低熱變性米蛋白與米糠蛋白之品質分析

現今稻米在食品工業的應用，主要以高溫澱粉酶萃取的方式生產米糖漿，而為了最佳化原料的利用，剩餘的米渣常用以製備米蛋白。然而製程上的高溫設定雖有利於米糖漿生產，卻容易使米蛋白產生熱變性，不利於米蛋白後續的加工利用。而米糠為碾米副產物，其中之米糠蛋白組成以水溶性蛋白為多，因此具備食品之應用潛力。本研究第一部分參考前人文獻中四大蛋白的熱變性溫度，搭配澱粉糊化溫度與酵素活性的檢測結果，設定低熱變性米蛋白之製程溫度為70℃。調整製程之澱粉酶濃度與反應時間，最後以凱氏定氮法得出米蛋白之蛋白質含量結果作為篩選製程條件的指標。另一方面，以 1% 澱粉酶，於 99℃ 高溫下反應1.5小時，能獲得蛋白質含量86.42%、回收率 84.03% 之熱變性米蛋白 (簡稱 RP99)。添加5% 澱粉酶，於70℃ 下反應 3.5 小時，能提取出蛋白質含量 67.85% 之低熱變性米蛋白 (簡稱 RP70)。第二部分為開發製備米糠蛋白 (簡稱 RBP) 之製程，分為酵素萃取法、鹼萃取法和酵素輔助鹼萃取法，以鹼萃取法可獲得蛋白質含量 60.29% 之米糠蛋白。第三部分比較 RP99、RP70 和 RBP 之理化特性與加工功能性差異，結果顯示RBP 之雙硫鍵數量與表面疏水性為三者中最低，RP70 較 RP99 含有較低之雙硫鍵數量與表面疏水性。RBP 於 pH7 條件下之水溶性25.27%，為三者中表現最佳，甚至優於黃豆蛋白。RP70 之水溶性較 RP99 佳，於 pH 7 條件下，RP70之水溶性 (6.72%)約為 RP99 水溶性 (3.58%) 的兩倍。本研究結果發現，熱變性程度影響米蛋白之理化特性與功能性展現。RP70 之蛋白變性程度低，理化特性與功能性較 RP99 為佳，且具備蛋白柔韌性，有助於蛋白後續修飾與加工應用。另外，RBP 之水溶性佳，具發展潛力，然而本研究生產RBP 之蛋白質含量過低，尚需後續研究提升。

關鍵字

米蛋白；米糠蛋白；植物蛋白； α-澱粉酶；低熱變性米蛋白

並列摘要

Rice is mainly used to produce rice syrup in food industry, and the remaining rice residue is commonly recycled to prepare rice protein. However, the high-temperature used in rice syrup production leads to the denaturation of rice protein, which is adverse to the subsequent processing and utilization of rice protein. Rice bran is a by-product after milling rice, and rice bran protein is composed mainly of water-soluble proteins, indicating that rice bran protein has the potential to be applied in food industry. In the first part of this study, the process temperature of less thermally-denatured rice protein (RP70) was set to 70℃ based on previous study and the assessment results of starch gelatinization level and enzyme activity under different temperature. On the other hand, amylase concentration and reaction time were optimized according to the protein content of rice protein which is determined by Kjeldahl method. The results show that thermally denatured rice protein (RP99) with the protein content of 86.42% and protein recovery of 84.03% could be obtained by adding 1% amylase and reacting at 99℃ for 1.5 hours. RP70 with protein content of 67.85% could be obtained by adding 5% amylase and reacting at 70℃ for 3.5 hours. The second part was to establish a process for preparing rice bran protein (RBP), which was divided into enzyme extraction method, alkaline extraction method and enzyme-assisted alkaline extraction method. Sixty point two nine percent of RBP was obtained by using alkaline extraction method. The physical, chemical characteristics and processing functionality of RP99, RP70 and RBP were compared in the third part of this study. RBP had the lowest number of disulfide bonds and surface hydrophobicity. The water solubility of RBP at pH 7 was 25.27%, which was the best among the samples, and even better than soybean protein. RP99 contained less disulfide bonds and surface hydrophobicity than RP70, and RP70 had better water solubility than RP99. At pH 7, the water solubility of RP70 (6.72%) was about twice of RP99 (3.58%). The results of this study indicated that the degree of thermal denaturation influenced the physical and chemical properties and functionality of rice protein. RP70, with less degree of protein denaturation, showed better physicochemical properties, functionality, and protein flexibility, which was suitable for subsequent modification and processing applications. In addition, RBP had good water solubility and showed great development potential. However, the protein content of RBP in this study needed to be improved by further research.