DEHYDRATION OF FOODS: REVIEW AND UPDATE

摘要

Today food dehydration is still one of the most relevant and challenging unit operations in food processing, although the art of food preservation by partially removing some of its water content dates from several centuries ago. The dehydration of foods started with the use of solar energy, and with time artificial dryers replaced the sun drying process. Major improvements in this process took place during and after the world wars. Among the first generation of dryers are the cabinet and bed variety that work either in continuous or batch modes; some examples in this category include the kiln, tray, double cone, rotary flow conveyor, and tunnel. Hot air flowing over an extensive area of a product was used to remove water from the surface, making these dryers most suitable for solid materials such as grains. The second generation is assigned to spray dryers, and was developed for the dehydration of liquids. In this operation a liquid food is atomized and a hot stream of air removes the surface water of the small-sprayed drops, resulting in a fine powder. Freeze dehydration can be considered part of the third generation since it was developed to overcome structural damages due to crushing or shredding and to minimize losses of flavor and aroma compounds. Interest in the preservation of fruits gave birth to the osmotic dehydration method; where the water removal takes place without a phase change. High-vacuum high-temperature, extrusion, fluidized beds, microwaves, and radio frequency dryers belong to the last generation of food dehydration technologies. Each of these has specific applications and should be considered based on product characteristics and the final quality desired for the dried food. Among the methods of the last generation, microwave and radio frequency drying are gaining increasing interest in the food industry and research community because of the possible energy savings they might represent. Conversely to hot air drying where the process may take a considerable amount of time due to water migration limitations, microwaves cause internal heating which generates a positive vapor pressure that pushes the vapor out and speeds the drying process. Therefore, microwaves in combination with hot air present energy advantages over traditional hot-air drying.