Physical Properties of Rennet-Induced Skim Milk Gels

Abstract: The aim of this work was to predict the effect of technological parameters on the physical processes of cheesemaking. To this end we developed and modified techniques with which to measure the physical properties of the curd, such as its permeability and its rheological properties. The effects of different factors, such as pH, temperature, the concentration of casein, the geometry of the curd, applied compressive stress and of the deformation on the physical properties of the curd were studied. Further modifications and development of the method to measure the syneresis of curd grains by estimating the progressive dilution of added tracer were made. The grain size was the most important factor determining the shrinkage of the curd grain in the initial stage of syneresis, whereas the pH was the dominating factor in the later stage. A laser distance meter was used to follow continuously the course of one-dimensional syneresis of rennet-induced skim milk gels. The method made it to follow the initial stage of the syneresis within 10 s after the surface of the curd slab was moistened and the initial syneresis rate was found to be dependent on the thickness of curd slabs in the range of 3-35 mm. Further development of the method for measuring the fusion of curd grains, using two plan-parallel surfaces of known area, that were exposed to fusion, and a means of controlling directly the fusion pressure, was undertaken. In this way the total deformation occurring during fusion could be followed, which had not been possible using earlier methods. A novel finding was that there was an optimum amount of syneresis prior to fusion, which led to the fastest fusion. Total fusion could be achieved in a fusion time of 20-25 min. We followed the compression behavior with different sample sizes over wide ranges of compression velocities. A key experimental finding was that the force increased almost linearly with the deformation at constant deformation rate, a finding which implied that curd permeability did not control the curd compression behavior. We have for the first time developed a method to determine, at least approximately, the fracture properties of cheese curd with no syneresis. The experiments showed that the time course of the behavior in tension did not replicate the behavior in shear or compression.