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Rheology of Mung Bean Starch Treatedby High Hydrostatic PressureBin Jianga, Wenhao Lib, Xiaosong Hua, Jihong Wua & Qun Shena

a College of Food Science and Nutritional Engineering, ChinaAgricultural University, National Engineering Research Center forFruits & Vegetables Processing, Beijing, Chinab College of Food Science and Engineering, Northwest A & FUniversity, Yangling, ChinaPublished online: 01 Aug 2014.

To cite this article: Bin Jiang, Wenhao Li, Xiaosong Hu, Jihong Wu & Qun Shen (2015) Rheology ofMung Bean Starch Treated by High Hydrostatic Pressure, International Journal of Food Properties,18:1, 81-92, DOI: 10.1080/10942912.2013.819363

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International Journal of Food Properties, 18:81–92, 2015Copyright © Taylor & Francis Group, LLCISSN: 1094-2912 print/1532-2386 onlineDOI: 10.1080/10942912.2013.819363

Rheology of Mung Bean Starch Treated by High HydrostaticPressure

Bin Jiang1, Wenhao Li2, Xiaosong Hu1, Jihong Wu1, and Qun Shen1

1College of Food Science and Nutritional Engineering, China Agricultural University, NationalEngineering Research Center for Fruits & Vegetables Processing, Beijing, China

2College of Food Science and Engineering, Northwest A & F University, Yangling, China

Mung bean starch water suspension (20%, w/w) was subjected to high pressure at 120, 240, 360, 480,and 600 MPa for 30 min, and the rheological properties were investigated by a rheometer. The storagemodulus and loss modulus increased with the increased pressure. The HHP-treated mung bean starchwere weak gels; their rigidity and viscoelasticity increased with the increased pressure. Pressurized mungbean starch gels were pseudoplastic non-Newtonian fluids and displayed shear thinning. The rheogramfitted the power-law model adequately. The mung bean starch gels were thixotropic. The hysteresis looparea of mung bean starch gel treated by 480 MPa was the highest.

Keywords: High hydrostatic pressure, Rheology, Mung bean, Starch.

INTRODUCTION

The rheological response of food depends on its chemical components, molecular structures, intra-and intermolecular interaction, and dispersion. The rheological properties of starch are closelyrelated with the quality of starch-based foods, such as hardness, stickiness, and chewiness. So theyare crucial in transportation, agitation, mixing, and energy consumption. These properties are some-times measured as an indicator of product quality (e.g., indication of total solids or change inmolecular size). Rheological data are required for calculation in any process involving fluid flow(e.g., pump sizing, extraction, filtration, extrusion, purification) and play an important role indescribing the heat transfer or in the design, evaluation or/and modeling of the continuous treatmentsuch as pasteurization, evaporation, drying, and aseptic processing.[1,2]

High hydrostatic pressure (HHP) is a non-thermal processing technology. It could be used as amethod to modify starch. Over 25 starches processed by HHP have been investigated. Starch canbe classified into “A”, “B”, and “C” types according to their X-ray diffraction patterns, such asA-type starch of wheat, corn, rice, B-type starch of potato, canna, and C-type starch of pea, mung

Received 9 September 2012; accepted 21 June 2013.Address correspondence to Qun Shen, College of Food Science & Nutritional Engineering, China Agricultural University,

No. 17, Qinghua East Road, Haidian District, Beijing 100083, China. E-mail: shenqun@cau.edu.cnColor versions of one or more of the figures in the article can be found online at www.tandfonline.com/ljfp.

