Техника и технология пищевых производств (Jun 2015)
DEPENDENCE OF VISCOELASTIC PROPERTIES OF RENNET GELS ON CONCENTRATIONS OF MILK FAT AND SOLIDS
Abstract
From the technological point of view, one of urgent problems is the study of milk coagulation for samples containing a close to natural amount of fat. This paper is devoted to theoretical and experimental study of flocculation and gelation processes in model samples of reconstituted milk containing different amounts of casein (2.5% and 5% by weight) and fat (0%, 2.5% and 5% by weight). Experimental study of the viscoelastic properties of milk clot during its formation was made with a dynamic rheometer of our own design characterized by reciprocal translational displacement of the cell relative to the stationary probe. A simplified kinetic mod el of the clot formation process is developed. Flocculation stage of this process is described as growth of fractal aggregates with fractal dimension D = 2.22. In this case, the average size of the aggregates increases faster than the average distance between them, and if the initial concentration of the casein micelles is sufficient, the system reaches percolation stage, i.e. formation of continuum gel. The next stage is strengthening of the clot due to formation of additional bonds. Based on the proposed model the presence of min imum micelle concentration for gelation is explained. A possible explanation for the proportionality of the elastic modulus and th e loss modulus for gel network is suggested. It is found that decreasing the micelle concentration leads to the lowering of the number of additional bonds per unit volume at the clot formation stage and proportional decrease of the clot strength. For example, at any fat concentration the clot strength wherein the initial micelle concentration twice as much is approximately two times higher. The fat concentration increase also leads to the increase in the clot strength both by reducing the volume available to micelles and by increasing the stiffness of the casein chains due to decreasing of their lengths. In addition, protein-coated fat globule surfaces may seem to become the seed centers for flocculation and the formation of additional bonds, thereby increasing the rates of these processes.
