For many food products, vacuum freeze drying (VFD) has clear product quality advantages above conventional thermal drying techniques. However, VFD is capital and energy intensive as the process takes place at deep vacuum and is relatively slow. Its application is therefore limited to products with a high added value like instant coffee. A relative new technology is atmospheric freeze drying (AFD), which operates at ambient pressure and temperatures just below 0°C (-5°C to -10°C). Because of the operating pressure and temperature, AFD promises to be less capital and energy intensive, provided the large drying air flow in the process is laid out smartly and is recirculated.
So far and despite its promises, AFD is not yet used much in industry. Current technological challenges relate to the efficient dehumidification of the air and the critical balance between transfer rates and resulting product structure. This project will therefore improve on the existing AFD technology by integrating: adsorptive techniques (using zeolite to remove moisture selectively), intensified air-product contact using a tumbling or fluidized bed and potentially intensified drying techniques (using microwaves to supply sublimation energy).
While improving the technology, the product quality has to be maintained or improved. For that reason the crucial development of the product quality /structure and the link to process parameters will be addressed in this project. This involves the understanding of the temperature dependent product properties (Aw, Tg) and resulting product structure during drying. The latter will be studied using a lab scale AFD in X-ray Tomography. Since it is well known that the freeze conditions have a large influence on the product quality, the freezing process prior to drying will be part of the project as well.