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82 JIANG ET AL.

bean, etc. Type “A” crystalline patterns, mainly associated with cereal starches, is dense and con-sists of starch chains in a monoclinic lattice; type “B” crystalline patterns, usually occur in tuberand amylose-rich starch, and have a large void in which up to 36 water molecules can be accom-modated; type “C” crystalline patterns is a mixture of both “A” and “B” patterns which appear inlegume starches.[3–5] HHP could significantly influence the structure and the rheological propertiesof starch, which would thus influence the application of starch in a food system.[6] Effect of HHPon the alteration of physicochemical properties of various starches has been studied on structure,retrogradation, and chemical reaction characteristics in the past several decades.[6−12] Barley starchsuspensions with 10 and 25% concentrations treated by 400–550 MPa were reported to be pressure-and time-dependent. For 10% suspension, only a slight increase in viscosity was observed, while astrong paste with a creamy texture and a maximum storage modulus of 23 kPa was formed duringpressurization of a 25% suspension.[8] The rheological properties and microstructure of the pressure-induced gelatinised samples were different from those of heat-induced gelatinised samples.[9] Theηinitial value of normal rice starch and waxy rice starch increased with the increase in pressure.[10,11]

The endset complex viscosity of 25% (w/w) HHP-treated sorghum starch suspensions showed nosignificant difference (P < 0.01) between samples treated at 300, 400, 500, and 600 MPa as well asthe controlled one.[12]

Mung bean (Vignu radiata L.) is native to India, and now commonly cultivated in SoutheastAsia. China and Burmer are the major exporter of mung beans. It is consumed as a vegetablein the form of cooked beans or as mung bean sprouts. Mung bean is similar in composition toother members of the legume family, with 24% protein, 1% fat, 63% carbohydrate, and 16%dietary fiber.[13] Mung bean has been reported as being the best raw material for starch noodlepreparation.[14] Consequently, the mung bean starch (MBS) noodle is regarded as the best of allkinds of starch noodles.[15] Zhu et al.[16] reported that noodles manufactured from mung beansare white and smooth, pliable, and have good cooking quality. And extruded MBS is also used inthe production of vermicelli or glass noodles. Thus, MBS is a valuable area for research. MBS-water suspension with 20% (w/w) was subjected to HHP treatment for different treatment time.A decrease in gelatinization temperatures and gelatinization enthalpy upon HHP treatments wasobserved, and the C-type MBS converted to the B-type pattern. A complete gelatinization achievedafter treatment at 600 MPa for 30 min.[7] However, a comprehensive and systematic study inrheological properties of the pressured MBS hasn’t been conducted according to literature. Thepurpose of this work was to investigate the effect of different high hydrostatic pressures on therheological properties of 20% (w/w) MBS suspension, as well as to analyze the characteristics ofHHP-treated MBS.

MATERIALS AND METHODS

Materials

Mung bean variety Zhonglv No. 1 was supplied by the Institute of Crop Science, Chinese Academyof Agricultural Sciences, Beijing, China.

Sample Preparation

MBS was extracted according to the method of Li.[7] Mung bean seeds were soaked in deionizedwater in a ratio of 1:3 w/v for 18 h at 30◦C. The mung bean and water were then blended completely.Starch slurry was passed through 100-mesh nylon cloth by granity, and left to stand in a beaker for4–5 h at room temperature (25 ± 2◦C) and then the supernatant was removed. Then deionized waterwas added to the sediment and blended thoroughly. The supernatant was discarded and the starch

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HHP ON MUNG BEAN STARCH RHEOLOGY 83

was washed five times with deionized water until the outflow became clear. The starch was driedin a cabinet drier at 40◦C and then ground by a grinder (Joyong Electric Appliance Co., Shandong,China) into a fine powder through 100 gauge mesh sieve. The starch powder was kept in an airtightcontainer at room temperature for further analysis.

HHP Treatment

A starch-water suspension (20% w/w) containing 20 g MBS was vacuum-packed in 200 mLpolyethylene bags with a poly heat bag sealer. The reading of the vacuum meter was –0.1 MPa,which indicated that there was almost no air trapped in the PE bags. Pressure treatments were per-formed using a HHP device (high pressure press type HHP-750, produced by Kefa High PressureFood Processing Inc., Baotou, China) with a cylindrical pressure chamber as described by Li.[7]

The pressure-transmitting medium was water, and the maximum working pressure of the unit was600 MPa. The samples were treated at pressures of 120, 240, 360, 480, and 600 MPa for 30 min atroom temperature 25◦C. Pressure was increased at an approximate rate of 2.4 MPa s−1. The timecourse of the experiment began when the desired pressure was reached. After the high-pressuretreatment, the sample bags were opened and vacuum filtered, frozen by liquid nitrogen, and thenfreeze-dried. Each dried starch sample was ground into a fine powder through 100 gauge meshsieve after fully pulverizing in a laboratory blender, and then kept in an airtight container at roomtemperature for further analysis.

