For the food conservation method, see drying (food). Drying is a method of Food preservation that works by removing Water from the food which prevents the growth of Microorganisms and Decay.

Drying is aㅜㅜ mass transfer process resulting in the removal of water moisture or moisture from another solvent, by evaporation from a solid, semi-solid or liquid (hereafter product) to end in a solid state. Mass transfer is the phrase commonly used in engineering for physical processes that involve molecular and convective transport of Atoms and Molecules A solvent is a liquid or gas that dissolves a solid liquid or gaseous Solute, resulting in a Solution. Evaporation is the process by which Molecules in a Liquid state (e A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of To achieve this, there must be a source of heat, and a sink of the vapor thus produced. In Physics, heat, symbolized by Q, is Energy transferred from one body or system to another due to a difference in Temperature A vapor or vapour (see Spelling differences) is a substance in the Gas phase at a Temperature lower than its Critical temperature

In the most common case, a gas stream, e. g. , air, applies the heat by convection and carries away the vapor as humidity. Humidity is the amount of water vapor in the air In daily language the term "humidity" is normally taken to mean Relative humidity. Other possibilities are vacuum drying, where heat is supplied by contact conduction or radiation (or microwaves) while the produced vapor is removed by the vacuum system. Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter from a region of higher Temperature to a region of lower Radiation, as in Physics, is Energy in the form of waves or moving Subatomic particles emitted by an atom or other body as it changes from a higher energy Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. Another indirect technique is drum drying, where a heated surface is used to provide the energy and aspirators draw the vapor outside the room.

Freeze drying or lyophilization is a drying method where the solvent is frozen prior to drying and is then sublimed, i. e. , passed to the gas phase directly from the solid phase, below the melting point of the solvent. Freeze drying is often carried out under high vacuum to allow drying to proceed at a reasonable rate. This process avoids collapse of the solid structure, leading to a low density, highly porous product, able to regain the solvent quickly. In biological materials or foods, freeze drying is regarded as one of the best if not the best method to retain the initial properties. It was first used industrially to produce dehydrated vaccines, and to bring dehydrated blood to assist war casualties. Now freeze drying is increasingly used to preserve some foods, especially for backpackers going to remote areas. The method may keep protein quality intact, the same as the activity of vitamins and bioactive compounds.

In turn, the mechanical extraction of the solvent, e. g. , water, by centrifugation, is not considered "drying". Centrifugation is a process that involves the use of the Centrifugal force for the separation of mixtures used in industry and in laboratory settings The ubiquitous term dehydration may mean drying of water-containing products as foods, but its meaning is more vague, as it is also applied for water removal by osmotic drive from a salt or sugar solution. Dehydration ( hypohydration) is the removal of Water ( hydro in ancient Greek) from an object Osmosis is the Diffusion of a solvent (frequently water through a semi-permeable membrane, from a solution of low solute concentration (high water potential In medicine, dehydration is the situation by which a person loses water by respiration, sweating and evaporation and does not incorporate, for whatever reason, the "make-up" water required to keep the normal physiological behavior of the body. In living organisms a respiratory system functions to allow Gas exchange. Evaporation is the process by which Molecules in a Liquid state (e Physiology (from Greek grc φύσις physis, "nature origin" and grc -λογία -logia) is the study of the mechanical physical

Drying may be either a natural or an intentional process.

The process of extreme drying is called desiccation. Desiccation is the state of extreme dryness or the process of extreme drying

There is very extensive technical literature on this subject, including several major textbooks and a dedicated scientific journal (Drying Technology [1]).

Methods of drying

In a typical phase diagram, the boundary between gas and liquid runs from the triple point to the critical point. In Physical chemistry, Thermodynamics, Chemistry and Condensed matter physics, a critical point, also called a critical state Regular drying is the green arrow, while supercritical drying is the red arrow and freeze drying is the blue. Supercritical drying is a process to remove liquid in a precisely controlled way similar to Freeze drying. Freeze drying (also known as lyophilization or cryodesiccation) is a Dehydration process typically used to preserve a perishable material

• Application of heated air (convective or direct drying). Convection in the most general terms refers to the movement of molecules within Fluids (i Air heating reduces air relative humidity, which is the driving force for drying. Besides, higher temperatures speed up diffusion of water inside the solids, so drying is faster. However, product quality considerations limit the applicable rise to air temperature. Too hot air almost completely dehydrates the solid surface, so internal pores shrink and almost close, leading to crust formation or "case hardening".
• Indirect or contact drying (heating through a hot wall), as drum drying, vacuum drying.
• Dielectric drying (radiofrequency or microwaves being absorbed inside the material) It is the focus of intense research nowadays. It may be used to assist air drying or vacuum drying.
• Freeze drying Is increasingly applied to dry foods, beyond its already classical pharmaceutical or medical applications. Freeze drying (also known as lyophilization or cryodesiccation) is a Dehydration process typically used to preserve a perishable material It keeps biological properties of proteins, and retains vitamins and bioactive compounds. Pressure may be reduced by a vacuum pump. If using a vacuum pump, the vapor produced by sublimation is removed from the system by converting it into ice in a condenser, operating at very low temperatures, outside the freeze drying chamber.
• Supercritical drying (superheated steam drying) involves steam drying of products containing water. Supercritical drying is a process to remove liquid in a precisely controlled way similar to Freeze drying. Strange as it seems, this is possible because the water in the product is boiled off, and joined with the drying medium, increasing its flow. It is usually employed in closed circuit and allows a proportion of latent heat to be recovered by recompression, a feature which is not possible with conventional air drying, for instance. May have potential for foods if carried out at reduced pressure, to lower the boiling point.
• Natural air drying takes place when materials are dried with unheated forced air, taking advantage of its natural drying potential. The process is slow and weather-dependent, so a wise strategy "fan off-fan on" must be devised considering the following conditions: Air temperature, relative humidity and moisture content and temperature of the material being dried. Grains are increasingly dried with this technique, and the total time (including fan off and on periods) may last from one week to various months, if a winter rest can be tolerated in cold areas.

