Refractance window drying of foods: A review

Vegar-Mercado, Gongora-Nieto and have grouped these drying methods into four generations noting that the latest developments in drying have produced drying technologies that fall in the fourth generation. Also opined that the fourth generation drying techniques give food products with greater retention of Refractance Window Dryer food quality and they include microwave drying, infrared drying, Refractance Window drying etc. Changes in color, glass transition, specific heat, and surface morphology of the dried fruits were also evaluated. The quality retention in the dried fruits was found to be product and drying method specific.
Unlike hot air tray or tunnel dryers that require several hours or freeze dryers that take from 12 to 72 hours, Refractance Window dryers require only 2 to 6 minutes to dry most products. Among these methods, RW™ drying is a recent non-thermal method for drying products (Forero et al., 2015) including heat-sensitive purees (Nindo et al., 2003a), and slices (Ochoa-Martínez  et al., 2012) of fruits and vegetables. Recently, Rostami et al. reported the potential of using RW™ drying for manufacturing meat powders. With numerous advantages, this direct drying technology has found several applications in the algae, pharmaceutical, nutraceutical, cosmetic and pigment handling industries (GW dryers, 2017; Nindo et al., 2003b, Caparino et al., 2012, Ortiz-Jerez et al., 2015).

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. Forced air convection in RW dryer causes lower product temperature during the process and higher moisture loss values in comparison with natural convection. In a recent study, a conjugate heat and mass transfer model was developed to simulate the RW drying of pumpkin slices.
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.
With the growing production of walnuts, the demand for new drying technologies in the industry on a large scale is increasing day by day. For efficient drying, on lab scale, different technologies were studied in recent years such as radiofrequency drying , infrared drying , and intermittent oven drying . Many investigations have been reported on drying methods which include microwave, convective, vacuum drying, freeze, and solar drying based on product properties and quality like carrot, resins, etc. More than 85% of industrial dryers perform the heat transfer as convection in hot air or combustion gases medium, resulting in higher drying times resulting in poor quality products. RWD systems are a drying technique designed to eliminate the existing disadvantages (installation cost, drying cost, drying time, component protection, etc.) in drying systems.

In this research, the objective is to compare the RW sample to the sun-dried sample. RW drying was done at different temperatures of walnut kernels. Various critical parameters such as moisture content and drying time were observed. The physicochemical properties of the RW-dried sample and sun-dried sample were analyzed and a comparative study was done.
Optimization of Refractance Window drying in terms of energy consumption and process design. RW dryer can preserve color (Castoldi, Zotarelli, Durigon, Carciofi, & Laurindo,  2014), heat sensitive vitamins and phytochemicals that some of them will be discussed in the following section. & Kushad, M. The effect of drying method and storage on color characteristics  of paprika.

Drying reduces moisture content and lowers water activity in perishable products  to safe levels, thereby prolonging shelf life and adding value. Compared to products produced with other preservation methods, dehydrated products have almost unlimited shelf-life and substantially lower transportation, handling and storage costs. In the United States, they provide opportunities for maximum convenience, flexibility, and economics as industrial and food service ingredients, whereas in other countries they are also frequently consumer products, as well. In this work, unripe green banana flour was dried by using Refractance window drying and Hot Air oven drying, and various properties were evaluated.
Spreading thickness did not play a critical role on the viability of yoghurt bacteria in dried samples. The viability of yoghurt bacteria in yoghurt powders was observed during 90 days of storage at 4 °C. At the end of storage period, the survival of Lactobacillus delbrueckii subsp.
This product was featured at a recent Technology and Market Assessment Forum sponsored by the Energy Solutions Center. In this dryer design, a slurry of liquid product is evenly applied to the top surface of a continuous sheet of transparent plastic. This impervious conveyor belt floats on a surface of hot water at temperatures of 210°F or less.