18 May 2002
Drying is an energy intensive process and considerable wastage of energy occurs in the traditional method of drying. The quality, in terms of appearance and nutrients, of the product is also affected by drying conditions. In a traditional dryer, it is difficult to maintain temperature and relative humidity of the drying medium independently. Heat pump enables to conserve energy, and control temperature, humidity and flow rate independently and accurately. A two-stage heat pump dryer has been developed and extensive studies, both analytical and experimental, have been conducted to determine the performance under different operating conditions with various food materials.
Pieces of banana were dried in a two-stage heat pump dryer capable of precise control of air humidity with predetermined cyclic variations of air temperature entering the drying chamber. The air temperature variations tested were : a cosine, a reversed cosine and three different square wave profiles with peak-to-valley variations from 20 ?C to 40 ?C. The cycle time was about 60 minutes with drying time of approximately 300 minutes. The drying samples were placed on trays in a thin layer. With appropriate choice of temperature-time variation, it is possible to reduce the overall color change while maintaining high drying rates.