Heat pump (HP) dryers have been used as one of the useful and promising drying techniques in food industries owing to their low energy consumption and costs, high coefficient of performance (COP), high energy efficiency, high drying efficiency, low drying temperature, and time, and less quality loss of dried products. It can be operated over a wide range of temperatures and humidity, providing the optimum conditions for drying of heat-sensitive food materials (vitamins). Many studies were done on food products drying by various HP drying systems such as simple HP, solar-assisted HP, infrared- assisted HP, vacuum HP and ultrasound intensified HP dryers. This paper reviews the effect of different and recent HP drying methods on the drying performance and quality of dried apple, banana, grape, jujube, kiwifruit, and pineapple. Also, this study reported the suitable mathematical models for HP drying modeling of various fruit crops. The HP dryer provides high-quality dried fruits as their drying conditions can be controlled. In addition, combination of solar/infrared/vacuum/ultrasound techniques and HP dryer has been used as an efficient and rapid drying method compared to HP drying alone. The effective moisture diffusivity (Deff) values lie within in range of 10−7 to 10−11 m2/s for some fruit crops during drying by HP dryer.
Abbreviations: COP: Coefficient of performance; Deff: Effective moisture diffusivity; HP: Heat pump.