Introduction:
Drying is a critical unit-of-production used to extend the shelf life of fresh and perishable goods. During the process of drying, moisture is removed from products, which prevents spoilage-causing microorganisms from growing as well as slows down enzyme activity. Drying involves heating up produce by conduction, convection or radiation, except for freeze-drying, which requires sublimation to remove moisture. Cost of dehydration, energy consumption, reduced drying time and preservation of final quality of the desiccated product are important factors that are taken into consideration while making the selection of an optimal drying method. Though, the preservation of fruits and vegetables through drying has existed for many centuries based on sun and solar drying technologies, it is mostly used today to increase storage stability, reduce packaging requirements as well as transport weight. By utilizing the above-mentioned techniques, it was possible to detect products of inferior quality as well as the presence of any contamination, including being able to detect the requirement for more space and weather dependency. Convective Tray Drying is among the commonly used alternative technique for drying of perishables, medicinal plants, spices and herbs. However, this method is already known to change key physical properties of products, such as colour, texture, taste and nutrients as well as shrinkage, due to long drying time and slow drying rate (Karadbhajne et al., 2019; Ratti & Mujumdar, 1995; Sontakke & Salve, 2015).
In recent years, a variety of technological advances have been made in the area of industrial drying of food, including the development of techniques, equipment, and quality. Microwave drying, heat pump drying, Refractance window drying, fluidized bed drying and radio frequency drying are among the novel methods for improving the efficiency and efficacy of drying, thus reducing energy use and preserving product quality simultaneously. Despite these proven successful technologies, commercial practitioners are often unaware of the techniques with the potential for the industry. In this article, we explore the process of the fourth-generation dryers, such as heat pumps, refractance windows, microwaves, fluidized beds and radiofrequency dryers to promote sustainability in the food industry (Sagar & P Kumar, 2010; Sagar & Suresh Kumar, 2010; Vega-Mercado et al., 2001).
Heat Pump Drying (HPD)
In the early 1950s, experimentation with heat pumps was conducted to dry clothes; the concept was mechanically feasible, but wasn’t appealing because of low fuel prices. As fuel prices rose, heat pumps became popular. In conventional dryers, high-quality energy (such as electricity or fuel) would be used to heat the air and a stream of moist, hot air would be expelled at the exhaust. This represented a significant amount of low-grade energy that was being lost during the process. In order to reduce this loss, heat pumps were introduced to the systems to recover the latent heat of evaporation of water lost in the exhaust from the dryer. With the heat pump evaporator placed in the exhaust stream of the dryer, the air is cooled (to recover the sensible heat component) and then dehumidified (to recover latent heat) by the refrigerant. The heat added to the refrigerant is then transferred to the air stream entering the dryer at the condenser of the heat pump, thus raising its temperature. The added benefit of dehumidifying the drying air is also realized when the air leaves the dryer and is recirculated, increasing its capacity to achieve better drying (Alishah et al., 2018; Patel & Kar, 2012; Sosle et al., 2003).
Microwave Drying (MD)
During Microwave Drying, electrical energy is used at frequencies between 300 MHz and 300 GHz, with 2,450 MHz being the most commonly used frequency. In a microwave oven, alternating current is stepped up from 60 Hz to 2,450 MHz using alternating current from a domestic power line. This is accomplished by utilizing a device known as the magnetron. The use of microwave energy for drying has shown to be relatively energy-efficient. Microwaves are an attractive source of thermal energy, due to their volumetric heating and short processing times. It generates rapid volumetric heating of materials in food products by altering their electromagnetic fields to interact with primarily water molecules and ions. However, as microwaves alone cannot complete the drying process, it is recommended to combine other techniques, such as forced air or vacuum, to further enhance the efficiency of the microwave. Drying of food by the use of Intermittent Microwave Convection Drying (IMCD) being an advanced technique improves energy efficiency and food quality at the same time (Dev et al., 2011; Giri & Prasad, 2007; Sharma & Zalpouri, 2022).
Refractance Window Dryer (RWD)
During the Refractance window drying process, circulating water at 95-97°C transfers thermal energy to the materials being dried. An evenly distributed plastic conveyer belt passes over a hot water trough as the pureed products are distributed. When the dried product reaches the cold-water section, it hardens, making it easier to separate from the belt using a scraper. Ideally, this technology is suitable for products with a distinct aroma and vibrant colour that are pureed or semi-solid. Using this method, fruits, vegetables and herbs with high moisture content can be dried in 3-5 minutes, while retaining their colour, vitamins and antioxidants. The RW drying method has been found to be a viable and low-cost method for producing edible films with adequate technological characteristics and high nutritional value. The dehydrated products can be consumed directly or used in the development of food products (Clarke, 2004; Raghavi et al., 2018; Zalpouri et al., 2020).
Radio-frequency (RF) Drying
Researchers are becoming more interested in RF drying, owing to the increased penetration depth, homogeneity of heating and control of product temperature. The method is also known as dielectric heating. The RF method of heating food is faster and more efficient, because internal heat is generated in the treated food, due to ionic conductance and dipole rotation of molecules. Hence, food quality could be preserved by evaporating only the water and heating it only minimally. RF heaters are used frequently in the final stages of drying to improve energy efficiency and product quality. Conventional hot-air drying methods for solid or semi-solid foods are inefficient at removing moisture during the falling rate period. Moreover, traditional drying involves adding heat from the inside to the outside of the food, which results in cracks and hard shells in the final product. Aside from its selective and volumetric heating capabilities, RF drying has two major advantages over hot-air drying. While RF drying is quite useful in the food industry, where high throughput is needed, the operating costs of such a process are not usually economically viable (Altemimi et al., 2019; Wei et al., 2021; Zhou & Wang, 2019).
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About the Authors:
Kulwinder Kaur, Damanpreet Kaur & Ruchika Zalpouri*
Department of Processing and Food Engineering,
Punjab Agricultural University, Ludhiana, India.
*Corresponding Author Email ID: zalpouri28@gmail.com
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