Expert reviews for assessment of recent developments and future prospectives of global drying R&D

With the explosive growth in archival scientific and technical literature, it is increasingly difficult for researchers as well as users of such literature to access and then assimilate the essential knowledge useful for their purposes. This journal has therefore recognized the need for the publication of critical and comprehensive reviews of selected topics of current and future interest to assist with and even guide further R&D. Here, we attempt to make it easier for our global readership to obtain a quick overview of review papers published in Drying Technology in the previous years (2021-2022). We hope this simplifies access to reviews available in the journal, which benefits researchers and users of this valuable knowledge.

In order to enable access to experts’ reviews that have appeared in this journal in recent years, we have devoted similar Guest Editorials in the past, along with a bibliometric analysis of publications in Drying Technology in selected drying areas and the relevance of drying science and technology in climate change.^[1–6] A capsule summary of the 22 review articles published in 2021-2022 is given in this Guest Editorial under broad theme areas enlisted hereunder.

Computer-aided drying techniques

Almost all existing applications of fuzzy logic in drying literature can be categorized into three main streams: modeling, clustering/classification and control. Both benefits and shortcomings of fuzzy logic applications are systematically reviewed by Hosseinpour and Martynenko.^[7] A roadmap for future studies based on the current state and significant underexplored potentials of fuzzy logic in drying technology is proposed. A comprehensive review of the application of machine learning (ML)-based approaches in food drying modeling, properties prediction, microstructural characterization and process parameters optimization, along with the possibilities and challenges of applying ML-based algorithms in multiscale modeling and microwave-based hybrid drying, is given by Khan et al.^[8]

X-ray micro-computed tomography represents a versatile technique for three-dimensional investigation of processes at a microscopic level. Basic principles of X-ray micro-computed tomography and an overview of the existing literature is systematically presented by Haide et al.^[9]

Renewable based drying techniques

Strict regulations imposed by the European Union, with the target of reducing greenhouse gas emissions, oblige biogas plant operators to manage digestate in a proper manner. Drying of digestate faces many difficulties because of material specific properties as well as environmental issues. A literature on drying of digestate, drying kinetics and emission behavior of digestate, is thoroughly discussed by Salamat et al.^[10] Furthermore, a comprehensive review of recent advances in drying technologies and energy consumption as well as efficiency and economic performances of several alternative drying technologies is critically discussed by Acar et al.^[11]

Lignite has great potential for use as one of the major sources of both energy and chemical feedstock. High moisture content nevertheless extremely restricts its use. The issue of high moisture re-adsorption capacity of lignite dehydrated using various drying techniques is critically reviewed, along with advise on future investigations on inhibiting moisture re-adsorption of dehydrated lignite by Liu et al.^[12] Literature addressing drying theory and underlying mass, species and heat transfer phenomena, identification of the most energy-consuming steps in the process, assessment of the potential for energy savings and energy saving techniques are considered and critically discussed by El Fil and Garimella^[13] in order to identify the optimal techniques to minimize energy consumption and drying time for various drying scenarios.^[13]

Role of drying techniques in encapsulation

Use of different wall materials, conditions as well as comparison among different microencapsulation techniques on encapsulation efficiency is discussed by Maroof et al.,^[14] along with pertinent industrial applications of spray-dried propolis. Microencapsulation as a way to enhance viability of probiotics during different processing techniques and under gastrointestinal conditions is also reviewed by Sehrawat et al.^[15]

Drying of food, plant-based, agricultural and textile products

A systematic review of the effects of protein-based drying aids on the quantity and quality of sugar-rich spray-dried powders, with the conclusion that ‘the protein drying agents are yield-effective ingredients compared to commercial alternatives’ is given by Mirlohi et al.^[16] Their use as wall materials for spray drying sugar-rich foods is nevertheless not always the best choice.^[16]

Production of insects for food and feed is rapidly emerging in Europe, filling an important niche of locally supplied protein and fat sources with improved environmental sustainability. The summary of recent activities performed by different interdisciplinary research groups dealing with insect drying and selected legal aspects concerning usage of dried insects as food is presented by Parniakov et al.^[17]

Based on the United Nations (UN) Environment Programme’s Food Waste Index Report 2021, the wastage of foods reaches 931 million tonnes each year. To align with the UN Sustainable Development Goals, which aim to halve food waste by 2030, food waste, especially that from fruits and vegetables that have short shelf life, can be processed into stable value-added products by different drying processes.^[18] Current trends in the development of drying techniques for plant materials in terms of the retention of their bioactive compounds is critically analyzed by Belwal et al.,^[19] with recommendations for better use of drying techniques for both plant materials and retention of their bioactive compounds.

