ABSTRACT

Nomenclature ....................................................................................................... 58 Variables ................................................................................................... 58 Greek Letters ............................................................................................ 58

References ............................................................................................................ 58

Drying is one of the major operations in food processing. There are several drying techniques, the most common being hot-air (fi xed and fl uidized bed), freeze-, and spray drying. In most drying conditions, a signifi cant proportion of the dried product remains in an amorphous state. In some slow drying processes such as tray and fl uidized bed drying, the high molecular weight carbohydrates and proteins normally remain amorphous but other low molecular weight sugars or organic acids may be partially crystallized. This crystallization can be intentional or undesirable. In the low temperature process (freeze-drying), almost all of the ingredients become amorphous because of the rapid cooling or prefreezing. Subsequent sublimation is followed by low temperature drying. The majority of spray-dried products are amorphous because this process is rapid enough to hinder the relatively slower crystallization process. In some cases pre-or postspray-drying crystallization of sugars is also practiced. In all drying processes, the degree of amorphous fractions in the powder is infl uenced by the processing conditions, compositions, and properties of the individual ingredients.