Progressive freeze concentration of skimmed milk in an agitated vessel: Effect of the coolant temperature and stirring rate on process performance.

The aim of this study was to investigate the freeze-concentration of skimmed milk by a progressive freeze concentration process. The progressive freeze concentration procedure was performed at three different temperatures (-5, -10, and -15 ℃) and stirring rates (0, 500, and 1000 r/min). The solids concentration was determined and used for calculations of the efficiency of the process, concentrated yield, and experimental results validation. A general linear model was applied to determine the influence of the two factors studied, namely coolant temperature and agitation speed. In all tests, it was possible to concentrated skimmed milk with total solids content higher (P < 0.05) than ultra-high temperature skimmed milk. The highest concentration (P < 0.05) was achieved at low coolant temperature (-15 ℃) and high agitation speed (1000 r/min). The coolant temperature and the stirring rate both had a significant effect (P < 0.05) on the results of efficiency of the process and concentrated yield. Nevertheless, the parameter that showed the most significant effect in our study was the stirring rate. The tests presented a good fit since the root mean square values were below 25%. The freezing point temperatures of the concentrated milk fractions were lower than that of skimmed milk. Finally, the best-operating conditions in our study were achieved using a high coolant temperature (-5 ℃) and high mechanical stirring (1000 r/min), which was also the variable with the lowest (P < 0.05) retention of solids in the ice fraction. In our study, the progressive freeze concentration technique showed promise as an alternative for the dairy industry since it makes the development of new dairy products possible.

Multi-plate freeze concentration: Recovery of solutes occluded in the ice and determination of thawing time.

The retention of solutes in the ice formed in a falling-film freeze concentrator (multi-plate freeze-concentrator) was analysed. Solutions of fructose, glucose and sucrose and a simulated juice with initial concentrations of 5, 10, 15 and 20 °Brix were freeze concentrated. The ice produced in the four steps of the process retains solutes at levels of 1.0-8.8 °Brix (expressed as solute mass fraction in the ice). The recovery of these solutes during thawing can increase overall system efficiency. All thawing steps were carried out dividing the sample in 10 fractions at 20 ℃. The first thawed fractions showed solute concentrations that were 1.9-3.3 times higher than the mean solute mass fraction in the ice, while the last fractions of ice showed very low levels of retained solutes, less than 0.2 times the mean solute mass fraction in the ice. It was found that fractionated thawing can recover most of the solute content in the ice. The procedure presented in the present study allows the determination of the solute concentration achieved in the various thawing fractions and predicts the thawing time required for a given form factor, melting temperature and initial solute mass fraction in the ice.