Water mobility in amorphous lactose below and close to the glass transition temperature

The water sorption behaviour of amorphous lactose has been investigated gravimetrically. II was found that the kinetics of absorption al (especially) 40% and (also) 50% RH were bi-phasic. Although we have no explanation for this behaviour, it is noted that the inflection point between the two processes is at a 1:1 mole ratio of water:lactose. Equilibration 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 the experiment (T). Following from this, desorption is rapid and the rate 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 structure, but not in instantaneous recrystallisation. Exposure to higher 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 original expanded amorphous form. The rate of water desorption from the collapsed amorphous structure is slow and follows square root of time dependency. The rate of this diffusion controlled process is not altered by changing the external RH. The duration of exposure to 50% RH alters the extent of collapse, and hence alters the amount of water which is free to leave the sample rapidly and that which is released by the slow diffusion through the solid. After reducing the RH the water content of the collapsed structure remains high, but the recrystallisation is greatly delayed, These studies show that water can be held in different ways within amorphous lactose and this has implications for physical, chemical and potentially even microbiological stability of products.