Effect of Diffusion on Component Segregation During Drying of Aqueous Solutions Containing Protein and Sugar

Water content profiles within thin aqueous films of lactose and bovine serum albumin (BSA) were tracked during drying using inverse microscope Raman spectroscopy (IMRS). These film drying experiments provide useful insights into component diffusion within droplets during spray drying in the food industry. Virtually no segregation of lactose and BSA occurred in the films during drying at 30 degrees C. Thus, in film drying simulations at this temperature, lactose and BSA can be assumed to be a single homogenous solute, and binary diffusion of water and this single solute can be modeled. A solute-fixed coordinate system drying model properly predicted the water content profiles and shrinkage of the films during the constant rate period. This represents a partial validation of the model. The drying model predicted significant gradients in water content in films dried at a higher temperature of 80 degrees C, which have the potential to drive segregation of lactose and BSA. However, IMRS analysis of films dried at 80 degrees C revealed no segregation of lactose and BSA. We propose that lactose and BSA are relatively immobile in the bulk of the film during drying, forming a homogenous gel that allows the significantly smaller water molecules to diffuse through. The observed accumulation of BSA as a monolayer at the surface of the dried films appears to be a local phenomenon most likely driven by the surface activity of BSA.