Ultrasonic and optical profiling studies of mass transport phenomena in food emulsions
Mass transport processes in food emulsions were investigated by low intensity ultrasonic spectroscopy and optical profiling techniques, e.g., droplet creaming and small molecule diffusion. Gravitational separation of droplets was observed in non-flocculating and flocculating oil-in-water emulsions. Flocculation was induced by either a depletion mechanism or by electrostatic screening.
The creaming velocity of oil-in-water emulsions stabilized by a non-ionic surfactant is strongly influenced by droplet flocculation and droplet size and concentration. The creaming velocity increased with droplet flocculation because of the increase in particle size, and decreased with droplet concentration due to particle crowding effects. The creaming velocity of flocculated emulsions could be approximately modeled using equations developed to describe creaming in non-flocculated systems, provided the increase in the effective volume fraction due to flocculation was taken into consideration.
The diffusion of small molecules, such as sugars, into a biopolymer gel and a colloidal dispersion was studied in a xanthan solution and an oil-in-water emulsion. It was found that ultrasonic profiling is an efficient and useful method for non-destructively monitoring molecular diffusion in optically opaque biopolymer solutions and emulsions. The biopolymer tested (1% xanthan) had little influence on the diffusion coefficient of sucrose in aqueous solutions because the sugar molecules movement through the biopolymer network was not significantly retarded. Oil droplets of up to 20 wt% a caused slight decrease in the diffusion coefficient of sucrose through oil-in-water emulsions, which was in agreement with theory and previous studies of molecular diffusion in colloidal dispersions.
All studies were conducted with both non-destructive analytical techniques; ultrasonic spectroscopy and optical profiling. It is essential to prove that monitoring the kinetics of mass transport on a single sample is time and cost efficient. It was found that ultrasonic profiling spectroscopy had major advantages for studies on; on optically opaque systems, whereas optical profiling was more sensitive for studies on optically transparent systems. The application of both techniques on model systems and real foods were demonstrated.