Effect of dynamic high pressure microfluidization modified insoluble dietary fiber on gelatinization and rheology of rice starch

Food Hydrocolloids 57 (2016) 55e61

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Effect of dynamic high pressure microfluidization modified insoluble dietary fiber on gelatinization and rheology of rice starch Cheng-mei Liu, Rui-hong Liang, Tao-tao Dai, Jiang-ping Ye, Zi-cong Zeng, Shun-jing Luo, Jun Chen* State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang 330047, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 November 2015 Received in revised form 16 January 2016 Accepted 19 January 2016 Available online 21 January 2016

Modification of insoluble dietary fiber (IDF) for facilitating its applications has been encouraged in food industry. IDF from soybean residues was treated by dynamic high pressure microfluidization (DHPM), and effect of modified IDF (MIDF) addition on gelatinization and rheology of rice starch (RS) was investigated. It was found that DHPM could effectively reduce particle size of IDF, induce puffed morphology, and increase their water holding capacity. Addition of IDF/MIDF to RS increased peak and final viscosity of paste, and MIDF decreased breakdown and setback value, indicating MIDF may be a great candidate for increasing stability of paste and restraining short-term retrogradation of starch gels. Dynamic rheology indicated that supplementing MIDF changed rheological properties of RS less than IDF did. The results suggested that DHPM would provide an opportunity to change the physicochemical properties of IDF, and the resulting MIDF may be more suitable for designing fiber-enriched products. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Insoluble dietary fiber Dynamic high pressure microfluidization Gelatinization Rheology Rice starch

1. Introduction Dietary fiber (DF) has attracted considerable interest in current researches. It can typically be divided into two categories, soluble dietary fiber (SDF) and insoluble dietary fiber (IDF). Many studies have stated that sufficient consumption of DF is beneficial to normal gastrointestinal and physiological functions, including a reduced risk of coronary heart disease, diabetes, obesity, and some cancers (Mann & Cummings, 2009). As such, food processing industries are spending a huge amount of money on developing new generation of food products by incorporating DFs. IDF were the predominant fiber fraction in byproducts of many major crops such as soybean residue and wheat bran. However, SDF rather than IDF was commonly incorporated into products, partly because introduction of IDF may cause undesirable sensory and unsuitable technological properties of the supplemented foods (Aravind, Sissons, Egan, & Fellows, 2012). Efforts have been made to modify various IDFs using different techniques, such as alkaline hydrogen peroxide treatment (Sangnark & Noomhorm, 2003), micronization (Chau, Wang, & Wen, 2007), enzymatic treatment

* Corresponding author. Nanchang University, 235 Nanjing East Road, Nanchang, China. E-mail address: [email protected] (J. Chen). http://dx.doi.org/10.1016/j.foodhyd.2016.01.015 0268-005X/© 2016 Elsevier Ltd. All rights reserved.

(Napolitano et al., 2006), microwave cooking (Zia-ur-Rehman, Islam, & Shah, 2003). These processing technologies may effectively improve physiochemical and health-rated properties of various types of DF. For example, reduction of particle sizes of carrot IDF by different micronizing technologies was found to effectively enhance its in vitro hypoglycemic potential (Chau et al., 2007) and cholesterol-lowering activities (Chou, Chien, & Chau, 2008). Incorporation of size reduced IDF has decreased loaf volume and softness of bread (Sangnark & Noomhorm, 2003). Therefore, modification of IDF and application of modified IDF (MIDF) in food matrix have drawn a great interest. Starchy foods are important source of calories in our diet, but it's often suggested that starchy foods are low in essential nutrients and fattening. There were a number of reports concerning supplementation of natural SDF to modify properties of starch (Banchathanakij & Suphantharika, 2009; Zhou, Wang, Zhang, Du, & Zhou, 2008). However, applications of IDF to starchy foods were unpopular. Because adding a high level of IDF to foods may adversely affect color, texture, flavor and taste of the supplemented foods (Robin, Schuchmann, & Palzer, 2012). Studying the effect of IDF or MIDF on starch would definitely help explaining some phenomenon of starchy food that contains IDF, and may widen applications of IDF in food industry, as well as open new possibilities for designing fiber-enriched products. In this paper, IDF was modified by dynamic high pressure

