Calorimetric measurement of the heat of desorption of water vapor from amorphous and crystalline lactose

The heat required to release and vaporize bound H₂O from crystalline α-lactose monohydrate and from lactose glass, as determined by differential scanning calorimetry is 12.3±0.7 and 10.8±0.5 kcal·mole⁻¹ of H₂O, respectively. Water vapor sorption by anhydrous α-lactose leads to the formation of the α-monohydrate. The isotherm, obtained gravimetrically for this process is Langmuir type. β-Lactose is completely non-hygroscopic below 97% relative humidity. Thereafter, it sorbs H₂O rapidly to form a concentrated solution wherein the lactose is capable of mutarotation. Densites of lactose forms determined pycnometrically by helium displacement are: 1.535 g/cm³ for α-lactose·H₂O; 1.547 g/cm³ for anhydrous α-lactose; and 1.576 g/cm³ for β-lactose.