Steady and oscillatory shear behaviour of semi-concentrated starch suspensions

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1 Procedia Food Science 1 (2011) th International Congress on Engineering and Food (ICEF11) Steady and oscillatory shear behaviour of semi-concentrated starch suspensions Monica Mironescu a *, Ion Dan Mironescu b, Letitia Oprean a a Department of Food Biotechnology, Faculty of Agricultural Sciences, Food Engineering and Environmental Protection, University Lucian Blaga of Sibiu, Romania, monica.mironescu@ulbsibiu.ro, letitia.oprean@ulbsibiu.ro b Department of Chemistry and Food Engineering, Faculty of Agricultural Sciences, Food Engineering and Environmental Protection, University Lucian Blaga of Sibiu, Romania, ion.mironescu@ulbsibiu.ro Abstract The viscoelastic moduli G and G of aqueous suspensions with 40% (w/v) normal corn starch (NCS) and waxy corn starch (WCS) were determined by oscillation rheometry. The oscillatory shear flow experiments at heating from 30 to 75 C and maintaining at this temperature showed changes from a behaviour predominant viscous (G >>G ) to predominant elastic (G >G ) for both starches at 60.5 C for WCS, respectively 70,85 C for NCS, WCS having higher values of G and G as NCS. After the gelatinisation temperature was attired, NCS showed no significant changes, both moduli remaining relatively constant. Peaks of both moduli G and G were obtained for WCS at its maintaining at 75 o C, these changes being attributed to the changes in the amylopectin structure in the absence of amylose for this starch type. The frequency influenced the results; analysis at constant low frequency (10s -1 ) gave big oscillations during the measurements and made the analysis impossible, whereas frequencies as 50s -1 or 100s -1 gaves reproducible and similar results. The shear flow measurements realised at angular frequencies ω from10-1 to 10 3 s -1 at 25 o C showed that changes from a behaviour predominant elastic (G >G ) to predominant viscous (G >>G ) occurred when ω attired the values 10s -1 for WCS and 3s -1 for NCS. The calculation of the Power-Law parameter B showed that NCS forms a physical gel structure, whereas WCS behaves as a covalent gel in the frequency domain 10-1 to 10 s -1 and as physical gel in the frequency domain 10 to 10 2 s Published Published by Elsevier by Elsevier B.V. Selection Ltd. and/or Selection peer-review and/or under peer-review responsibility of under 11th International responsibility Congress of ICEF11 on Executive Engineering Committee and Food (ICEF Members 11) Executive Committee. Keywords: corn starch;waxy corn starch; steady shear; oscillatory shear; viscoelastic moduli * Corresponding author. Tel.: ; fax: address: monica.mironescu@ulbsibiu.ro X 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of 11th International Congress on Engineering and Food (ICEF 11) Executive Committee. doi: /j.profoo

