (25.73% RDS, 38.63% SDS, 35.64% RS).
Fig. 1. (a) Starch nutritional fraction in waxy potato starch product stored at a constant temperature of 4 ◦ C, 25 ◦ C or cycles of 4/25 ◦ C (24 h each) for 3 days, (b) The yield of SDS from waxy potato starch prepared at a constant temperature 4 ◦ C, 25 ◦ C, and cycles of 4/25 ◦ C (24 h each) for 1–7 days. 4 ◦ C and 25 ◦ C indicate waxy potato starch product stored under isothermal conditions. 4/25 ◦ C indicates waxy potato starch product stored under temperature-cycled condition.
cycles of 4 and 25 ◦ C for 3 days with an interval of 24 h (Fig. 1). It was suggested that temperature-cycled retrogradation was beneficial to the formation of SDS, but the amount of SDS was not linearly correlated with the retrogradation time. This was similar to the results from waxy rice starch reported by Zhang et al. [15], who found that the maximum SDS yield of 51.62% was obtained by temperature-cycled retrogradation for 7 day at cycles of 4/25 ◦ C. The susceptibility of starch product to digestive enzymes may depend not only on the amorphous structure, but also on the crystalline structure of the starch product [12]. Under the temperature-cycled retrogradation condition at 4/25 ◦ C, numerous imperfect crystallites were formed in the crystalline matrix, hence resulting in the increase in SDS. However, after isothermal retrogradation treatment at 25 ◦ C, a large number of perfect crystallites were formed in the crystalline matrix, leading to higher content of RS [32]. In addition, as shown in Fig. 1b, a significant increase in the yield of SDS was observed when retrogradation time was 3 days. This indicated that retrogradation time is another main factor for slowly digestible starch preparation. 3.2. Thermal properties of starch products The DSC thermograms of starch products stored at 4, 25 ◦ C and cycles of 4/25 ◦ C (24 h each) for 3 days are presented in Fig. 2. Their corresponding thermal parameters onset (To ), peak (Tp ), and conclusion (Tc ) temperatures, melting temperature range (Tc − To ), and melting enthalpy (H) are summarized in Table 1. Variations in To , Tp , Tc and H have been shown by various researchers to reflect the crystallinity, structure and composition of starches. Compared to the waxy potato starch stored at 4 ◦ C,
the increases in To , Tp , Tc , Tc − To , and H were observed in the waxy potato starch stored at 25 ◦ C and cycles of 4/25 ◦ C. Tester and Morrison [23] reported that the low-gelatinization temperature starches had less crystallinity and less perfect crystallites than the high-gelatinization temperature starches due to minor structural differences in their amylopectins. Eerlingen et al. [24] also reported that increased retrogradation extents (high melting temperatures, melting enthalpies, and higher crystallinity levels) caused reduced enzyme susceptibility to pancreatic ␣-amylase and amyloglucosidase at 37 ◦ C, since these were associated with slow digestion property of starch. The RDS content of waxy potato starch products stored at a constant temperature of 4, 25 ◦ C or cycles of 4/25 ◦ C (24 h each) for 3 days were 32.89%, 29.09% and 25.73%, respectively. These results indicated that the retrogradation treatment of waxy potato starch stored at 4 ◦ C would inhibit the generation of crystallites in modified starch products and numerous amorphous regions were formed in starch product, resulting in the high content of RDS [12]. Furthermore, the melting temperature range of the waxy potato starch products stored at cycles of 4/25 ◦ C had the shift toward lower value compared with that stored at 25 ◦ C. This result suggests that more imperfect crystallites are formed under temperature-cycled retrogradation conditions, resulting in the high content of SDS [16,25,26]. 3.3. Fourier transform-infrared spectroscopy (FT-IR) The IR absorbance bands at 1047 and 1022 cm−1 are sensitive to ordered or crystalline structures and amorphous structures in starch, respectively, and the ratio of 1047/1022 cm−1 from deconvoluted FT-IR spectrum has been used to express the amount of ordered crystalline to amorphous domains in starches [27]. The deconvoluted spectra of waxy potato starch stored at 4, 25 ◦ C and cycles of 4/25 ◦ C for 3 days are presented in Fig. 3. The absorbance ratio of waxy potato starch stored at 4, 25 ◦ C and cycles of 4/25 ◦ C were 0.536, 0.701, 0.889, respectively. Compared to the waxy potato starch stored at 4 and 25 ◦ C, the waxy potato starch stored at cycles of 4/25 ◦ C has the highest absorbance ratio, suggesting that temperature-cycled retrogradation is beneficial to the packing of double helices within crystallinity. Zhang et al. [15] also reported that the absorbance ratio for waxy rice starch stored at
