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Chemical Engineering and Processing 64 (2013) 10– 16

Contents lists available at SciVerse ScienceDirect

Chemical Engineering and Processing:Process Intensification

j ourna l h o me pa ge: www.elsev ier .com/ locate /cep

heological and viscoelastic properties of corn starch suspension modified byydrothermal process: Impacts of process intensification

eyed Amir Bahrania,b,∗, Catherine Loisel c, Zoulikha Maache-Rezzouga, Dominique Della Valled,id-Ahmed Rezzouga

LaSIE, FRE-CNRS 3474, La Rochelle University, Avenue Michel Crépeau, 17042 La Rochelle, FranceIRTES-SeT, University of Technology Belfort-Montbéliard, 90010 Belfot Cedex, FranceGEPEA, UMR CNRS 6144, ONIRIS, Rue de la Géraudière, 44322 Nantes, FranceLTN, CNRS UMR 6607, École Polytechnique-University of Nantes, Rue Christian Pauc, 44306 Nantes, France

r t i c l e i n f o

rticle history:eceived 29 March 2011eceived in revised form7 November 2012ccepted 28 November 2012vailable online 11 December 2012

eywords:ydrothermal process (treatment)

a b s t r a c t

Rheological properties of starch suspension and particle size distribution of treated starch granules underthe same conditions (2 bar and 20 min) by direct vapour-heat moisture treatment (DV-HMT), Reduced-Pressurized Heat Moisture Treatment (RP-HMT) and Instantaneous Controlled Pressure Drop Process(DIC) were investigated. DV-HMT and RP-HMT were compared to DIC process in order to determine therole of the vacuum pressure steps before and after injection of live steam on the starch powder. For alltreated starches an increase of the particle size of granules was observed, a reduction of the consistencyindex and of the conservative modulus, compared to the untreated one. This difference becomes soimportant with increasing of process intensification. Relaxation spectra evaluated from the generalized

orn starchranule size distributionlow behavioriscoelasticityelaxation spectra

Maxwell model implied a drift to shorter relaxation time for treated starch suspension than the nativeone. A decrease in peak intensity H(�) of relaxation spectra with increase of intensification of processeswas observed which conclude that the modified starch suspension moving away from a stable state likegel according to the following order DV-HMT < RP-HMT < DIC. The effect of DIC process seems to be moreintense than the two other treatments. This difference can be attributed to the mechanical effect inducedby the abrupt decompression towards vacuum after treatment.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

Starch is considered as a renewable biopolymer and one of theost abundant reserve carbohydrates. Pregelatinized starch is used

n drilling mud as a viscosity increaser, in pharmaceutical tabletss an excipient, in building materials as an additive, in cosmet-cs as a thickening agent and especially in the food industry as ahickener, gelling agent, bulking agent and water retention agent.he rheological properties of starch suspensions can be character-zed by steady-state flow and dynamic oscillatory tests. The testedydrothermal processes yielded an increased fluidity and a loss ofhe elastic response of pastes, as a result of partial gelatinization oftarch granules.

In this study, we compared three hydrothermal processespplied to corn starch, for which the starch powder is heated byirect contact with saturated steam under pressure. In a recent

∗ Corresponding author at: University of Technology Belfort-Montbéliard, 90010elfort Cedex, France. Tel.: +33 03 84 58 39 64; fax: +33 03 84 58 31 41.

E-mail address: amir.bahrani@utbm.fr (S.A. Bahrani).

255-2701/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cep.2012.11.011

study [1], we studied two different hydrothermal processes in orderto show the intensification of heat transfer phenomena due tothe presence of an initial vacuum before injection of live steaminto the reactor. The author was proposed a phenomenologicalapproach, which describes the simultaneous heat and moisturetransfer within the corn starch bed. The principal heating sourceof starch results from the transfer of latent heat of steam conden-sation into the starch granules [2]. This heating raises temperatureof granules, from room temperature to steam equilibrium temper-ature. As the variation in temperature is important, the quantity ofcondensed water is significant and causes a rapid increase in themoisture content of the matter.

