Freeze concentration of skim milk

Abstract

Freeze concentration is an excellent alternative to evaporation and reverse osmosis for concentration of many liquid foods. Product quality is generally high since low temperatures are used and no vapor-liquid interface occurs. However, current commercial freeze concentration technology is not economically competitive with the more established alternatives. Understanding the principles by which ice crystals grow in fluid foods would aid in furthering freeze concentration technology. In particular, if optimal heat balance conditions can be maintained throughout the freeze concentration process, large, easily separated crystals can be grown in short times. The parameters influencing heat balance conditions in a suspension crystallizer were studied to determine the conditions at which optimal crystal growth could be attained in skim milk. The refrigerant temperature, the agitation rate and the surface area of crystals in the crystallizer were studied for their effects on ice crystal growth rates in seeded, batch crystallization. Experiments were conducted at either constant refrigerant temperature, constant refrigeration subcooling (bulk temperature minus regrigerant temperature) or constant refrigerant subcooling with slurry removal from the crystallizer. The effects of seed addition level were also studied. At constant refrigerant temperature, ice crystal seeds initially grew rapidly; however, growth slowed considerably as time progressed and the system approached a thermal equilibrium. Initial growth rates were higher at lower refrigerant temperatures and lower seed addition levels. When the refrigerant subcooling was maintained constant by lowering the refrigerant temperature, crystal size increased more rapidly, with crystal size c. 20% higher than at constant refrigerant temperature. Again, lower seed addition levels resulted in larger crystal sizes. When crystal slurry was removed during crystallization, the mean crystal size increased significantly for longer periods of time since optimal heat balance conditions were maintained for longer times. Crystals c. 80% larger were obtained than for equivalent batch conditions. Spherical ice crystals of nearly 800 μm in diameter with smooth surfaces were grown in about 5 h under moderate heat balance conditions. However, when crystal growth rates were too high, irregularly shaped crystals, with many surface pockets were formed. These results suggest that large crystals can be grown in reasonably short times in batch crystallizers when the heat balance conditions are maintained at high levels. The most important parameters for maintaining optimal heat balance conditions were crystal surface area and refrigerant temperature. By controlling these parameters during ice crystallization, a competitive freeze concentration technology may be developed.