Guest editorial: Reviews on drying science and technologies

Empirical understanding of drying processes and invention of associated technologies have served human civilization ever since the hunting-gathering age. Once human civilization entered the agriculture age, the volume of production grew and further understanding of drying process and improvisation of drying technologies became much more pressing as most of the agricultural produce are seasonal and perishable. Sun drying and more sophisticated hot air drying, backed by empirical understanding were predominantly developed in this age. Transfer of this empirical knowledge on hot air drying occurred primarily through apprenticeship.

The industrial age of human civilization saw great advances in fundamental science and engineering. Increased understanding of heat and mass transfer processes and their governing laws such as Fourier’s Law and Fick’s law helped advance drying science to a new level. In several decades after 1940s, scientists and engineers further deepened drying research to single droplet, single grain, microscopic and even molecular levels. A.V. Luikov’s theories on heat and mass transfer provided the early analytical and theoretical footing for drying science. The concept of vapor-liquid equilibrium or concept of ‘water activity’ and understanding of free and bound water, sorption isotherms, activation energies helped link the drying process to shelf-life prediction and preservation.

Manufacture of newer and cutting-edge dryers progressed hand-in-hand with the advancing science. The ability to create sub-atmospheric pressure gave rise to vacuum dryers. The understanding and application of refrigeration system together with sub-atmospheric pressure gave birth to freeze drying. Soon afterwards, drying technologies and dryers were used to produce high tonnage of high value food ingredients such as spray-dried lactose, spray-dried whey proteins. Spray-dried infant formula powders and coffee powders and freeze-dried coffee powders became house-hold products. Drying technology is responsible for giving the world highly specialized space foods. Drying science and technologies then crossed the agri-food sector to other fields such as the pharmaceutical industry, which led the production of powdered, tableted and encapsulated drugs and excipients. Pharmaceutical industry now routinely uses sophisticated dryers and produces high value dried products. Drying technologies are also encountered in other industrial sectors, e.g., pulp and paper, wood, ceramics, mineral processing, polymer processing.

In the last decade of the 20th century, we saw drying research and application moving to hybrid systems. Research on combination of newer energy sources such as microwave, ultrasound, infrared proliferated and industry soon started manufacturing dryers that combined hot-air drying or freeze-drying with one or more of the above-mentioned novel energy sources. Some of these hybrid-drying systems are now manufactured in industrial scale and used commercially. Many of these hybrid systems are still confined to the laboratory; however, the innovations reflected in these hybrid dryers are a testament of advances this field is making.

The development of many advanced characterization systems and instruments including dynamic vapor sorption system, porosimeter, thermal scanning calorimeter, scanning electron and transmission electron microscopes, X-ray photoelectron microscopic systems and others enabled characterization of dried products to molecular level. Such information, together with mathematical and numerical modeling tools, helped make the drying systems more efficient.

Systematic publication of drying research in the form of journal started in 1983 with the arrival of Drying Technology. One can now download the digitized version of all articles published in this journal through various databases. Drying-related research is also extensively published in numerous other journals and books on science, agriculture and engineering. A search using keywords ‘drying’ and ‘review’ as ‘article title’ indicates that review articles dedicated to critical evaluation and analysis of contemporary knowledge on drying science and technology also has a long history as arguably the first archival review paper was published in 1951 on freeze drying. We nevertheless realized that no journal has published a special issue that solely includes high-quality review articles dealing with various aspects of drying, holistically covering fundamentals, modeling and applications in multiple fields. We also felt that a dedicated issue on the above, carried out by highly regarded experts in the field with special focus on current state of knowledge, would enable current and future researchers to obtain the gist of drying research in a single issue.

This issue covers up-to-date knowledge on fundamentals and application aspects of drying in spray drying of protein pharmaceuticals. This is important as drying of proteins is sophisticated, owing to their unique secondary and tertiary structures and sensitivity to heat and desiccation. The issue also documents how particle engineering approach is applied to various drying technologies including spray- and freeze-drying to produce highly soluble and dispersible pharmaceutical drugs and excipients. The issue brings thorough and substantial in-depth reviews on the ingredients, formulations and storage aspects of infant formula powders written by young scientists. These reviews are complemented with a review on encapsulation of protein hydrolysates. The issue also brings in comprehensive reviews on hybrid technologies that are commercially available and/or in R&D stage. This is complemented in tandem with systematic reviews on mathematical modeling and fundamental approaches used for achieving energy efficiency in drying systems, particularly in spray drying systems. Interesting and unique reviews on out-of-ordinary fields, including application of adsorptive materials, drying and dehydration of minerals as well as value-addition of agricultural by-products through the use of drying technologies are also included.

The authors who have authored these high-quality critical reviews are internationally well-known experts in their fields. The Editorial Team wishes to sincerely thank the authors for their valuable contributions, especially during current uncertain times. We are happy to be able to bring this special issue to our valued readers.

Benu Adhikari
RMIT University, Melbourne, Australia
[email protected]
Shivanand S. Shirkole
Institute of Chemical Technology Mumbai
ICT-IOC Odisha Campus Bhubaneshwar, India
[email protected]
Hong-Wei Xiao
China Agricultural University, Beijing, China
[email protected]