It is intended to obtain a water-soluble polysaccharide material, which has such an ability to stabilize dispersion as never observed in the existing ones and from a soybean seed material and provide an acidic milk drink having a refreshing feel without stickiness by using the water-soluble polysaccharide material thus obtained. It is found out that water-soluble polysaccharides and which are excellent in the ability to stabilize the dispersion of protein particles within the pH range around the isoelectric point, are obtained by extracting a soybean seed material (in particular, okara and i.e., bean curd refuse obtained as a by-product in processing soybeans) in the presence of a sequestering agent under neutral conditions at a high pressure and elevated pressure. Then and an acidic protein food is obtained by using the same.