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microfluidization (DHPM), which uses combined forces of highvelocity impact, high-frequency vibration, instantaneous pressure drop, intense shear, cavitation, and ultra-high pressures up to 200 MPa with a short treatment time (less than 5 s) (Liu et al., 2009). The unique working mechanism results in powerful energy so that the DHPM would be used to modify properties of many macromolecules such as whey protein (Liu et al., 2011), pectin (Chen et al., 2012) and starch (Kasemwong, Ruktanonchai, Srinuanchai, Itthisoponkul, & Sriroth, 2011). Wang, Sun, Zhou, and Chen (2012) also have suggested that DHPM process would provide an effective method to change physicochemical properties of wheat bran, but the feasibility of this technique on purified IDFs or IDFs from other resources needs to be verified, and application of MIDF also needs to be exploited. Therefore, purpose of this paper is on the one hand to investigate the effects of DHPM on physicochemical properties of IDF from soybean residues, on the other hand to evaluate the potential use of IDF/MIDFs as fiber enriching ingredient in starchy food.

suspension of the first pass was discarded to eliminate the dilution effect of water which was used between passes, 300 mL for the third pass because of the same reason. The temperature of the treated sample was 22e26
C because there was a machine cooling system on the microfluidizer. Samples just dispersed but without DHPM were taken as control (IDF). Each experiment was done three times in order to verify the repeatability of the DHPM process. Parts of solutions were applied to particle size determination. Others were collected and freeze-dried. 2.5. Determination of particle size

2. Materials and methods

The particle size of IDF and MIDFs was determined by Nicomp 380/ZLS Zeta potential/Particle Sizer (PSS Nicomp, Santa Barbara, USA) based on dynamic light scattering (Chen, Wu et al., 2014). Samples were diluted 1: 10 (v/v) with deionized water. The dispersion was shaken on a vortex mixer for 3 min, and then applied to Particle Sizer immediately. Particle size refers to the corresponding intensity distribution calculated by NICOMP Distribution Analysis. All measurements were carried out at 25
C.

2.1. Materials

2.6. Scanning microscopy analysis

Rice starch (RS), bought from Puer Yongji Biological & Technique Co. Ltd., China, contained 9.6% moisture, 28.9% amylose; 0.4% protein, 0.3% ash, and 0.6% fat. IDF isolated from soybean residues were purchased from Lion Biological Technology Co. Ltd., China.

IDF and MIDFs were taken after freeze-drying, and samples were prepared by sticking them to one side of double-sided adhesive tape attached to a circular specimen stub at the other end. The samples were viewed using an environmental scanning electron microscope (ESEM) (Quanta200F, FEI Deutschland GmbH, Kassel, Germany) at 30 kV voltages and 3.0 spot size. Low vacuum mode was used while operating the ESEM.

2.2. Purification of IDF IDF was purified using acid-base method of Selvendran and Susan Pont (1980) with some modifications. 10 g IDF samples were added to 400 mL 1 mol/L sodium hydroxide. The dispersion was subsequently stirred at 1500 rpm, 40
C for 100 min. Filtration was then carried out, and the obtained solid residues were washed with distilled water to neutral pH, followed by draining to dry. The solid residues were then stirred at 1500 rpm with 400 mL sulfuric acid (pH ¼ 2) at 60
C for 120 min, filtrated. The resulting residues were washed with distilled water again to neutral pH. Finally, residues were lyophilized and stored in hermetically sealed glass bottles. 2.3. Characterization of IDF Moisture of IDF was determined using a halogen moisture analyzer (Model HR83, Mettler-Toledo, Switzerland). Proteins were analyzed as total nitrogen content by the Kjeldahl procedure (AOAC, 2002), and the conversion factor used to transform nitrogen into protein was 5.71. Ash content was determined by incineration of samples at 550
C in a muffle furnace (AOAC, 2002). Sugar composition of IDFs was determined according to our previous procedures adapted for analyzing alcohol insoluble solids from Premna microphylla turcz leaves (Chen, Liang et al., 2014). 2.4. Dynamic high pressure microfluidization Dynamic high pressure microfluidization experiments were performed on laboratory scale microfluidizer M-100EH-30 (channel diameter ¼ 75 mm) (Microfluidics Co., Newton, MA). The purified IDF was dispersed in water to a solid to water ratio 1: 35 and stirred gently at room temperature overnight, and treated under the DHPM pressures of 80, 120 and 170 MPa for 3 passes, respectively, the resultant modified sample (MIDF) was named IDF80, IDF120 and IDF170 accordingly. 500 mL suspension was used for the first pass, 400 mL for the second pass because initial 100 mL