Rheological Measurements

The pasting properties of starch samples were obtained with a strain/stress controlled rheometerAR500 (TA Instrument, Waters Co., Ltd., Surrey, UK) equipped with a cone-plate configurationusing a cone geometry (40 mm dia., angle 2◦) at gap 0.05 mm. Starch-water slurry of 20% (w/w)was deposited in the middle of the testing plate. The exposed sample edge was covered with a thinlayer of light paraffin oil to prevent evaporation or absorption of atmospheric moisture during mea-surements. The temperature ramp measurement procedure was selected. Procedure of temperatureincrease was from 20 to 95◦C, which enables a starch slurry system to become a paste and the tem-perature decreased from 95 to 25◦C. The heating rate was 4◦C s−1 and the cooling rate was 5◦C s−1.The shear rate was set to 200 s−1.

In order to minimize the effects of transmission time, temperature, agitating, and shear stress onsamples during the pasting process on Rapid Visco Analyzer (RVA), the MBS was tested for pastingproperties first and then the static and dynamic rheological properties together by rheometer.

Dynamic-shear properties were obtained from frequency sweeps over the range of 0.1–100 rads−1 at 0.5% strain. The 0.5% strain was in the linear viscoelastic region. Frequency sweep tests wereperformed at 25◦C. Dynamic moduli G′ (elastic or storage modulus), G′′ (viscous or loss modulus)and tanδ (G′/G′′) (related with the overall viscoelastic response) were obtained. The shear thinningdata were obtained from steady state flow step over the shear rate range of 0–300 s−1 at 25◦C.The thixotropic properties were obtained from steady state flow step over the shear rate range of300–0 s−1 at 25◦C.

Data Analysis and Calculations

In order to test the flow behavior of starch suspension samples, the data were fitted to the followingmodels.

The power-law model:

τ = K · γ̇ n (1)

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84 JIANG ET AL.

Where τ is the shear stress (Pa), γ̇ the shear rate (s−1), K the consistency coefficient (Pa sn), and nthe flow behavior index (dimensionless).[17]

Herschel–Bulkley model:

τ = τ0 + K · γ̇ n (2)

Where τ0 is the yield stress (Pa).[18]

Casson model:

τ 0.5 = τ0 + K · γ̇ 0.5 (3)

Cross model:

σ = σ0 + η∞ · γ̇ +[η0 · γ̇

/1 + (λ · γ̇ )1−n

](4)

Where η0 and η∞ (Pa s) are the zero-shear viscosity (when σ0 = 0) and the infinite-shear viscosity,respectively, λ (s) is a characteristic time.[18]

Hysteresis Loop

Hysteresis loop data were obtained by registering shear stress at shear rates from 1 to 300 s−1. Areasunder the upstream data points (Aup) and under the downstream data points (Adown) as well as thehysteresis area (Aup – Adown) were obtained using Microcal Origin 7.5 (Microcal Software, Inc.,Northampton, MA). The percentage of relative hysteresis area was calculated by:

Ar = (Aup − Adown

)/Aup × 100[19] (5)

All statistical computations and analyses were conducted using SPSS 16.0 for Windows andMicrosoft Excel 2003. Experimental data were expressed as mean value ± standard deviation.Duncan’s multiple range test was used to separate mean values (P < 0.05). The data were plottedusing the software Microcal Origin 7.5. All measurements were conducted in triplicate.