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Table 1 Melting parameters of waxy potato starch products stored at 4 ◦ C, 25 ◦ C, and cycles of 4/25 ◦ C (24 h each) for 1-7 days. Samples
To (◦ C)
Tp (◦ C)
Tc (◦ C)
Tc − To (◦ C)
H (J/g)
NS 4 ◦ C-1 d 4 ◦ C-3 d 4 ◦ C-5 d 4 ◦ C-7 d 25 ◦ C-1 d 25 ◦ C-3 d 25 ◦ C-5 d 25 ◦ C-7 d 4/25 ◦ C-1 d 4/25 ◦ C-3 d 4/25 ◦ C-5 d 4/25 ◦ C-7 d
65.49 ± 0.25 48.32 ± 0.12 49.87 ± 0.11 50.24 ± 0.06 50.57 ± 0.04 51.54 ± 0.26 52.46 ± 0.40 52.59 ± 0.46 52.88 ± 0.60 48.37 ± 0.34 51.27 ± 0.06 51.60 ± 0.35 52.36 ± 0.25
70.31 ± 0.25 58.75 ± 0.15 59.79 ± 0.13 59.62 ± 0.13 59.95 ± 0.62 59.74 ± 0.24 61.80 ± 0.15 62.36 ± 0.11 63.39 ± 0.29 58.77 ± 0.13 61.51 ± 0.18 61.72 ± 0.02 61.28 ± 0.04
75.84 ± 0.13 70.16 ± 0.12 74.41 ± 0.16 74.46 ± 0.05 74.49 ± 0.07 73.65 ± 0.23 80.59 ± 0.15 80.42 ± 0.10 80.17 ± 0.14 70.15 ± 0.12 76.64 ± 0.15 77.71 ± 0.11 77.81 ± 0.60
10.35 ± 0.12 21.84 ± 0.24 24.54 ± 0.12 24.22 ± 0.01 23.92 ± 0.03 22.11 ± 0.44 28.13 ± 0.26 27.83 ± 0.52 27.28 ± 0.72 21.78 ± 0.46 25.36 ± 0.11 26.11 ± 0.32 25.44 ± 0.39
15.66 ± 0.12 3.16 ± 0.10 4.66 ± 0.10 5.30 ± 0.05 6.53 ± 0.05 4.32 ± 0.14 5.28 ± 0.20 8.24 ± 0.08 8.34 ± 0.11 3.17 ± 0.11 6.72 ± 0.02 8.31 ± 0.15 9.45 ± 0.06
NS indicates native waxy potato starch. 4 ◦ C and 25 ◦ C indicate waxy potato starch product stored under isothermal conditions. 4/25 ◦ C indicates waxy potato starch product stored under temperature-cycled conditions. To , Tp , Tc , Tc − To , H indicate melting parameters onset, peak, conclusion temperature, melting temperature range and enthalpy of melting, respectively. Means ± SD (n = 3).
cycles of 4/25 ◦ C was increased to a maximum value (0.899) with the storage time up to 7 days, which indicated that ordered structure was generated in crystallites. Furthermore, the retrogradation of amylopectin under the temperature-cycled conditions reduced the enzyme susceptibility, resulting in a great amount of SDS and RS [12]. The variations in nutritional fractions of waxy potato starch stored at 4, 25 ◦ C and cycles of 4/25 ◦ C for 3 days are shown in Fig. 1a. Temperature-cycled retrogradation gives rise to a decrease in RDS, but the increase in the amount of SDS and RS as compared with isothermal retrogradation. A possible explanation for this behavior could be due to the increase in the amount of ordered regions to amorphous regions of starch granule under the temperature-cycled conditions, which was consistent with the result of the absorbance ratio of 1047/1022 cm−1 . 3.4. Crystalline properties of starch products The area between the experimental data and baseline area under the crystalline peak (Fig. 4) is amorphous area. The area above amorphous area is crystalline area. The relative crystallinity is the ratio of Ac /(Ac + Aa ), where Ac and Aa represent crystalline area and amorphous area, respectively. The X-ray diffraction patterns
for waxy potato starch stored at 4, 25 ◦ C and cycles of 4/25 ◦ C for 3 days are presented in Fig. 4. The relative crystallinity and the type of diffraction pattern of waxy potato starch stored at 4, 25 ◦ C and cycles of 4/25 ◦ C are summarized in Table 2. The different types of semi-crystalline supramolecular structure have been classified into A, B, or C types, which are the representations of the non-random arrangement and length of each linear chain in the molecule [28]. In our study, native waxy potato starch displayed the B-type pattern with reflection intensities at values of 5.6◦ , 14.4◦ , 17.2◦ , 19.5◦ , 22.1◦ and 23.8◦ , respectively. All starch products stored at 4, 25 ◦ C and cycles of 4/25 ◦ C for 3 days showed diffraction peaks at 5.7◦ , 16.9◦ , 17.2◦ , 22.2◦ , suggesting that the crystal type of starch products is a characteristic C-type, which consists of a mixture of A- and B-types. Heat-moisture treatment has been reported to change the X-ray diffraction patterns of potato starch and waxy potato starch from B-type to C-type [8,29]. In this study, we also found that different retrogradation treatment changed the X-ray diffraction pattern of waxy potato starch from B-type to C-type, indicating that the molecular and structure organization of starch products have been further modified by retrogradation treatment [30]. As shown in Table 2, the relative crystallinity of native waxy potato starch is 36.74%, which is similar to the result reported by Lee et al. [8]. However, compared with native waxy potato starch, the relative crystallinity of starch products by retrogradation treatment under different temperature conditions decreased. Shin et al. [10] found that hydrothermal treatment disrupted crystalline region of sweet potato starch, resulting in the decrease
Table 2 The relative crystallinity and type of diffraction patterns of waxy potato starch products stored at 4 ◦ C, 25 ◦ C, and cycles of 4/25 ◦ C (24 h each) for 1–7 days.