According to Doublier et al. [3], the rheological behavior ofpasted starch suspensions results from its biphasic structure. Thedispersed phase consists of swollen granules (mainly amylopectin)and is characterized by the volume fraction of starch granulesresulting from the pasting conditions. The continuous phase is

composed of soluble macromolecules (amylose) that have diffusedout of the starch granules upon pasting. The rheological proper-ties of starch suspensions can be characterized by steady-stateflow and dynamic oscillatory tests. Starch suspensions exhibit a

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S.A. Bahrani et al. / Chemical Engin

ime-dependant shear-thinning behavior that may be thixotropic4–6] or antithixotropic [7,8]. The rheological behavior of pastedorn starch suspensions depends on the thermal treatment con-itions. Bahrani et al. [4] exhibited that the values of the elasticodulus decreased with the increase of the treatment conditions;

his could be mainly linked to the lower elasticity of the continu-us phase. Indeed, the more intensive the treatment conditions,he higher the tendency of the starch suspensions to lose theirigidity and to reach a Newtonian behavior. According to Loiselt al. [9] the reduced elasticity could be related to lower abilityo form a gel of the amylose in the continuous phase. The aimf the contribution was to study of the rheological and struc-ural properties of standard maize starch treated by DV-HMT,P-HMT and DIC hydrothermal treatment. This study was basedn rheological (viscosity and viscoelasticity) and size distributioneasurements.However, there have been few studies that have investigated the

elaxation spectrum from mathematical models on starchy mate-ials. Ptaszek et al. [10–12] and Sodhi et al. [13] have been applied aethodology proposed by Honerkamp and Weese [14] and Weese

15] which predicted the relaxation spectra based on Tikhonov reg-larization. They reported an increase in peak intensity of H(�) ofpectra with increase in concentration of starch gels is an indi-ation of the transition of the system to a more stable state likeel.

The purpose of this study was focused to impact of vacuumressure in order to determine role of intensification of transferhenomena during hydrothermal process. In this objective, weave analyzed the rheological and morphological properties of corn

tarch modified by DV-HMT, RP-HMT and DIC hydrothermal pro-esses. This study was based on rheological properties such as flowehavior and viscoelasticity analysis through the relaxation spectraf corn starch suspension.

Fig. 1. Schematic of the experimental setup and typical pressure–time

g and Processing 64 (2013) 10– 16 11

2. Materials and methods

2.1. Raw material

Standard corn starch at moisture content of 14% (dry basis) wassupplied by Roquette Frères (Lestrem, France).

2.2. Hydrothermal processes

Three hydrothermal processes (Fig. 1) applied to corn starch(14% dry basis) were referred to as DV-HMT (direct vapour-heatmoisture treatment), RP-HMT (reduced-pressurized heat moisturetreatment) and DIC (Détente Instantanée Contrôlée). DV-HMT con-sists in heating up the starch by direct contact with saturatedsteam, injected from atmospheric pressure up to processing pres-sure, while for RP-HMT a reduced pressure (50 mbar) is created inthe reactor before the injection of steam. As for RPHMT, the DICtreatment contains a vacuum step before the injection of steam.But the difference with this one is that the abrupt decompressionis carried out towards the vacuum pressure after the maintainingat 2 bar (120 ◦C) during 20 min. The experimental setup (Fig. 1) waspreviously detailed by Bahrani [16].

2.3. Granule size distribution

Granule size distribution was carried out at room temperatureusing a Malvern Master Sizer (Malvern Instruments, Ltd) laser scat-tering analyser with a 300 mm Fourier cell (range 0.05–879 �m).

The pasted starch were diluted (1/10) with demineralized water at20 ◦C then dispersed into the sample dispersion unit (1 ml/100 mlwater). From each distribution, the median volume diameter Dv,0.5was presented and the swelling ratio was defined as (D/D0)3, with

profile of a DIC, RP-HMT and DV-HMT hydrothermal processes.

1 eering and Processing 64 (2013) 10– 16

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2

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2

si1twm(

2

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G

d

G

sa

h

G

G

3

3

ss

Fig. 2. Particle size distribution of native and treated corn starch.

2 S.A. Bahrani et al. / Chemical Engin

and D0 the median diameters of treated and untreated starch,espectively [9].

.4. Scanning electron microscopy (SEM)

The morphology of the native and treated starch granules wasnalyzed using a field emission scanning electron microscope (JoelSM-6400, Japan). A small quantity of each sample was spreadirectly on double-sided adhesive tape attached to a circular alu-inum stub and then were coated with gold under vacuum. The

amples were viewed and photographed with a magnification of000× and 3000×.

