A critical review on drying of food materials: Recent progress and key challenges

One important problem that is closely related to developing and refining novel drying techniques is the deterioration of aromatic herbs, fruits, and vegetables during the drying process. Traditional drying techniques are significantly less energy-efficient, take longer time, or both. This also occasionally leads to decrease of product quality. However, using sophisticated drying techniques might result in inconsistent food product drying and increased capital expenses. Hence, a multifaceted strategy that offers a foundation for choosing a specific drying method is of absolute need. Drying also involves a variety of pretreatment and hybrid drying procedures which increases the complexity of the entire process. Some of the pretreatment methods discussed include osmotic dehydration (OD), ultrasound, pulsed electric field (PEF), cold plasma and pulsed vacuum treatment (PV). Also hybrid or combined drying methods have been discussed like electromagnetic waves assisted drying methods, solar assisted heat pump drying methods, convective drying followed by vacuum microwave drying technique to name a few. This review has considered a variety of factors, including the amount of time needed for drying, energy utilization and process complexity. It has also taken into account qualitative aspects of the drying material, such as color, structure, chemical composition, and sensory qualities. This study also highlights several comparison schemes and nondestructive ways of quality check. Additionally, it offers a special technique called "The Drying Triangle," which plots the various drying techniques according to their respective utility in a single plot. Different drying techniques have been allocated to Seven different zones namely zone 1 to zone 7 in "The Drying Triangle" considering the factors as discussed above. A better quality of dried product is obtained at the expense of higher method complexities. Drying time and energy utilization varies from one method to another. A good drying technique should minimize drying time, boost energy utilization, reduce process complexity and improve the quality characteristics of the item being dried. Thus, "The Drying Triangle" can used to select the best drying technique for a particular item to be dried. Various pretreatment techniques like ultrasound and osmotic dehydration reduces drying time and improves product quality. Use of electromagnetic waves though reduces time to large extent but deteriorates product quality when used alone, hence they are used in conjugation which sometimes reduces the energy utilization of the whole process. Hybrid drying techniques such as vaccumspray drying, spray-freeze drying and convective drying followed by vaccum -microwave drying reduce time, improve energy efficiency but increase process complexity. Solar power as low cost energy source can be utilized to further enhance the economic prospects in drying.