FREEZE DRYING TIME AT VARIOUS SURFACE TEMPERATURE PROFILES

Abstract

In our country, the food produce is not distributed evenly throughout. So the transportation of food from one place to another becomes necessary to fulfil* the need of consumers. Some good food preservation techniques are essential to preserve the food stuff during this time of transportation and upto it's consumption. Freeze drying is an advanced food preservation technique by which we can arrest the aroma and texture within the food stuff. In freeze drying process, water content of frozen food stuff is sublimated under vacuum. In freeze drying the rate of dehydration is controlled by the transfer of heat which may be supplied by either a radiant heat source, or heating porous platens sandwiching the product. For economic freeze drying process and for the better quality of f, eze dried product, it is essary, to know the freeze drying time. The largest source of error in methods given in literature for calculation of freeze drying time is in generalizing the surface temperature and assuming it to be constant. In actual practice, as the dried layer thickness grows, the surface temperature has to be increased to overcome the increased thermal resistance. Simlarly the conductivty of the dried layer is also affected with the change in temperature. iii iv A finite element program have been developed to calculate the freeze drying time of rectangular slab of food stuff taking in the effect of growing dried layer thickness and variation of temperature into account., The results obtained from the proposed method have been compared with the experimental data and other methods available in literature. An error analysis has been done which shows that proposed method gives better results than the reported methods. The freeze drying time at linear surface temperature profile and at parabolic surface temperature profile has also been calculated and it has been found that for parabolic surface temperature profile freeze drying time reduced considerably at constant pressure during freeze drying.