THE USE OF THERMAL TECHNIQUES TO ASSESS THE IMPACT OF FEED CONCENTRATION ON THE AMORPHOUS CONTENT AND POLYMORPHIC FORMS PRESENT IN SPRAY-DRIED LACTOSE

The influence of feed concentration (covering solutions and suspension s) on the physical forms of lactose obtained by spray drying was inves tigated. Isothermal microcalorimetry was used to assess the heats of c rystallisation of the amorphous materials, which enabled the determina tion of the % amorphous content. Differential scanning calorimetry (DS C) provided qualitative data for the lactose polymorphs that were pres ent in the spray dried products. Lactose monohydrate content was deter mined thermogravimetrically. The lactose which was dissolved was solid ified as the amorphous form, due to the rapid drying conditions. The a morphous contents for the suspension feed concentrations were higher t han the amount of lactose dissolved, which was due to a milling effect on the suspended lactose particles in the atomiser. The milling resul ted in formation of amorphous material by solid state transition, or e nhanced solubility or more likely a combination of both. Only the samp le with the highest feed concentration contained small amounts of lact ose monohydrate due to incomplete dehydration of the lactose in suspen sion. The presence of anhydrous lactose was due to the high inlet air temperatures causing dehydration of the lactose monohydrate which was in suspension. Variation of feed concentration during spray drying lea ds to products with different % amorphous contents and different propo rtions of crystalline forms. Beta lactose was not detected, either bec ause it was absent or present in quantities below the detection limits of the thermal methods. The spray drying process is now better unders tood as a process that leads to loss of crystallinity in materials, po ssibly by a combination of rapid solidification of dissolved material and solid state transitions due to milling effects in the atomiser. (C ) 1997 Elsevier Science B.V.