Textural properties of coagulated milk products containing soy protein

The textural properties of coagulated milk products were studied by Scanning Electron Microscopy and Texture Profile Analysis. Soy protein and casein interactions were explored with chemically modified soy proteins. The microstructure of process cheese was changed from fine to coarse network by either native or boiled soy proteins (10 percent as total protein). Free fat appeared on the surface of remelted cheeses containing soy proteins but not on control cheese; the fat emulsion of cheese was broken by soy proteins. The hardness and cohesiveness of cheese were lowered 20 and 10 percent, respectively, by soy proteins. Native soy protein (20 percent as total protein) in rennet coagulated milk curd (stored for 2 A hr) was adhered onto casein strings and was aggregated. The aggregated soy protein interfered with the contraction of casein and decreased the hardness and cohesiveness of milk curd by 26 and 22 percent, respectively. Boiled soy protein was more aggregated than native soy protein and lowered both hardness and cohesiveness of milk curd by 50 percent; it also lowered the springiness by 18 percent. The 24 hr old rennet coagulated caseinate curd was composed of a network of casein submicelles. Hardness and cohesiveness of caseinate curd were 49 and 84 percent, respectively, of those of milk curd. Both added native and boiled soy proteins (20 percent as total protein) were indistinguishable from caseinate in micrographs. Hardness of caseinate curd was lowered 14 and 42 percent by native and boiled soy proteins, respectively; cohesiveness was lowered 19 percent by boiled soy protein. Soy protein (20 percent as total protein) in nonfat milk was adsorbed onto casein micelles and lowered the strength of nonfat milk coagulum, which was measured by back-extrusion test before syneresis had occurred. Soy protein modified to increase negative charges (acetylation, succinylation and alkylation) or modified by bulky neutral reagents (N-ethylmaleimide, glycine methyl ester and carbodiimide) lowered the strength of milk coagulum more than did native soy protein, whereas, the opposite was true for soy proteins modified to decrease negative charges or treated by alkali. Thus, soy protein in milk interfered with the electrostatic and hydrophobic interactions among rennet treated casein micelles.