History of Food Drying
The preservation of food materials by drying has been carried out since the early recorded history of human civilization. The history of drying foods is a long one and goes back as far as 20 000 BC (Hayashi, 1989). The readers may refer further to Hayashi (1989) for detailed chronological information on the history of food drying. Evidence shows that Middle East and oriental cultures actively dried foods as early as 12 000 BC in the hot sun. During the mesolithic age (around 10 000 BC), fish were caught and dried by people in Solvieux in southern France. In ancient Egypt (around 2800 BC), fruits and nuts (e.g., apples, grapes, apricots, figs, and almonds) were sun-dried. During the middle ages (AD 630 to AD 1630), sun-dried powdered milk was produced by the Mongolian army, while cocoa leaves and fruits were sun-dried in Mexico and Peru. In the late 1700s, the French successfully developed a dehydration unit to dehydrate fruits and vegetables at a controlled temperature.The French team sliced the fruits and vegetables, then dried at hot air (40 C), pressed, and then sealed in tinfoil. However, it was not until the turn of the twentieth century that mechanical drying finally begun to replace natural sun drying. It was particularly at the end of World War II that the mechanical drying has since taken off at great pace. The early methods of drying such as trucked-tray and drum drying were soon improved. This was followed by the developments of new methods (at that time), including spray drying, fluidized bed drying, vacuum drying, and freeze drying. A significant effort continues to the present day to improve these drying methods. In addition, new challenges are constantly emerging as new drying requirements appear for new products. This continues to drive further efforts to develop new, innovative, and novel drying techniques.
Common Applications
Today, drying is employed in various industrial sectors (e.g., paper, wood, food, agriculture, waste management, etc.) utilizing different techniques. As far as industrial sectors are concerned, food and agriculture remain the most dominant sectors with respect to the critical importance of drying to these industries (Mujumdar, 2010). Numerous food products are routinely preserved by drying, which include grains, marine products, meat products, dairy products, as well as fruits and vegetables (Jangam, 2011). There are many different methods of drying food materials, each with their own advantages and disadvantages for particular applications. A vast number of dryer designs reported in the literature are due to the differences in the physical attributes of the product, modes of heat input, operating temperatures and pressures, quality specifications on the dried product, and so on. The majority of dryers used in the food industry are of convective type, i.e., hot air is used both to supply heat for evaporation of water and to carry away the evaporated moisture from the product. This is by far the most common drying method used at industrial scale because it is simple and easy to operate, in addition to relatively low capital costs (for the time being), although it is poor in energy efficiency. Another type of dryers involves supplying of heat to the drying material through contact with heated metallic/nonmetallic solids (molecular vibration) or stationery fluids (primarily by molecular collision) by conduction (e.g., drum dryers). Dryers are also classified according to the radiant energy supplied in various forms of electromagnetic waves, categorized according to the region of the electromagnetic spectrum (e.g., radio frequency (RF), infrared (IR), microwave (MW)). In food drying applications, the main types of radiation applied are IR, MW, and RF, all of which employ very different heating mechanisms. These can be
used either alone, or as combined technologies (e.g., combinations of MW and vacuum application, MW and IR application, among others). The details of the various drying techniques for food materials can be found elsewhere. The readers may refer to Sabarez (2015), Jangam (2011), and Bansal and Chung (2007) for additional information on the variety and classification of dryers applicable for drying food materials, as well as to other articles in this and other sections of the Food Science Reference Module. This section briefly describes some of the most commonly used methods for drying food materials.Tunnel Dehydration Tunnel dehydration is the most widely used method in industrial dehydration of fruits and vegetables. This method of drying has been commercially used in Australia and other parts of the world for drying of fruits (mainly prunes, apricots, grapes, and tomatoes) (Unadi et al., 1996; Sabarez, 2007). The tunnel dehydrator basically consists of a tunnel (as a drying chamber) containing trays of product that are placed on mobile trolleys (also referred to as trucks) moving along the tunnel, a fan to circulate the he