| Rice starch retrogradation restrict the development of food industry. Rice starch is the major components of rice foods, the retrogradation and textural properties of rice foods are depended upon rice starch retrogradation. Up to now, a lot of works were focused on rice starch retrogradation at low and room temperature, however, the rertogradaion of rice starch precooling with low and ultra-low temperatures has not been studied in detail. Therefore, in this paper, these projects will be investigated: (1) Physicochemical properties of rice starch and rice flour, (2) The retrogradation properties and retrogradation mechanism of rice amylopectin, rice starch and flour by low and ultra-low temperature precooling, (3) The effects of low and ultralow temperature precooling on the quality and retrogradation properties of cooked rice. The work in this dissertation is devoted to help the food industry to produce high quality instant ready meal.We investigated the physicochemical properties and correlation of starch and flour of intermediate amylose content rice (SM), low amylose content rice (DH), very low amylose content rice (TML) and waxy rice (SN2). The results of SEM indicated that the granule of rice starch and flour was polygonal shape, the size was 2~8μm. The X-ray results indicated that the rice starch and flour was A pattern, the strong peak of 2θwas 15°, 17°, 18°and 23°. The results of swelling power, solubility, turbidity, pasting properties, gel textural properties and retrogradation enthalpy were correlated to the ratio of rice amylose to amylopectin.The higher the ratio value, the higher of the turbidity, pasting temperature, hardness and gelatinization enthalpy, however, the lower of swelling power and solubility. The retrogradation properties of rice amylopectin precooling with low and ultra-low temperatures were investigated by Differential Scanning Calorimeter (DSC).Rapid cooling can retard amylopectin retrogradation effectively, and the rice amylopectin didn't retrograde after precooling immediately. However, the rice amylopectin retrograded rapidly within 1 day, the retrogradation enthalpy continuely increased from 1~11 days, and the rice amylopectin of SM, DH and TML reached the highest retrogradation enthalpy values of 11.0 J/g (SM), 7.0 J/g (DH) and 10.0 J/g (TML) in the 11th day, respectively. From 14 to 21 days, the retrogradation enthalpy of rice amylopectin changed slowly during chill storage. The retrogradation process of waxy rice (SN2) amylopectin was very different with SM, DH, and TML. The SN2 amylopectin retrograded slowly in 0~11 days, increased rapidly from 11 to 14 days, and reached the highest retrogradation enthalpy value of 9.0 J/g in the 21th day of chill storage. Furher study indicated that low and ultralow temperature precooling and frozen effectively retrard amylopectin retrogradation, there was no amylopectin retrogradation when amylopectin frozen for 1~5 months. The retrogradation properties of rice starch precooling with low and ultralow temperatures were investigated by DSC and texture analyzer (TA). Rice starch retrograded rapidly from 1 to 7 days, increased slowly from 7 to 14 days, and the retrogradation process finished within 21 days during chill storage. Low and ultra-low temperature precooling and frozen effectively retrard rice starch retrogradation, there was no starch retrogradation when frozen for 1~5 months. The textural properties of rice starch increased rapidly from 1 to 7 days, increased slow from 7 to 21 days, and the textural properties changed slowly during frozen storage. The amylose-lipid complex was not changed during chill or frozen storage, and the properties of rice amylose-lipid complex were related to amylose variety and molecule structure.The retrogradation properties of rice flour precooling with low and ultra-low temperatures were investigated by DSC and TA. The rice flour retrograded rapidly from 1 to 7 days, retrogradation enthalpy increased slowly from 7 to 14 days, and the retrogradation process was finished within 21 days during chill storage. The hardness and adhesiveness of rice flour paste changed rapidly from 0 to 7 days, changed slowly from 7 to 14 days, and changed slowly from 14 to 21 days. The resuts indicated that low and ultra-low temperature precooling combined with frozen method can effectively retard rice flour retrogradation, there was no starch retrogradation when frozen for 1~5 months. The amylose-lipid complex in rice flour was not changed during chill or frozen storage.Rcie amyloepctin, rice starch and flour retrogradation kinetics were accorded with Avrami equation. The SM amylopectin, rice starch and flour retrogradation were instant crystallization. The SN2 amylopectin, rice starch and flour retrogradation were spontaneous crystallization process. The DH and TML amlopectin, rice starch and flour retrogradation were both instant crystallization and spontaneous crystallization, which were depended upon freezing or cooling rate. The retrogradation of rice starch and flour mainly caused by amylopetin and amylose retrogradation, the two process contributed to rice starch and flour retrogradation.The effects of low and ultra-low temperature precooling on the eating quality and retrogradation properties of cooked rice were investigated. Rice starch retrogradation contributed to hardness increase and adhesiveness decrease of cooked rice, higher amylose content of cooked rice had higher hardness and low adhesiveness value. The textural properties of cooked rice were related to starch retrogradation and cooling rate. Rapid precooling could effectively retard cooked rice staling and textural properties loss during a short time chill storage. Freezing rate and storage temperature showed significantly effects on retrogradation and textrual properties of cooked rice, the textural properties of cooked rice were positively related to freezing rate and starch retrogradation. Rapid freezing (1.45℃/min) can produce a high quality of cooked rice, i.e. lower hardness and higher adhesiveness value, however, slow freezing increased the staling of cooked rice. The advantages (lower hardness and higher adhesiveness, less starch retrogradaton) of cooked rice gained by rapid freezing, were lost quickly within 3 days of storage at 4℃. However, rapid freezing combined with -18℃frozen storage can effectively retard starch retrogradation and maintain the textural properties of cooked rice for at least 7 months. Therefore, high quality cooked rice can be produced by combined rapid freezing with frozen storage. The work in this dissertation is devoted to help the food industry to produce high quality instant cooked rice.
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