The influence of heating/drying on the crystallisation of amorphous lactose after structural collapse

Abstract

This study was designed to investigate the influence of collapse of amorphous lactose on its subsequent behaviour during drying, or other processes which cause increases in the temperature of the material. Amorphous lactose was prepared by spray drying from aqueous solution. The solid was dried and then exposed to 50% RH for various times in order to induce different amounts of collapse in the amorphous structure. All the samples remained amorphous for the range of exposure times used. During heating in a differential scanning calorimeter, the non-collapsed material crystallised at ca. 180°C to give mostly α-lactose, with some β-lactose present. The collapsed lactose crystallised at ca. 70°C and yielded mostly β-lactose, with some α-lactose monohydrate present. It can be concluded that the collapsed structure will crystallise on drying at lower temperatures than the non-collapsed lactose. The non-collapsed material rapidly loses its sorbed water (this would occur during the early stages of drying), whilst the collapsed lactose loses its water suddenly during crystallisation. Thermogravimetric analysis revealed (generally) three distinct water loss peaks for the collapsed structure, two of which were believed to be due to crystallisation occurring and the final one being the loss of water of crystallisation. The sudden loss of water from the collapsed material will make a substantial contribution to the free water content of a formulation and as such could cause confusion during drying processes. Material which was partially collapsed behaved in an intermediate manner between non-collapsed and totally collapsed samples.