Retrogradation Mechanism Of Rice Starch Mixtures And Extruded Rice Pellets

Retrogradation of starches is vitally important topic, particularly inthis age when foods are prepared and stored for length periods of time.Physicochemical properties of native and physically modified rice starcheswere investigated by differential scanning calorimetry, dynamic rheometer,X-ray diffraction, texture analyzer, gel permeation chromatography, andBrabender Micro viscoamylograph. The results are highlighted in thepresent work.One attempt in the present study (section two) was the mixing (atvarying blending ratios) of different starches from different rice cultivarsto clarify the rheological changes and their behavior in terms ofretrogradation potency. Firstly, starches isolated from four rice cultivarswith well-defined physicochemical properties, classified as low,intermediate, and high apparent amylose content (AC) and common waxywere chosen to cover a broad range in AC and thermal characteristics.Waxy starches influenced the rheological properties of mixedstarches and the increase in storage modulus (G') was lower in commonwaxy starch and in some of its blends. These results show that TG'max ,G'5 an tan δ5 are certainly governed by AC. Depending on the proportion ofthe native rice starch in the mixture, the G' peak maxima and G'5 for ricestarch blends were generally between those observed for their individualstarches components i.e. they showed an intermediate change in gelrigidity upon heating and cooling. Waxy blends of 25% were mosteffective at reducing the rate of firmness as shown by the values ofhardness when mixed with low and intermediate AC nonwaxy starches. Asaging progressed over several days, significant hardness was observed inthe individual components samples compared with their blends. Storagemodulus (G') was always higher than loss modulus (G'').Incorporating waxy rice starch (25%) significantly decreased the AC.AC correlated negatively with swelling power (SP) (r = -0.925, P<0.01).SP exhibited non-linear relationship (r2=0.820) with water solubility (WS)and both increased with temperature. The correlation showed that WS isalso an index of starch characteristics and the granules rigidity affectedthe granule swelling potential. The results suggest that the leached-outamylose from non-waxy rice starches in the mixture was likely entrappedin the network of solubilized waxy starch and we concluded that fewmolecules were available for recrystallization.______________________________________________________________________________________The third part of this work discusses the physical modification ofstarch with particular reference to extrusion cooking with the aim ofchanging the properties and the integrity of the starch granules, in orderto modify its behavior in terms of gelatinization, pasting, andretrogradation. A response surface methodology (RSM) was firstly used tostudy the effect of extrusion process parameters (screw speed, barreltemperature, and feed moisture content) on some functional, physical,pasting, and digestibility characteristics of rice extrudates. Viscosityvalues of extruded rice flours were far less than those of theircorresponding unprocessed rice flour indicating that the starches had beentotally or partially pregelatinized by extrusion process. Peak viscosityindicated a high positive correlation with hot paste viscosity (HPV) andcold paste viscosity (CPV) with r>0.700, (P<0.01).The effects of processing on nutritionally starch fractions weretested for in vitro digestibility using controlled enzymatic hydrolysis withα-amylase and glucoamylase in rice extrudates. The starch-digestion ratevaried depending mainly on the processing conditions. Rapidly digestiblestarch (RDS) was found to correlate negatively with slowly digestiblestarch (SDS) (r = -0.964, P<0.01) and with resistance starch (RS) (r =0.793, P<0.01), respectively. Whereas SDS correlated positively with RS(r = 0.712, P<0.01). Such information was useful for further studies topredict and to optimize the expected performance of extruded materials innovel foods development.Further studies were conducted on extrusion cooking of rice flouras raw material fortified with soy protein concentrate (SPC) to improve itsnutritional value. The blends were processed in a twin-screw extruderwith factorial combinations (2 level factorial designs). Addition of SPC intorice flour has increased the protein content of the extrudates i.e. theprotein content of rice flour increased by about 34-80%.Rheological properties in terms of steady state flow behavior ofextruded dispersions (rice flour / Soy Protein Concentrate blend) wereinvestigated using dynamic rheometry. The effect of concentration (2, 5, 7,9 and 11 %) and temperature (25-70?C) on the rheological parameters(yield stress, flow behavior index) of non-expanded pellet blend weredetermined using common rheological models and the extent of fitting to amodel was judged by finding the lowest standard error. From typicalcurves on the dependence of shear stress on shear rate it could beobserved that all suspensions exhibited a non-Newtonian andpseudoplastic behaviour. The model that best fitted the experimental dataat all temperatures and concentrations was the Herschel-Bulkeley model.Differential scanning calorimetry studies of gelatinization andretrogradation revealed that extrusion reduced the onset temperature (To),peak (Tp) and conclusion temperatures (Tc). The retrogradation kinetic ofuntreated rice flour (E) and three types of extruded blends of riceflour/soy protein concentrate (SPC) (namely, sample A, B and C) wasinvestigated by differential scanning calorimetry (DSC) during storage at4?C for 25 days. The results showed that enthalpy of retrogradation (?H)was highest for the untreated rice flour and the rate of retrogradation inextruded blends should be likely attributed to the extent of starchdegradation as a result of extrusion cooking. Thermal analysis revealedthat extruded blends exhibited a slower increase in crystallinty with timecompared to their counterpart native rice flour. Maximum retrogradationwas 65% after storage at 4?C for 25 days. The setback (SB) tendency ofthe native starch was reduced significantly after extrusion cooking.The values of Avrami exponent (n), which is a combined function ofthe modes of nucleation and growth of crystals, and the rate constant (k)were used to describe the mechanism of starch retrogradation. A possiblemechanism of retrogradation was presented while different retrogradationbehaviors were likely due to smaller size amylopectin fragments,narrowed linear chain length distribution and decreased branching. Weassume that the solubilized sugars molecules may also interact with starchmolecular chains to stabilize the amorphous and entangled matrix ofgelatinized starch.Molecular degradation of starch during extrusion cooking wasstudied by GPC on Sepharose CL-2B (as the gel filtration medium). Twomain fractions namely amylopectin (Fr-I), which eluted first due to itslarge size and amylose (Fr-II) should be distinguished from the elutionpattern. The absorption maxima of iodine complex (λmax) of the Fr-Ishowed an increase while that of Fr-II exhibited a decreasing dueprobably to the increase of branched structures. Kav Fr-II was lower thanthat of native rice flour.In vitro starch digestibility varied between 70-85% and increasedwith the barrel temperature and with decreasing SPC ratio. Proteindigestibility using a multi-enzyme method ranged 67-72%. Modelsdeveloped for the indices gave statistically a good fit to experimental datawith R2 adj. and R2 higher than 85%. This suggested that the optimalprocess variables for the production of simulated rice grain with enhancednutrition and non spongy structure from a rice-soy protein concentrateblend are high moisture of 30-34%, screw speed evaluated from 60-80rpm, temperature 90-110?C and final protein content 10% in the mixture.The linear components alone significantly explained most of thevariation in responses examined. Increasing SPC content affected thevarious shades of color in the product. The loss of solids in gruel uponcooking of the developed instant product was estimated between 5 and 6%without presoaking prior cooking. It was observed that hardness wascorrelated negatively with most of amylograph viscosities PT, PV, HPV,CPV, and BD (r>0.450), and positively with SB (r=0.566). However, therehydration rate of extruded simulated rice pellets decreased with thestorage time suggesting that retrogradation of amylopectin occurred. Theextruded simulated rice pellets developed can offer preparation flexibilityto suit the consumer in hurry even in limited preparation facilities.The process developed in the present study can be easily andconveniently scaled up to industrial needs.