Utilization of Combined Processes in Freeze-Drying of Shrimps

This paper presents experimental results obtained during freeze-drying of shrimps under vacuum and at atmospheric pressure. The latter technique is based on the utilization of a fluidized bed of flne adsorbent particles, operating at atmospheric pressure, into which the product to be freeze-dried is fed. The effects of the following variables on the freezedrying process are investigated: product size, air velocity, bed temperature and freezing temperature. Due to the very long time required to achieve the complete dehydration of samples in atmospheric freeze-drying, higher than that needed in the conventional process under vacuum, two pre-treatments of samples are proposed: vacuum thermal drying and osmotic dehydration. The effect of these pre-treatments on freeze-drying under vacuum and at atmospheric pressure is analysed and discussed. The quality of the product dried in a single stage and in combined processes is also evaluated. In particular, rehydration capacity, texture and porosity are determined in different process conditions and compared.