2.7. Water holding capacity Water holding capacity (WHC) was determined according to the method of Raghavendra, Rastogi, Raghavarao, and Tharanathan (2004). An accurately weighted dry sample (1 g) was hydrated in a graduated test tube with 20 mL distilled water for 18 h. The supernatant was removed by passing through a sintered glass crucible under vacuum. The weight of hydrated residue was recorded after 1 h of draining and then the sample was dried at 105
C overnight to obtain the residual dry weight. WHC was calculated according to the following Equation (1):

WHC ðg=gÞ ¼

Residue hydrated weight  Residue dry weight Initial weight of sample (1)

2.8. Determination of pasting properties by Rapid Visco-Analyzer (RVA) Pasting and paste properties of samples were determined by a Rapid Visco-Analyzer (RVA-TecMaster, Newport Scientific Pt. Ltd., Australia) according to Wu et al. (2016) with some modifications. Starch (2.85 g, dry basis) and IDF/MIDF samples (0.15 g, 5 wt.% on a starch basis) were directly weighted into the RVA canister and deionized water was added to obtain a total constant sample weight of 28 g. The heating and cooling cycles were programmed according to an inherent thermal program in the apparatus, namely ‘rice rapid’, in which the sample was held at 50
C for 1 min, then heated to 95
C at a constant rate of 12
C/min, and held at 95
C for 2.5 min. It was subsequently cooled to 50
C with the rate of 6
C/ min and held at 50
C for 1.4 min. Agitation speed was fixed at 960 rpm for the first 10 s to ensure the uniformity of the dispersion,

C.-m. Liu et al. / Food Hydrocolloids 57 (2016) 55e61

and then at 160 rpm throughout the rest of measurement. Five characteristic parameters were measured from the RVA curve: (i) Peak viscosity e maximum viscosity of material developed soon after the heating portion of the test; (ii) Trough viscosity e the lowest viscosity after the peak viscosity just before it begins to increase again; (iii) Breakdown e peak viscosity minus trough viscosity; (iv) Final viscosity e viscosity at the end of the test; (v) Setback (setback from trough) e final viscosity minus trough viscosity.

2.9. Oscillatory rheological measurements Oscillatory rheological measurements were performed by an AR 2000 Rheometer (TA Instruments, New Castle, DE, USA) using parallel metal plates with a diameter of 40 mm and a gap of 1 mm. Experimental conditions followed a previous description (Zhu & Wang, 2012) with some modifications. 500 mg starch þ2.0 mL H2O, or 475 mg starch þ25 mg IDF or MIDF þ2.0 mL H2O was weighted individually. All the mixtures with water were vortex-shaken for 5 min before being loaded on the plate of the rheometer. After loading, samples were conditioned at 25
C for 1 min. At the strain of 2% and frequency of 1 Hz, sample was equilibrated for 1 min at 25
C. Then the temperature was ramped from 25 to 95
C and from 95 to 25
C with a ramp rate of 5
C/min. The resulting gel was equilibrated at 25
C for 1 min. Two steps of rheological measurements were performed at 25
C: (1) deformation sweeps at a constant frequency (1 Hz) to determine the maximum deformation attainable by a sample in the linear viscoelastic range and (2) frequency sweeps over a range of 0.06e20 Hz at a constant deformation (2% strain) within the linear viscoelastic region of all gel samples. During the determination, edge of the gap was covered with a thin layer of low-density silicon oil (dimethylpolysiloxane, 50 cP viscosity) to minimize evaporation. The parameters of storage modulus (G0 ) and loss modulus (G00 ) were obtained directly from the manufacturer supplied computer software (Rheology Advantage Data Analysis Program, TA Instruments, New Castle, DE, USA). The deviation did not exceed 5% between duplicate runs, as the experiment was repeated. The average of the duplicate runs was reported as the measured value. 2.10. Statistical analysis All the experiments were done in triplicate. Statistical analysis was carried out using SPSS (version 16.0, Chicago, United States). The results were expressed as mean ± standard deviations and compared using the Tukey test at 5% confidence level. 3. Results and discussion 3.1. IDF composition The composition of IDF was analyzed. The moisture, ash and protein content of IDF were 9.65, 5.06, 1.63 g/100 g, respectively. While the content of IDF, as the sum of its monomeric constituents (uronic acid, rhamnose, fructose, arabinose, xylose, mannose, galactose, and glucose), was 83.87 g/100 g. The major monomers of IDF was glucose (27.29 g/100 g), accounting for 32.54%, indicating that cellulose was the predominant polysaccharide. There were also significant amounts of uronic acids (17.38 g/100 g) and galactose (15.44 g/100 g) and moderate contents of arabinose (9.70 g/100 g) and xylose (9.91 g/100 g). According to the literature, the high percentages of xylose could be due to the presence of xyloglucans

