PURPOSE: To make the foaming ability of an emulsifying agent for a food containing a monoglyceride as an active constituent keep long, by incorporating a specific amount of a trans acid in a constituent fatty acid.

CONSTITUTION: The crystal of a solid monoglyceride transforms generally from the liquid to the α→β'→β form, and among them the α-crystal or hydrated α- crystal gel has improved foaming ability. If the monoglyceride transforms to the β-crystal by the change with time, the foaming ability is extermely reduced. A monoglyceride containing 45% or more, based on the constituent fatty acid, trans acid needs to be contained as an active constituent to correct the disadvantages of the emulsifying agent containing the monoglyceride. The crystal transformation of the monoglyceride depends on the type and chain length of the constituent fatty acid, and generally the transformation rate is lower with increasing chain length.