Time-resolved near-infrared spectroscopic study of the dissolution of crystalline lactose

In the present study, initial dissolution of crystalline α-lactose monohydrate was investigated via time-resolved Fourier Transform near-infrared (FT-NIR) spectroscopy using the liquid penetration method. The process of water penetration is classically characterized by a two-step increment in the combination bands between OH stretching and bending modes of water; however, only one-step increment in the combination bands was induced by penetration of lactose saturated solution. These results suggest that a first phase occurs in which penetrating water adsorbs to solid lactose surface creating a saturated layer, followed by a second phase of dissolution of the hydrated lactose. Decomposition of differential spectra at 2-min intervals supports the existence of an adsorption-diffusion model and gives a threshold as the amount of adsorbed water to dissolve solid lactose.