Outlook in Refractance Window Drying Technology
Erasmo Herman y Lara
Department of Chemical and Biochemical Engineering, Technological Institute of Tuxtepec, Mexico
Corresponding author: Erasmo Herman y Lara, Department of Chemical and Biochemical Engineering, Technological Institute of Tuxtepec, Mexico. E-mail: email@example.com
Citation: Herman-Lara E (2019) Outlook in Refractance Window Drying Technology. Recent Adv Food Sci Nutr Res 2019: 01-02. doi:https://doi.org/10.29199/2638-9770/FSNR-201019
Received Date: 26 November, 2018; Accepted Date: 11 January, 2019; Published Date: 11 January, 2019
Today people demand increasingly fewer perishable foods, where the lengthening of the shelf life is fundamental, especially for those foods that need to be transported for long periods of time from one province, country or from one continent to another. Therefore, drying methods are frequently used in order to reduce the water content of these products and achieve this goal. However, the processes or technologies of drying have been evolving to generate higher quality dry foods; so far four periods or generations of types of dryers have been developed to obtain these kinds of products.
Initially, foods, such as seeds, grains, vegetables, and fruits, were dried with first generation equipments, such as solar, tunnel, cabinet, rotary flow, and trays dryers, in which the use of hot air, hot surfaces and radiation induce to processes convective, conductive and radiative, respectively. However, the excessive energy expenditure and the poor drying quality of these products make them currently inconvenient in some cases. Subsequently, second-generation dryers, such as drum and spray dryers, ideal for pastes, creams, suspensions, and concentrated solutions of solids produce food powders and flakes. Even when these devices are still used and substantial improvements have been made, they are limited to a certain specification of the food shape to dry. Osmotic dehydration and freeze-drying belong to the third group of drying technologies. The first method, despite being protective of the quality of dehydrated foods, only reaches a level of water removal not very low and only limited to the dehydration of solid foods, such as fruits and vegetables with a high consumption of salts, sugars or combinations of them. On the other hand, the second process is a technology that achieves lower levels of water content, obtaining foods with a long shelf life. Despite this, it is an expensive technology that is limited only to a considerable number of foods such as tea, coffee, spices, among others. Finally, fourth-generation drying technologies, such as fluidized bed, microwave, radio frequency, heat pump and drying technology by Refractance window (RW™) are emerging. This last methodology uses contact transparent films inducing both conductive and infrared radiation processes.
All the drying technologies have had as the main point to increase the shelf life of food products, in such a way that if the water content in them is reduced, the main form of microbial and enzymatic activation is eliminated; therefore, the undesirable biochemical reactions are diminished. In addition, the volume and weight of dry products are reduced obtaining lower storage and transportation costs. However, due to the use of operating conditions with excessive hot, aggregation of dehydrating sugars and salts, electromagnetic radiations, phase changes from solids to gas and liquid to gas produce undesired effects such as severe changes in chemical, physical, physicochemical, and nutritional properties. In addition, many of these technologies are only used to dry certain types of food, whether in liquid, semi-solid or solid form, or a certain texture.
Then, the question is: What makes that RW™ drying technology attractive? Being a new technology, which initially started drying suspensions, sauces and purees, why is not it a limited drying technology? RW™ formally is a technology patented by MCD Technologies, Inc. in Tacoma, Washington. RW™ technology is a process where conductive thermal energy is transmitted with infrared radiation by using a Mylar film (DuPont TM Wilmington polyester film, Cleveland, OH, USA) on a surface of hot water less than 100ºC.
After diverse research works with this type of drying, it has been possible to obtain dry powder products very similar in final moisture content to those obtained by freeze-drying, but with some advantages such as: lower cost, greater energy efficiency, a less complex process, shorter drying time, greater capacity of production that directly influence the conservation of nutrients (vitamins and proteins mainly), functional properties (antioxidants, rehydration capacity, porosity, density, solubility, foam capacity), sensory (degree of acceptance by the consumer in terms of colour and taste) and retention of bioactive food compounds (polyphenols, tocopherols, carotenes, ascorbic acid and others). Therefore, it was ideal to dehydrate heat sensitive foods, enzymes and pharmaceutical products. It was, therefore, essential to research the use of RW ™ in completely solid complexion foods such as fruits, vegetables and meats, in order to obtain dry products without adverse effects such as deterioration of texture, a substantial reduction in size, loss of colour and the other properties that were important in liquid and semi-solid foods. A substantial success in the retention of the physical, chemical, physicochemical, functional, nutritive properties and, other properties, were obtained only when these foods were used between 1 to 2 mm thick and when the temperatures of RW™ were between 70 to 90 ºC.
This technology is consequently more versatile to be used with food products of different textures and shapes, obtaining a reduction of the drying time between 40 to 70% in foods that when is used other types drying methods. The use of different thicknesses in Mylar films and other transparent plastic films is currently being investigated. Currently, scientific works are carried out to test the yield and energy efficiency of food drying using RW™. Other materials besides the Mylar film have been tested as energy conducting materials: sheets of stainless steel, copper, iron and other plastics with a certain degree of heat resistance in order to test its scalability, energy efficiency, costs, and end-product quality of the food. New research is being directed to the use of RW™ drying technologies combined with the use of microwaves, with convective drying with direct air flow/reversal airflow and drying of trays. A specific current case is the evaluation of the combined effect of osmotic dehydration as an initial pre-treatment and later drying by RW™ specifically in hard skin fruits with a high content of pigments and bioactive compounds such as carrot, chili, aubergine, papaya and beet, among others, considering different thicknesses of the food, different temperatures and concentrations of the dehydrating agent in the osmotic dehydration and different temperatures in the RW™.
Consequently, it is important to continue researching the transmission forms of infrared radiation and conductive of various materials replacing the Mylar film, as well as the study of various food products in which the greater retention of physical, chemical, functional-nutritional properties can be obtained, that allow consumers more time for consumption