.5. Rheological measurements

A Brabender Viscograph was employed to prepare the starchuspension (6% w/w). Starch (485 g) was slurried in demineral-zed water at room temperature, then heated from 30 to 96 ◦C at.5 ◦C min−1, held at 96 ◦C for 10 min and cooled at 1.5 ◦C min−1

o 70 ◦C. Flow behavior and viscoelastic properties of starch pastesere characterized using the cone/plate geometry (6 cm/2◦) and alleasurements were carried out in a stress-controlled rheometer

TA Instrument AR1000).

.6. Relaxation spectra analysis

The viscoelastic properties of starch suspension were performedy the experience of relaxation which consists of applying a con-tant strain in the linear zone during the relaxation time. Fromhese dynamic measurements two independent parameters cane obtained. The storage modulus G′ (Pa), which is a measure ofhe energy stored and subsequently released per cycle of deforma-ion and per unit of volume, i.e. the elastic response. The secondarameter is the loss modulus G′′ (Pa), which is a measure of thenergy dissipated as heat per cycle of deformation, i.e. the viscousesponse:

∗(ω) = G′(ω) + iG′′(ω) (1)

The generalized Maxwell model is the most currently used forescribe the viscoelastic behavior:

∗(ω) = Ge +∫ +∞

0

H(�) · (�ω)2

1 + (�ω)2d� + i

∫ +∞

0

H(�) · �ω

1 + (�ω)2d�

(2)

Where Ge is the equilibrium modulus (Pa), H(�), the peak inten-ity of the relaxation spectra (Pa), �, relaxation time (s) and ω is thengular frequency (Hz).

For the generalized Maxwell model consisting of N elements, weave:

′ = Ge +N∑

j=1

Hj · (�jω)2

1 + (�jω)2(3)

′′ =N∑

j=1

Hj · �jω

1 + (�jω)2(4)

. Results and discussion

.1. Impact of process intensification in granule size distribution

The Fig. 2 illustrates the size distribution of native and treatedtarch granules before pasting. The native starch showed a narrowize distribution curve from 4 to 35 �m with a median diameter

in volume (Dv,0.5) of 13.7 �m, corresponding to usual value of cornstarch. Maache-Rezzoug et al. [17] have reported a similar value foruntreated maize starch (12.9 �m) with the same tendency of gran-ule size distribution curve. The distribution curves of treated starchwere shifted towards higher sizes and broadened according to thefollowing order; DV-HMT < RP-HMT < DIC processes. This enlarge-ment is partially due to the improvement of swelling capacity ofstarch granules induced by the intensification of hydrothermal pro-cess and to the presence of agglomerates particles which can beexplained by the third peak at approximately 300 �m observedwith DIC treated (Fig. 2). Dv,0.5 starch increased (Fig. 2) from 13.7 �mfor native toward 16.9, 17.8 and 22.7 �m with a swelling ratio(D/D0)3 of 1.9, 2.2 and 4.5 for DV-HMT, RP-HMT and DIC processes,respectively.

As seen from morphological properties (Fig. 3), native gran-ules show oval and regular granule shape, with different sizes, asmooth surface. This morphology was also detected for treatedstarch by DV-HMT and RP-HMT process, with presence ofagglomerates particle. The form of granules observed follow-ing DIC process shows the presence of alveoles and cavitieswith modification of particles contour caused by the partial ortotal melting of the macromolecular network, the formation ofagglomerates and the presence of fragmented particles. Con-sequently, the amylose which is responsible for the formationof a gel in the macromolecular network disappeared and theviscosity of treated starch suspension (Table 1) decreased. Accord-ing to Herrera-Gómez et al. [18] the spherical shape suggeststhat the granule is not gelatinized while an irregular shapeindicates that the granule has been gelatinized, at least par-tially.

This significant difference (swelling capacity) between DV-HMT,RP-HMT and DIC processes clearly underlined the mechanical effectwhich occurred during the abrupt decompression towards vacuumpressure with DIC process.