2 Monica Mironescu et al. / Procedia Food Science 1 (2011) Introduction Corn starch is a valuable raw material at the obtaining of starch hydrolysates as maltodextrins, glucose or maltose syrups by using enzymes, acid or by combined acid-enzymatic hydrolysis [1]. The enzymatic hydrolysis of starch occurs in three steps: gelatinisation, liquefaction and saccharification. Starch gelatinisation is commonly de ned as an irreversible order disorder transition of starch granules from a structured state to a disordered state (starch pastes) upon heating [2]. Upon heating of a starch suspension, the granules swell due to absorption of moisture in the amorphous regions of the granule and amylose (AMY) and water start diffusing out of the granular entities [3]. At higher temperatures, amylopectin (AMP) melting begins and enables the release of the important amount of AMY that was trapped between AMP layers and of the small part that was confined within crystalline amylopectin; the crystalline order of AMP losses and so the birefringence [3][4]. Finally, a gel or a pastelike mass is formed [5]. The structure of gel or paste is affected by the starch concentration and the structure of swollen starch granules, the amounts and the types of AMY and AMP leached out, and heating conditions such as temperature, heating time and heating rate [6]. Starch exhibits unique rheological behaviour with change of temperature, concentration and shear rate., rheology being a valuable tool to predict the product properties and select the raw materials for food production [7]. Rheological behaviour of concentrated starch suspensions was reported to be affected by AMY [8]. Amylose molecules begin to leach from the granules and the viscosity increases, maximal viscosity corresponding to the point when the number of swollen but still intact starch granules is maximal. The maximum is followed by a decrease in paste viscosity, as the granules rupture and starch molecules are dispersed in the aqueous phase. The rate and extent of breakdown are dependent on the type and amount of starch, the temperature gradient, shear force and the composition of the mixture [8]. The gelatinization temperature of most starches is between 60 and 80oC. In general, there is a negative relationship between the AMY content of starch and the gelatinization temperature and peak viscosity [9]. This research investigated the dynamic rheological behaviour of concentrated corn starch suspension by small amplitude oscillatory shear tests and steady shear tests. Waxy corn starch consist of essentially 100% AMP, effects of leached-out AMY on the gelation of starch dispersions are negligible. Therefore, two starch types: normal corn starch (NCS) and waxy corn starch (WCS) are used in this study for elucidating the effect of composition on the change of viscoelastic properties during heating at temperatures corresponding to the gelatinisation. 2. Materials and Methods Two types of corn starch were used: native corn starch (NCS) and waxy corn starch (WCS), with 78% AMP and µm granule mean diameter, respectively with 99% AMP and 9.5 µm granule mean diameter. 40% (w/v) aqueous starch solutions were prepared by stirring at 25 o C. Small amplitude oscillatory rheological measurements were performed using a rotational rheometer Rotovisco RV100 (Haake, Germany) with temperature controlling system TCP/P and cone-plate geometry (C60/1). Two types of measurements were done: oscillatory shear flow and steady shear flow. In the oscillatory shear flow experiments the sample is exposed to a forced oscillation and the transmitted stress is measured. Derived parameters include storage and loss moduli (G and G ), where the storage parameter is related to elastic energy and the loss parameter to viscous flow. For the analysis of starch suspensions in the oscillatory mode, measurements at three constant frequencies ω (10, 50 and 100 s -1 ) were made. The heating rate was linear from 30 to 75 C with heating rate 0.18 C/sec. For structural fluids, such as starch suspensions or starch gels, small amplitude oscillatory shear flow may be the only way to find their rheological characteristics. By working with suspensions, small amplitude tests are recommended [10], because of the possibility of structure breakdown at large shear

3 324 Monica Mironescu et al. / Procedia Food Science 1 (2011) amplitude. Taking into account these observations, for the analysis of starch suspensions in the steady shear mode, measurements at angular frequencies ω from10-1 to 10 3 s -1 were made. The gap between cone and plate was 0.5 mm and the deformation was constant (γ=1%) for all measurements. The type of gel formed was determined by using the Power-Law function [11]: G = A. ω B where G is the storage modulus, ω is the oscillation frequency and A is a constant. The exponent B is the slope in a log log plot of G versus ω. B = 0 for a covalent gel, whereas for a physical gel B > 0, the B-value being related to the strength and nature of the gel [11]. 3. Results and Discussions Typical results obtained at the oscillatory shear flow analysis of the two starches are presented in Figure 1. In both starches the viscoelastic moduli remain constants below a temperature characteristic for each starch type, the elastic character being more pronounced. The loss modulus G (viscous character) is parallel to the storage modulus G, but five decades lower, this phenomenon being typical for starch suspensions [12]. During the thermal treatment structural changes occur. At temperatures higher as 60.5 C for WCS, respectively 70,85 C for NCS, a change in the viscoelastic characteristics of starches is registered and the viscous character becomes dominant (G > G ). This change can be attributed to begin of starch gelatinization. The differences between the values of the two moduli are much smaller as in the case of unchanged starch suspensions, even the viscous character remains predominant after these temperatures. The difference is bigger for NCS having the ratio AMY/AMP bigger as WCS. 1,00E+07 1,00E+06 1,00E+05 1,00E+04 1,00E+03 G' NCS, Pa G' WCS, Pa G" NCS, Pa G" WCS, Pa Temperature, oc G', G", Pa 1,00E+02 1,00E+01 1,00E+00 1,00E Temperature, o C 1,00E-02 1,00E-03 1,00E-04 1,00E ,00E Time, sec Fig. 1. Evolution of storage (G ) and loss (G ) moduli measured with a cone-plate geometry (gap 0.5 mm) at the thermal treatment of 40% (w/v) normal corn starch NCS and waxy corn starch WCS from 30 to 75 C (heating rate 0.18 C/sec) at constant frequency (50s -1 )