Fig. 3. Typical deconvoluted FT-IR spectra curves of waxy potato starch products stored at a constant temperature of 4 ◦ C, 25 ◦ C or cycles of 4/25 ◦ C (24 h each) for 3 days: (a) waxy potato starch product stored at 4 ◦ C for 3 days, (b) waxy potato starch product stored at 25 ◦ C for 3 days, (c) waxy potato starch product stored at cycles of 4/25 ◦ C for 3 days. The absorbance ratio of 1047 cm−1 /1022 cm−1 for waxy potato starch product stored at 4 ◦ C, 25 ◦ C and cycles of 4/25 ◦ C were 0.536 (0.7194/1.3408), 0.701 (0.8318/1.1873), 0.889 (2.3079/2.5949), respectively.
Samples
Relative crystallinity (%)
The type of diffraction pattern
NS 4 ◦ C-1 d 4 ◦ C-3 d 4 ◦ C-5 d 4 ◦ C-7 d 25 ◦ C-1 d 25 ◦ C-3 d 25 ◦ C-5 d 25 ◦ C-7 d 4/25 ◦ C-1 d 4/25 ◦ C-3 d 4/25 ◦ C-5 d 4/25 ◦ C-7 d
36.74 10.11 13.74 14.03 15.69 10.34 20.46 21.40 23.05 10.69 25.55 26.02 27.42
B-type C-type C-type C-type C-type C-type C-type C-type C-type C-type C-type C-type C-type
NS indicates native waxy potato starch. 4 ◦ C and 25 ◦ C indicate waxy potato starch product stored under isothermal conditions. 4/25 ◦ C indicates waxy potato starch product stored under temperature-cycled conditions.
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Fig. 4. X-ray diffraction patterns of waxy potato starch products stored at a constant temperature of 4 ◦ C, 25 ◦ C or cycles of 4/25 ◦ C (24 h each) for 3 days: (a) native waxy potato starch, (b) waxy potato starch product stored at 4 ◦ C for 3 days, (c) waxy potato starch product stored at 25 ◦ C for 3 days, (d) waxy potato starch product stored at cycles of 4/25 ◦ C for 3 days.
in relative crystallinity. The relative crystallinity of waxy potato starch products stored at 4, 25 ◦ C and cycles of 4/25 ◦ C for 3 days were 13.74%, 20.46%, 25.55%, respectively. This behavior showed a good correlation between the increase in the relative crystallinity and the absorbance ratio of 1047/1022 cm−1 . The crystallinity of starch product has been shown to be influenced by the amount of double helices that are arranged into a crystalline array, amylose content, crystalline size, and extent of disruption of amylopectin crystallites by amylase [31]. It could be speculated through combining these results with those obtained from relative crystallinity and the absorbance ratio of 1047/1022 cm−1 stated above, temperature-cycled retrogradation treatment would be beneficial to improving the crystallinity or the content of double helical structure in modified starch granules. As shown in Fig. 1a, starch products with higher relative crystallinity did not have higher RS content, but had lower RDS content. This was because that the RDS consisted of amorphous structure and appeared to be located in the outer regions. As opposite to this nutritional fraction, SDS and RS mostly consisted of imperfect crystallites and perfect crystallites, which was located in the inner regions [32,33].