Herrera-Gómez et al. [19] defined 5 aggregate classes dependingon the cooking degree of corn starch. According to this classifica-tion, the presence of the single particles at the native, modifiedstarches by DV-HMT and RP-HMT (Fig. 3), correspond to class

0. The starch treated by DIC process can be classified in class 1aggregates which correspond to a fully gelatinized granule sur-rounded only by a limited number of partially or non-gelatinizedgranules.

S.A. Bahrani et al. / Chemical Engineering and Processing 64 (2013) 10– 16 13

native

3

3

ocbosg

sat

r

�

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TH

6a

Fig. 3. Scanning electron micrographs of native of (A)

.2. Impact of process intensification in rheological properties

.2.1. Flow propertiesFig. 4a shows the flow curves (linear scale) measured at 60 ◦C

f starch suspensions before (native) and after hydrothermal pro-esses. The results exhibited a non-Newtonian shear-thinningehavior, and a clockwise loop (thixotropic behavior). We alsobserve the persistence of the thixotropic behavior of the treatedtarch by DIC process, which may result from a disruption of starchranules aggregates under shearing.

The equilibrium flow curves in logarithmic coordinates of sheartress versus shear rate of native and treated starch dispersionsfter two consecutive up-down shear cycles to eliminate thehixotropic phenomenon are presented in Fig. 4b.

The Herschel–Bulkley model (Eq. (5)) was used to describe theheological behavior of starch suspension.

= �o + K �̇n (5)

Where � is the shear stress (Pa), �̇ is the shear rate (s−1), �o ishe yield stress (Pa), K is the consistency index (Pa sn) and n is theow behavior index (dimensionless). The parameters of the model

able 1erschel–Bulkley and viscoelastic parameters of generalized Maxwell model, at 60 ◦C.

Starch treated Rheological properties (T = 60 ◦C) Vi

�o (Pa) K (Pa sn) n �̇ = 600 s−1 ω

� (mPa s) G′

Native 3.23 ± 0.2 2.59 ± 0.3 0.59 ± 0.01 193 ± 2.7 94DV-HMT 1.22 1.02 0.6 81.0 66RP-HMT 0.69 1.14 0.6 89.4 25DIC 0.00 0.23 0.68 29.7 30

% (w/w) standard corn starch suspension; �o: yield stress; K: consistency index; n: flowpparent viscosity measured at shear rate of 600 s−1 at 60 ◦C.

; (B) DV-HMT; (C) RP-HMT and (D) DIC treated starch.

(�o, K and n) and the apparent viscosity (�) calculated for a shearrate of 600 s−1 are given in Table 1.

We observed a shear-thinning behavior (n < 1) with a yield stress(�o) for native, DV-HMT and RP-HMT modified starches suspension.For DIC treated starch the shear-thinning behavior was maintainedbut without a yield stress (�o = 0). As seen from Table 1, inten-sification of DIC process produces a more pronounced effect onthe decrease of apparent viscosity than the RP-HMT and DV-HMTprocesses. This difference is due only to the presence of vacuumstep after steam injection in DIC process. In a recent study [4],we showed that for severe processes conditions (≥2.5 bar) theintensification of heat transfer by the initial vacuum becomes soimportant that the mechanical effect, induced by the final vacuumonly present in DIC treatment is masked. To summarize, the moreintensive the process conditions, the higher the tendency of thestarch suspensions to lose their rigidity and to reach a Newtonianviscous liquid behavior.

3.2.2. Viscoelastic properties (analysis of the relaxation spectra)The viscoelastic behavior of native, DV-HMT, RP-HMT and

DIC treated starch dispersions are illustrated in Fig. 6. The

scoelastic properties (T = 60 ◦C)

= 6.3 (rad s−1) Parameters of generalizedMaxwell model

(Pa) G′′ (Pa) tan ı Ge (Pa) H(�)(Pa) � (s)

.1 ± 7.2 18.79 ± 0.78 0.19 ± 0.01 70 105 0.006

.1 9.24 0.12 60 80 0.009

.2 7.21 0.28 56 60 0.009

.1 7.43 0.25 28 36 0.020

behavior index (�o , K and n were determined from Herschel–Bulkley model); �:

14 S.A. Bahrani et al. / Chemical Engineering and Processing 64 (2013) 10– 16

F scales

maaAett

ig. 4. (a) Flow curves (linear scales) and (b). Equilibrium flow curves (logarithmic

easurements of mechanical spectra were performed at 60 ◦C tovoid retrogradation of amylose, through the variations of the stor-ge modulus G′ and the loss modulus G′′ as a function of frequency.

