A review on mechanism, quality preservation and energy efficiency in Refractance Window drying: a conductive hydro-drying technique

Thermal drying consumes up to 25% of the industrial energy consumption in developed countries. Refractance Window (RW) is a relativelynew drying methodthat it is characterized by maintaining a relatively low temperature inside the food and short drying times. A RW dryer uses circulating hot water as a means to convey thermal energy to materials to be dehydrated. A RW dryer can produce high-quality products with retaining heat sensitive vitamins, color, phytochemicals content and antioxidant activity close to the freeze-dried products, while the cost of the RW equipment is less than 30 percent of the cost of a freeze-dryer and the energy consumption of RW is less than 50 percent of the energy consumption for a freeze-dryer. Early understandings heat transfer mechanism in RW drying suggested that the moist material on a thin plastic sheet over hot water, creates a “window” for infrared radiation(IR) and as the material dries, the “window” gradually cuts off the radiation. Recently a conjugate heat and mass transfer model showed that a major portion of thermal energy is transferred via conduction; some of the researchers proposed the term “conductive hydro-drying” for the name of this technology. Another important parameter in RW drying is air convection. Forced air convection causes lower product temperature and higher moisture loss in comparison with natural convection. Although many studies have been conducted on this technology, there are still many types of food materials for which RW drying and process optimization have not yet been investigated.