57

as the main hemicellulose polysaccharides in soybean cell walls, frequently linked to other sugars, such as galactose, fructose and arabinose (Huisman, 2000). Very low values were found on mannose (2.98 g/100 g), rhamnose (1.17 g/100 g) and fructose (1.32 g/100 g). Redondo-Cuenca, Villanueva-Su
arez, RodríguezSevilla, and Mateos-Aparicio (2007) reported similar levels of insoluble fiber monomers of soybeans to those of IDF found in this paper. 3.2. Effect of DHPM on physicochemical properties of IDF 3.2.1. Effect of DHPM on average particle size of IDF The average particle size of IDF before DHPM was 25.9 mm (Table 1). DHPM dramatically reduced the average particle sizes of IDF with increase of pressure. 80 MPa DHPM reduced the average particle size to 5.8 mm, and the value was further reduced to 1.6 mm by increasing the pressure to 120 MPa. The reduction of particle size may be attributed to breakage and disruption of particles by fluid shear stress in the DHPM process, as suggested by Clarke, Prescott, Khan, and Olabi (2010). However, the average particle size of samples treated at 170 MPa was 3.0 mm, slightly larger than that of IDF120. Similar result was found in the study of Tu et al. (2013), who also reported an increment of particle size of maize amylose when pressure was increased above 80 MPa. This phenomenon may result from re-aggregation of small particles. It was reported that the surface power with van der Waals force and electrostatic attraction of the granules may be increased when the particle size was lower than a certain value. The fine particles then may be easier to attract one another, causing the phenomenon of reaggregation (Visser, 1989). 3.2.2. Effect of DHPM on microstructure of IDF As shown in Fig. 1A, the surface morphology of the purified IDF was observed to be in irregular shape with compact surface layers. DHPM-treated fibers had more open structures with a high degree of porosity. The fiber structure was disrupted with the increase of applied DHPM pressure, as indicated by increase in surface porosity (Fig. 1BeD) as well as reduction in particle size. This change of morphology by DHPM would be due to rapid release of pressure at the exit of the interaction chamber of machine leading to expansion of particles. The expansion would be likely to loosen microstructure of the particles and create pores or cavities inside the particles, as suggested by Brookman (1975). Such microstructure changes along with size reduction could give rise to a larger surface area. Our previous studies also demonstrated that DHPM process could effectively degrade another SDF (high-methoxyl pectin), and lead to porous surface morphology (Chen et al., 2012). 3.2.3. Effect of DHPM on water holding capacity of IDF As discussed above, DHPM induced particle size reduction, along with more porous microstructure but no change of primary structure of IDF. Such changes may lead to a larger surface area and changes in the IDF's physicochemical properties including water holding capacity (WHC). The WHC value for the original IDF sample was 6.0 g/g (Table 1), which was significantly (p < 0.05) lower than those of DHPM treated samples. The WHC value of IDF80, IDF120, and IDF170 was 13.5, 15.1 and 14.8 g/g, respectively. Particle size, specific surface area, porosity and microstructure of the IDF matrix are important factors influencing fiber's hydration properties. Many authors have tried to relate the WHC with particle size of IDF. However, results from different studies were inconsistent. Gupta and Premavalli (2010) reported the value of WHC decreased with reduction of particle size. However, in our study, the value of WHC increased with the decrease of particle size. A linear relationship can be used to describe the influence of particle

