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The Impacts of the Entanglement Concentration on the Hydrodynamic Properties of Kudzu and Lotus Rhizome Starch Aqueous Solutions

Li Guo*, Shuilin Wang, Chenchen Zhu, Jian Hu, Juanjuan Zhang, Xianfeng Du*

Department of Food Sciences, Anhui Agricultural University, Hefei, ChinaDepartment of Food Sciences, Anhui Agricultural University, Hefei, China

Abstract: With the rapid development of consumer demands for health, kudzu and lotus rhizome starches have been widely utilized as the nutritiously and naturally medicinal drinks after they are suspended in aqueous solutions. However, it is difficult to control the suitable concentrations to obtain the ideal textures of the kudzu and lotus rhizome starch solutions. In this study, on the basis of starch structure characteristics, the hydrodynamic properties of the kudzu and lotus rhizome starch aqueous solutions around entanglement concentration (the boundary between the semi-dilute regime and the concentrated regime of a polymer solution, ce) were studied. The results indicated that the two starch solutions showed a clear up-turn curve of the ηsp/c versus c curves in dilute solutions. The ce values of the kudzu and lotus rhizome starch aqueous solutions were determined to be 1.56% and 0.6%, respectively. The impact of the ce value on the network formation of the kudzu starch solutions was much more significant compared with the impact on the lotus rhizome starch solutions. Shear thinning behaviour hardly occurs when the concentrations of the kudzu and lotus rhizome starch aqueous solutions were lower than ce, and shear thinning behaviour develops when the concentrations are equal to or greater than ce. The pseudoplastic behaviour became obvious when the concentrations rose above ce.

Keywords: Starch entanglement concentration in aqueous solutions, Aqueous starch solution hydrodynamic properties, kudzu and lotus rhizome starches

Citation: Li Guo, Shuilin Wang, Chenchen Zhu, Jian Hu, Juanjuan Zhang, Xianfeng Du, The Impacts of the Entanglement Concentration on the Hydrodynamic Properties of Kudzu and Lotus Rhizome Starch Aqueous Solutions. Journal of Food and Nutrition Research, 4 (11). 750-759. January 2016. doi:10.12691/jfnr-4-11-8

1. Introduction

The starches isolated from such as kudzu and lotus rhizome are under intensive research. Both kudzu and lotus have been considered the root plants of high nutritional value used for health care in Asian for a long time, such as diuretic, antiemetic, antidote in the treatment of tissue inflammation and cancer, as well as treating fever, diarrhoea, dysentery, diabetes, dizziness and cerebrovascular diseases, etc [1,2,3]. The microstructure and physicochemical characteristics of kudzu and lotus rhizome starches have been extensively investigated and they have been applied to prepare functional foods. In previous researches [4,5,6,7], the characteristics of kudzu starch were investigated, such as the morphology, the molecular structure, the thermodynamics and the digestibility properties. It was found that these characteristics of kudzu starch mainly attributed to the associations between amylose and amylopectin chains including intra- or inter-molecular interactions. Recently, with the rapid development of consumer demands for health, kudzu and lotus rhizome starches have been widely utilized as the nutritiously and naturally medicinal drinks after they are suspended in aqueous solutions. However, it is difficult to control the suitable concentrations to obtain the ideal textures of the kudzu and lotus rhizome starch solutions, such as homogeneous solution system, high viscosity and consistency, etc. [8]. The pasting behaviour is very important and is usually investigated by observing the change in viscosity of starches. A hot starch paste is a mixture of swollen granules and granule fragments, together with colloidally and molecularly dispersed starch granules. The starch pasting behaviour in aqueous solutions depends on the size distribution of a starch granule, water content and the ratio of amylose to amylopectin, etc. [8,9]. Starch hydrodynamic properties in aqueous solutions are a valuable tool to gain insight into the rheological properties of starch paste, which is of considerable technological importance in foods, pharmaceuticals, adhesives, etc. [10]. To date, many studies have focused on the effects that high concentrations have on the physicochemical properties of kudzu and lotus rhizome starches, but their hydrodynamic properties in low concentrations remain unclear, especially in dilute and semi-dilute aqueous solutions.