RESULTS AND DISCUSSION

Pasting Properties of Starch Samples

All pasting parameters variations including peak viscosity (PV), trough viscosity (TV), breakdown(BD, the difference between PV and TV), final viscosity (FV), setback, and peak time (PT) of 20%w/w MBS HHP-treated for 30 min at different pressures were summarized in Table 1. From 0 to480 MPa, the PV, BD, and FV values of MBS were significantly increased with the increase inpressure (P < 0.05). From 480 to 600 MPa, they decreased sharply (P < 0.05). The TV and PTvalues were increased with the increase in pressure (P < 0.05). At 600 MPa, the BD, FV, and SB ofMBS reached a minimum; however, PT value was significantly higher than that of samples treatedby the other pressures as shown in Table 1.

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HHP ON MUNG BEAN STARCH RHEOLOGY 85

TABLE 1Pasting properties of 20% w/w mung bean starch HHP-treated for 30 min at different pressures from rotational

rheometers1,2

Pressure (MPa) PV (Pa. s) TV (Pa. s) BD (Pa. s) FV (Pa. s) SB (Pa. s) PT (min)

0.1 2.61 ± 0.01e 1.38 ± 0.05e 1.23 ± 0.01d 3.84 ± 0.02c 1.23 ± 0.01ab 13.01 ± 0.00b

120 4.39 ± 0.02c 2.06 ± 0.01d 2.32 ± 0.04c 5.52 ± 0.53b 1.13 ± 0.01b 12.32 ± 0.11c

240 4.78 ± 0.02c 2.12 ± 0.03c 2.68 ± 0.04c 5.57 ± 0.17b 0.78 ± 0.01c 12.23 ± 0.01c

360 5.24 ± 0.02b 2.23 ± 0.01c 3.01 ± 0.02b 5.95 ± 0.13b 0.71 ± 0.03c 12.24 ± 0.20c

480 6.05 ± 0.03a 2.42 ± 0.02b 3.63 ± 0.03a 7.27 ± 0.18a 1.22 ± 0.01a 12.30 ± 0.33c

600 3.12 ± 0.05d 2.65 ± 0.02a 0.47 ± 0.00e 2.60 ± 0.08d 0.52 ± 0.00d 14.81 ± 0.40a

1All values are means of triplicate determinations ± SD. Means within columns with different letters are significantlydifferent (p < 0.05);

2PV: peak viscosity; TV: trough viscosity; BD: breakdown; FV: final viscosity; SB: setback; GT: pasting temperature;PT: peak time.

Before 480 MPa, as the pressure increased the amorphous regions of starch granules were sup-pressed to swell; the growth ring structure of the granule starts to disintegrate and the crystallineregions undergo melting simultaneously with a progressively increasing hydration,[20] resulting aweaker resistance to heat and shear. Meanwhile, because of pressure more water molecules wereassociated with the amorphous regions by Van der Waals forces to reach the FV, Consequently, thePV, FV, and BD value increased. The peak time irreversibly swollen granules under pressure was themain contribution for the increasing viscosity.[20] At 600 MPa, a decrease of PV maybe attributed tothat starch granules were already gelatinised and thus cannot absorb much more water. A decreaseof the FV was speculated to result from that starch granules have lost their ability to fully gelatiniseand to form a network structure. This was proved by the scanning electron micrographs and X-rayresults of MBS treated by high pressure.[7] Because of the fully gelatinization, it took a longer timefor the starch suspension treated by 600 MPa to attain the PV. Therefore, the MBS granule has abetter shear and heat resistance after HHP treatment. The difference of pasting parameters amongdifferent HHP-treated MBS was speculated to be attributed to starch granule structure change causedby pressure.

Dynamic Rheological Behavior Measurements

The variation of (a) storage modulus and (b) loss modulus of 20% w/w MBS HHP-treated for30 min at different pressures was shown in Fig. 1. It was shown that the storage modulus (G′) wassignificantly larger than the loss modulus (G′′) (P < 0.05) and there was no crossover between thetwo moduli throughout the whole frequency sweep range, indicating that a typical weak gel structurewas formed after HHP treatment.[21] Both G′ and G′′ increased with the increase in pressure (Fig.1a and 1b). They increased rapidly first at low frequency and then increased slowly, except that theG′ of MBS gel at 480 MPa decreased after a rapid increase.