4. Conclusions The results showed that temperature-cycled retrogradation was favorable for preparing the slowly digestible starch from waxy potato starch. The maximum yield of SDS reached 38.63% under temperature cycles of 4/25 ◦ C for 3 days with an interval of 24 h. Compared to isothermal retrogradation treatment, numerous imperfect crystallites were formed in the crystalline matrix under temperature-cycled retrogradation at 4/25 ◦ C. This could be explained by the evidence obtained from a higher relative crystallinity, a higher absorbance ratio of 1047/1022 cm−1 and melting enthalpy, but a narrower melting temperature range. Therefore, temperature-cycled retrogradation treatment would be applied for preparing the slowly digestible starch product in food industry. Acknowledgements This study was supported by National High Technology Research and Development Program of China (863 Program) (No. 2013AA102201), the Key Program of National Natural Science
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Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Effect of temperature-cycled retrogradation on in vitro digestibility and structural characteristics of waxy potato starch Yao-Yu Xie a , Xiao-Pei Hu a , Zheng-Yu Jin b , Xue-Ming Xu b , Han-Qing Chen a,∗ a
School of Biotechnology and Food Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei, Anhui 230009, PR China State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, 1800 Lihu Road, Wuxi, Jiangsu 214122, PR China b
a r t i c l e
i n f o
Article history: Received 10 February 2014 Received in revised form 3 March 2014 Accepted 8 March 2014 Available online 14 March 2014 Keywords: Waxy potato starch Slowly digestible starch Temperature-cycled retrogradation Structure Digestibility
a b s t r a c t The effects of temperature-cycled retrogradation treatment on the structural characteristics and in vitro digestibility of waxy potato starch were investigated in this study. The results showed that the maximum yield of slowly digestible starch (SDS) in waxy potato starch reached 38.63% by retrogradation treatment under temperature cycles of 4/25 ◦ C for 3 days with an interval of 24 h. The starch products prepared under the temperature cycles of 4/25 ◦ C exhibited a narrower melting temperature range (Tc − To ), a higher melting enthalpy (H) and a higher IR absorbance ratio (1047 cm−1 /1022 cm−1 ) than that prepared at a constant temperature of 25 ◦ C. Compared to native starch, X-ray diffraction pattern of treated starch was altered from B-type to C-type. Furthermore, the relative crystallinity of the starch products prepared under temperature-cycled retrogradation was the highest. This study suggests that more imperfect crystallites are formed in the crystalline matrix under temperature-cycled retrogradation, resulting in a high yield of SDS. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Starch is composed of essentially linear amylose and highly branched amylopectin with ␣-d-glucopyranose as the structure unit, and plays a very important role in supplying metabolic energy and nutrition for humans [1]. For specifying the nutritional fraction of starch or starch products, starch is generally classified into rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS) [2]. RDS is the starch fraction digested rapidly and completely in the small intestine, which is related to a series of health complications such as diabetes and cardiovascular diseases [3]. RS is not digested in the upper gastrointestinal tract that provides function control of glycemic index (GI) which is beneficial to the prevention of cardiovascular diseases [4]. SDS, as an intermediate starch fraction between RDS and RS, is digested slowly throughout the small intestine to provide a slow and prolonged release of glucose, and it has potential application in controlling and preventing hyperglycaemia-related diseases [5,6]. Therefore, SDS received more attention as a new functional food component or ingredient in novel food product development in recent years.
∗ Corresponding author. Tel.: +86 551 62901516; fax: +86 551 62901516. E-mail addresses: [email protected], [email protected] (H.-Q. Chen). http://dx.doi.org/10.1016/j.ijbiomac.2014.03.007 0141-8130/© 2014 Elsevier B.V. All rights reserved.
Recently, a number of techniques for preparing SDS products have been developed. These techniques include physical, chemical, enzymatic, and combined modification. Due to safety, cost-effectiveness, and more suitable for commercial use, therefore, physical method is widely used in starch modification. Physical method includes, but are not limited to, thermal treatment [7–11] and starch retrogradation [12–16]. Compared to thermal treatment, starch retrogradation is a relatively simple method, therefore, it receives more attention in recent years. Starch retrogradation, including short-term retrogradation and long-term retrogradation, is an unavoidable phenomenon and occurs readily during the storage of heat-processed starchy food. Through the process of retrogradation, the gelatinized starch is transformed from an amorphous state to a more ordered or crystalline state [17,18]. Its occurring could reduce the digestibility of starch by amylase and it was suitable for preparing RS and SDS. Park et al. [12] reported that temperature-cycled retrogradation could induce a great amount of RS from waxy maize starch gel. Zhou and Lim [16] indicated that the retrogradation increased SDS content in waxy and normal corn starch powders more effectively by the temperature cycling (4/30 ◦ C) than the isothermal storage (4 ◦ C). Zhang et al. [15] successfully prepared the SDS products with a high yield of 51.62% from waxy rice starch by temperature-cycled retrogradation. Moreover, Tian et al. [14] further confirmed that temperature-cycled retrogradation significantly increased the SDS
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yield and the slow digestibility of waxy rice starch, the maximum yield of SDS reached 54.5%. At present, hydrothermal treatment has been used to prepare slowly digestible starch from potato starch and waxy potato starch [8,9]. Waxy potato starch is a kind of newly developed starch source that lacks amylose, which is regarded as a good source of RS because of its B-type crystalline structure [9]. However, few studies reported the effects of temperature-cycled retrogradation treatment on the structural characteristics and digestibility of waxy potato starch. In order to explore its applications in food industry, we investigated the effects of temperature-cycled retrogradation on in vitro digestibility and structural characteristics of waxy potato starch.