ll the starch suspensions including the native one have the prop-rties of a weak gel. The elastic modulus G′ is significantly higherhan the viscous modulus G′′ (between G′ > 4G′′ and G′ > 7G′′) andhey are practically independent of frequency. As seen from Table 1,

Fig. 5. Evolution of loss angle tangent of suspensions durin

) of native, DV-HMT, RP-HMT and DIC treated corn starch at a temperature of 60 ◦C.

for a frequency of 6.3 rad s−1, the values of G′ and G′′ decrease andthe tangent of loss angle (ı) increase for all treated starch suspen-sion as the process conditions became more intense.We presented

in Fig. 5 the time dependence of the mechanical loss tangent tan ıwhich can observed during gelation of starch suspension at 25 ◦C.For all the treated starches, the values of tan ı varied with the time(10 h) from ∼0.19 down to ∼0.06 (tan ı < 1). The increase in tan ı of

g gelation at 25 ◦C of native and treated corn starch.

S.A. Bahrani et al. / Chemical Engineerin

Fig. 6. Relaxation and mechanical spectra of (A) native; (B) DV-HMT; (C) RP-HMTand D) DIC treated composite starches gel at 60 ◦C.

g and Processing 64 (2013) 10– 16 15

modified starch suspension was observed with increasing of pro-cess intensification. These values indicate that the modified starchsuspension presents a solid like viscoelastic behavior.

According to generalized Maxwell model, the peak intensityH(�) values of the spectra decreased from 105 for native toward80, 60 and 36 Pa for DV-HMT, RP-HMT and DIC treated starchesat the relaxation time (�) of the 0.006, 0.009, 0.009 and 0.02 s,respectively (Fig. 6). The decrease in peak intensity of relaxationspectra with increase of intensification of processes conclude thatthe modified starch suspension moving away from a stable statelike gel. Moreover, the reduction of relaxation time spectra exhib-ited a rapid relaxation process which concluded an increase in thefluidity of starch gel [10–13].

Furthermore, the equilibrium modulus parameter Ge gave addi-tional information regarding the nature of starch suspension. Inthis study, a Ge value of 70 Pa was observed for native corn starchsuspension. This parameter decreased with increase of intensifica-tion of processes to 60, 56 and 28 Pa for DV-HMT, RP-HMT and DICtreated starches suspension, respectively (Table 1). This responsecan be indicated that the modified starches suspension tends topresent a solid like viscoelastic behavior in the intense processes.While in the intense conditions, the values of Ge tending toward azero which can be significant a transition to a liquid like behavior[20].

4. Conclusion

The results of this study showed that the extent of rheologi-cal modifications depends on the intensification of hydrothermalprocess, which can be due to the presence of a pressure vac-uum before and after injection of live steam into the reactor.The distribution curves of treated starch were shifted towardshigher sizes and broadened according to the following order; DV-HMT < RP-HMT < DIC processes. This enlargement is partially due tothe improvement of swelling capacity of starch granules inducedby the intensification of hydrothermal process and to the presenceof agglomerates particles.

The form of observed granules of DIC treated starches wasfragmented to the difference of the native, DV-HMT and RP-HMTtreated starch. This can be explained by the partial or total meltingof the macromolecular network, the formation of agglomerates andthe presence of fragmented particles. Consequently, the amylosewhich is responsible for the formation of a gel in the macro-molecular network disappeared and the viscosity of treated starchsuspension decreased.

Intensification of DIC process produces a more pronouncedeffect on the decrease of apparent viscosity than the RP-HMTand DV-HMT processes. We also observe the persistence of thethixotropic behavior of the treated starch by DIC process, whichmay result from a disruption of starch granules aggregates undershearing. This difference is due only to the presence of vacuumstep (mechanical effect) after steam injection in DIC process. Thedecrease in peak intensity H (�) of relaxation spectra with increaseof intensification of processes conclude that the modified starchsuspension moving away from a stable state like gel.

Acknowledgement

The authors would like to thank Nicolas Stephan (IMN, Nantes,France) for assistance in scanning electron microscopy experi-ments.

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