Effect of freezing with temperature fluctuations on microstructure and dissolution behavior of freeze-dried high solid systems

This study demonstrates the potential to control ice crystal formation in highly concentrated system for process design of energy efficient freeze-drying technology. I, the effect of freezing with temperature fluctuations on the microstructure and dissolution of aerated and non-aerated freeze-dried concentrated coffee (50 and 60% w/w) is presented. Samples were either frozen at -40°C or subjected to fluctuating temperatures between -40 and -20°C prior to drying. Total freezing time was kept constant for both freezing profiles. Microstructural characteristics of the freeze-dried solids were revealed using scanning electron microscopy (SEM) and high speed camera was used to record the dissolution process. Solution with 60% solids nucleated at lower temperatures (<-20°C) and developed smaller pore sizes (by 70%) than 50% concentration. SEM micrographs of the freeze-dried solids showed the formation of voids similar to the shape of crystal dendrites in less concentrated coffee sample. Aerated systems were characterized by growth of small dendrites (20-40 μm) within large air bubbles which diameter varied from 200-1000 μm. Meanwhile, collapsed microstructure was observed in 60% coffee but was not visible on aerated sample. Temperature fluctuations doubled the observed pore sizes in both concentrations which effectively accelerated the dissolution process. Morphology of ice crystals is also affected by the periodic temperature fluctuations with growth of hexagonal ice crystals (50-100μm) was evidence from the SEM images.