Refractance window drying of foods: A review
Introduction
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). Nevertheless, dried products are more stable to microbial contamination and other deteriorative chemical reactions, apart from facilitating storage and minimizing transportation costs.
Over 85% of industrial dryers are convective, with either hot air or combustion gases as the media for heat transfer (Zarein et al., 2015); often resulting in significant levels of changes in product quality from the initial ‘fresh-like’ form. Basically, these are gas-solid contact operations working on gradients in water vapour pressures. Further, in most cases, conventional drying methods produce inferior quality products and require higher drying times (Moses et al., 2013, Moses et al., 2014b. There is a strong need for developing alternative drying technologies considering operational capacity, process control, time requirements, cost economics, product quality, safety and environmental aspects. Novel drying technologies are intelligent combinations of conventional technologies necessitated by changes in consumer and market requirement.
Drying technologies belong to first, second, third or fourth generation. Cabinet and bed type dryers such as kiln, tray, truck tray, rotary flow conveyor and tunnel using hot air as the medium of heat transfer belong to first generation and better suit solid materials including food grains and horticultural commodities. Spray and drum dryers are second generation technologies and better suit slurries and pastes that require to be dried in the form of flakes and powders. Dreeze dehydration and osmotic dehydration belong to third generation of drying technologies (Vega-Mercado et al., 2001). Microwave drying, infrared drying, heat pump drying, fluidized bed drying, radio frequency drying and refractance window (RW™) drying are considered as fourth generation drying technologies based on the type of raw materials they can handle and the retention of quality attributes of the intended products (Chou and Chua, 2001).
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. (2017) 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).
The technique is already reportedly used for commercial applications by MCD Technologies, Tacoma and Mt. Capra, Washington for drying of whey and other products (Mt Capra, 2017). Commercial dryers also dry a range of products; including brine shrimps, micro-algae and herbal formulations (Food Online, 2017). Unlike certain other drying technologies, RW™ drying does not require high operational pressures. Also, there is limited scope to oxidation and free radical formation and RW™ dried products offer improved shelf life. (Mt Capra, 2017).
The objective of this work is to present the recent trends in refractance window drying of foods. Detailed descriptions about typical dryer components and the mechanisms of heat and mass transfer during the drying process are presented. This review also includes an exhaustive summary on changes in product quality during RW™ drying of foods as compared to other drying methods such as tray, spray and freeze drying, based on available published works till date. In comparison with several conventional drying approaches, RW™ drying is known to offer superior quality powders and flakes at relatively lower operation costs and time requirements. To give a complete understanding on the topic, aspects of energy requirements and thermal efficiency calculations are also presented.
Section snippets
Components and working of a RW™ drying system
“Drying of viscous solutions and suspensions spread in flexible supports was named as Cast-Tape Drying – CTD” (Durigon et al., 2016, Durigon et al., 2017). The RW™ drying technique is a variant of the CTD technique (Zotarelli et al., 2017) patented by Magoon (1986) and developed by MCD Technologies Inc. (Tacoma, WA) (MCD Technologies, 2017). The major components of a typical RW™ drying system are presented in Fig. 1. Essentially, evenly applied food materials over a thin infrared transparent
The contact film
During RW™ drying, all three modes of heat transfer are active. Though in typical cases the heat transfer due to conduction is dominant, the relative role of each mode of heat transfer depends on the resistance of water each mode offers (Ortiz-Jerez et al., 2015). While conduction, convection and radiation occur at the hot water-film interface, conduction and radiation occur through the film and convection occurs at the air-film interface.
The transport of thermal radiation through the plastic
Energy consumption in RW™ drying
At the film-product interface, intense evaporation takes place as transmitted thermal energy gets absorbed by the high-moisture product. This is supported by evaporation of product moisture at the product-air interface (Ortiz-Jerez and Ochoa-Martínez, 2015) also suggested the possibility of evaporative cooling at the film-product interface due to air gap formations and associated effects. Thermal efficiency of RW™ drying systems are usually in the range of 52–77%; comparable to drum drying. On
Thermal efficiency of RW™ drying
RW™ drying is characterized with high evaporation capacities of up to 10 kg m−2 h−1 (as observed in studies conducted on 2 mm thick mango pulp spread over a film in contact with water at 95 °C) (Zotarelli et al., 2015). Typical RW™ drying times range from 3 to 5 min (Celli et al., 2016). The approach used by Nindo et al. (2003a) for an accurate energy audit considering a RW™ dryer operated with natural gas supplied steam boiler is presented below. Total sensible heating of product during
Changes in product quality during RW™ drying
All known designs of RW™ drying systems for foods operate under atmospheric pressure. In general, the product to be dried requires being milled/chopped as suspensions, pastes or slurries before being uniformly spread on the film's surface. With minimal effects of the physico-chemical properties, the technique has also been for rapid reduction in moisture content of oil reduced avocado pulp (Da Silva and Da Silva, 2015). With lesser product temperatures, RW™ drying is known to offer dried
Case study on RW™ drying of mango powder
In their work, Caparino et al. (2012) used a RW™ drying system with temperature of circulating hot water maintained between 95 and 97 °C; freeze drying carried out at 20 °C under 20 Pa pressure after an initial freezing stage at −25 °C for 24 h; drum drying done in metal drums with temperatures raised to 152 ± 2 °C using steam and spray drying done at feed rates of 50 ± 2 g/min through a drying chamber maintained at 190 °C (at inlet) and 90 °C (at outlet). Maltodextrin at different
Conclusion
RW™ is a variant of hydro-drying and has the potential to produce high quality dried slices, purees and juices. RW™ can be considered as a novel alternative to dry foods and research infers that active aromatic and pigment compounds responsible for characteristic sensory and nutritional attributes can be retained through this technique. To supplement existing research findings, extensive research may be taken up to understand the underlying intricate mechanisms of heat and mass transfer;
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