4 Monica Mironescu et al. / Procedia Food Science 1 (2011) WCS behaves differently from NCS during the maintaining at 75 o C. After approximately 2 minutes of heating, when both moduli register a slightly decrease from the maximal value, corresponding to the granules rupture and starch molecules dispersion in the aqueous phase, G and G increase very fast to a higher value (ten times higher) and then decrease to the former value. The duration of the first increase is relatively short (around 40sec). Another similar behaviour is obtained immediately after the system restabilises, the second peak being smaller as the first one. Because the main difference between WCS and NCS is the much higher amount of AMP in WCS, these peaks could be attributed to this component. AMP begins to melt at high temperatures and, in the absence of AMY, forms a new structure which is shortly destabilised. The frequency has a big influence on the results; tests made at lower frequencies show big oscillations during the measurements, making the analysis at low frequency impossible. Application of higher frequencies as 50s -1 (figure 1) or 100s -1 gives reproducible results; the results obtained by using the two frequencies are similar. Data for G and G as a function of frequency provide information on structure. In Figure 2 the variation of G and G of two corn starches containing 40% starch as a function of frequency at 25 C are presented. 1,00E+02 1,00E+01 1,00E+00 1,00E-01 G', G", Pa 1,00E-02 1,00E-03 1,00E-04 1,00E-05 1,00E-06 1,00E-07 G' NCS G' WCS G" NCS G" WCS 1,00E-08 1,00E-01 1,00E+00 1,00E+01 1,00E+02 Frequency, 1/s Fig. 2. Storage modulus G and, loss modulus G of two corn starch types (NCS and WCS) containing 40% starch (w/v) as function of frequency at 25 C The magnitudes of G and G decreased in the case of the two starches with an increase in ω, and G is not very great compared with G at values of ω smaller as 10s -1 for WCS and 3s -1 for NCS, showing a slight frequency dependency and the dominancy of the elastic behaviour. Both G and G are much smaller for NCS as for WCS, most probably due to their different content in AMP and AMY. When a critical value of ω is attired (10s -1 for WCS and 3s -1 for NCS), the two moduli changes their ratio, the viscous behaviour becoming dominant. These modifications are most probably due to structural

5 326 Monica Mironescu et al. / Procedia Food Science 1 (2011) changes, which are not strongly influenced by the composition. Even AMY/AMP ratio is very different for the two starch types analysed, the evolution of the viscoelastic moduli is parallel after their switch. As the Power-Law parameter B given in Table 1 shows, NCS forms a physical gel structure. WCS behaves as a covalent gel in the frequency domain 10-1 to 10 s -1 and as physical gel in the frequency domain 10 to 10 2 s -1. Table 1. Power Law constants for relationship between storage modulus and frequency for NCS and WCS Starch type Frequency domain A B R-squared value NCS 10-1 to 3 s -1 0,0670 0,0863 0,59 3 to 10 2 s ,4935 0,9251 WCS 10-1 to 10 s -1 5,5428-0,4829 0, to 10 2 s ,4334 0, Conclusions The oscillatory shear flow experiments realised with heating from 30 to 75 C and maintaining at this temperature show structural changes of both starches from a behaviour predominant viscous (G >>G ) to predominant elastic (G >G ). These changes are observed at lower temperatures in the case of WCS (60.5 C) as for NCS (70,85 C). In both states WCS has higher values of G and G as NCS. After the gelatinisation temperature is attired, NCS show no significant changes, both moduli remain parallel and relatively constant. At the maintaining of WCS at 75 o C, G and G continues to increase, two peaks for each modulus being obtained; after these short increases of viscoelastic moduli, they come back to values near the maximal values obtained at the switch between G and G. The changes evidenced by the peaks could be attributed to the changes in the amylopectin structure in the absence of amylose for this starch type. The frequency influences the oscillatory shear flow measurements. Analysis at constant low frequency (10s -1 ) gives big oscillations during the measurements and makes the analysis impossible, whereas frequencies as 50s -1 or 100s -1 give reproducible and similar results. The shear flow measurements realised at angular frequencies ω from 10-1 to 10 3 s -1 at 25 o C show that changes from a behaviour predominant elastic (G >G ) to predominant viscous (G >>G ) occur when frequency attires the values 10s -1 for WCS and 3s -1 for NCS. The calculation of the Power-Law parameter B evidences that NCS forms a physical gel structure, whereas WCS behaves as a covalent gel in the frequency domain 10-1 to 10 s -1 and as physical gel in the frequency domain 10 to 10 2 s -1. Acknowledgements This work was co-financed from the European Social Fund through Sectoral Operational Programme Human Resources Development , project number POSDRU/89/1.5/S/63258 Postdoctoral school for zootechnical biodiversity and food biotechnology based on the eco-economy and the bio-economy required by eco-san-genesys.

