Freezing Point Depression

Solutes depress the freezing point of water. Therefore, as ice forms and solute concentrations increase, the freezing point decreases. Figure 1 shows how the freezing points of wine and some juices depend on concentration (1). The freezing point depression is roughly inversely proportional to the solute's molecular weight. Thus, at equal concentrations, freezing point depressions for juices (eg, apple juice) in which the main solutes are fructose and glucose are greater than those for juices (eg, orange juice) containing greater amounts of sucrose. Depending on the fruit juice involved, solute concentrations in the 50% range can be obtained by cooling juice to —8 to - 14°C. Cooling to -13 to — 18°C is required to obtain 60% concentration.

When simple binary solutions (eg, NaCl in water) undergo freezing, the solute involved will precipitate at a certain temperature and corresponding concentration, the eutectic temperature and concentration. As the solute precipitates, the remaining water simultaneously freezes. It is extremely difficult to separate the components of the resulting solid mixture. For such solutions, freeze concentration beyond the eutectic concentration is impossible. Liquid foods contain complex mixtures of solutes that inhibit each others' crystallization. Therefore, eutectics rarely

Eutectic Freeze Crystallization

Figure 1. Freezing point versus concentration for wine and various fruit juices. Source: Ref. 1. Courtesy of Marcel Dekker.

Weight fraction solute

Figure 1. Freezing point versus concentration for wine and various fruit juices. Source: Ref. 1. Courtesy of Marcel Dekker.

form when liquid foods freeze. Around 65% solute concentration, the remaining unfrozen water in foods so strongly associates with the solutes, or is otherwise prevented from freezing, that freezing stops or radically slows down. Further, at such concentrations, concentrates become so viscous that they cannot be readily separated from ice. Therefore, 65% represents the upper concentration limit for freeze concentration of liquid foods. Final concentrations obtained in most food freeze concentration systems are lower (45 or 50%). In some cases, only a modest increase in concentration is provided, such as when low-sugar-content wine grape juices are freeze concentrated to permit production of wine that contains adequate amounts of alcohol. Large increases in concentration are used when wine grape juice is freeze concentrated so that it can be stored for subsequent use in wine production.

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