Due to the increased knowledge of nutrients and functional ingredients and their positive effects on health, people are tending to choose healthy food products, including snacks, rich in nutrients and bioactive ingredients. The characteristics of freeze drying and novel processing technologies based on freeze drying as applied to the production of healthy snacks and meal replacement products are critically reviewed. Also, new freeze-dried healthy snacks and meal replacement products enriched with special nutrition and function are discussed by Chen et al.^[20]

Viability of probiotics is known to be affected by environmental conditions and biochemical changes during storage and digestion. Drying of probiotics into powder form exerts various benefits (e.g., ease of handling, high stability, less space needed for storage). A comprehensive summary of probiotics drying techniques, protective material as well as mathematical models to predict probiotics survival and powder properties is given by Ermis.^[21]

Drying temperature, air velocity and thickness of material exerts significant influences on drying kinetics and quality attributes of fruits and vegetables. Automatic RH control and adjustment has been investigated based on material temperature. Such RH control drying strategy provides theoretical foundation for enhancing drying efficiency and quality of fruits and vegetable materials and is discussed by Zhang et al.^[22]

Due to the fast pace of life around the globe and to the increasing demand for high-quality convenience foods by consumers, instant foods have attracted widespread attention. Microwave freeze drying, infrared drying, among others, are relatively new processing technologies that are suitable for the production of instant foods. An overview of recent developments in processing technologies for the production of compound formula instant foods is systematically given by Du et al.^[23]

Therapeutic protein preparations are usually stored in a solid state to reduce physical and chemical degradations. The stabilization mechanisms of proteins during freeze drying, with a focus on factors that induce protein instability, and the relationship between protein structural changes and instability, are expected to provide references for solving the instability problems during protein freeze-drying and storage. These topics are thoroughly discussed by Wang et al.^[24]

The current trend in microwave-assisted drying of fish is given by Shahriar et al.,^[25] with the conclusion that pulsed microwave convective drying can be effectively implemented in fish drying, considering the critical need for improving energy efficiency and quality retention in fish drying.

Drying of pharmaceutical ingredients, vaccines, and biomolecules

Vaccines are the most important life-saving prophylactic medicines administered in maintaining global health. In a pandemic situation, manufacturing, formulation, which is followed by preservation, of vaccines are challenging areas of concern. A comparative discussion of various technological challenges in drying of vaccines in support of the current global health scenario affected by COVID-19 is given by Thorat et al.^[26]

Active pharmaceutical ingredients are often present in more than one polymorphic form. Most of the time, one of these polymorphs exhibit superior properties to the others and is therefore the desired form. One of the unit operations that may produce undesired polymorph is drying. A thorough account of past and current research, which indicate future directions is comprehensively discussed by Chakraborty et al.^[27]

Recent advances in sludge dewatering

Dewatering and drying play significant roles in minimizing sludge volume, facilitating transportation, increasing calorific value and reducing leachate production in landfill sites. Various new technologies are emerging and existing ones are always evolving because of the sharp growth in the generation of sewage sludge. The present dewatering and drying technologies are reviewed and compared in order to provide useful guidelines for sludge disposal. Future demands and opportunities for sludge dewatering and drying are also pointed out from industrial perspective.^[28]

Aparupa Pani http://orcid.org/0000-0002-1457-2152
Kalinga Institute of Industrial Technology
Bhubaneswar, India
[email protected]
Shivanand S. Shirkole http://orcid.org/0000-0001-8324-2846
Institute of Chemical Technology Mumbai
ICT-IOC Odisha Campus, Bhubaneswar, India
[email protected]
Arun S. Mujumdar
McGill University
Ste. Anne de Bellevue, Quebec, Canada
[email protected]