The storage modulus G′ is directly related to the cross-link density of the network in a gel.[22] TheG′ rheogram of 480 and 600 MPa gels intersected, showed that the gelling property was strengthenedby 480 MPa. The rheological measurements indicated the rigidity and viscoelasticity of the gelsincreased as the pressure increased, and they were all larger than those of the native ones. Therefore,the strength of mung bean gel significantly increased with the increase in pressure (P < 0.05).During pressurizing, the compression effect prevailed over the swelling effect at lower pressure.The inner-structure was compact. Thus the rigid increased as the pressure increased, as well as

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86 JIANG ET AL.

–20 0 20 40 60 80 100 1201000

1200

1400

1600

1800

2000

2200

2400

G' (

Pa)

Frequency ( rad s–1)

Native 120 MPa 240 MPa360 MPa 480 MPa 600 MPa

0 20 40 60 80 100

50

100

150

200

250

300

350

400

450

500

550

G''(

Pa)

Frequency (rad s–1)

Native 120 MPa 240 MPa360 MPa 480 MPa 600 MPa

a

b

FIGURE 1 Variation of (a) storage modulus and (b) loss modulus of 20% w/w mung bean starch HHP-treated for30 min at different pressures. Measurements were made at 25◦C and 0.5% strain.

the gelatinization. When the MBS suspension became fully gelatinised, the swelling effect wasdominant. It was speculated that the crystalline region remaining in the swollen starch granule maysmelt, which deforms and loosens the particles[23] and the granule structure collapsed. As a result,G′′ became the largest.

Effect of HHP treatment on rheological properties of different types of starch has been inves-tigated by some researchers. G′ value of barley starch was reported to increase with extending intreatment time.[8] Also G′ of sticky corn starch first increased and then decreased with prolongingin treatment time at pressures higher than 500 MPa.[22] G′ of 10% potato starch and 25% barleystarch were explored to increase with the increase in treatment time.[24,25] Changes in dynamicviscoelastic properties of starch were affected by process parameters including pressure ranges,treatment time, solvent, etc.[26−28] Therefore, the effect of holding time, temperature, pH, solvent,and starch/water ratio on rheological properties of MBS suspension by HHP treatment needs furtherstudy.

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HHP ON MUNG BEAN STARCH RHEOLOGY 87

Rheological Model of Starch

Starch paste belonged to non-Newtonian fluid. It displayed shear thinning and its apparent viscositydecreased with the increase in shear rate.[29] Flow curves of 20% w/w MBS HHP-treated for 30 minat different pressures were shown in Fig. 2. Curves of HHP-treated MBS were convex to the ShearStress Axis. It was judged to be non–Newtonian flow. At the same shear rate, shear stress of MBSincreased with the increase in pressure.

In order to perform a quantitative comparison of the starch samples, shear stress-shear rate datawere tested based on various rheological models (The power law, Herschel-Bulkley, Casson, andCross). It was found that both upward and downward curves of rheogram fitted power-law model(Eq. (1)) adequately.

The power-law model flow curve parameters of different HHP-treated MBS obtained with regres-sion analysis were listed in Table 2. When the shear rate was between 0–300 s−1, the correlationcoefficients of MBS were between 0.953 and 0.987, suggesting that the curves of HHP-treated MBShad a good correlation with power-law equation. The consistency coefficient (K) increased with theincrease in treatment pressure. For native samples and HHP-treated samples, an increase in pressurefrom 0 to 480 MPa was accompanied with a decrease in pseudoplasticity, shown by a decrease invalues of the flow behavior index (n).

0 50 100 150 200 250 300

0

100

200

300

400

500

Shea

r st

ress

(Pa

)

Shear rate (s–1)

Native 120 MPa 240 MPa

360 MPa 480 MPa 600 MPa

FIGURE 2 Flow curves of 20% w/w mung bean starch HHP-treated for 30 min at different pressures.

