Comparison of Protein Extraction Solutions for Rice Starch Isolation and Effects of Residual Protein Content on Starch Pasting Properties

Rice starch is used as an additive in various food and industrial products. With the inherent merits of small and uniform size distribution of rice starch and its white color and clean odor, deserts and bakery products are some of the favorable applications among processed foods [1]. The small granular size of the starch imparts it soft texture, high water retention and low syneresis, suggesting the possibility in using rice starch as fat substitute [2]. Rice protein in the endosperm, however, tightly associates on the surface of starch granules and the difficulty in removing the protein makes the starch isolation more costly compared to other starches [3]. Rice protein which accounts for 7–8 % (DB) in the milled rice kernel is classified into four types: alkali-soluble glutelins (80 %), water-soluble albumins (9–11 %), salt-soluble globulins (7–15 %) and alcohol-soluble prolamins (2–4 %) [4]. Among those, albumin and globulin existing in the aleurone layer are usually removed during milling. But heterogeneous large molecules of glutelins exist inside the rice endosperm in the forms of protein bodies [5–8]. These spherically shaped protein bodies bind strongly to the compound starch granules with strong disulfide bonds and/or hydrophobic bonds [9, 10]. To isolate rice starch, alkaline solvents, surfactants or protein hydrolyzing enzymes could be used to remove rice protein from rice flour [11]. Alkaline solvents such as NaOH and surfactants such as dodecylbenzene sulfonate (DoBS) and sodium lauryl sulfate (SLS) are commonly used in the protein extraction for starch isolation. These solvents destruct the oligomeric protein structures and transform them