percentages of RDS, SDS, and RS in the product were calculated by following equations: RDS (%) = SDS (%) = RS (%) =
(G − FG) × 0.9 20
(G
120
TS
× 100
− G20 ) × 0.9 × 100 TS
(TS − RDS − SDS) TS
(1)
× 100
(2) (3)
where G20 and G120 are glucose content released after 20 and 120 min, respectively; FG is the free glucose; TS is the weight of total starch sample used for each test. 2.4. Differential scanning calorimetry (DSC)
2. Materials and methods 2.1. Materials Waxy potato starch was obtained from National Starch Specialties (Shanghai) Ltd. (Shanghai, China). ␣-amylase type VI-B from porcine pancreas (EC 3.2.1.1, A3176) and amyloglucosidase (EC 3.2.1.3) were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA) and Shanghai Yuanye Bio-Technology Co., Ltd. (Shanghai, China), respectively. All other chemicals (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
2.2. Preparation of slowly digestible starch products Waxy potato starch (5.0 g) and distilled water (10 mL) were placed in a 150 mL glass container, sealed, and heated in boiling water for 30 min. The resultant gels were cooled to room temperature, then hermetically sealed and stored for 1, 3, 5, 7 days under three different temperature conditions: temperature cycles of 4 and 25 ◦ C with an interval of 24 h, or constant temperatures of 4 and 25 ◦ C. After treatment, each of resulting gels was cut into pieces (less than 5 cm in length and thickness), then freeze-dried. All samples were milled (Model LG-04, Baixin Yaoji Corporation, Ruian, China) to pass through a 100-mesh sieve to obtain the starch products.
2.3. Measurement of in vitro digestibility of starch products The in vitro digestibility of the starch products were analyzed by previously described methods [19,20] with some modification. Briefly, starch sample (200 mg, dry solids) was hydrolyzed by a mixed enzyme solution of porcine pancreatic ␣-amylase (290 U/mL) and amyloglucosidase (15 U/mL), where 1 U is defined as the amount of enzyme that liberates 1.0 mg glucose from starch in 1 min at pH 5.2 and 37 ◦ C. Phosphate buffer (15 mL, 0.2 mol/L, pH 5.2) and five glass balls (10 mm in diameter) were added to each of the conical tubes containing starch samples (200 mg, dry base). After equilibration at 37 ◦ C for 5 min, the enzyme solution (5 mL) was added to the sample tube, followed by incubation in a water bath at 37 ◦ C with shaking (170 r/min). Aliquots (0.5 mL) were taken at intervals of 20 and 120 min and mixed with 4 mL of 80% ethanol to deactivate the enzymes. The mixed solution was centrifuged at 2000 r/min for 10 min, and the glucose content in the supernatant was measured using the 3,5dinitrosalicylic acid (DNS) assay [21]. Percentage of hydrolyzed starch was calculated by multiplying a factor of 0.9 with the glucose content [20]. Each sample was analyzed in triplicate. The
The thermal properties of each starch sample were determined by a TA Q200 (TA Instruments, New Castle, DE, USA). Approximately, 3 mg anhydrous starch sample was mixed with 6 L deionized water and hermetically sealed in an aluminum pan [15]. Then the pan was equilibrated at 4 ◦ C for 24 h. After equilibration, the pan was heated from 20 to 85 ◦ C at a rate of 8 ◦ C/min. An empty pan was used as a reference. Onset temperature (To ), peak temperature (Tp ), conclusion temperature (Tc ) and melting enthalpy (H) were calculated using DSC software (TA Instruments, New Castle, DE, USA). Experiments were conducted in triplicate. 2.5. Fourier transform-infrared spectroscopy (FT-IR) The FT-IR spectra were obtained on a Nicolet 6700 spectrometer (Thermo Electric Corporation, Waltham, MA, USA). A spectral resolution of 4 cm−1 was employed and 64 scans were acquired for each spectrum. Spectra were baseline-corrected and deconvoluted by drawing a straight line at 1200 and 800 cm−1 . The absorbance ratio of 1047 cm−1 /1022 cm−1 was obtained from the deconvoluted spectra using Omnic version 8.0 software (Thermo Fisher Scientific Inc. Waltham, MA, USA). The ratio of 1047 cm−1 /1022 cm−1 from deconvoluted FT-IR spectrum has been used to express the amount of ordered crystalline to amorphous domains in starches [27]. 2.6. X-ray diffraction X-ray diffraction analysis was performed with an X-ray diffractometer (D/MAX 2500 V, Rigaku Corporation, Japan), operating at ˚ The starch 40 kV and 40 mA with Cu K␣ radiation (k = 1.5406 A). sample was scanned at a rate of 2/min from 4◦ to 35◦ at room temperature. The degree of relative crystallinity was calculated according to the method of Nara and Komiya [22] using MDI-Jade 6.0 software (Material Date, Inc. Livermore, California, USA). 2.7. Statistical analysis Results are expressed as the mean ± standard deviation of the triplicate experiments. Data were analyzed by one-way analysis of variance (ANOVA), followed by Duncan’s multiple range test using SPSS 17.0 Statistical Software Program (SPSS Incorporated, Chicago). A value of P < 0.05 was considered statistically significant. 3. Results and discussion 3.1. In vitro digestibility of starch products The in vitro digestibility of waxy potato starch products is significantly affected by the storage conditions. The yield of SDS in waxy potato starch increased as the retrogradation time increased, reaching the maximum content (38.63%) after retrogradation at
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Fig. 2. Melting thermograms of waxy potato starch products stored at a constant temperature of 4 ◦ C, 25 ◦ C or cycles of 4/25 ◦ C (24 h each) for 3 days. (a) Native waxy potato starch (18.37% RDS, 8.64% SDS, 72.99% RS), (b) waxy potato starch product stored at 4 ◦ C for 3 days (32.89% RDS, 31.57% SDS, 35.54% RS), (c) waxy potato starch product stored at 25 ◦ C for 3 days (29.09% RDS, 32.65% SDS, 38.26% RS), (d) waxy potato starch product stored at cycles of 4/25 ◦ C(24 h each) for 3 days (25.73% RDS, 38.63% SDS, 35.64% RS).