In a dilute solution, the polymer chains in the form of solvated random coils do not overlap. With increasing the concentration in the dilute region, the random coils begin to come into contact with one another, reaching the critical concentration (c*), and then overlapping at the entanglement concentration (ce) [11]. The ce is referred as the boundary between the semi-dilute regime and the concentrated regime of a polymer solution. In the semi-dilute regime, polymer chains overlap with each other but do not entangle, whereas in the concentrated regime, polymer chains significantly overlap each other such that individual chain motion is constrained [12]. Thus, ce marks the onset of significant coil overlap and interpenetration. The viscometric behaviour of the starch solutions will change when the concentrations are more or less than ce. The value of ce is influenced by some factors such as relative molecular weight, properties of solvent, the internal branched chains of amylopectin, etc [11]. In this paper, water was as solvent, starch structure characteristics including starch granule size distribution and the internal branched chain distribution of amylopectin were first analysed using a laser light scattering particle size analyzer, high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD). Secondly, the entanglement concentration (ce) was determined using a Ubbelohde viscometer to study the dimensions of the kudzu and lotus rhizome starch chains in dilute solutions. The third objective was to investigate the network formation changes in microstructure during starch gelatinization and retrogradation, as evaluated using a transmission electron microscope (TEM). Finally, the starch solution dynamics in dilute and semi-dilute aqueous solutions were analyzed using steady shear rheological measurements. These insights may help to accurately regulate the concentration in order to obtain the optimum processing properties of the kudzu and lotus rhizome starch solutions.

2. Materials and Methods

2.1. Materials

Kudzu starch was obtained from Geye Starch Co. (Anhui, China). Lotus rhizome starch came from Zhoushi Food Co. (Guangxi, China). α-amylase from porcine pancreatin (50 U/mg), barley β-amylase (53 U/mg) and isoamylase (3,000,000 U/mg) from pseudomonas amylodermosa were ordered from Sigma-Aldrich. The starches were defatted using methanol (85%, v/v) and were deproteinized using chloroform/n-Butyl alcohol (4:1, v/v) as solvents. Total starch content and amylose content were determined [13,14], and the contents of the moisture, protein, lipid and ash of starch were measured [15].

2.2. Starch Structure Characteristics

2.2.1. Starch Granule Size Distribution

Particle size analysis was carried out using a laser light scattering particle size analyzer (Malvern Mastersizer Hydro 2000 MU, Malvern Instruments Ltd., UK). Starch granule was suspended in distilled water and stirred at 2,000 rpm. A general analysis model was used with particle refractive and absorption indices of 1.53 and 0.01, respectively, and the refractive index of water as the dispersant was 1.33. Starch particles were examined within the range of 0.02-2,000 μm. The obscuration in all the measurements ranged from 9 to 13%. Particle size was defined in terms of the volume weighted mean [D(4, 3)], 10th percentile [d(0.1)], 50th or median [d(0.5)], 90th percentile [d(0.9)] and surface weighted mean [D(3, 2)]. The average granule size was expressed as the surface weighted mean value, i.e. the diameter of a sphere that has the same volume/surface area ratio. This was used to determine the speciﬁc surface area (m²/g) assuming spherical granules of uniform density.

2.2.2. The internal Branched Chain Distribution of Amylopectin

The internal branched chain distribution of amylopectin was carried out as described previously with proper modiﬁcation [16,17]. Amylopectin (0.2 g, dry basis) was dissolved in deionized water (40 mL) at 30°C for 20 min with constant magnetic stirring to completely dissolve the amylopectin samples. The 0.4 mL α-amylase (10 U/mL) was added in 0.01M NaOAc buffer with a pH of 6.9 containing 0.2 mL of 10% sodium azide and then incubated in a shaking water bath at 37°C for 12h before the enzyme was deactivated by boiling for 10 min. Subsequently, the hydrolysate obtained by α-amylase was treated with 0.45 mL β-amylase (9 U/mL) in 0.01M NaOAc buffer with a pH of 4.8 and incubated in a shaking water bath at 40°C for 5 h to completely remove any external chain segments, after which the hydrolysate solution was boiled for 10 min to make the enzyme inactive. Finally, the hydrolysate was hydrolyzed again by 4 mL α-amylase (6 U/mL) in NaOAc buffer (0.01 M, pH 6.9) and incubated in a shaking water bath at 37°C for 12h to produce the building blocks (describing the internal branched chains of amylopectin). The building blocks in solution were frozen and stored at -18°C before analysis. Double distilled water was added to dilute the samples to a concentration of 1.0 mg/mL. An aliquot (50 μL) was filtered through a membrane filter (0.45 μm) and then analyzed by HPAEC.

