A process for preparing a composition comprising encapsulated gas microbubbles, the process comprising: providing an aqueous protein solution of a heat-denaturable protein at a temperature necessary to achieve incipient denaturation； heating a gas by using heat from the heated protein solution； mixing the heated gas and the heated protein solution to obtain a gas/liquid mixture； and dispersing the gas into the protein solution by subjecting the gas/liquid mixture to mechanical shear forces to form a composition of gas microbubbles encapsulated by denatured protein.
