Mechanism of the chemical composition changes of yuba prepared by a laboratory processing method.

Yuba is a filmlike soybean food made from heated soymilk that contains oil bodies (average diameter, 270 nm), particulate protein (>40 nm; average diameter, 70 nm), soluble protein (<40 nm), and carbohydrate (molecular size). Three varieties of soybean were used to make yuba. The carbohydrate in the remaining soymilk increased sharply while lipid increased a little. The particle size distributions of oil body showed the trend that smaller oil bodies were concentrated in the remaining soymilk, and the percentage of soluble protein in whole protein increased in the remaining soymilk. These results could be explained well with diffusion theory. The temperature gradient and concentration gradient originating from the heat treatment were considered to cause the net particle diffusion from the surface to the bottom soymilk. Lipids, which mainly exist as oil bodies, are easily incorporated into yuba films because a few of these less dense droplets diffuse downward, causing the lipid concentration in the soymilk to change a little. Carbohydrate at the surface quickly diffuses downward, causing the carbohydrate concentration increase in the soymilk beneath the developing yuba. Protein (particulate and soluble) in the soymilk was intermediate between lipid and carbohydrate.