In Vivo Digestibility of Rice Prolamin/Protein Body-I Particle Is Decreased by Cooking

Abstract

Rice is a staple cereal and a major source of both protein and energy in most Asian countries. Thus, the true digestibility of cooked rice protein is very important for human nutrition. The proteins in rice endosperm consist mainly of storage proteins, e.g., glutelin, prolamin and globulin. Two major proteins, glutelin and prolamin, can be separated into several protein bands by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Glutelin separates into three bands: proglutelin (57 kDa), an acidic subunit (37-39 kDa), and a basic subunit (22-23 kDa) (1). Prolamin also separates into three bands: 16, 13 and 10 kDa prolamin (2). These storage proteins accumulate in organelles called protein bodies (PBs). Rice endosperm has two different types of PBs; prolamin accumulates in type-I protein bodies (PB-I), while glutelin and globulin accumulate in type-II protein bodies (PB-II) (3). Tanaka et al. (4) reported that fecal protein particles (FPP), such as PBs, were excreted in the feces of humans who had ingested cooked rice (CR). Ogawa et al. (2) demonstrated that the 13 kDa prolamin band in raw rice (RR) remained after pepsin digestion, although the glutelin subunit bands (37-39 and 22-23 kDa) disappeared. These results imply that prolamin is an indigestible protein. However, we recently reported that structural changes induced by alkali extraction can improve the in vivo digestibility of prolamin (5, 6). A study by Eggum et al. showed that cooking decreased the true protein digestibility (TD) of rice from 99.7 to 88.6% (7). However, no reports have directly demonstrated whether rice prolamin is rendered indigestible in vivo by cooking.In the present study, we monitored the gastrointestinal (GI) transit and fecal excretion of rice glutelin and prolamin in rats by using anti-23 kDa glutelin (23G) and anti-13 kDa prolamin (13P) antibodies to estimate their digestibility in CR and RR. Additionally, morphological observations of PBs in the feces of rats fed the CR or RR diets were conducted using electron microscopy to confirm the effects of cooking on PB structure. Although the digestibility of 23G was not affected by cooking, the digestibility of 13P was clearly reduced. Almost intact PB-Is were observed in the feces of rats fed the CR diet. Our results clearly indicate that rice prolamin is not indigestible in its raw form, but is rendered indigestible by cooking. Materials and MethodsMaterials and animal experiment. Rice (Koshihikari, 10% polished fraction) was purchased from Niigata Seifun Co., Ltd. (Tainai, Japan). RR flour was prepared using a jet mill (KV-3-4S, Yakushinkikai Seisakusho, Japan). CR flour was prepared by rinsing the rice first and then cooking it in twice its volume of water in an electric rice cooker (JAG-A100 TM, Tiger Co., Ltd., Osaka, Japan). Summary Rice has storage proteins, e.g., glutelin, globulin and prolamin, in the seeds, which are used as nitrogen sources during germination. Rice prolamin has been reported to be an indigestible protein that decreases the nutritional ...

Effect of domestic cooking on rice protein digestibility

“…This condition might prove the hypothesis of Kubota et al. (), who suggested that cooking might lead to hydrophobic interactions and disulfide linkages in PB‐Is. Rice protein solubility in the solvent without urea was almost constant during cooking processes, suggested that changes of hydrogen bonds did not occur during cooking‐induced protein interactions despite its important role in the native structure of rice protein.…”

“…Kubota et al. () also reported that rice prolamin indigestibility was associated with PB‐I, which could be degraded or weakened by alkali, and thus increased protein digestibility of rice prolamin (Kubota et al., ). Therefore, in the present study, the hydrolysis of the 16 kDa prolamin might be due to the IEF buffer corrosion on PB‐I structure.…”

“…Kubota et al. () assumed that rice PB‐I structure can be strengthened as a result of hydrophobicity interactions and disulfide bonds during cooking, possibly reducing rice protein solubility. However, in this study, cooking‐induced formation of disulfide bonds reduced rice protein digestibility, whereas similar result was not detected in hydrophobicity interactions occurring during cooking.…”

“…In some Asian countries, pre‐cooking processes, such as washing and soaking, are performed to remove remaining dust, hull, and bran on the rice kernel surface and improved cooking quality (Yu, Turner, Fitzgerald, Stokes, & Witt, ). However, cooking, especially heat‐moisture treatment, can reduce protein digestibility of many cereals, such as rice (Kubota et al., ), wheat (Wu, Taylor, Nebl, Ng, & Bennett, ), sorghum (Vu, Bean, Hsieh, & Shi, ), and millet (Gulati et al., ). Factors affecting cereals protein digestibility can be categorized into exogenous and endogenous factors.…”

“…Kubota et al. () found that rice prolamin/protein body‐I (PB‐I) cannot be indigestive in cooked rice. However, Zhang et al.…”