Measurement of the Effective Diffusion Coefficient of Water in Spray Dried Amorphous Lactose Particles

Abstract

Stickiness and caking phenomena in dairy powders have been attributed to the amorphous lactose component in dairy powders. The effect of water on the glass transition temperature of amorphous lactose is a key to understanding these phenomena. The speed at which the powder particles take up water is critical when modelling caking or sticking processes. There is little in the literature on the measurement of this. This paper presents a method that uses the absorption of water vapour into a monolayer of particles of mixed size to estimate the diffusion coefficient of water in amorphous lactose. The aim was to measure the diffusion coefficient of water in amorphous lactose. Amorphous lactose particles were produced by spray drying and freeze drying and residual free moisture removed by further drying in an oven at 105°C. A monolayer of the particles was spread over a Petri dish and the dish exposed to 30% RH air at 30°C. The change in weight with time was recorded. The particle size distribution was measured using a Malvern Mastersizer S. The size distribution was combined with a mathematical model for the absorption of water into a sphere, applied to each particle size simultaneously, to estimate the weight increase with time. The diffusion coefficient that minimised the sum of squares of the difference between the predicted and experimental values was taken as the diffusion coefficient of water in amorphous lactose. The diffusion coefficient of water in amorphous lactose was found to vary depending on how the particle was made. Values were (3.4±1.7) *10-14 and (6.6±0.7)*10-14 m2s-1 when made by spray drying from 30 wt% and 10 wt% solutions respectively, compared to (4.5±2.5)*10-11 m2s−1 for freeze dried particles. This result indicates that the diffusion rate into amorphous lactose occurs faster than previously thought in freeze dried products.