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.