WATER MOBILITY IN AMORPHOUS LACTOSE BELOW AND CLOSE TO THE GLASS-TRANSITION TEMPERATURE

The water sorption behaviour of amorphous lactose has been investigate d gravimetrically. II was found that the kinetics of absorption al (es pecially) 40% and (also) 50% RH were bi-phasic. Although we have no ex planation for this behaviour, it is noted that the inflection point be tween the two processes is at a 1:1 mole ratio of water:lactose. Equil ibration at 40% RH results in an equilibrium uptake of 7% water, which is not sufficient to lower the T-g of lactose to the temperature of t he experiment (T). Following from this, desorption is rapid and the ra te proportional to the extent to which the RH has been lowered. If the sample is equilibrated to 50% RH the water content exceeds that which lowers the T-g below T, this results in a collapse of the amorphous s tructure, but not in instantaneous recrystallisation. Exposure to high er humidities in an isothermal microcalorimeter revealed that the heat output for recrystallisation of the collapsed amorphous structure was indistinguishable from that produced on recrystallisation of the orig inal expanded amorphous form. The rate of water desorption from the co llapsed amorphous structure is slow and follows square root of time de pendency. The rate of this diffusion controlled process is not altered by changing the external RH. The duration of exposure to 50% RH alter s the extent of collapse, and hence alters the amount of water which i s free to leave the sample rapidly and that which is released by the s low diffusion through the solid. After reducing the RH the water conte nt of the collapsed structure remains high, but the recrystallisation is greatly delayed, These studies show that water can be held in diffe rent ways within amorphous lactose and this has implications for physi cal, chemical and potentially even microbiological stability of produc ts.