Fig. 1. (a) Starch nutritional fraction in waxy potato starch product stored at a constant temperature of 4 ◦ C, 25 ◦ C or cycles of 4/25 ◦ C (24 h each) for 3 days, (b) The yield of SDS from waxy potato starch prepared at a constant temperature 4 ◦ C, 25 ◦ C, and cycles of 4/25 ◦ C (24 h each) for 1–7 days. 4 ◦ C and 25 ◦ C indicate waxy potato starch product stored under isothermal conditions. 4/25 ◦ C indicates waxy potato starch product stored under temperature-cycled condition.
cycles of 4 and 25 ◦ C for 3 days with an interval of 24 h (Fig. 1). It was suggested that temperature-cycled retrogradation was beneficial to the formation of SDS, but the amount of SDS was not linearly correlated with the retrogradation time. This was similar to the results from waxy rice starch reported by Zhang et al. [15], who found that the maximum SDS yield of 51.62% was obtained by temperature-cycled retrogradation for 7 day at cycles of 4/25 ◦ C. The susceptibility of starch product to digestive enzymes may depend not only on the amorphous structure, but also on the crystalline structure of the starch product [12]. Under the temperature-cycled retrogradation condition at 4/25 ◦ C, numerous imperfect crystallites were formed in the crystalline matrix, hence resulting in the increase in SDS. However, after isothermal retrogradation treatment at 25 ◦ C, a large number of perfect crystallites were formed in the crystalline matrix, leading to higher content of RS [32]. In addition, as shown in Fig. 1b, a significant increase in the yield of SDS was observed when retrogradation time was 3 days. This indicated that retrogradation time is another main factor for slowly digestible starch preparation. 3.2. Thermal properties of starch products The DSC thermograms of starch products stored at 4, 25 ◦ C and cycles of 4/25 ◦ C (24 h each) for 3 days are presented in Fig. 2. Their corresponding thermal parameters onset (To ), peak (Tp ), and conclusion (Tc ) temperatures, melting temperature range (Tc − To ), and melting enthalpy (H) are summarized in Table 1. Variations in To , Tp , Tc and H have been shown by various researchers to reflect the crystallinity, structure and composition of starches. Compared to the waxy potato starch stored at 4 ◦ C,
the increases in To , Tp , Tc , Tc − To , and H were observed in the waxy potato starch stored at 25 ◦ C and cycles of 4/25 ◦ C. Tester and Morrison [23] reported that the low-gelatinization temperature starches had less crystallinity and less perfect crystallites than the high-gelatinization temperature starches due to minor structural differences in their amylopectins. Eerlingen et al. [24] also reported that increased retrogradation extents (high melting temperatures, melting enthalpies, and higher crystallinity levels) caused reduced enzyme susceptibility to pancreatic ␣-amylase and amyloglucosidase at 37 ◦ C, since these were associated with slow digestion property of starch. The RDS content of waxy potato starch products stored at a constant temperature of 4, 25 ◦ C or cycles of 4/25 ◦ C (24 h each) for 3 days were 32.89%, 29.09% and 25.73%, respectively. These results indicated that the retrogradation treatment of waxy potato starch stored at 4 ◦ C would inhibit the generation of crystallites in modified starch products and numerous amorphous regions were formed in starch product, resulting in the high content of RDS [12]. Furthermore, the melting temperature range of the waxy potato starch products stored at cycles of 4/25 ◦ C had the shift toward lower value compared with that stored at 25 ◦ C. This result suggests that more imperfect crystallites are formed under temperature-cycled retrogradation conditions, resulting in the high content of SDS [16,25,26]. 3.3. Fourier transform-infrared spectroscopy (FT-IR) The IR absorbance bands at 1047 and 1022 cm−1 are sensitive to ordered or crystalline structures and amorphous structures in starch, respectively, and the ratio of 1047/1022 cm−1 from deconvoluted FT-IR spectrum has been used to express the amount of ordered crystalline to amorphous domains in starches [27]. The deconvoluted spectra of waxy potato starch stored at 4, 25 ◦ C and cycles of 4/25 ◦ C for 3 days are presented in Fig. 3. The absorbance ratio of waxy potato starch stored at 4, 25 ◦ C and cycles of 4/25 ◦ C were 0.536, 0.701, 0.889, respectively. Compared to the waxy potato starch stored at 4 and 25 ◦ C, the waxy potato starch stored at cycles of 4/25 ◦ C has the highest absorbance ratio, suggesting that temperature-cycled retrogradation is beneficial to the packing of double helices within crystallinity. Zhang et al. [15] also reported that the absorbance ratio for waxy rice starch stored at
