Ice Crystal Size Changes During Ripening in Freeze Concentration

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Abstract
Crystal sizes were measured photographically versus elapsed time for non-uniform ice crystal populations mixed at equilibrium temperatures with sugar solutions. The mass of ice did not change, but crystals smaller than a “neutral diameter,” Dn, tended to melt because of their excess surface energy. This subcooled the solution with respect to larger crystals, which consequently grew. This process, “ripening,” which slowly increases mean crystal sizes and Dn is used to produce ice crystals large enough to be cleanly separated from freeze concentrated solutions. “Sequential analysis” was used to determine Dn and follow changes in the diameters, Di, of fractions of the ice crystal populations. Rates of change in Di for both growth and melting were roughly proportional to (I/Dn - 1/Di. Overall mass-transfer coefficients were not markedly different for growth and melting, and decreased from roughly 2 mm/s to roughly 0.5 mm/s as sugar concentration increased 10% to 42%. In 10% sucrose solutions, mass-transfer coefficients decreased from roughly 2 mm/s to roughly 1 mm/s when gelatin was added at levels which increased from 0% to 0.5%. Ripening in freeze concentrated liquid foods apparently can be accelerated by manipulating initial crystal size distributions and temporarily removing high molecular weight components.

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