Use of solute fixed coordinate system and method of lines for prediction of drying kinetics and surface stickiness of single droplet during convective drying

The distribution of moisture within fructose and lactose droplets, subjected to convective drying conditions, was predicted using a convective-diffusion equation based on a solute fixed coordinate system. The partial differential equation was solved using the method of lines. Neumann and Robbins type boundary conditions were imposed at the centre and at the droplet–air interface, respectively. The temperature history was predicted by coupling the heat balance equation with the rate of change in average moisture. The distribution of glass transition temperature (Tg) within the droplet and at the surface layer was predicted. The concept of sticky point temperature (Tsticky) was introduced and used as an indicator of the development of stickiness at the droplet surface. Single droplets were dried in a controlled air stream and their moisture and temperature history were measured experimentally. These data were used to compare with the model predictions. A custom built in situ surface stickiness testing instrument was used to experimentally determine the development of surface stickiness at the droplet surface. These data were used to validate the model on surface stickiness. The model predicted the experimental moisture and temperature histories of fructose and lactose droplets within ±5–7% absolute error and ±1–2 °C accuracy. It also predicted the development of surface stickiness of fructose and lactose droplets reasonably well. In test conditions, lactose surface attained non-sticky state through crystallization or transformation into glassy state. Fructose droplets remained sticky even in a bone dry state due to their low Tg.