The unit chain distribution was analyzed by HPAEC-PAD on a Dionex ICS 3000 instrument (Sunnyvale, CA, USA). The analytical column was CarboPac PA-100 anion-exchange column (4×250 mm), combined with a CarboPac PA-100 guard column (4×50 mm). The flow rate was 0.5 mL/min and the injection volume was 25 μL with a carbohydrate concentration of 1 mg/mL. The samples were eluted at 0.5 mL/min with a gradient of NaOAc made by mixing eluent B (1 M NaOAc) into eluent A (0.25 M NaOH) as follows: 0-15 min from 15-34%; 15-26 min from 34-40%; 26-52 min from 40-49%; 52-54 min from 49-100%; and finally return to start mixture at 58-60 min from 100-15%. PAD signal was converted to carbohydrate contents.

2.3. Starch Viscosity Characteristics

Capillary viscometry was performed using an Ubbelohde viscometer, immersed in a thermostated bath at 65±0.1°C to prevent starch retrogradation from occurring. The kudzu and lotus rhizome starch aqueous solutions (0.01-5.0 g/dL) were prepared and completely gelatinized prior to viscosity measurements. Relative viscosity (ηr) and specific viscosity (ηsp) were calculated by the following equations [18]:

[picture] | (1)

[picture] | (2)

where t and t₀ are the flow times of polymer solution and of the pure solvent through the Ubbelohde viscometer. The ce values are determined by plots of ηsp versus starch concentrations (c) for the starches in aqueous solution.

2.4. TEM

Based on the ce values, the kudzu and lotus rhizome starches were dispersed in deionized water to prepare the starch dispersions with different concentrations (0.1-2%, W/V). The dispersions were heated at 90°C with magnetic stirring occurring at a constant rotating speed (100 rpm) for 30 min to gelatinize the starch granules. The gelatinized starch dispersions were then cooled to room temperature and were stored at 4±1°C for 7 days. Images were taken with a HT7700 transmission electron microscope, TEM (Hitachi, Japan) at an accelerating voltage of 80 kV. A 50 μL aliquot of gelatinized or retrograded starch dispersion was deposited on glow-discharged carbon-coated grids. The liquid in excess was blotted away with filter paper, and 40μL of 2% phosphotungstic acid negative stain was added prior to drying. After 1 min, excess staining was removed with filter paper, and the thin remaining liquid film was allowed to dry. The unstained dry specimens were shadowed using W/Ta. The samples were observed in low-dose conditions, at a magnification of 40000×, using a Philips CM200“Cryo” microscope operated at 80 kV. Images were recorded on Formvar films.

2.5. Steady Shear Rheological Measurements

The kudzu and lotus rhizome starch solutions with different concentrations (0.1-5.0%, w/w) were dispersed in deionized water, and gently stirred and then heated at 90°C for 30 min to allow complete gelatinization of the starch granules. The gelatinized starch dispersions were cooled to 30°C and immediately used for steady rheological measurements. The steady shear rheological properties were obtained with a R/S plus rheometer (Brookfield Instruments, USA) using a cone-plate geometry (40 mm diameter) at a gap of 0.05 mm and a cone angle of 2°. The apparent viscosity-shear rate behaviour was determined was determined by increasing the shear rate from 0 to 300 s⁻¹ at 30°C. The sample was loaded onto the platen of the rheometer, and then the exposed sample edge was covered with a thin layer of light paraffin oil to prevent evaporation during measurements.

2.6. Statistical Analysis

Results were expressed as the means±standard deviations (SD) of triplicate analyses for each sample and the means were separated using a least significant difference (LSD) test. A one-way analysis of variance (ANOVA) using the ORIGIN 7.5 (OriginLab Inc. USA) and Tukey’s test were used to establish the significance in differences among the mean values at a 0.05 level of confidence.

3. Results and Discussion

In this study, the chemical compositions (%) of kudzu starch are as follows: starch 96.92 ± 0.04 (amylose: 23.6±0.03); moisture 2.98±0.02; protein 0.03±0.01; lipid 0.02±0.01; ash 0.05±0.02. The chemical compositions (%) of lotus rhizome starch are as follows: starch 93.42 ± 0.01 (amylose: 20.29± 0.01); moisture 6.37±0.02; protein 0.12±0.03; lipid 0.02±0.01; ash 0.07±0.01.

