Production of rice protein by alkaline extraction improves its digestibility

Rice seed endosperm has two types of protein bodies (PB). Type I protein body (PB-I) accumulates prolamin and is hard to digest, while type II protein body (PB-II) mainly consists of glutelin, an easily digestible protein. A simple method to process rice protein and improve its digestibility was tested from the viewpoint of its application to food manufacturing. Rice protein prepared by alkaline extraction followed by neutralization sedimentation (AE-RP) was compared with that prepared by starch degradation by alpha-amylase (SD-RP). The crude protein content of AE-RP and SD-RP was 84.7% and 78.2%, respectively. There were no major differences in protein composition among AE-RP, SD-RP and rice flour by SDS-PAGE, except 16 kDa polypeptide. With respect to amino acids, all the groups showed quite similar compositions, although cysteine and methionine were lower in AE-RP. In an in vitro digestion study with pepsin and pancreatin, both the SDS-PAGE analysis of protein pattern and the crude protein content of undigested residue clearly demonstrated that AE-RP has a higher digestibility than SD-RP. To find the cause of the difference in digestibility, the structural property of protein bodies by two production methods was compared using electron microscopy. PB-II of AE-RP was transformed into small, amorphous granules, while that of SD-RP was still kept partial protein body structures. PB-I of AE-RP kept its protein body structure, but produced double layers. From the finding that glutelin-gold was detected by immunochemistry not only in small, amorphous granules but also in PB-I, mainly the cortex layer, in AE-RP, it became clear that PB-I was swollen and fragile as a result of alkali treatment. These results strongly indicate that the improvement in digestibility of AE-RP is a result of the structural change of PB-I and -II caused by alkaline extraction.