Rheological and structural properties of enzyme-induced gelation of milk proteins by ficin and Polyporus badius

The rheological and microstructural characteristics of ewes' milk curd obtained by coagulating with milk-clotting enzymes, including ficin extract and Polyporus badius were evaluated. The gelation of milk was examined by small amplitude oscillatory shear measurements (SAOS). Different concentrations of ficin and P. badius extracts (1, 3, and 5%) were utilized to coagulate milk proteins. The ewes' samples containing ficin and mushroom enzymes were heated from 25 to 45 degrees C at a heating rate 1 degrees C/min and kept for 30min. Then, the curds were cooled down to 25 degrees C with the same heating rate. The ficin extract could induce stronger gels at 45 degrees C and 5% ficin. Similar results were also found for 5% P. badius extract and incubation at 45 degrees C. However, P.badius gels achieved a network with more viscous characteristics and had a softer texture than ficin gels. Therefore, it may be concluded the induced gels with mushroom had higher moisture and lower protein contents, which related to the high proteolytic activity of P. badius. The microstructure survey showed that the mushroom-induced gel had a more compact structure. By increasing enzyme concentration, both gels showed a coarser and more compact protein network. Whereas, the P. badius gels had more fusions and folds which indicate the greater proteolysis occurred during gelation and there was greater breakdown of protein. Our findings suggest the application of ficin and P. badius enzymes to develop a novel procedure to coagulate milk proteins and providing new structures in food systems.