porridge-Rheology of Broken Rice-Foxtail Millet-Maize Flour Blends Related to Extrusion Cooking

Rheological properties of the various flours from the broken rice, foxtail millet and maize used during extrusion conditions were investigated for pasting properties and dynamic oscillation using Rheometer. Results pointed out that each of the different flour might be utilized for specific applications in food processing. In addition, the functional properties of individual flours were measured. The swelling power of broken rice flour showed the highest values, indicated that the viscoelastic properties of pastes were dependent on the swelled starch–protein complex granules and the formation of new cross-links in the network. All formulation flours exhibited variable pasting behavior, among different temperatures and formulations at 90°C and 20:60:20 had the highest peak viscosity 6741 MPa.s and final viscosity 6842 MPa.s and was found to be more effective than other temperature and formulation combinations. Viscosity decreased with the increase of temperature and formulation combinations. The storage modulus was invariably higher than loss modulus at all stages during heating, holding and cooling of the sample, showing that the paste has a tendency to behave like weak gel. G´ was the most primary dynamic rheological parameter that reflected rheological properties, G´ represented the elastic properties of rice starch paste.

Rice (Oryza sativa) is one of the major cereal crops worldwide and it is consumed principally as a grain obtained from specific varieties. The understanding of their rheological behavior is very important for optimizing industrial applications and allowing consumer to select appropriate types for different culinary recipes (Correa et al., 2013). In the food industry, starch based products like cereal flours are cooked during extrusion processes. Therefore, the knowledge of their viscous behaviour is required to process them because the choice of the optimal processing conditions and the quality of end products depends on the rheological properties of the material.
During extrusion, protein structures are disrupted and altered under high shear, pressure, and temperature. Protein solubility decreases and crosslinking reactions occur possibly due to some covalent bonds formed at high temperature as well as protein denaturation and formation of complexes between starch and lipids and between protein and lipids. Among all flour components, starch plays a key role. The extrusion of starchy foods results in gelatinization, partial or complete destruction of the crystalline structure and molecular fragmentation of starch polymers (Perez et al., 2008).
The rheological properties of extruded blend dispersions, which influence the breakfast porridge-making process, are essential for producing highquality products. Recent studies concentrated on extrusion cooking processing effects for creating new products and evaluation of physical and chemical properties. However, there are very few literatures investigating thoroughly on dynamic rheological properties of extruded product pastes.
The objective of this work was to examine the effects of temperature (90-110°C) and formulation combinations [broken rice flour: foxtail millet: maize flour, 10:80:10 (Mix 1); 20:60:20 (Mix 2); 30:40:30 (Mix 3); 10:60:30 (Mix 4) and 30:60:10 (Mix 5)] on the rheological properties of flours used during the extrusion study (employing a dynamic rheometer). The purpose of this study was to investigate the use of temperatures and formulation combinations used during extrusion cooking on flours and to provide basic information for the further application of broken rice, millets and maize in food extrusion industry. a flourmill. The flour was sieved in ISS 40 sieve and used in the experiments.

Swelling power and solubility index
The flour samples of (broken rice flour, foxtail millet flour and maize flour) at 1 % w/v (dry basis) were prepared in a centrifuge tubes with closed screw caps and heated in a water bath for 30 min at 80°C with minimum shear condition. After heating, the centrifuge tubes were immediately immersed in an ice bath to quickly cool the dispersion to room temperature. After cooling in ice for 5 min, samples were centrifuged at 4500 rpm at 5°C for 15 min and then the supernatant was removed for the measurement of solubilized starch. The supernatant was dried to constant weight in a hot air oven at 105°C. Precipitated paste and dried supernatant were weighed. All measurements were done in triplicate. The swelling power (SP) and solubility index (SOL) were calculated as follows (Abdel-Rahman et al., 2008). tan ä as a function of frequency. G' is the dynamic elastic or storage modulus, related to the material response as a solid. G" is the dynamic viscous or loss modulus, related to the material response as a fluid. The data of all rheological measurements were analyzed with the supporting software Rheoplus/322 v2.81 (Anton Paar).

Statistical analysis
The data reported in the tables are an average of triplicate observations. The data were subjected to statistical analysis using IRRISTAT Version 3/93, Biometrics Unit, International Rice Research Institute, Philippines, Completely Randomized Design with Analysis of Variance (ANOVA) was carried out on each of the variables and the Least Significant Difference (LSD) and Duncun Multiple Range Test (DMRT). Significant differences were reported for P<0.05.

