Studies On Isolation Process And Physicochemical Properties Of Rice Starch

Rice starch is unique in its small granule size,large specific surface area,white color and good digestibility.To develop an efficient,environmental and economical isolation process of rice starch from milled rice is the precondition to product high quality rice starch in industry. To study the relationship between rice starch functional properties and molecule structure and develop the rice starch products with high qualities can provide theoretical basis for rice starch investigation and guide further processing and application of rice starch.The physicochemical properties of rice starches isolated from Alkaline steeping(NaOH), sodium dodecyl sulfate(SDS) and enzymatic process were compared.The rice starch isolation process from milled rice with low damage to starch granules was established.The pasting property measurement was set up using a rotational rheometer instead of Rapid Visco Analyzer(RVA).The results indicated that the pasting curve of the rice starch from enzymatic isolation was similar to that of native rice flour,while the pasting peak temperature of the starches isolated from NaOH or SDS treatments was significant lower than that of native rice flour,which suggested that the nature structure of rice starch granules was retained using enzymatic process.The starch pastes from enzymatic treatments were more elastic and viscous at low shear stresses.The damaged starch content showed that the rice starch from enzymatic isolation was around 2%,while it was from 2.4-3.9%for the starches isolated from NaOH or SDS treatments.The enzymatic isolation also produced the smallest granule size of rice starch,which was around 5μm.The efficiency of starch isolations from milled rice with different protease preparations was evaluated.Alcalase and Protease N "Amano"(Protease N) were found to provide for a better separation of the rice starch from protein.The preferred condition for producing the rice starch with low residual protein content was 60 units/mL Cellulase pre treatment for 2 h followed by the treatment with 100 units/mL Alcalase at initial pH 10.5 or 100 units/mL ProteaseN at initial pH 8.5 and digestion for 4 h at 50℃.The residual protein content was 0.53%with the damaged starch content of 2.01%for Alcalase treatment,and 0.52%residual protein content with 1.86%damaged starch content for Protease N treatment.There were no notable differences in terms of the thermal properties among isolated rice starches obtained from different protease treatments as determined with DSC.The granule size of the rice starches isolated with the two proteases was smaller than that of the native rice flour and no apparent surface damage was found.In evaluating the pasting quality of the starches,it was found that Protease N treatments produceu rice starch with similar pasting viscosities to the native rice flour,while the trough and final viscosities of 0.1 Pa.s and 0.24 Pa.s were dramatically lower than those of rice flour,which were 0.3 Pa.s and 0.57 Pa.s,respectively.The effects of Alcalase and Protease N treatments on the physicochemical properties, granule structure and molecular weight distributions of rice starches with different ratios of amylose to amylopection were studied.The rice starches produced from Protease N exhibited higher pasting trough and final viscosities than those produced from Alcalase.The hot pastes of the starches produced from Protease N also showed higher elastic moduli,zero-order Newtonian viscosities and yield stresses than those produced from Alcalase.No differences were found in the crystalline pattern,thermal properties,granules appearance,and average molecular weight(Mw) of the rice starches between the two proteases treatments.It indicated that neither Alcalase nor Protease N would damage the structure of the rice starch during the isolation process.But the Mw of the pasted starch produced from Protease N was significantly higher than that produced from Alcalase.When additional protease was added to the isolated starches and the mixture pasted,the Mw of the starches pasted with added Alcalase was 47-92%lower than that of the pasted starches without protease.The reduction in molecular weight suggested that Alcalase had modified the starch molecules,hydrolyzed part of the starch molecule during pasting,resulted in the reduce in pasting viscosity and elastic moduli.Although protease treatments provide an effective method for the purification of native rice starches,commercial protease should be selected carefully in order to avoid the damage to the starch structure by the contamination in the protease during or after isolation process.The pasting properties of rice starches isolated from protease treatment were studied.The effects of starch granules and leached amylose on the viscosity of rice starch dispersions during heating were investigated.It was found out that starch granule swelling increased rice starch dispersion viscosity during heating.The viscosities of the starch dispersions during heating were principally dependent of granular volume fraction and independent of starch variety and leached amylose.Granular swelling was necessary before amylose leaching.High initial amylose concentrations in starch granules reduced swelling during heating thereby reducing rice dispersion viscosities.Amylose played an indirect role in affecting rice dispersion viscosity during heating by reducing granular swelling.The change of crystallinity of rice starches during pasting was investigated using attenuated total reflectance Fourier-transform IR spectroscopy(FTIR) and X-ray diffraction(XRD).The results indicated that the loss of crystallinity for the starch granules was significant when the temperature reached the pasting onset temperature.The amylose would inhibit the disruption of the crystalline areas of rice starch granules during pasting.Although the definitions of "crystallinity" were different for FTIR and XRD,the results were correlative and comparable and could provide useful information for the study of starch pasting.Aqueous leaching of amylose from rice starch at different temperatures during pasting was carried out using rotational rheometer.The molecular structure of amylose from rice starch was analyzed by a high performance size exclusion chromatography system.The range of average molecular weight(Mw) of amylose from different rice starches is from 3.65×10~5 to 4.48×10~6.There are no significant differences in Mw and RMS of the amylose leached at different temperatures.The Mw and RMS correlated negatively to the amylose content in the rice starch.The application of rice starch to edible film was investigated.It indicated that the water vapor and oxygen permeability,the solubility the tensile strength and G' of rice starch-based films were correlated to the amylose content in the rice starch of the rice starch edible films were natively correlated to the amylose content in rice starch.Both water vapor and oxygen permeability were smaller of rice starch-based films as compared to those of corn or potato starch-based films.The solubility of rice starch-based films was higher than those of corn or potato starch-based films.But the tensile strength and elastic module of rice starch-based films were lower as compared to other starch-based films.In order to improve the mechanical properties of rice-starch based film,the rice starch was heat-treated in a dry state after being impregnated with carboxymethylcellulose(CMC).The modified rice starch-based films showed higher tensile strength and elastic module and lower water vapor and oxygen permeability than other modified starch-based films.In order to address the reaction mechanism between the rice starch and CMC,pasting properties and particle size distributions of rice starches before and after modifications were determined.It indicated that the starch particle size increased after heat treatment with CMC.The ester bonds were supposed to be formed between the hydroxyl groups in amylopectin branches of rice starch and carboxylate acid groups of CMC.The change in pasting properties of the modified rice starch depended on the reaction level of the combination between the starch and CMC.It suggested that crosslinkage occurred between the starch and CMC.The crosslinkage level depended on the amount of CMC and starch,which participated in the forming of ester linkage,as well as the viscosity of CMC.The heating process with different types of CMC at different concentrations could be used as a modification method for rice starch and provide desirable properties of starch-based films.