Refractance window drying: principles, applications, and emerging innovations

In another research, time–temperature evolution of drying mango pulp surface was determined by thermography during the RW process performed with 2 mm-thick pulp and water at 95 °C. After spreading, pulp temperatures quickly increased up to 70 °C and remained almost steady during the constant drying rate period. From10 to 15 min after the RW process started the temperature began to increase in some areas, owing to their low moisture content (Zotarelli et al., 2015).
In conclusion, this study revealed that RWD provides an appropriate drying temperature as an alternative to freeze-drying. RWD may improve dragon fruit drying, adding value to the food industry. Refractance Window Dryer has an immense advantage in terms of final product quality , energy consumption, and drying time over other conventional dryers. RW is a thin film drying system and a technologically evolving drying process. RW drying is an energy-efficient (re-circulation of water) short drying process as the drying of food materials occurs due to a combined mode of heat transfer conduction, radiation, and convection .

The micrographs showed that USAD created more porous structures that allowed water to migrate outward from inside the FSI tissue. Processing conditions of 600 W ultrasonic power and 5 min ultrasonication time produced better antioxidant activity in dried products than in other conditions. The highest characteristic properties were DPPH (75.51%), ABTS (95.14%), FRAP (83.05%), acceptable drying time , and low colour difference (1.09).
Boukouvalas C.J., Krokida M.K., Maroulis Z., Marinos-Kouris D. Effect of material moisture content and temperature on the true density of foods. Wojdyło A., Figiel A., Lech K., Nowicka P., Oszmiański J. Effect of convective and vacuum–microwave drying on the bioactive compounds, color, and antioxidant capacity of sour cherries. Ahmed M., Akter M.S., Eun J.-B. Optimisation of drying conditions for the extraction of β -carotene, phenolic and ascorbic acid content from yellow-fleshed sweet potato using response surface methodology. Pham N.D., Khan M., Karim M. A mathematical model for predicting the transport process and quality changes during intermittent microwave convective drying. Wojdyło A., Lech K., Nowicka P., Hernández F., Figiel A., Carbonell-Barrachina Á. Influence of different drying techniques on phenolic compounds, antioxidant capacity and colour of ziziphus jujube Mill.
A reasonable depth of sweet potato slices is critical to circulate drying air through the voidage from channels, particularly in a fixed bed, conventional, low-capacity tray batch dryer. Thin packable layers are commonly employed to disperse food ingredients. In other words, the liquid diffusion mechanism is extremely complex due to the diversity of the chemical composition and physical structure of different products. Silva, N.C.; Duarte, C.R.; Barrozo, M.A.S. Effects of dehydration methods on quality characteristics of yellow passion fruit coproducts. Durigon, A.; Parisotto, E.I.B.;  Carciofi, B.A.M.; Laurindo, J.B. Heat transfer and drying kinetics of tomato pulp processed by cast-tape drying. The total phenolic content was determined by the Folin-Ciocalteau method , using gallic acid as a standard and performing spectrophotometric reading at 622 nm .

Firstly, it was assumed that the main heat transfer mechanism of RW drying system was thermal radiation via the plastic sheet through a transparent window which was created by the contact of wet material on the surface of the sheet. Another important parameter in a RW drying is air convection. Forced air convection in a RW dryer causes lower product temperature during the process and higher moisture loss values in comparison with natural convection. Recently, refractive window drying has received considerable scientific attention due to its potential to produce high-quality dried products. However, understanding the drying mechanism and comparing it with conventional drying methods are important to expand its potential applications.
The system has a drying belt directed through the drying chamber. A feed application tray at a first end of the drying belt applies a liquid to the drying belt. The system has an exhaust manifold located at the first end of the drying belt. Open sun, Tunnel + Solar Photovoltaic Mint The system consists of a solar tunnel dryer , photovoltaic system, and flat solar collector. Time to reach equilibrium moisture thickness I 210 min, thickness II 270 min and thickness III 360 min and 270 min, 360 min and 420 min . The highest drying efficiency of 30.71% and overall  efficiency of 16.32% were recorded in the layer thickness of mint II.

K. Fellman, “Effect of drying methods on the physical properties and microstructures of mango (Philippine 'Carabao' var.) powder,” Journal of Food Engineering, vol. S. Bawa, “Degradation kinetics of colour and total carotenoids in jackfruit bulb slices during hot air drying,” Food and Bioprocess Technology, vol. C. K. Pua, N. S. A. Hamid, C. P. Tan, H. Mirhosseini, R. B. A. Rahman, and G. Rusul, “Optimization of drum drying processing parameters for production of jackfruit powder using response surface methodology,” LWT-Food Science and Technology, vol. A powdered material’s flowability, as determined by the Carr index and Hausner ratio, is a significant characteristic. The physical properties of the powder, such as particle size and shape, surface structure, particle density, and bulk density, all influence flowability .
This reduction in drying time highlights RWD’s unique and speedy heat transfer mechanism. Table 1 shows the drying times of the dragon fruit samples. Ascorbic acid is an important vitamin and an essential nutrient for humans and some animal species. It is found in many fruits and vegetables but it is very sensitive to heat and oxidation (Burdurlu, Koca, & Karadeniz, 2006). Several studies have shown that RW drying can preserve ascorbic acid content in the products.
Refractance window drying is a novel drying technique to preserve precious nutrients in dried products at relatively low processing temperatures with reasonable capital costs (Nindo et al. 2003a). The main goal in the production of yoghurt powder is to store the product in a stable and readily utilisable form (Tamime & Robinson, 1999). Nevertheless, it is desired to have high content of yoghurt bacteria and good physical properties for yoghurt powder. Increase in FIR temperature from 50 to 60 °C reduced drying time by 10–19% without having any deleterious effect on quality.

The shrinkage results from the reduction of particles porosity, while dehydration increases the bulk density of the material. Experimental data on shrinkage of fruits during hot air-drying reported in previous studies showed products with higher shrinkage and dense structure. Constant porosity and minimal shrinkage are often stated as key assumptions in the model for dryer design. Freeze-dried fruits and vegetables are usually characterized by minimal shrinkage, while the product quality of hot air-dried products is relatively low, mainly due to the hard texture of the products dried by this method .