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C.-m. Liu et al. / Food Hydrocolloids 57 (2016) 55e61

Table 1 Characteristics of IDF and MIDF.a Samples

Particle size (mm)

IDF IDF80 IDF120 IDF170

25.9 5.8 1.6 3.0

a b c

± ± ± ±

1.2 0.4 0.2 0.1

ac b c c

Water holding capacity (g/g) 6.0 13.5 15.1 14.8

± ± ± ±

0.3 0.7 0.5 0.3

a b c c

G0 at 1 Hz (Pa)b 5883.5 5507.1 5367.8 5186.2

± ± ± ±

167.2 135.7 106.6 113.5

G00 at 1 Hz (Pa)b a b b b

159.6 121.4 135.1 145.5

± ± ± ±

9.3 7.6 8.5 8.1

ac b bc c

Values are means ± standard deviations. G0 (storage modules) and G00 (loss modules) of RS/IDF or RS/MIDF gel at 1 Hz. Values followed by different letter in the same column are significantly different (p < 0.05).

Fig. 1. Scanning electron micrograph of IDF treated by DHPM at different pressures (A) original IDF; (B) 80 MPa; (C) 120 MPa; (D) 170 MPa.

size (S) on the WHC: WHC ¼ 0.38 S þ 15.8 (R2 ¼ 0.99). This trend was consistent with the reports of Wang et al. (2012) and Ahmed, Al-Jassar, and Thomas (2015). The increase of WHC in our study may be due to the smaller particle sizes and porous structure of DHPM-treated IDFs (Fig. 1BeD), which could increase the surface area and could expose more polar groups and more water binding sites to the surrounding water, as suggested by Chau et al. (2007), so to facilitate penetration and absorption of water inside the fiber matrix, leading to the significant improvement of WHC. Previous researches suggested that DHPM could effectively improve the functionalities of DF (Li et al., 2013) and adding DFs into kinds of food products has been of great interests to food industries. However, there is relatively little information concerning the effect of IDF addition on physicochemical properties of food matrix. Therefore, IDF or MIDF was tentatively added to rice starch for investigating the possibility of its application in starchy foods in this study.

3.3. Effect of MIDF addition on gelatinization and rheology of rice starch 3.3.1. RVA pasting profile As shown in Table 2, the peak viscosity (PV) of RS was 3395.3 cP. This value was significantly (p < 0.05) increased to 3857.3 cP for RS added with IDF. This result was quite similar to those of starch/SDF systems, such as starch/galactomannans system (Funami et al., 2005) and starch/inulin system (Peressini & Sensidoni, 2009). The incorporation of DF often resulted in increase of PV, which may be due to increases in the work required to move swollen starch granules past each other (as compared to water alone or water containing dissolved/leached starch polymer molecules) (Hongsprabhas, Israkarn, & Rattanawattanaprakit, 2007), or changes in their shape during swelling, making the forces exerted on starch much greater which resulted in more granule disintegration and starch molecule solubilization (Christianson, Hodge, Osborne, & Detroy, 1981). Even though the addition of MIDF also increased the PV, the magnitude of increase was less than that of IDF/RS. The PV of IDF80, IDF120 and IDF170 was 3767.5, 3586.7 and 3647.6 cP, respectively. The decrease in PV might result primarily

C.-m. Liu et al. / Food Hydrocolloids 57 (2016) 55e61

59

Table 2 Pasting properties of RS, RS/IDF, RS/IDF80, RS/IDF120 and RS/IDF170.a Samples

Peak viscosity (cP)

