This technology is being applied to heat sensitive products such as fruits and vegetables in order to retain its various qualities (colour, texture, aroma, sensory) and nutritional (antioxidant, polyphenol, flavour compounds) 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 (conduction, convection and radiation). In Refractance Window Drying, the temperature of drying, including the cost, time and energy consumption are low, but the product’s quality as well as thermal efficiency remains higher, as compared to various conventional drying techniques such as hot air drying, spray drying, tray drying and drum drying. 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. The Mylar film takes the heat from the hot water (temperature below 99°C) at atmospheric pressure. The product slurry is spread over a transparent film evenly and the film permits the infrared energy at the speed of light to the product. The energy transfer due to conduction or radiation depends on the thermal resistance offered by the plastic film and the evaporation of moisture is by convection of air above the product. The air convection helps to lower the temperature during drying. Again, a cooling system arrangement is usually done by cooling the water in a tank below the conveyer belt of the dryer.

Refractance Window Drying system has been successfully applied by Da Silva and Da Silva, 2015 for rapid reduction in the moisture content of oil reduced avocado pulp with minimal change in physico-chemical properties. RW drying offers high quality dried products at a low drying temperature at a minimum time. The RW process had dried raspberry puree from 92% to 4% moisture content in 5 minutes. (Raghavi et al. 2018)

Schematic Diagram of Refractance Window Dryer

Just as in most of the drying processes, during RW drying the product that is being dried initially goes through a constant drying rate period which is also mainly dependent on the product thickness. RW drying mainly follows type III patterns of moisture isotherm (Zotarelli et al. 2017). Nindo et al. (2003) reported the reduction in moisture content of pumpkin puree by around 80% at 95°C hot water temperature with 2.98 m/min conveyor belt speed in 4.5 minutes of drying. The reflectivity value of pulp increases and the absorptivity values drop during the drying process (Nindo and Tang, 2007). No chemical additives are used in RW drying.

Advantages of Refractance Window Drying

Higher product quality retention happens within a lower drying time. This can be justified with the research study that was reported by Abonyi et al. (2002) during strawberry puree Refractance Window drying. The strawberry puree moisture content reduced from 4.8 kg H2O/kg solid to less than 0.2 kg H2O/kg solid in less than 4 minutes. The colour is an important sensory attribute and RW drying results in minimum total change in colour values, as compared to other conventional drying methods. (Vithu and Moses, 2016)

Nindo and Tang (2007) reported that RW can effectively dry high sugar products that are difficult to dry by following other drying techniques, which also includes spray drying. Other research findings from Nindo and Tang (2007) and Nindo et al. (2003) on RW drying system are given below:
(1) It can be suitable for drying microbial cultures with acceptable viability and other bioactive compounds, as the product can be dried at a lower temperature (not exceeding 30°C).
(2) It has the capacity to reduce E.coli and coliforms inoculums population by 106 CFU/ml.

The product temperature is the critical factor to decide the end quality of the dried product and the moisture transfer, due to natural or forced convection that affects the temperature profiles of the product at different sublayers on the film. Generally, for the same temperature level in the contact medium (water in RW and air in convection drying), the relative temperature gain is lower in RW which results in improved product quality (Sojak et al., 2014). Some research study using circulating water baths at 90°C in RW drying reported the temperature difference across the Mylar® films to be 10°C lower than the hot water bath temperature. Moisture content and temperature of storage greatly affects the storage stability of RW dried powders. Pavan et al. (2012) found RW dried powder to be microbiologically safe, because of low water activity with good storage stability.

Recent Improvements

RW drying is combined with the different drying methods to improve drying efficiency and minimize product quality degradation. Recently, RW drying is combined with infrared radiation to accelerate the drying process, as it can be easily implementable. Another research study reported infrared assisted RW drying on drying characteristics, colour changes and microstructural changes of Physalis fruit purée at a different drying temperature and different Mylar film thickness. The drying kinetics behaviour was strongly influenced by the drying temperature and the IR-assistance, whilst slightly by Mylar thickness. The research study revealed that IR-assisted RW drying reduces the drying time by 60% to attain 10% equilibrium moisture content. They recommended RW drying combined with IR to be a better drying process to dry Physalis fruit puree, due to reduction in energy consumption and low operational cost during the industrial-scale drying process. (Luis Puente-Díaz et al. 2020)

IR-assisted RW drying set up
Fig. 3: IR-assisted RW drying set up

Hybrid drying of RW has still not yet been explored so far. Therefore, in the future, microwave, ultrasound and osmotic drying could be combined with RW drying in order to obtain a better drying quality.

1. Abonyi, B.I., Feng, H., Tang, J., Edwards, C.G., Chew, B.P., Mattinson, D.S. and Fellman, J.K., 2002. Quality retention in strawberry and carrot purees dried with Refractance WindowTM system. Journal of Food Science, 67(3), pp.1051-1056.

2. Da Silva, C., and Da Silva, C. (2015). Recovery of avocado paste from avocado oil milling process or guacamole processing. U.S. Patent Application No. 14/ 846,744.

3. Pavan, M.A., Schmidt, S.J. and Feng, H., 2012. Water sorption behavior and thermal analysis of freeze-dried, Refractance Window-dried and hot-air dried açaí (Euterpe oleracea Martius) juice. LWT-Food Science and Technology, 48(1), pp.75-81.

4. Nindo, C., Sun, T., Wang, S.W., Tang, J. and Powers, J.R., 2003. Evaluation of drying technologies for retention of physical quality and antioxidants in asparagus (Asparagus officinalis, L.). LWT-Food Science and Technology, 36(5), pp.507-516.

5. Nindo, C.I. and Tang, J., 2007. Refractance window dehydration technology: a novel contact drying method. Drying technology, 25(1), pp.37-48.

6. Puente-Díaz, L., Spolmann, O., Nocetti, D., Zura-Bravo, L. and Lemus-Mondaca, R., 2020. Effects of infrared-assisted Refractance Window™ drying on the drying kinetics, microstructure, and color of physalis fruit purée. Foods, 9(3), p.343.

7. Raghavi, L.M., Moses, J.A. and Anandharamakrishnan, C., 2018. Refractance window drying of foods: A review. Journal of food engineering, 222, pp.267-275.

8. Sojak, M.J., Jaros, M. and Głowacki, S., 2014. Analysis of giant pumpkin (Cucurbita maxima) quality parameters in various technologies of convective drying after long-term storage. Drying Technology, 32(1), pp.106-116

9. Vithu, P. and Moses, J.A., 2016. Machine vision system for food grain quality evaluation: A review. Trends in Food Science & Technology, 56, pp.13-20.

10. Zotarelli, M.F., da Silva, V.M., Durigon, A., Hubinger, M.D. and Laurindo, J.B., 2017. Production of mango powder by spray drying and cast-tape drying. Powder technology, 305, pp.447-454.

About the Author:
Sudarshanna Kar
Assistant Professor, Department of Agricultural Engineering,
Institute of Agricultural Sciences, SOADU.
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