The freeze-drying technique is one of the most useful processes for drying thermosensitive substances that are unstable in aqueous solutions. Because of the rapid evolution of biotechnologically obtained materials, increased attention has been focused on this process, especially in pharmaceutical technology, during recent years. Actually, freeze-drying techniques are used to dry many kinds of mainly biological materials. Food products, in particular, are processed on a large scale with considerable success. As an alternative to the classical freeze-drying process in a vacuum, the feasibility of dehydrating frozen pharmaceutical solutions and liquid foods at atmospheric pressure was investigated. An apparatus and a technique for spray-freezing aqueous solutions in situ at very low temperatures (almost-equal-to -90-degrees-C) and for subsequent dehydration of the resulting frozen particles in a stream of cold, desiccated air was developed. The influence of various process variables and of certain product characteristics on the drying kinetics as well as on the quality properties of the respective lyophilizates is discussed. Compared to the classical freeze-drying process the following differences can be pointed out: (1) improved heat and mass transfer between the circulating drying medium and the frozen sample; (2) high and homogeneous quality properties of the dry product with an increased retention of volatile aromatic compounds in foods; (3) instead of a cake, a fine, free-flowing powder with a large inner surface area and good instant, i.e. wetting and solubility, properties was obtained.