RS RS RS RS RS

3395.3 3857.3 3767.5 3586.7 3647.6

a b

þ þ þ þ

IDF IDF80 IDF120 IDF170

± ± ± ± ±

29.5 39.3 25.6 41.0 33.2

ab b b c c

Trough viscosity (cP) 2445.5 3008.7 3205 3330.3 3444.0

± ± ± ± ±

36.3 40.2 37.6 39.4 42.5

a b c d e

Final viscosity (cP) 4950.0 5433.0 5449.0 5462.0 5605.0

± ± ± ± ±

11.3 54.1 46.7 62.5 79.5

a b bc bc c

Breakdown (cP) 950.0 849.5 562.5 256.4 203.6

± ± ± ± ±

6.8 a 0.9 b 12.0 c 1.6 d 9.3 e

Setback (cP) 2504.5 2425.2 2244.0 2131.7 2161.0

± ± ± ± ±

25.0 a 15.2 b 9.1 c 23.3 d 37.0 d

Values are means ± standard deviations. Values followed by different English alphabet in the same column are significantly different (p < 0.05).

from the smaller particle size, higher water competition ability of MIDF than IDF. The MIDF could have more opened structures with a higher degree of porosity and expose more surface area and other water-binding sites than IDF. Therefore, the presence of MIDF reduced the volume fraction of water available for granule swelling and gelatinization, leading to smaller PV. This mechanism was quite similar to the one proposed by Khanna and Tester (2006) for explaining the gelatinization of konjac glucomannan/starch systems. After the samples were gelatinized, the pastes were subjected to both thermal and shear stresses at the holding temperature (95
C), resulting in loss of granule integrity and subsequent disruption, further causing a reduction in the paste viscosity. This reduction was defined as breakdown (BD), which reflected the stability of the paste. The BD for RS without additive was 950.0 cP. After added with IDF or MIDF, the BD of paste was significantly (p < 0.05) lowered to 849.5, 562.5, 256.4 and 203.6 cP of RS/IDF, RS/IDF80, RS/ IDF120 and RS/IDF170, respectively (Table 2), which indicated that addition of MIDF significantly increased the stability of paste. The results of BD may be also due to interactions between starch and IDF/MIDF. The MIDF with smaller particle size, higher specific surface area, and more opened structure was likely to facilitate interactions and network formation, resulting in less decrease in viscosity when temperature was kept at 95
C. In the RVA measurement, setback (ST) value obtained from the increase in viscosity of pastes from holding strength value to final viscosity after cooling down to 50
C, usually indicates short-term retrogradation of starch (Pongsawatmanit, Temsiripong, Ikeda, & Nishinari, 2006). ST of RS was 2504.5 cP, while addition of IDF decreased ST to 2425.2 cP. Addition of IDF120 and IDF170 further significantly reduced (p < 0.05) ST to 2131.7 and 2161.0 cP, respectively, indicating MIDFs were good choices to restrain shortterm retrogradation of RS. This result may be because MIDF had small particle size, porous structure and high WHC, thus bound water, reduced the mobility of the starch chains, and thereby retarded retrogradation, as suggested by Satrapai and Suphantharika (2007). A linear relationship was found between ST and particle size (S) of samples: ST ¼ 11.38S þ 2137.06 (R2 ¼ 0.93), indicating significant influence of S change induced by DHPM on pasting properties of RS. 3.3.2. Dynamic rheological properties The dynamic rheology studied starch gelatinization from the view different from RVA. Fig. 2A showed the change of storage modulus (G0 , elasticity) and loss modulus (G00 , plasticity) during change of temperature from 65 to 95
C. It can be seen that at the beginning of heating G00 was higher in magnitude than G0 reflecting the liquid-like behavior. However, by increasing temperature, the G0 and G00 increased markedly and the cross-over of G0 and G00 was reached. Gel point, most commonly defined as the time at which G0 and G00 intersect, generally occurs as a result of the early stage of starch gelatinization. For RS alone, the crossover temperature (Tc) was 76.25
C, in the range of RS between 64.2 and 77.15
C as

Fig. 2. Dynamic mechanical spectra of pastes of RS, and RS blended with IDF or IDF120. (A) Change of storage modulus (G0 ) and loss modulus (G00 ) during temperature sweep from 65 to 95
C. (B) Change of G0 and G00 during frequency sweep from 0.06 to 20 Hz.

reported by Vandeputte, Derycke, Geeroms, and Delcour (2003). When IDF was added into RS, it was found that Tc was significantly decreased to 67.75
C. The decrease of Tc for RS/IDF may be owing to the phase separation due to thermodynamic incompatibilities between IDF and RS, which made the amylose molecules being close to each other, so initial gelation was accelerated, as suggested by Kim and Yoo (2006). In the case of RS/MIDF systems, their Tc was also reduced, but the magnitude of reduction was less than that of RS/IDF. The Tc for RS/IDF80, RS/IDF120 and RS/IDF170 was 74.70, 74.05 and 70.60
C, respectively. This was possibly because that MIDFs

