Because high temperature will denature and inactivate protein in jujube, denatured protein will precipitate out of the solution. Because the concentration of protein is not very high, and it has a certain viscosity, it will not sink, but form a foam floating on hot water.

To remove the foam, just add cooking oil and stir. Theoretically, this white foam is saponin (also called saponin), which is beneficial to human body. It is a special glycoside widely existing in the plant kingdom. Its aqueous solution can produce soap-like foam after shaking, hence the name.