TABLE 2The power-law parameters of 20% w/w mung bean starch HHP-treated for 30 min at different

pressures

Pressure (MPa) K (Pa.sn) n (–) r2

0.1 28.23 ± 2.02 0.24 ± 0.02 0.953120 40.63 ± 2.01 0.22 ± 0.01 0.970240 52.31 ± 1.79 0.21 ± 0.01 0.984360 52.52 ± 1.74 0.22 ± 0.01 0.986480 72.97 ± 3.29 0.20 ± 0.01 0.970600 86.81 ± 3.86 0.28 ± 0.01 0.987

K: consistency coefficient; n: flow behavior index; r2: coefficients of determination.

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88 JIANG ET AL.

Consistency coefficient, K, from the power-law model (Eq. (1)) can be taken as a viscosity crite-rion. An increase in consistency coefficient was observed with the increasing pressure indicating anincrease in apparent viscosity at higher pressures. For all native and HHP-treated starch samples, nvalues were below 1 (Table 2). Therefore, the rheological behavior of MBS samples was suited tonon-Newtonian fluid and performed shear thinning behavior.[30–33]

Comparatively, the magnitude of flow behavior index (n) was the lowest for starch suspensiontreated by 480 MPa and the highest for starch suspension treated by 600 MPa within the pressurerange studied. It was showed that gum solutions with a high value of n tend to feel slimy in themouth. When good mouthfeel characteristics are desired, the choice should be a gum system havinga low n value.[34] It has also been reported that, for a given gum type, the value of flow behaviorindex (n) are highly dependent on the molecular size.[35,36]

It was postulated that after the initial stress was applied to the starch paste, the arrangementof macromolecules inside the matrix was disorganized by the stress and it started to flow morefreely.[37] The shear stress increased while the apparent viscosity decreased at a high shear rate.During the steady flow test, the shear stress increased with the increase in shear rate; when thestarch gel of network structure flowed, friction appeared between the walls in the network. More andmore starch molecules began to flow instead of maintaining a network, thus the viscosity decreased.The MBS had a spur form in flow curves at the shear rate of 50 s−1. After the peak, the shearstress increased gently with the increase in shear rate. This was in accordance with the study ofBanchathanakij and Suphantharika.[21] The first peak of the flow curves was caused by the increaseof shear rate. Large shear stress was needed to destroy the network structure of the starch gel; thenmore and more starch molecules performed the “liquid-like” behavior instead of the previous “gel-like” behavior.[38] The structure of MBS was ruptured; the swelling of the granules was limited; thecontent of soluble starch was low. At the same shear rate, the strength of starch gel was higher thanthat of the native starch, which led to the change of shear stress versus shear rate profile.

Apparent Viscosity of HHP-Treated Starch

The apparent viscosity of 20% w/w MBS HHP-treated for 30 min at different pressures was shownin Fig. 3. HHP had a notable impact on apparent viscosity of MBS paste. At the same pressure,apparent viscosity declined with the increase in shear rate. When the shear rate was fixed, the appar-ent viscosity of MBS treated at 120 MPa was slightly lower than that of the native; while the apparent

0 120 240 360 480 6000

200

400

600

800

1000

1200

1400

App

aren

t vis

cosi

ty (

Pa.s

)

Pressure (MPa)

0.3 s–1 0.5 s–10.7 s–1 1.2 s–1 1.8 s–1

FIGURE 3 Apparent viscosity of 20% w/w mung bean starch HHP-treated for 30 min at different pressures.

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HHP ON MUNG BEAN STARCH RHEOLOGY 89

viscosities of MBS treated at 240 and 480 MPa were all higher than those of the native starch. Theapparent viscosity increased as the pressure increased before 480 MPa. But the apparent viscosityof the MBS treated at 600 MPa dropped to the bottom.

