| Rice, abundant in China, has the annual yield of 130-140 million ton, of which 10-million-ton byproducts such as rice residue, bran and germ were obtained in the rice manufacturing process. Rice residue contains 35%-40% protein and rice protein is a kind of protein with high biological availability and low allergy, but the application of rice protein is limited in the food processing industry because of the poor solubility, emusifying and foaming capability. Enzymatic hydolysis is a means of modifying the properties of food protein. However, in most cases, bitter peptides are generated and some functional properties of hydrolysates such as oli holding capacity decreased by such treatment. One effective method for solving this problem is the plastein reaction, as it can not only enhance the functional properties of protein but also improve the nutritional properties and eliminate the bitterness. Effects of hydrolyzing conditions on hydrolysis of protein in rice residue, effects of hydrolyzing course on molecular weight distribution and the plastein reaction, effects of synthesis conditions on the plastein reaction were studied and molecular, functional and nutritional chracteristics of rice protein, alcalase-hydrolysate and plastein were compared. The results were as follows:1. Effects of hydrolyzing conditions on hydrolysis of rice protein were studied. Hydrolyzing degree and nitrogen recovery of hydrolysates were related to hydrolyzing conditions. The optimum temperature of hydrolysis with trypsin, alcalase and pepsin were 50℃, 60℃and 60℃, pH 10.0, 11.0 and 1.0, substrate concentration 7%, 5% and 5%, E/S 1%, 3% and 7%, and reacting time 4h, 4h and 4h, respectively, when hydrolyzing degree of rice protein hydrolyzed by trypsin,alcalase and pepsin, 54.0%, 71.4% and 57.3%, and their nitrogen recovery, 56.3%, 73.8% and 58.1%, were obtained, respectively. Hydrolyzing degree and nitrogen recovery of alcalase-hydrolysate were obviously higher than those oftrypsin-hydrolysate and pepsin-hydrolysate (P＜0.05).2. Effects of hydrolyzing course on molecular weight distribution of hydrolysates were studied. Two fractions of hydrolysates of trypsin, alcalase and pepsin were obtained through Sephadex G-25 Gel Permeation Chromatography. As the hydrolyzing time prolonged, the larger molecular weight fractions of hydrolysates by trypsin and pepsin decreased and the lower molecular weight fractions increased gradually, while lower molecular weight fraction of alcalase-hydrolysate increased remarkably. 3. Factors influencing the plastein reaction were studied (P＜0.05). The yield of the plastein reaction was obviously influenced by hydrolyzing degree of hydrolysates, type of proteases and conditions of the plastein reaction. For different substrates, the highest yield was obtained when the plastein was synthesized by pepsin. And the optimum yield, 2.59%, 17.49% and 15.22%, were obtained by pepsin with hydrolysates of trypsin, alcalase and pepsin as the substrate whose hydrolyzing degree were 54.01%, 71.37% and 69.14%, respectively. The yield of the plastein reaction using hydrolysates of alcalase and pepsin as the substrate was obviously higher than that of trypsin-hydrolysate, respectively. And the yield of plastein reaction using alcalase-hydrolysate was the highest. The plastein reaction was influenced by temperature, pH and substrate concentration remarkably (P＜0.05). The optimum yield of plastein reaction, 54.85%, was obtained when the alcalase-hydrolysate at substrate concentration of 150% (w/v) was incubated with 5% (E/S) pepsin at 65℃, pH 6.0 for 6 h.4. Molecular characteristics of rice alcalase-hydrolysate and plastein were compared. Molecular characteristics of rice alcalase-hydrolysate and plastein were studied through GPC, SDS-PAGE, HPLC and TOF-MASS. And that the plastein reaction was a process of molecular weight increase of rice protein was proved. The increase of larger molecular weight fraction and the decrease of lower molecular weight fraction were caused by the plastein reaction. And the formation of some peptides with higher molecular weight different from those of alcalase-hydrolysate was observed in the plastein.5. Functional properties of rice alcalase-hydrolysate and plastein were compared. Solubility of rice plastein changed slightly as the pH increased, and it was bigger than that of rice protein. Heating stability of plastein, about 82%, was better than that of rice protein. Heating stability of plastein under 100℃for 60min was 14.08 times as high as that of rice protein. EAI, FC and FS of the plastein were better than those of rice protein and alcalase-hydrolysate. And oil holding capability of the plastein was between that of rice protein and that of alcalase-hydrolysate.6. Nutritional properties of rice protein, alcalase-hydrolysate and plastein were compared. Amino acid composition of the plastein was different from that of rice protein and alcalase-hydrolysate. The essential amino acid such as Thr, Met, Leu, Ile, Phe and Lys of the plastein were higher than those of rice protein and alealase-hydrolysate, and the content of lysine was 1.38 and 1.27 times as high as that of rice protein and alcalase-hydrolysate. The content of essential amino acid in the plastein was most similar to that of FAO/WHO standard mode. Digestibility of rice protein was influenced slightly by the plastein reaction. Digestibility of the plastein (89.01%) was between that of rice alcalase-hydrolysate (100%) and that of rice protein (88.9%). Thus the nutritional property of rice protein could be improved by the plastein reaction.
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