3.1. Intrinsic Viscosities in Dilute Regime

Figure 1 shows the plot of the reduced viscosity (ηred=ηsp/c) versus the concentrations (c) for kudzu and lotus rhizome starch solutions ranging from 0.01 to 2.00 g/dL. It can be seen that the kudzu and lotus rhizome starch solutions present a non-linear shape with an upturn, which indicates a characteristic transition concentration. As was reported by Dondos’ group, when random coils approach closely each other, the coils shrink due to the mutual repulsion between polymer chains, which made the hydrodynamic volume of coils and viscosity in the solution to decrease [19,20]. Figure 1 indicates the slope of the linear curve begins to significantly increase when the concentrations of lotus rhizome starch solution increase to 0.075 g/dL. The slope of the linear curve hardly changes when the concentrations of the kudzu starch solution are greater than 0.05 g/dL. So, compared to the kudzu starch solutions, the behaviour of the lotus rhizome starch solutions possesses more pronounced intermolecular hydrophobic interactions [21].

Figure 1. The plot of the ηred vs. c for the gelatinized kudzu and lotus rhizome starches in dilute and semi-dilute aqueous solutions at 65°C

3.2. Entanglement Concentrations of Starch Dispersions

The ce values (w/v) in the kudzu and lotus rhizome starch aqueous solutions were determined from the intercept of the ﬁtted slopes using the plots of ηsp versus c (Figure 2). The ce values of kudzu and lotus rhizome starch aqueous solutions are 1.56 and 0.6%, respectively. The ce value (1.56%, w/v) of kudzu starch is greater than the ce value (0.6%, w/v) of lotus rhizome starch. This may be attributed to the internal components of starch granules. In this study, the amylose contents (%) of kudzu and lotus rhizome starches were 23.6±0.03 and 20.29 ± 0.01, respectively. It was reported that amylose is the major contributor that is responsible for making the molecules interpenetrate into one another and become well entangled, while amylopectin behaves as a hard ellipsoid. Entanglements are formed in the semidilute entangled regime, and they become weaker as as the amylopectin content increases [12]. So, the ce value is directly proportional to the amylose content, that is, the higher amylose content is, the larger ce value is. This explains why the ce value of kudzu starch solutions is greater than that of lotus rhizome starch solutions. The chain distributions of building blocks of amylopectins of kudzu and lotus rhizome starches were performed by HPAEC (Figure 3). Table 1 shows the parameters of the chain distributions of of the building blocks. The degree of polymerization (DP, 10.2), the chain lengths (CL, 4.7), the internal chain length (ICL, 3.9), the number of chains (NC, 2.2) and the density of branches (DB, 11.6) of the amylopectin of kudzu starch were generally similar to DP 11, CL 4.8, ICL 3.9, NC 2.3 and DB 11.9 of the amylopectin of lotus rhizome starch. The internal chains consist of B-chains (substituted with other chains), whereas all A-chains (unsubstituted and completely external) appear as maltosyl stubs. The B-chains are divided into B1-, B2- and B3-chains. Short B1-chains are designated B1a(s)-chains at DP 3-7 and B1a(l)-chains at DP 8-17, and B1b-chains at DP 18-22. Long B-chains at DP ≥ 23 are subdivided into B2-chains at DP 23-52 and B3-chains at DP ≥ 53 [22,23]. The molar ratio (5.4) of short:long B-chains (BS:BL) of amylopectin from kudzu starch is far less than that (29.7) of amylopectins from lotus rhizome starch. It demonstrates that lotus rhizome starch has the higher numbers of short-chains with a greater DB (11.9), which results in more compact structure and high molecular density, high molar mass and small molecular size of amylopectin molecules [24]. It is the more compact structure and high molecular density, high molar mass and small molecular size of amylopectin that lead to the low ce value (0.6%, w/v) of lotus rhizome starch. In the semidilute unentangled regime, the ηsp values of the kudzu and lotus rhizome starch solutions are proportional to c1.09 and c1.10, respectively; while in the semidilute entangled regime, they are proportional to c3.49 and c2.66, respectively. Similarly, the exponent of the kudzu starch concentrations is more than that of the lotus rhizome starch concentrations. The small exponent demonstrates that starch molecules are entangled, but can not interact strongly. The exponents of the concentrations in the entangled regime increase as the amylose contents increase [12].

