Refractance Window Dryer

Heating water transmits heat energy within itself by conduction, convection, and radiation. Among them, radiation is the most rapid process. In RW dryer, the circulating water temperature is maintained between 95 and 98°C. During drying, the hot Refractance Window Dryer water surface is covered by a Mylar film which blocks the evaporation of water and its heat loss. Hence, the product temperature remains at 60-72°C. At this reduced temperature, the pharmaceutical and bioactive products can be dried efficiently.
It gently extracts water from our foods while preserving vitamins, minerals, and micronutrients. Despite our best efforts to do so , it is difficult to keep a food’s full nutritional content after its typical shelf life. In other words, as soon as the apple is picked from the tree, its advantages start to fade. When it comes to retaining the quality of the food at hand, some preservation techniques, such as refrigeration, drying, canning, and freezing, have proven to be more successful than others in the modern world.

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). Since the plastic conveyor is very thin, it reaches the temperature of hot water flowing beneath it almost instantly.
By extending shelf life and safety, drying also facilitates transport and storage. Refractance Window is a relatively new film drying technique and it is characterized by high-quality products, short drying times, low cost, high energy efficiency and high evaporation capacity. RW dryer can retain product quality in terms of heat sensitive vitamins, phytochemicals content, color and antioxidant activity equal or greater than those of the freeze-dried products. Early understandings heat transfer mechanism in RW drying have concluded that thermal radiation from hot water transfers to the moist material by the use of a thin plastic sheet that allows creates a “window” for transfer of infrared radiation. This “window” gradually closes as the material dries out cutting off radiation. Drying technologies belong to first, second, third or fourth generation.
Finally, new insights into quality and energy modelling using CFD are discussed. Based on the research achievements, future challenges of CFD modelling and their contribution to improving the drying process in terms of high quality and low energy consumption and decarbonisation are addressed. Drying describes a complex heat and mass transfer process of removing moisture from food materials. The final product may be in the form of sheet, flakes, film, powder, or granules. An additional size reduction step may be involved based on product requirements. Drying is an energy intensive unit operation, accounting to around 12–20% of the total energy consumed in the manufacturing industry (Raghavan et al., 2005).

The physiochemical characteristics of the dried apple slices including flavonoid content, total phenolic compounds, antioxidant activity, vitamin C content and color were measured. In addition, the energy consumption and energy efficiency of the dying methods were evaluated. Results showed that combining ultrasound and infrared with conductive hydro-drying can result in higher drying rates  and lower product moisture content. Quality of UIACHD dried apple slices was close to the freeze-dried products and it was significantly better than the cabinet dried products. Moreover, the energy efficiency of UIACHD was considerably better than the cabinet dryer and the freeze-dryer. The results of this study showed that combining ultrasound and infrared with conductive hydro-drying can lead to an energy-efficient process with good quality retention ability.
Enzyme activity was calculated in katal, which is the conversion rate of 1 mol of substrate per second and is given in µ katals per milligram. The breaking strength of the kernel samples was measured objectively by using a Texture Analyser as adopted by the standard method by AACC . The compression (70%) test was performed using a 36 mm P/36 cylindrical aluminum probe at the pre and post-test speed of 2 mm/s and a test speed of 1 mm/s.

Figure1 shows a laboratory-scale refractance window dryer that was used for this study. The main components of this refractance window dryer were a rectangular double wall water bath and a Mylar sheet 10 Mil (250  μm) thickness having properties such as high thermal resistance, water impermeability, and infrared waves permeability. Water bath temperatures were set from 50 to 90 °C and all samples were dried in triplicate at the above-mentioned temperatures. The temperature of the mylar sheet and walnut kernels was checked constantly.
During RW drying, the three modes of heat transfer are active which include conduction , convection , and radiation (C. I. Nindo & Tang, 2007). Although RW drying is comparable with freeze drying and other low temperature drying methods in terms of physiochemical quality retention, it can also reduce microbial load in its dried products. In a study on RW drying of pumpkin puree from 80% to 5% moisture content which was completed in about 5 minutes (with water at 95ºC). RW drying of inoculated pumpkin purees resulted in 4.6, 6.1, 6.0, and 5.5 log reductions of total aerobic plate counts , coliforms, Escherichia coli, and Listeria innocua, respectively. The RW dryer in this study demonstrated up to 70% energy efficiency (C. I. Nindo, Feng, et al., 2003). Using response surface methodology for studying the effect of emulsion formulation drying conditions resulted in mathematical models for predicting the pattern of the variables analysed in this work.
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. This technology is being applied to heat sensitive products such as fruits and vegetables in order to retain its various qualities and nutritional characteristics. In the refractance window dryer, the slurry of food product is placed over a thin infrared transparent film which serves as a window for drying. The product temperature is kept extremely low and the rapid drying occurs due to involvement of all three modes of heat transfer . In RW drying, circulating hot water provides thermal energy for drying of agricultural products through transparent contact film (Mylar®), which is the heart of Refractance Window Drying System.