Swelling power and solubility index
The swelling power (SP) and solubility index (SOL) of broken rice flour, foxtail flour and maize flour are shown in Figs. 1 and 2, respectively. The

Pasting properties
Pasting properties were determined using a starch cell C-ETD 160/ST (Physica Smart, Starch analyzer-Anton Paar) attached to a rheometer (MCR 52, Anton Paar, GmbH, Germany) and established methodology (Jayakody et al., 2007). A sample (7% w/w) was equilibrated at 50°C for 1 min, then heated from 50 to 90°C, 100°C, 110°C at 6°C/min, held at 90°C, 100°C, 110°C for 5 min, cooled to 50°C at 6°C/min, and held at 50°C for 2 min. The speed was 960 rpm for the first 10s, then 160 rpm for the reminder of the experiment. The pasting properties of each sample were inferred from acquired diagrams including the peak time, peak viscosity, holding strength, setback, and final viscosity.

Viscoelastic behavior
Fresh pastes obtained from the Rheometer were used for dynamic oscillatory rheological measurement by a rheometer (Physica MCR 52, Anton Paar GmbH, Stuttgart, Germany). Samples were placed into the rheometer measuring system (cone and plate geometry, 24.983 mm diameter, 1.993° cone angle, and 0.106 mm gap) which was equilibrated to 25°C. All samples were subjected to a shear rate sweep at 20°C, from 0.01 to 100/s. Two steps of rheological measurements were performed: (a) amplitude sweeps at a constant frequency (10 rad/s) to determine the maximum deformation attainable by a sample in the linear viscoelastic range and (b) frequency sweeps at a constant deformation (0.5% strain) within the linear viscoelastic range. The mechanical spectra were obtained recording the dynamic moduli G', G" and Aliquot volume (ml) x sample weight (g) x 100 Vertical bar represents ± SD from the mean Vertical bar represents ± SD from the mean g/g) showed the highest swelling power compared to foxtail millet flour (2.90 g/g) and maize flour (2.69 g/g) with less difference between foxtail millet flour and maize flour. Swelling was regulated by the degree of crystalinity of the starch granules and the swelling power was determined by the ability of starch granules to swell in the presence of excess water when heated. Generally speaking, swelling power of starches reflects the interactions between water molecules and starch chains in amorphous and crystalline domains, respectively (Kim et al., 2012). The process of pasting can be expressed as the state that is to a large extent associated with gelatinization of starch and retrogradation to a minor researchers have suggested a relationship between swelling power and solubility. Statistical analysis was performed comparing SP and SOL values of flours. The SP and SOL profiles of flours were nearly the same. The broken rice flour, however, exhibited a significantly (Pd"0.05) higher SP and lower SOL values than those of the foxtail millet flour and maize flour tested. This result demonstrated that broken rice flour inhibited starch swelling and prevented amylose leach out, whereas foxtail millet flour and maize flour seemed to have no effect on these properties.

Pasting profiles
The pasting behavior of samples is reported in terms of degree of gelatinization, gelatinization temperature, PV, BDV, setback and final paste viscosities, and the set back and breakdown ratios ( Table 1). The data obtained from the rheograms indicated that viscosity of all temperature and formulation combination flours studied increased during heating, started decreasing just before holding, and increased again during the cooling cycle (Fig. 3). The viscosity values observed ranged from 111.6 to 1552, 384.5 to 6741, 56.07 to 1348, 30.27 to 1260 and 283.7 to 1242 MPa.s, for the formulations of broken rice flour: foxtail millet flour: maize flour combinations 10:80:10, 20:60:20, 30:40:30, 10:60:30 and 30:60:10, respectively. The changes taking place during heating of starch in excess water have been studied in detail by Shinojet al. (2006). Starch granules are insoluble in water, but when a starch suspension in water is heated above critical temperature, the hydrogen bond responsible for structural integrity of the granules weakens allowing the penetration of water, and hydration of the linear segments of amylopectin.
In this study, the increase in temperatures, broken rice and maize flours synergistically increased the viscosity observed during pasting (Fig.3) reflected the contribution of media viscosity, these observations were in good agreement with the results obtained from the extrusion of rice flour  for mung bean starch. This can also make the shear forces exerted on granules much larger than those encountered in starch-water suspensions.
This significantly affects the breakdown of granules leading to a significantly (Pd"0.05) higher breakdown viscosity of 20:60:20 formulation combination systems as compared with all temperatures but formulation combinations are not significantly different at the 5% level by DMRT. Breakdown viscosity measures the tendency of swollen starch granules to rupture when held at high temperatures and continuous shearing. The maximum rise observed before the completion of heating cycle for all the temperature, formulation combinations of flours was due to the changing temperatures and behavioural characteristics of starch and protein components present in flours. extent. The apparent viscosity of starch dispersions in water is strongly influenced by the extent of swelling of starch granules. Starch granules swell radially in the beginning of heat induced pasting, and when the temperature is increased, the amylopectin-rich granules swell tangentially. As a consequence, the granules get deformed and lose their original shape. The presence of amylose in the continuous phase surrounding the swollen granules results in the formation of a gel on cooling.
Data from the present solubility studies revealed significant difference (Pd"0.05) between broken rice flour, foxtail millet flour and maize flour. Most previous 90°C 100°C 110°C starch suspensions. On cooling, the viscosity of starch rises due to the high retrogradation tendency of the amylase fraction.

