Atmospheric or convective freeze drying (AFD) is a dehydration process mostly employed in the food industry. It is also a promising process for the freeze drying of high-end products in the biological and pharmaceutical industry. Proteolytic and lipolytic activity as well as changes in lipid classes and lipid oxidation for products from AFD were investigated and the general potential of the drying method is described.
The drying rate in AFD is generally low. In order to accelerate the drying process, the application of high intensity, airborne ultrasound in AFD (US-AFD) was investigated for different products and under different drying conditions. The drying rates obtained in AFD and US-AFD show that both processes occur in the falling drying rate period and no drying period with a constant drying rate occurs. A modification of the Weibull model was developed and showed high accuracy for modelling and evaluating drying rates in AFD and US-AFD.
In general, US-AFD investigations showed a faster drying rate than AFD. The effective mass transfer for the sublimation of pure ice particles increased by 38% and the effective diffusion for peas is increased by 24.7% at an approach velocity of 1 m sec-1. Airborne ultrasound increased AFD processes especially at low temperatures and low approach velocities during the first hours of drying. Even a minor ultrasonic field increased the effective diffusion significantly. The product quality (shrinkage and colour) is not affected by the application of the ultrasonic field.
A higher mass transfer rate at the solid-gas interface, caused by a reduced boundary layer due to a higher turbulent interface, is identified as the cause for the higher effective diffusion in US-AFD. Hence high intensity, airborne ultrasound has great potential to accelerate drying rates and reduce investment and production costs associated with AFD. It can be expected that other low temperature processes that are based on convective heat and/or mass transfer (such as blast freezing) can be improved in presence of airborne ultrasound.