Guest Editorial: Research on Drying

I am pleased to congratulate Arun Mujumdar on the occasion of the twenty-fifth anniversary of the founding of Drying Technology. With the start of this journal and the earlier launching of the International Drying Symposia (IDS), Professor Mujumdar created vehicles that invigorated the field of drying research and brought it to a much greater and broader level of activity. When one considers the very large capacity and myriad applications of drying, the amount of activity on drying research still seems less than is warranted by the importance of the field, but the research would be far less without Drying Technology and IDS.

Drying processes are among the largest consumers of energy in industry. There are good avenues for research to lessen energy consumption in drying. The volume of research on energy conservation in drying over the years has been much less than, for example, that on energy conservation in distillation. Yet the importance is comparable. Possible reasons for this striking difference are that the equipment for distillation is less varied than that for drying and that distillation research classically received much attention from large petroleum and chemical companies, as well as from the Fractionation Research Institute. Applications of drying and types of dryers are more diverse and scattered than those for distillation, with the result that the private sector has not invested as much in research. There is therefore a cogent argument for more public sector support of research on drying. The U.S. Department of Energy and U.S. Department of Agriculture and comparable organizations in other countries are logical venues for that purpose.

Research on drying has tended to be product specific and dryer specific. This fact has limited the growth of general understanding. There can and should be more generic research on drying. For example, the mechanisms of water movement through most media being dried are still not well understood, yet with knowledge of transport mechanisms one has a much better insight into how to control structure, migration of solutes, drying rates, and the loss or retention of properties such as texture, flavor, aroma, and color. Since drying is one of the simplest, yet effective, means of preservation of foods, pharmaceuticals, beverages, and other consumer products, such knowledge can extend and improve applications considerably.

Another general challenge in food and beverage applications is to retain flavor and aroma in substances that are far more volatile than water, even though the water is being vaporized. Since equilibrium is not favorable, the approach must be through selective transport, taking advantage of the much smaller size of the water molecule and creating local environments that trade upon molecular size for mobility. Much has been learned about how to do this in freeze and spray drying, but many opportunities for understanding and advances remain for those and other applications. The inverse need pertains when an odorous substance is being dried to a product that should be odor free, leading to approaches that minimize transport barriers to enable full escape with the water vapor.

Yet another need is to know how to control the structure and morphology (shape, form, and configuration) of a dried product. This knowledge is useful to control bulk density, porosity, lightness or darkness of appearance as relates to exposure of pores at the surface, layering of substances for products that are mixtures, tendencies of the product to be crystalline or amorphous, flowing properties of powders as they relate to surface smoothness, loss or retention of non-aqueous volatile substances during bursting, wetting properties, and rehydration tendencies.

Rehydration can also occur in unwanted situations. For amorphous materials, the phenomena of stickiness and structural collapse have been explained in terms of viscous flow.^{[ 1 2 ]} With this understanding, more research could be carried out on product formulation and on ways to control drying conditions to result in desired structural morphology and lack of stickiness, with applications particularly to specialized biological and pharmaceutical products and/or to freeze drying.

These are but a few of the avenues of drying research that are promising and that are not product specific or dryer specific. More research of this sort will provide an underpinning of fundamental understanding that will be powerful in improving and controlling product quality, enabling new and better designs and configurations of dryers, saving energy, and extending the use of drying as a method of stabilization and preservation.