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Table 1 Melting parameters of waxy potato starch products stored at 4 ◦ C, 25 ◦ C, and cycles of 4/25 ◦ C (24 h each) for 1-7 days. Samples
To (◦ C)
Tp (◦ C)
Tc (◦ C)
Tc − To (◦ C)
H (J/g)
NS 4 ◦ C-1 d 4 ◦ C-3 d 4 ◦ C-5 d 4 ◦ C-7 d 25 ◦ C-1 d 25 ◦ C-3 d 25 ◦ C-5 d 25 ◦ C-7 d 4/25 ◦ C-1 d 4/25 ◦ C-3 d 4/25 ◦ C-5 d 4/25 ◦ C-7 d
65.49 ± 0.25 48.32 ± 0.12 49.87 ± 0.11 50.24 ± 0.06 50.57 ± 0.04 51.54 ± 0.26 52.46 ± 0.40 52.59 ± 0.46 52.88 ± 0.60 48.37 ± 0.34 51.27 ± 0.06 51.60 ± 0.35 52.36 ± 0.25
70.31 ± 0.25 58.75 ± 0.15 59.79 ± 0.13 59.62 ± 0.13 59.95 ± 0.62 59.74 ± 0.24 61.80 ± 0.15 62.36 ± 0.11 63.39 ± 0.29 58.77 ± 0.13 61.51 ± 0.18 61.72 ± 0.02 61.28 ± 0.04
75.84 ± 0.13 70.16 ± 0.12 74.41 ± 0.16 74.46 ± 0.05 74.49 ± 0.07 73.65 ± 0.23 80.59 ± 0.15 80.42 ± 0.10 80.17 ± 0.14 70.15 ± 0.12 76.64 ± 0.15 77.71 ± 0.11 77.81 ± 0.60
10.35 ± 0.12 21.84 ± 0.24 24.54 ± 0.12 24.22 ± 0.01 23.92 ± 0.03 22.11 ± 0.44 28.13 ± 0.26 27.83 ± 0.52 27.28 ± 0.72 21.78 ± 0.46 25.36 ± 0.11 26.11 ± 0.32 25.44 ± 0.39
15.66 ± 0.12 3.16 ± 0.10 4.66 ± 0.10 5.30 ± 0.05 6.53 ± 0.05 4.32 ± 0.14 5.28 ± 0.20 8.24 ± 0.08 8.34 ± 0.11 3.17 ± 0.11 6.72 ± 0.02 8.31 ± 0.15 9.45 ± 0.06
NS indicates native waxy potato starch. 4 ◦ C and 25 ◦ C indicate waxy potato starch product stored under isothermal conditions. 4/25 ◦ C indicates waxy potato starch product stored under temperature-cycled conditions. To , Tp , Tc , Tc − To , H indicate melting parameters onset, peak, conclusion temperature, melting temperature range and enthalpy of melting, respectively. Means ± SD (n = 3).
cycles of 4/25 ◦ C was increased to a maximum value (0.899) with the storage time up to 7 days, which indicated that ordered structure was generated in crystallites. Furthermore, the retrogradation of amylopectin under the temperature-cycled conditions reduced the enzyme susceptibility, resulting in a great amount of SDS and RS [12]. The variations in nutritional fractions of waxy potato starch stored at 4, 25 ◦ C and cycles of 4/25 ◦ C for 3 days are shown in Fig. 1a. Temperature-cycled retrogradation gives rise to a decrease in RDS, but the increase in the amount of SDS and RS as compared with isothermal retrogradation. A possible explanation for this behavior could be due to the increase in the amount of ordered regions to amorphous regions of starch granule under the temperature-cycled conditions, which was consistent with the result of the absorbance ratio of 1047/1022 cm−1 . 3.4. Crystalline properties of starch products The area between the experimental data and baseline area under the crystalline peak (Fig. 4) is amorphous area. The area above amorphous area is crystalline area. The relative crystallinity is the ratio of Ac /(Ac + Aa ), where Ac and Aa represent crystalline area and amorphous area, respectively. The X-ray diffraction patterns
for waxy potato starch stored at 4, 25 ◦ C and cycles of 4/25 ◦ C for 3 days are presented in Fig. 4. The relative crystallinity and the type of diffraction pattern of waxy potato starch stored at 4, 25 ◦ C and cycles of 4/25 ◦ C are summarized in Table 2. The different types of semi-crystalline supramolecular structure have been classified into A, B, or C types, which are the representations of the non-random arrangement and length of each linear chain in the molecule [28]. In our study, native waxy potato starch displayed the B-type pattern with reflection intensities at values of 5.6◦ , 14.4◦ , 17.2◦ , 19.5◦ , 22.1◦ and 23.8◦ , respectively. All starch products stored at 4, 25 ◦ C and cycles of 4/25 ◦ C for 3 days showed diffraction peaks at 5.7◦ , 16.9◦ , 17.2◦ , 22.2◦ , suggesting that the crystal type of starch products is a characteristic C-type, which consists of a mixture of A- and B-types. Heat-moisture treatment has been reported to change the X-ray diffraction patterns of potato starch and waxy potato starch from B-type to C-type [8,29]. In this study, we also found that different retrogradation treatment changed the X-ray diffraction pattern of waxy potato starch from B-type to C-type, indicating that the molecular and structure organization of starch products have been further modified by retrogradation treatment [30]. As shown in Table 2, the relative crystallinity of native waxy potato starch is 36.74%, which is similar to the result reported by Lee et al. [8]. However, compared with native waxy potato starch, the relative crystallinity of starch products by retrogradation treatment under different temperature conditions decreased. Shin et al. [10] found that hydrothermal treatment disrupted crystalline region of sweet potato starch, resulting in the decrease
Table 2 The relative crystallinity and type of diffraction patterns of waxy potato starch products stored at 4 ◦ C, 25 ◦ C, and cycles of 4/25 ◦ C (24 h each) for 1–7 days.
