優化米糠蛋白鹼萃取製程以提升其功能性

Abstract

食品中的蛋白質決定其質地與結構，而蛋白質在食品系統中的水溶性與加工功能性高度相關。碾米過程生產之副產物米糠，為常用萃取植物性蛋白質之原料，其產量豐富且成本低廉，所萃取出之蛋白質營養價值與生物利用性高，然而其中高含量之脂肪與植酸，易對米糠蛋白之萃取與應用造成限制。米糠蛋白一般以鹼萃取之方式製備，於中性環境水溶性約為 20%。故本研究將優化米糠蛋白萃取流程，以期能提升蛋白質水溶性與功能性。首先調整蛋白質鹼萃取溫度並去除中性環境不溶物質，製備高溫鹼萃取米糠蛋白 (high temperature alkaline extracted protein, HTE)、室溫鹼萃取米糠蛋白 (room temperature alkaline extracted protein, RTE)、高溫鹼萃取水溶性米糠蛋白 (high temperature alkaline extracted soluble protein, HTES) 與室溫鹼萃取水溶性米糠蛋白 (room temperature alkaline extracted soluble protein, RTES)，並評估不同製程所得蛋白質之水溶性。RTES 在 pH 7 時，水溶性為 49.11%，是四者中最高，顯示進行室溫鹼萃取並去除中性條件不溶物質，可提高米糠蛋白之水溶性。接著為優化米糠蛋白製程，以前述結果為基礎，在鹼萃取前先去除米糠中部分磷化合物，製備去磷高溫鹼萃取米糠蛋白 (phosphorus removed high temperature alkaline extracted protein, PRHT)、去磷室溫鹼萃取米糠蛋白 (phosphorus removed room temperature alkaline extracted protein, PRRT)、去磷高溫水溶性米糠蛋白 (phosphorus removed high temperature alkaline extracted soluble protein, PRHTS) 與去磷室溫水溶性米糠蛋白 (phosphorus removed room temperature alkaline extracted soluble protein, PRRTS)。PRRTS 於 pH 7 之水溶性高達 60.81%，表示米糠先經過去磷處理，並於室溫鹼萃取再去除中性條件不溶物質，可得到水溶性最佳之米糠蛋白。而後評估八種米糠蛋白之萃取成效，PRRTS 蛋白質含量與回收率較低，分別為 57.59% 與 34.37%，若以提高米糠蛋白之含量與回收率為目標，蛋白質含量為 66.66% 且回收率為 39.33% 之 RTES 為最佳。雙硫鍵數量與表面疏水性測定結果以 PRRTS 最高，故推測其水溶性之提高與蛋白質組成最為相關，並未受蛋白質結構與表面特性影響。最後，分析不同萃取方法之米糠蛋白組成、理化性與功能性，PRRTS 含植酸 10.88 mg/g，為八種處理中最低，其蛋白質乳化性與乳化穩定性、起泡性與起泡穩定性及保油力皆最高；RTES 之植酸含量為 12.22 mg/g，整體功能性僅次於 PRRTS。此二種製程可生產高水溶性與功能性之米糠蛋白，未來可以本研究成果為基礎，進行米糠蛋白應用於食品系統之研究，以提升米糠之經濟價值與產業效益。

In food systems, proteins perform different roles by adding certain functional qualities to them, and also provide food with structure and texture. The solubility of food proteins is essential to food industry applications. Rice bran, with abundant production and low cost, is a by-product that derives from rice milling, and rice bran protein has high nutritional value and bioavailability. However, the high content of lipid and phytic acid in rice bran limits its extraction and application. Rice bran protein is generally prepared by alkaline extraction, and its water solubility is about 20% in neutrality condition. In this study, we modified rice bran protein extraction procedure in order to improve its water solubility and functionality. First, we adjusted the alkaline extraction temperature and removed insoluble components under neutral condition. High temperature alkaline extracted protein (HTE), room temperature alkaline extracted protein (RTE), high temperature alkaline extracted soluble protein (HTES) and room temperature alkaline extracted soluble protein (RTES) were prepared, and their water solubility were evaluated. RTES showed the highest water solubility under pH 7, which was 49.11% and it indicated that extracting by alkaline in room temperature with insoluble components under pH 7 removed could enhance the water solubility of rice bran protein. Based on the results of the first section, we further optimized the rice bran protein extracting process. Before alkaline extraction, we partially removed the phosphorus in rice bran, and prepared phosphorus removed high temperature alkaline extracted protein (PRHT), phosphorus removed room temperature alkaline extracted protein (PRRT), phosphorus removed high temperature alkaline extracted soluble protein (PRHTS) and phosphorus removed room temperature alkaline extracted soluble protein (PRRTS). The water solubility of PRRTS reached 60.81% under pH 7, which revealed that the rice bran protein with highest water solubility could obtained by removing phosphorous in rice bran before room temperature alkaline extraction, and then remove insoluble components in neutral condition. Next, the protein extraction efficiency of all eight kinds of rice bran protein was determined. The protein content and protein recovery of PRRTS were lower than RTES, which were 57.59% and 34.37%, and that of RTES were 66.66% and 39.33%, respectively. On the other hand, the disulfide bond contents and the surface hydrophobicity of PRRTS were the highest, thus we speculated that the improvement of rice bran protein water solubility was mainly attributed to the composition and properties of its proteins, which was less relevant to protein structure and surface characteristics. Furthermore, the protein composition, physical and functional properties of the rice bran protein were analyzed. The content of phytic acid in PRRTS was the lowest among rice bran protein extracted by different procedures, which was 10.88 mg/g, and PRRTS also performed better emulsifying properties, foaming properties, and oil holding capacity. The phytic acid content of RTES was 12.22 mg/g, and its functionalities was second to PRRTS. From these two extraction processes, rice bran protein with improved water solubility and functionality could be obtained. Further researches about the application of rice bran protein in food systems could be conducted base on the results of this study, which would have the potential to enhance the economic value of rice bran and improve its industrial competitiveness.