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Alpha Lactose and Crystallization Rate

Department of Food Science and Technology, University of California, Davis 95616

ABSTRACT

The effect of - and ß-lactose on crystal growth was measured by weight gain of crystal aggregates in solutions differing in alpha-beta content. Solutions were prepared from stable anhydrous alpha-lactose; the beta content was controlled by direct addition and by muta-rotation.

In 5-h tests, aggregates of alpha-hydrate crystals grew more if ß-lactose was present whereas for short test periods (5 min) ß-lactose made no difference. The growth of regular -hydrate crystals depended on the amount of -lactose in solution and was independent of the amount of beta. Growth in -stable solutions slows rapidly because of the rapid decrease in -lactose by mutarotation. Conversely, increased supersaturation of the solution increased the rate of crystal growth because the increased alpha was not decreased by mutarotation but was sustained. It also explains why large amounts of ß-lactose in highly supersaturated solutions do not slow crystallization.

Nucleation and growth of needle crystals were inhibited by ßlactose. Thus, the effect of beta on growth rate depended on the type of alpha-hydrate crystal being produced in the solution. We suggest that beta influences the type of crystal normally formed. Inhibiting the growth of the needle or prism form allows mutarotation time to reduce the amount of -lactose, thereby reducing the crystallization pressure and allowing the more commonly observed lactose crystal types to develop.