Effect Of Freezing And Thawing On The Properties Of Rice Starch And Its Control

Frozen sweet dumplings constitute main frozen wheat and rice products in China. Andsweet dumplings may crack after freezing and collapse during storage as well as occur muddysoup and poor taste when cooking. Several methods have been designed to improve thequality, for instance focusing on the addition of additives, such as hydrophilic colloid,modified starch, emulsifier and so on. The largest component of sweet dumplings is ricestarch, but little was considered from the properties change of rice starch to think about thedeterioration. Thus, samples of rice starch-water suspension were frozen at-18~-30℃andthawed at room temperature. After that soluble solids content of supernatant, particlemorphology, small and wide angle X-ray diffraction, pasting characterization, thermalcharacterization, and other physicochemical properties were determined, thus generatingtheoretical knowledge for quality controlling of sweet dumplings and other rice basedproducts.After starch-water suspension (starch to water ratio was1:1.5, w/v) was treated bymultiple freezing/thawing, soluble solids content of supernatant increased, after10freezing-thawing cycles, the value of waxy rice starch increased from0.13%to0.43%(of dryweight), and the value of indica rice starch increased from0.25%to1.22%(of dry weight);though the rice starch crystal type didn’t change, staying as A-type, the relative degree ofwaxy rice starch reduced from25.19%to21.34%and the relative degree of indica rice starchreduced from21.41%to16.14%, the relative degree of waxy rice starch was higher thanindica rice starch; the initial gelatinization temperature decreased by0.3～0.67℃, the pasteviscosity (hot paste viscosity, keeping paste viscosity, cold paste viscosity) also decreased; theglass transition temperature of frozen-thawed rice starch was5℃higher than native ricestarch. But between freezing-thawing cycle, there was minor difference; damaged starchcontent decreased.Also, after starch-water suspension (starch to water ratio was1:1.5, w/v) was treated byfrozen storage, soluble solids content of supernatant increased. After storing210d, the valueof waxy rice starch increased to1.23%, the value of indica rice starch increased to1.40%(ofdry weight), soluble solids content of frozen storage7days matched with frozen-thawed1time; rice starch crystal type still stayed as A-type, the relative degree of waxy rice starchreduced to21.87%and the relative degree of indica rice starch reduced to21.00%. Thereducing range of storage relative degree was less significant than frozen-thawed treatment;the initial gelatinization and thermal parameter both decreased; the glass transitiontemperature of frozen stored rice starch was5℃higher than native rice starch. But there wasno significant difference when rice starch was treated with more time.Higher water content of rice starch was, more significant the properties of rice starchchanged. Soluble content of supernatant was higher aligned with higher water content. Crystaltype of rice starch still stayed as A-type, the relative degree of crystallinity increased from25.19%to32.47%and31.65%(30%and40%w/w±, respectively). Higher water content ofrice starch was, both the gelatinization and the thermal parameter was lower; the glass transition temperature of moistured rice starch was3℃higher than native rice starch. Anddamaged starch content was lower with higher water content.Analyzing the soluble solids of content, it was found that the formation and melting ofice crystals may lead to the form of micro mechanical force. The force may expand theinternal passages of starch molecules and weaken the double helix structure of starchmolecules, resulting in leaching of fat, protein, amylose and amylopectin. Therefore increaseof soluble solids may reflect the properties change in some degree. After treating, the edges ofthe starch granules were injured, and with the increasing of freezing-thawing cycles, theparticle surface appeared more pit.By adding simplex additive can improve the properties of rice starch influenced byfreezing and thawing. The improving effect was0.1%guar gum>0.2%monoglyceride>0.2%CMC-Na>0.15%composite phosphate. And the most effective additive was0.1%guargum.