Mass Transfer Patterns during Low-Temperature Concentration of Skim Milk

Low-temperature concentration of milk and dairy products is a prospective alternative to such traditional concentration methods as evaporation or membrane technologies. This energy-effective method preserves the native properties of dairy raw materials. In this research, the kinetics of moisture freezing made it possible to determine the effect of time, temperature, and composition on the amount of frozen ice on the cooled surface. The study featured the main physical and chemical parameters of milk, i.e., water, fat, protein, and solids. Chromel-copel thermoelectric converters were used to identify the freezing temperature. Experimental laboratory equipment made it possible to describe the kinetics of low-temperature concentration. The ice growth rate curves were similar and not linear at temperatures between –2 and –8°C. At all temperature conditions, the solids increased together with the freezing time as the coolant temperature went down. The experiment yielded a regression equation that revealed the mass fraction of solids in concentrated skim milk depending on the temperature and freezing time. The temperature and freezing time affected the transition rate of milk solids into ice. The coolant temperature had the greatest effect on the cryoconcentration of skim milk. When the temperature of the coolant was reduced from –2 to –8℃, it increased the share of solids in the finished product and sped up its transition into ice. The optimal temperature for milk concentration in the experimental capacitive-type crystallizer proved to be –4°C.