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Journal of Dairy Science Vol. 56 No. 7 843-848
© 1973 by American Dairy Science Association ®
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Effect of Methanol on the Various Forms of Lactose

S. G. Lim and T. A. Nickerson

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

ABSTRACT

Methanol is suitable for removal of the water of crystallization from alpha-lactose hydrate to form new crystals of stable anhydrous alpha-lactose. The crystalline product is more bulky than the hydrated lactose and is not hygroscopic, as is the anhydrous form produced by heat and vacuum. Conversion from alpha-hydrate to the new anhydrous form can be completed in 1 h at refluxing temperature (64.6 C) at all proportions of alpha-lactose hydrate to dry methanol or in methanol containing up to 5% moisture.

The rate of production of the anhydrous form follows a first-order reaction at room temperature in methanol. Formation is rapid at first while the methanol is dry, but as the solvent is diluted by the water of crystallization from alpha-hydrate, the reaction gradually slows. For complete conversion, a ratio of 1 part alpha-hydrate to at least 15 parts dry methanol is required for 2 h at room temperature. This ratio of 1:15 provides the maximum limit of moisture (.4%) in the final solution where the rate remains maximum at room temperature. Between .4 and 2.0% moisture retards nucleus formation of the new crystal. Therefore, an induction period is observed since the reaction cannot proceed until nuclei develop. Seeding eliminates the induction period by furnishing nuclei. The conversion reaction ceases when moisture is increased above 2%.

The regular (hygroscopic) anhydrous alpha-lactose also is converted by methanol to the stable anhydrous form, indicating it is the more stable in low moisture methanol. The new crystal form is produced even in dry methanol, showing that moisture is unnecessary for the crystallization. Beta-lactose, however, is not changed by the action of methanol at either room or boiling temperature.

Noncrystalline lactose (lactose glass) readily dissolves in methanol forming a supersaturated solution which quickly crystallizes as an anhydrous mixture with a normal equilibrium ratio of 60% beta-lactose and 40% alpha-lactose. This same ratio is produced when lactose glass is added to methanol containing no more than 10% moisture; above this moisture, alpha-lactose hydrate will form. As the moisture content is increased further, the amount of alpha-hydrate increases until at 35% moisture content the entire product is alpha-hydrate.

Contrary to aqueous solutions, beta-lactose does not interfere with crystallization of alpha-lactose in methanol.






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Copyright © 1973 by the American Dairy Science Association ®.