Applications of drying

Grain Drying

Hundreds of millions of tonnes of wheat,corn, soybean, rice other grains as sorghum, sunflower seeds, rapeseed/canola, barley, oats, etc. , are dried in grain dryers. In the main agricultural countries, drying comprises the reduction of moisture from about 17-30%w/w to values between 8 and 15%w/w, depending on the grain. The final moisture content for drying must be adequate for storage. The more oil the grain has, the lower its storage moisture content will be (though its initial moisture for drying will also be lower). Cereals are often dried to 14% w/w, while oilseeds, to 12. 5% (soybeans), 8-9% (sunflower) and 9% (peanuts). Drying is carried out as a requisite for safe storage, in order to inhibit microbial growth. However, low temperatures in storage are also highly recommended to avoid degradative reactions and, especially, the growth of insects and mites. A good maximum storage temperature is about 18°C. The largest dryers are normally used "Off-farm", in elevators, and are of the continuous type: Mixed-flow dryers are preferred in Europe, while Cross-flow dryers in the USA. In Argentina, both types are usually found. Continuous flow dryers may produce up to 100 metric tonnes of dried grain per hour. The depth of grain the air must traverse in continuous dryers range from some 0. 15 m in Mixed flow dryers to some 0. 30 m in Cross-Flow. Batch dryers are mainly used "On-Farm", particularly in the USA and Europe. They normally consist of a bin, with heated air flowing horizontally from a narrow-diameter cylinder through a perforated metal sheet, placed in the center of the bin. Air passes through a path of grain some 0. 50 m deep in radial direction and leaves the system through another perforated sheet. The usual drying times range from 1 h to 4 h depending on how much water must be removed, the air temperature, and the grain depth. In the USA, continuous counterflow dryers may be found on-farm, adapting a bin to slowly drying the grain, and removing the dried product using an auger. Grain drying is an active area of manufacturing and research. Now it is possible to "simulate" the performance of a dryer with computer programs based on equations that represent the physics and physical chemistry of drying.

Drum Drying

The drum dryer technology has kept its position of importance. Today, in foods, potato puree is dehydrated as well as banana and tomato purees to produce dehydrated flakes

Spray drying

Spray drying is an important technique to produce dried powders. Spray drying is a commonly used method of drying a liquid feed through a hot gas The principle is that a pumpable feed is first atomized, i. e, converted in a fog of droplets of about 100 micrometers in diameter, which dry very fast while falling by gravity, accompanied by heated air. The dried particles eventually exit through the bottom of the dryer and is separated from the drying air by a cyclone, or a system based on cyclones plus bag filters or electrostatic precipitators. Milk powder is possibly the most popular product, and tomato powder is becoming very important. On the other hand, washing powder is an example product of the chemical process industry. The production of dehydrated natural flavors and essences is very important and is growing together with encapsulation, a technique devised to trap a volatile, but large molecule (as the flavor compound) inside a dry particle, the walls of which develop on drying and are permeable to water flux but not to the flux of the larger volatiles. This principle of selective diffusion was first developed by the Dutch researcher Thijssen, in Eindhoven, during the 1970's. Spray dryers differ in the type of atomizer, the relative directions of air and product flows, the chamber design, type of drying agent (air, nitrogen) in the system charactersitics ( closed or open circuit), among other features. Equipment can be very large, of up to 20 m tall.

• Devices commonly called dryers are used for efficient drying of various things: hair after a shower, candies at candy factories, semiconductor wafers
• Most processes giving a solid product involve a drying step
• Drying is often used to preserve food
• The production of anhydrous alcohol requires azeotropic distillation, or a membrane process. Hair is a keratinised protein filament that grows through the epidermis from follicles deep within the Dermis. Candy, specifically sugar candy, is a confection made from a concentrated solution of sugar in water to which a variety of flavorings and colorants is added A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that A wafer is a thin slice of Semiconductor material such as a Silicon crystal used in the fabrication of Integrated circuit and other microdevices Drying is a method of Food preservation that works by removing Water from the food which prevents the growth of Microorganisms and Decay. Food preservation is the process of treating and handling Food in a way that preserves its edibility and nutrition value The 96° mixture of ethanol-water cannot be separated by distillation, as it constitutes an azeotrope ("boiling without variation", from the Greek)
• Wood drying is an integral part of timber processing

See also

• Desiccation

External links

• European Drying Working Party; includes links to other sites worldwide
• Suppliers of Perforated Metal for Dryers
• Vacuum Kiln Drying Info