Viscoelastic behavior
The mechanical spectra for dispersions of three temperatures and five formulation flour During the isothermal holding at 90, 100 and 110°C, all flours exhibited a reducing tendency in viscosity, which has been reported for minor millets flours (Shinoj et al., 2006). Again during cooling, an increase in viscosity was observed, which can be related to the retrogradation tendency of starch. This behaviour is largely determined by the affinity of hydroxyl groups of one molecule to another. The amylose molecules being randomly dispersed can orient themselves in parallel fashion to form aggregates at low solubility leading to gel formation. The peak viscosity, peak time, pasting temperature, holding strength, breakdown, final viscosity, setback from peak and setback from trough were obtained (Table 1).
From the results it is seen that the peak viscosity and final viscosity of broken rice flour: foxtail millet flour: maize flour with the ratio of 20:60:20 at 90°C were higher than that of corresponding holding, breakdown, set back from peak and trough. A similar trend was reported by Musa et al. (2011) for rice  combinations are shown in Fig. 4 and 5. Viscoelastic properties can be used to characterize the three dimensional network structure of five formulation flour combinations. Dynamic moduli involves two parameters viz. storage modulus G' and loss modulus G", showing initial increase during heating cycle, followed by decrease during holding, then rise during cooling and remain near constant ( Fig. 4 and 5). The storage modulus was invariably higher than loss modulus at all stages during heating, holding and cooling of the sample, showing that the paste has a tendency to behave like weak gel. G' was the most primary dynamic rheological parameter that reflected rheological properties, G' represented the elastic properties of rice starch paste. These observations were in agreement with the results obtained from Indica rice flour (Qin-lu et al., 2011).
In the dynamic measurement with rheometer, the changes of G' can reflect the changes of hardness and strength of the gel, and the higher G' means higher hardness and strength (Lawal et al. 2011). The rheological parameters, G' and G" showed significant variation among three temperatures and five formulation flour combinations when subjected to oscillatory rheological assays of both amplitude and frequency sweep testing ranging from 1 to 100 rad/s under a deformation of 0.5% strain. For all samples, the magnitudes of G' were higher than those of G" over a range of oscillatory amplitudes and frequencies studied. The increase in G' was accompanied by a corresponding increase in G". The G' and G" curves of all combinations were almost parallel in the frequency range studied. Decreased temperatures and increased flour combinations the G' values of starch paste up to 14-and 5-fold at an angular frequency of 1 rad/s, respectively. Fig. 5 shows that except formulation 20:60:20 at 90°C others resulted in a decrease of the tan ä values from 0.261 for 20:60:20 to 0.290 at 1 rad/s frequency and remained roughly constant across the range of angular frequencies; and 0.217 to 3.76 at 10 rad/s angular frequency; whereas,other temperature and flour combinations exhibited slightly lower tan ä at low frequencies but higher at high frequencies due to the reduction of tan ä of formulation flours at high frequencies (Fig. 4). Therefore, the 20:60:20 formulation exhibited seemingly more solid like behavior than the other remaining formulation combinations.
The main properties of weak gels are given by Wu et al. (2010) as G'>G" throughout the accessible amplitude and frequency range, both G' and G" are slightly increased with increasing frequency and the separation of the two moduli (tan ä=G"/G') is smaller than 0.1 for typical polysaccharide gels. The increase of G' values were also related with the rate of amylose and amylose-gum associations that occurred during cooling. The increase of G' cannot be attributed to the aggregation of amylopectin because amylopectin retrogradation occurs over long time periods. From the rheological point of view, the decrease of tan ä corresponded to a more solid like behavior, that is, a stronger three-dimensional network was constructed by amylose and amylosegum system.

Conclusion
The formulation 20:60:20 at 90°C increased viscosity and viscoelasticity of broken rice flour: foxtail millet flour: maize flour suspensions during and after heating, respectively. The ionic interactions between starch and protein molecules play an important role on these properties. The strong electrostatic interactions between three temperatures and five formulation combination flours resulted in an instantaneous aggregation of granules. These observations can be related to the increase in swelling power, decrease in solubility index of broken rice flours, decrease in peak viscosity, and loss tangent (tan ä) but increase in pasting temperature and rheological parameters G' and G" of formulation 20:60:20 at 90°C with respected to the other temperature and formulation combinations.