Innovation in monitoring food freeze drying

This paper aims to extend the field of application of the pressure rise test technique, from freeze drying of pharmaceutical or biological products in vials to freeze drying of liquids or foodstuff in trays. The proposed method, which is based on DPE+ algorithm, has been adapted to monitor the drying of liquids in trays and of individually quick frozen products. Examples of results obtained in a small-scale plant wherein such a method was used for monitoring the freeze drying of spinach samples and solutions of sucrose and lactose are presented. Since in bulk freeze drying the radiant energy coming from the upper shelf might be not negligible, the effectiveness of this method has been tested in the presence of different radiating contributions. A more sophisticated algorithm to interpret the pressure rise curve, which uses a mathematical model accounting for both conduction heat transfer from the bottom and radiant energy coming from the upper shelf and including heat accumulation into the dried layer, is proposed and experimentally tested.