Hot air and freeze-drying of high-value foods: a review

Drying is an ancient process used to preserve foods. Conventional drying (hot air) offers dehydrated products that can have an extended life of a year. Unfortunately, the quality of a conventionally dried product is drastically reduced from that of the original foodstuff. Freeze-drying is based on the dehydration by sublimation of a frozen product. Due to the absence of liquid water and the low temperatures required for the process, most of deterioration and microbiological reactions are stopped which gives a final product of excellent quality. The comparison of both preservation processes, hot air and freeze-drying, was done taking into account several important characteristics such as shrinkage, glass transition temperature, process–quality interaction, drying kinetics, costs and new improvements. An updated bibliographic research served to compare both drying processes. Experimental data as well as theoretical results, from several years of research in the subject, were presented and compiled in order to support conclusions.