Refractance window dryers: A comprehensive review of the fourth-generation drying technology

Refractance Window Drying (RWD) is an emerging thin-film drying technology for heat-sensitive foods. In this process, liquid or semi-liquid materials are spread in a uniform thin layer on an infrared-transparent polymer film in contact with circulating hot water. This enables rapid moisture removal at moderate product surface temperatures (60–70 °C range). Compared with conventional hot-air drying (HAD), RWD reduces drying times from hours to minutes while effectively preserving color, thermolabile nutrients, antioxidants, and phenolic compounds. The resulting product quality is reported to be comparable to freeze-drying (FD), but with substantially lower energy consumption and capital investment. RWD has been successfully applied to fruit and vegetable purees, powders, dairy-based formulations, and plant- and animal-derived protein systems. Recent developments in hybrid RWD configurations, incorporating infrared heating, vacuum conditions, microwave assistance, and pretreatments such as osmotic dehydration, ultrasound, and cold plasma, have further enhanced RWD efficiency and product quality. Despite these advantages, challenges related to scale-up, feed-layer uniformity, and process control remain under active investigation. Overall, RWD represents a sustainable, high-performance drying technology with strong potential for high-value food and bioactive applications.