Fig. 3. Typical deconvoluted FT-IR spectra curves of waxy potato starch products stored at a constant temperature of 4 ◦ C, 25 ◦ C or cycles of 4/25 ◦ C (24 h each) for 3 days: (a) waxy potato starch product stored at 4 ◦ C for 3 days, (b) waxy potato starch product stored at 25 ◦ C for 3 days, (c) waxy potato starch product stored at cycles of 4/25 ◦ C for 3 days. The absorbance ratio of 1047 cm−1 /1022 cm−1 for waxy potato starch product stored at 4 ◦ C, 25 ◦ C and cycles of 4/25 ◦ C were 0.536 (0.7194/1.3408), 0.701 (0.8318/1.1873), 0.889 (2.3079/2.5949), respectively.
Samples
Relative crystallinity (%)
The type of diffraction pattern
NS 4 ◦ C-1 d 4 ◦ C-3 d 4 ◦ C-5 d 4 ◦ C-7 d 25 ◦ C-1 d 25 ◦ C-3 d 25 ◦ C-5 d 25 ◦ C-7 d 4/25 ◦ C-1 d 4/25 ◦ C-3 d 4/25 ◦ C-5 d 4/25 ◦ C-7 d
36.74 10.11 13.74 14.03 15.69 10.34 20.46 21.40 23.05 10.69 25.55 26.02 27.42
B-type C-type C-type C-type C-type C-type C-type C-type C-type C-type C-type C-type C-type
NS indicates native waxy potato starch. 4 ◦ C and 25 ◦ C indicate waxy potato starch product stored under isothermal conditions. 4/25 ◦ C indicates waxy potato starch product stored under temperature-cycled conditions.
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Fig. 4. X-ray diffraction patterns of waxy potato starch products stored at a constant temperature of 4 ◦ C, 25 ◦ C or cycles of 4/25 ◦ C (24 h each) for 3 days: (a) native waxy potato starch, (b) waxy potato starch product stored at 4 ◦ C for 3 days, (c) waxy potato starch product stored at 25 ◦ C for 3 days, (d) waxy potato starch product stored at cycles of 4/25 ◦ C for 3 days.
in relative crystallinity. The relative crystallinity of waxy potato starch products stored at 4, 25 ◦ C and cycles of 4/25 ◦ C for 3 days were 13.74%, 20.46%, 25.55%, respectively. This behavior showed a good correlation between the increase in the relative crystallinity and the absorbance ratio of 1047/1022 cm−1 . The crystallinity of starch product has been shown to be influenced by the amount of double helices that are arranged into a crystalline array, amylose content, crystalline size, and extent of disruption of amylopectin crystallites by amylase [31]. It could be speculated through combining these results with those obtained from relative crystallinity and the absorbance ratio of 1047/1022 cm−1 stated above, temperature-cycled retrogradation treatment would be beneficial to improving the crystallinity or the content of double helical structure in modified starch granules. As shown in Fig. 1a, starch products with higher relative crystallinity did not have higher RS content, but had lower RDS content. This was because that the RDS consisted of amorphous structure and appeared to be located in the outer regions. As opposite to this nutritional fraction, SDS and RS mostly consisted of imperfect crystallites and perfect crystallites, which was located in the inner regions [32,33].
4. Conclusions The results showed that temperature-cycled retrogradation was favorable for preparing the slowly digestible starch from waxy potato starch. The maximum yield of SDS reached 38.63% under temperature cycles of 4/25 ◦ C for 3 days with an interval of 24 h. Compared to isothermal retrogradation treatment, numerous imperfect crystallites were formed in the crystalline matrix under temperature-cycled retrogradation at 4/25 ◦ C. This could be explained by the evidence obtained from a higher relative crystallinity, a higher absorbance ratio of 1047/1022 cm−1 and melting enthalpy, but a narrower melting temperature range. Therefore, temperature-cycled retrogradation treatment would be applied for preparing the slowly digestible starch product in food industry. Acknowledgements This study was supported by National High Technology Research and Development Program of China (863 Program) (No. 2013AA102201), the Key Program of National Natural Science
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