6 Monica Mironescu et al. / Procedia Food Science 1 (2011) References [1] Hobbs L Sweeteners from starch: Production, properties and uses. In J. N. BeMiller & R. L. Whistler (eds.), Starch: Chemistry and Technology 3rd Ed. Academic Press, Elsevier, London, p [2] Tan I., Torley PJ, Halley PJ. Combined rheological and optical investigation of maize, barley and wheat starch gelatinization. Carbohydrate Polymers, 2008; 72: [3] Atkin NJ, Cheng SL, Abeysekera RM, Robards AW. Localisation of amylose and amylopectin in starch granules using enzyme-gold labelling. Starch/Staerke 1999; 51(5): [4] Atwell WA, Hood LF, Lineback DR, Varriano-Marston E, Zobel HE. The terminology and methodology associated with basic starch phenomena. Cereal Foods World 1988; 33: [5] Jenkins PJ, Donald AM. Gelatinisation of starch a combined WAXS/SAXS/DSC and SANS study. Carbohydrate Research 1998; 308: [6] Genovese DB, Rao MA. Role of starch granule characteristics (volume fraction, rigidity and fractal dimension) on rheology of starch dispersions with and without amylose. Cereal Chemistry 2003; 80: [7] Ognean M., Ognean CF, Danciu I. Technological effects of some xylanolytic preparation on white flour. Acta Universitatis Cibiniensis, series E Food Technology 2010; 14 (1): [8] Miles MJ, Morris VJ, Ring SG. Gelation of amylose. Carbohydrate Research 1985; 135: [9].Brouillet-Fourmann S, Carrot C, Mignard N. Gelatinization and gelation of corn starch followed by dynamic mechanical spectroscopy analysis. Rheologica Acta 2003; 42: [10] Hudson NE. The basics of rheometry. In I.Bauwens (ed.), Process Rheology. Lecture notes, Leatherhead, Surrey, 2-6 Nov1998. [11] Khondkar D, Tester RF, Hudson N, Karkalas J, Morrow J. Rheological behaviour of uncross-linked and cross-linked gelatinised waxy maize starch with pectin gels. Food Hydrocolloids 2007; 21: [12] Seidel C, Kulicke WM, Heß C. Hartman B, Lechner MD, Lazik W. Influence of the cross-linking agent on the gel structure of starch derivatives. Starch/Stärke 2001; 53: Presented at ICEF11 (May 22-26, Athens, Greece) as paper FMS866.

INFLUENCE OF WORKING PARAMETERS ON THE VISCOSITY OF THERMAL TREATED CORN STARCH SUSPENSIONS

INFLUENCE OF WORKING PARAMETERS ON THE VISCOSITY OF THERMAL TREATED CORN STARCH SUSPENSIONS MIRONESCU Monica*, SCHIERLE Claudia** *University Lucian Blaga of Sibiu, Romania, Faculty of Agricultural Sciences,