Harshita Sonarthi1* and Supreetha. S2


As we all know the shelf of any product depends chiefly on its moisture content. Microorganisms survive and multiply mostly because of moisture content. So, if we have to hinder the growth of microorganisms we have to think over the existence of moisture content in the product, so either we can make available moisture content unavailable, which is possible by freezing, but it is a cost-intensive process other technique is by removing water by the application of heat that is drying. Drying nowadays is a prevalent technique to preserve any food product. Refractance Window-Drying (RWD) is one such novel fourth-generation drying technology that has recently gained huge attention due to its several advantages. It is employed to dry heat-sensitive products like juices and purees to retain the product colour, aroma, antioxidant compounds, and nutritional properties. In this technology, drying of product is done using thin, transparent infrared film which eventually forms a “window” for drying, it assures very low-temperature drying with rapidity, with all modes of heat transfer. It has various advantages over other conventional drying methods like drying temperature, time, cost and energy consumption are lower in this technology, but if we talk about thermal efficiency and product quality it shows good results as compared to drum drying, freeze-drying, spray drying, tray drying, etc., which possess their own limitations.


The main feature of this technology is the use of hot water as a heat transfer medium at a temperature below its boiling point. The heart of refractance window drying technology is contact film (Mylar), which utilizes the hot water normally at a temperature of less than 99 °C and atmospheric pressure. The slurry of food product is evenly applied over the transparent film, this film allows infrared energy to the product at the speed of light. Even if the temperature of the water is at 99 to 92 °C the product temperature is limited to (60 to 70 °C), because of evaporative cooling and convective heat transfer to the ambient air above the drying material. The hot water beneath the film is recirculated to improve thermal efficiency. The belt moves at the desired speed while the drying process takes place.

Construction and working:

This drying technology totally depends on Mylar film, which is the heart of this technology. The film helps in transmitting the radiant energy to the product for drying the product and to conveying the product from one end to another. This film is supported over a two end pulley for moving the film during the drying process. Below this film, a shallow water bath is present in which hot water is coming from the water heating unit, besides this hot water bath there is a cold water bath, which is used for lowering the product’s temperature before leaving the dryer. In the hot water bath, the temperature of the water is maintained constant nearly 99 to 92℃ with the help of a heating unit. One pump is also provided to pump the hot water from the heating unit to the hot water bath. In between the hot water bath and heating unit, there is a valve that helps in pumping the hot water from the heating unit to the water bath if the temperature of the water bath decreases from the desired value. Puree which is having high moisture is evenly placed on one side of the film and removed from the other side after drying.  At the top of the film, a stainless steel hood is provided to exhaust the vapour that is evaporated from the product. 

Figure 1: Pictorial representation of RW drying along with components


  1. Colour of product:

Colour of food product plays an important role in the acceptance or rejection of the product by the consumer. A study was carried out for the colour characteristics of strawberry and carrot purees with or without additives by refractive window drying over the freeze drying, spray drying, drum drying. And the results show that the samples dried under RWD and FD are brighter and redder than the fresh sample in both the cases with and without additive. So from this study, we can say that the RWD helps in preserving the colour of the product.

  1. Microstructure:

Microstructure also plays a significant role in the transport properties, quality, and stability of processed products. Due to the uniform and control application of puree/slurry using the spreader over a film at the inlet side of the RWD resulted in uniform and smooth flakes of the dried product. The microstructure of mango powder obtained from RWD was smooth, flaky, and with uniform thickness according to one research.

  1. Anthocyanin content:

Anthocyanins are water soluble pigments belonging to the phenolic group. Research has demonstrated that RWD leads to low loss of anthocyanin. A study reported 23%, 41%, and 45% losses in TAC on in the coloured potato flakes following RWD, DD, and FD, respectively.

  1. Bulk density and Porosity:

The bulk density of RWD products is significantly higher than that of other advanced drying technologies, such as FD and SD, due to the flat structure of the dried product resulting in lower porosity. Due to the lower porosity and higher bulk density of the RWD product, less prone to the oxidative degradation processes during storage due to its lower surface area, therefore it can also be stored and transported easily.

Future :

Refractive window drying is a developing technology it has a positive effect on product cost and quality, energy efficiency, and scalability. RWD protects the thermal-labile compounds like colour, phytochemicals, and vitamins than other drying methodologies due to its forced air convection which decreased the product temperature and accordingly, moisture level also. But, till now only drying of juices and purees of fruit and vegetables are done. So, more research is needed for the drying of solid foods like meat, fruits, vegetables, etc without compromising their quality. Nowadays, efforts are done to combine RWD with other novel processing technologies like microwave, ultrasound, etc., for the pre-treatment of food products. A bottleneck to scaling up the RW drying process is the need for a wide area to dry and thermal exchange and the film’s thickness is kept on the low side, which reduces the processing capacity. The most developed RWD systems are bulky, requiring more space for installment in commercial and pilot plant applications. Therefore, further research is necessary for development and modification in the design of the RWD system for effective use of limited industrial floor space and to enhance its application in the food industry.

*Ph.D. Scholar, ICAR- NDRI, SRS, Bengaluru

*Senior Research Fellow, ICAR- NDRI, SRS, Bengaluru


*Corresponding author: hsonarthi@gmail.com