Table 1. The amylose contents and particle size parameters of the starchesA

Figure 2. Plots of ηsp versus c in aqueous solutions for the gelatinized (a) kudzu starch (b) lotus rhizome starch. The entanglement concentrations and slopes of fitted lines in two regimes are illustrated

Figure 3. HPAEC Chromatogram of building blocks of amylopectins: (A) The building blocks from kudzu starch and (B) The building blocks from lotus rhizome starch. The ﬁrst peak is an impurity in the enzyme preparation. Numbers on the peak indicate degree of polymerization (DP). (C) Standard samples (G1-G7) obtained by HPAEC-PAD

3.3. TEM

The TEM images further illustrates the chain shapes and dimensions of the gelatinized kudzu and lotus rhizome starch solutions, as shown in Figure 4. The specimens for TEM were prepared from the gelatinized starch dispersions around ce. Because they were revealed by the outer cast of heavy atoms from phosphotungstic acid, they appeared clear on a dark background. Figure 4a shows that the 0.1% kudzu starch solution distributes many homogeneous, stubby and curved strands with lengths ranging from 10 nm to 80 nm. When the concentration increases to ce (1.56%), many strands accumulate and densely entangle, which causes approximately circular and thick masses with about 700nm in diameter to form. When the concentration continues to increase to 2.0% (c> ce), almost all strands uniformly entangle and form the aggregates with different sizes and shapes. After cooling the gelatinized starch dispersions down to 4±1°C for 7 days, the different aggregate forms existing between the kudzu and lotus rhizome starch solutions, at different concentrations, can be observed in Figure 4b and Figure 4d. For the retrograded kudzu starch solutions, a homogeneous, well-distributed network with many uniformly pores develops when the concentrations are less than ce (0.1%), as can be seen in Figure 4b. When the concentration increases to ce (1.56%), the polydisperse wormlike strands become associated with forming obviously dense network with opaque and different-size particles. When the concentration is greater than ce, it seems that the dense network is more closely bound to form fibrous aggregates. From the pictures in the bottom left, it is obvious that the opalescent gels can form whether the concentrations are less than, equal to or greater than ce. It suggests that the gelatinized kudzu starch solutions readily recrystallize and retrograde even at diluted solutions on cooling. Actually, the retrogradation degree of the kudzu starch solutions is more significant when the concentrations are equal to or greater than ce. Figure 4c shows the gelatinized lotus rhizome starch solutions appear to be thick and curved strands with lengths ranging from 20 nm to 100 nm when the concentration is less than ce (0.1%). The polydisperse wormlike strands that were interwoven with each other form a dispersedly entangled network consisting of many circle pores of different sizes and shapes when the concentration increases to ce (0.6%). The strands in the entangled network further closed to each other to produce a relatively homogeneous network when the concentration increases to 2.0% (c> ce). Compared to the gelatinized kudzu starch solutions (Figure 4a), the molecular chains in the gelatinized lotus rhizome starch solutions entangle more intensely. This may be related to starch particle size [25]. The particle size parameters are shown in Table 1. The particle size of lotus rhizome starch is larger than that of kudzu starch. For lotus rhizome starch, the 10th [d(0.1)], 50th [d(0.5)], and 90th [d(0.9)] percentiles are 9.587 μm, 16.289 μm, and 28.266 μm, respectively. The volume weighted mean diameter (D[4,3]) is 17.886 μm, corresponding to surface area weighted mean diameter (D[3,2]) of 15.091 μm and specific surface area of 0.398 m²/g. For kudzu starch, the 10th [d(0.1)], 50th [d(0.5)], and 90th [d(0.9)] percentiles are 7.451 μm, 13.276 μm, and 23.592 μm, respectively. The D[4,3] is 14.558 μm, corresponding to D[3,2] of 12.061 μm and specific surface area of 0.497 m²/g. These data demonstrate that larger granules swell and paste more readily and the smaller granules are more resistant to that process, which is agree with the previous report [26]. For the retrograded lotus rhizome starch solutions (Figure 4d), when the concentration (0.1%) was lower than ce, the interpenetrated strands form a fractal-like network with a loose organization of long strands. When the concentrations are equal to or greater than ce, the fractal-like network becomes more intensive and thicker. The pictures in the bottom left demonstrate that the turbidity becomes cloudier and opalescent, and only a small number of gels form due to re-crystallizing of amylose chains when the concentrations are equal to, or greater than ce. This is due to slow reassociation of amylopectin from lotus rhizome starch with higher numbers of short-chains with a greater DB on cooling. The amount of long amylopectin chains is an important determinant in accelerating the retrogradation of starch. The amount of long amylopectin chains is correlated positively with gel ﬁrmness [27]. As depicted in Table 1, the greater molar ratio (29.7) of BS:BL of amylopectin from lotus rhizome starch prevents significantly starch from retrograding. The above results also show that the effects of ce on the network and aggregate formation were much more pronounced in kudzu starch solution than in the lotus rhizome starch solution, especially for retrograded kudzu starch solution. Thus, the ce value may be a proper reference concentration to regulate the suitable concentrations to obtain the ideal viscosity and consistency of the kudzu and lotus rhizome starch solutions.