Shear Thinning of Starch Paste

During the pasting process, the apparent viscosity of MBS first declined drastically then becomesmooth with the increase in shear rate, exhibiting a shear thinning behavior. Since for all samplesthe n value was less than one, the shear-thinning behavior was apparently attributed to the alignmentor disentanglement of the macromolecules and their chains with the increasing shear rate.[38,39]

The shear-thinning behavior is crucial in processing procedure. The declining apparent viscos-ity of starch by shear stress makes it easy for the thick fluid to filling, modeling, and leaking.Machinery wear and energy consumption can also be reduced. The starch paste usually consisted ofgelatinised swollen granules or, occasionally, granular fragments dispersed in a predominantly amy-lose solution.[40] Such suspensions are known to exhibit dilatant flow behavior.[39,41] The declineof starch gel apparent viscosity with the increase in shear rate indicated that the intermolecularhydrogen bonding was easier to be disrupted at a higher shear rate.[42−44]

Under shear stress, the friction in starch gel was decreased; the starch molecule was disintegrated;intra- and interchain hydrogen bonding and other sub bonding broke due to the molecules flow inthe fluid direction. Thus the gel system was destroyed and the apparent viscosity was reduced withincrease in shear rate. When the shear rate went above 120 s−1, the apparent viscosity remained aconstant. The apparent viscosity of MBS samples first increased and then declined as the shear stressincreased. The granule structure became more compact, which may resulted from the rearrangementof molecules caused by HHP. Thus the content of soluble starch increased; the gel become firmer;the space structure was difficult to be damaged and the apparent viscosity was higher at the sameshear rate.

Hysteresis Loop

A relative hysteresis loop area was observed in all pastes studied (Fig. 4). It can be interpretedas structural breakdown by the shear field to alter a structure or form a new structure, which

0.1 120 240 360 480 6000

10

20

30

40

50

60

70

80

Hys

tere

sis

loop

(%

)

Pressure (MPa)

FIGURE 4 Experimental hysteresis area values of 20% w/w mung bean starch HHP-treated for 30 min at differentpressures.

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90 JIANG ET AL.

then maintained a shear-thinning characteristic on following shear sweeps.[45] Generally, a largerhysteresis loop area suggests a greater extent of destruction in gel structure.

The hysteresis loop area of MBS paste increased with the increase in pressure from 0 to 480 MPabut dropped at 600 MPa as shown in Fig. 4. Assuming that a hysteresis loop area is an index ofthe energy needed to destroy the structure, the experimental data showed that MBS paste treated at480 MPa, whose apparent viscosity was the highest, required the highest energy for structure break-down. Tárrega et al.[19] reported that a high-viscosity thixotropic fluid may show a larger hysteresisarea than a lower viscosity one even if the latter undergoes a stronger structural destruction. Thusthe change of hysteresis loop was in accordance with that of the apparent viscosity of HHP-treatedMBS paste. Comparison of straight loop areas between differently viscous systems may not rendervalid conclusions on the extension of time-dependent structural breakdown.

CONCLUSIONS

MBS form weak gels after HHP treatment, belonging to non-Newtonian fluids. The rigidity, vis-coelasticity, and the gelling property were increased with the increase in pressure. The power-lawmodel (Eq. (1)) was found to best describe the shear stress versus shear rate data with R2 valuesbetween 0.953 and 0.987. At the same shear rate, the apparent viscosity declined with the increase inpressure. The MBS gels showed shear-thinning behavior. Consequently, the swelling of starch gran-ules under shear stress and heat was limited and the rheological properties of starch gel changed.It was anticipated that the starch granule structure was changed by high pressure, which includedthe rearrangment of starch molecules, and the disruption of crystalline region. This was proved withthe study of Li et al.[5] The study of rheological properties of MBS offered theoretical support forindustrial application of HHP-treated starch and food processing.

FUNDING

This work was supported by the Fund of National Natural Science Foundation (No. 30972067) andthe National Key Technology R&D Program of the Ministry of Science and Technology of People’sRepublic of China (No.2011AA100801).

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