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were more compatible with starch and had more opened structure, allowing them easily react with leached amylose/amylosepectin and earlier onset of network formation. However, it seems that the effect of phase separation was still the primary factor, since reduction of Tc in RS/IDF170 was still smaller than that of RS/IDF. Dynamic frequency sweep spectra of systems followed a frequency dependence of the G0 over the wide range of frequencies studied (Fig. 2B). This behavior reflected the existence of threedimensional networks among the starch gels with or without additives. The storage modulus was much larger than the loss modulus, indicating that elastic behavior dominated over viscous component throughout the frequency range. Addition of IDF or MIDF increased the G0 and G00 of starch gels, and changes in the G0 were greater than changes in G00 , indicating that the addition of IDF or MIDF greatly influenced the elastic property of RS. Generally, dietary fiber immobilizes water molecules. It was expected to increase the effective starch concentration, and elasticity of gels. An increase in G0 with fiber incorporation has previously been reported in many literature (Ahmed, Almusallam, Al-Salman, AbdulRahman, & Al-Salem, 2013; Bonnand-Ducasse, Della Valle, Lefebvre, & Saulnier, 2010). Even though the WHC of samples increased in the order of RS/IDF < RS/IDF80 < RS/IDF120 z RS/IDF170 in this study, we found that G0 decreased in the order of RS/ IDF > RS/IDF80 > RS/IDF120 > RS/IDF170. At a frequency of 1 Hz, G0 of RS/IDF, RS/IDF80, RS/IDF120, RS/IDF170 was 5883.5, 5507.1, 5367.8 and 5186.2 Pa, respectively (Table 1). The lower G0 of RS/MIDF may be due to other reasons such as change of physical interactions. The viscoelasticity of mixtures may be shaped by the characteristics of starch (permanent junction zones in the network) and the characteristics of IDF/MIDF (temporary entanglements between IDF/ MIDF and starch, as well as loops among IDF/MIDFs themselves in the network). With more open structure, MIDF's interaction with starch could be facilitated, increasing the number of temporary network points, while the number of permanent network points, represented by gel-forming junction zones of the starch, decrease, causing decrease in G0 with increasing magnitude of modification. This explanation was quite similar to the theory proposed by Kulicke, Eidam, Kath, Kix, and Kull (1996) to explain the decrease of elasticity with increasing amounts of galactomannan. It is interesting to find that RS/IDF and RS/IDF170 showed a similar G00 , and G00 in MIDF added systems showed an opposite trend to that of G0 . At a frequency of 1 Hz, G00 of RS/IDF, RS/IDF80, RS/IDF120, RS/IDF170 was 159.6, 121.4, 135.1 and 145.5 Pa, respectively (Table 1). Viscous properties of MIDF/RS systems decreased in the order of RS/ IDF170 > RS/IDF120 > RS/IDF80. This result can be attributed to their difference in the capability of absorbing water, the more liquid phase present, the viscous properties more obvious, as described by Li and Yeh (2001). Overall, the results of dynamic rheology may give a helpful recommendation that supplementing MIDF would be better than IDF in RS-based food products when less influencing rheological properties of RS is required.

4. Conclusion DHPM significantly changed particle size, morphology of IDF from soybean residues without altering their primary structure. The modified IDF showed puffed microstructure along with size reduction leading to significant improvement in WHC. Addition of IDF as well as the modified one had a significant effect on gelatinization and rheology of rice starch. MIDF may be a better candidate than IDF for increasing stability of paste, restraining shortterm retrogradation and maintaining rheological properties of RS gels. Its effect on long-term retrogradation of starch gels will be investigated in future paper.

Acknowledgments The authors thank Center of Analysis and Testing Nanchang University and State Key Laboratory of Food Science and Technology for expert technical assistance. This study was supported financially by the National Natural Science Foundation of China (Nr 31260386, 31571875 and 31401655), and the State Key Laboratory of Food Science and Technology Projects (SKLF-ZZA-201304).

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