Figure 4. The starch dispersions with different concentrations (w/v): (a) gelatinized kudzu starch (b) retrograded kudzu starch (c) gelatinized lotus rhizome starch (d) retrograded lotus rhizome starch. The specimens were negatively stained with 2% phosphotungstic acid at a magnification of 40000× (scale bars: 200nm). Bottom left is the pictures of the starch dispersions

3.4 Steady Shear Property

Flow curves of the kudzu and lotus rhizome starch aqueous solutions with concentrations around ce are shown in Figure 5. Figure 5A and B indicates that shear thinning behaviour hardly occurs when the concentrations of the kudzu and lotus rhizome starch solution are lower than ce (0.1%). The rheological behavior of starch pastes is related to the volumetric fractions that occupy the swollen granules. At low concentrations of starch (< 1%), rheological behavior is governed by the continuous phase (aqueous phase). At these concentrations, probably the swollen granules precipitated [26]. Compared to lotus rhizome starch solution, kudzu starch solution exhibits more significant shear thinning behavior at concentrations ≥ ce (1.56%). While for lotus rhizome starch solution, shear thinning behaviour only occurs when the concentrations are greater than ce (> 0.6%). This might be because the interwoven chains are broken into smaller chains with increasing shear rate when the concentrations are equal to or more than ce, which causes the intra- or inter-molecular entanglements to reduce [8]. The results are the same as the previous reports, or the ce concentration depends on polymer-polymer entanglement [12,28]. It can be seen from Figure 5a and Figure 5b that the plot of shear stress versus shear rate at different concentrations presents the non-Newtonian nature with yield stress. Shear stress increases when shear rate increases, and the gradient of the curve increases with increasing starch concentrations. It reveals from Figure 5a that the kudzu starch solution is obviously pseudoplastic when the concentrations are greater than ce (≥ 1.56%). The pseudoplastic behaviour is rarely obvious when the concentrations are less than ce. Similarly, it demonstrates from Figure 5b that the pseudoplastic behavior of the lotus rhizome starch solution is insignificant when the concentrations are equal to or less than ce (≤0.6%), and it becomes so obvious when the concentrations are higher than ce. These findings indicate that the impact of the ce value on steady shear property of the kudzu starch solutions is much more significant compared with the impact on the lotus rhizome starch solution.

Figure 5. Steady shear properties of the gelatinized starch dispersions ranging in concentration around ce. Apparent viscosity versus shear rate plot: (A) kudzu starch (B) lotus rhizome starch; Shear rate versus shear stress plot: (a) kudzu starch (b) lotus rhizome starch

4. Conclusions

The kudzu and lotus rhizome starch solutions present the non-linear shape of the ηsp/c versus c curves with a clear upturn in dilute solutions due to a mutual repulsion behaviour between polymer chains. The ce values of the kudzu and lotus rhizome starch aqueous solutions are determined to be 1.56% and 0.6%, respectively. The more amylose content and the higher numbers of short-chains with a greater density of branches are, the greater ce value is. Compared to the gelatinized kudzu starch solutions, the molecular chains in the gelatinized lotus rhizome starch solutions entangle more intensely due to larger particle size. Interestingly, the effects of ce on the retrograded kudzu starch dispersions are more pronounced at ce. It demonstrates that the impact of the ce value on the network formation of the kudzu starch solutions was much more significant compared with the impact on the lotus rhizome starch solutions. The steady shear properties of the gelatinized kudzu and lotus rhizome starch dispersions demonstrate that shear thinning behaviour hardly occurs when the concentrations are lower than ce, and shear thinning behaviour exists when the concentrations are equal to or greater than ce. Besides, the pseudoplastic behaviour is insignificant at lower concentrations (c< ce), and it becomes obvious when the concentrations rise above ce. The obtained information about ce will be used to regulate the kudzu and lotus rhizome starch concentrations to prepare the functional foods with the ideal viscosity, consistency, taste, texture, etc.

Acknowledgement

This research has been supported by National Natural Science Foundation of China (Grant No. 31371735 and Grant No. 31471700) and The Outstanding Youth Talent Support Program of Anhui Department of Education of China (Grant No. gxyqZD2016038).

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