Last updated

Drying is a mass transfer process consisting of the removal of water or another solvent [1] by evaporation from a solid, semi-solid or liquid. This process is often used as a final production step before selling or packaging products. To be considered "dried", the final product must be solid, in the form of a continuous sheet (e.g., paper), long pieces (e.g., wood), particles (e.g., cereal grains or corn flakes) or powder (e.g., sand, salt, washing powder, milk powder). A source of heat and an agent to remove the vapor produced by the process are often involved. In bioproducts like food, grains, and pharmaceuticals like vaccines, the solvent to be removed is almost invariably water. Desiccation may be synonymous with drying or considered an extreme form of drying.


In the most common case, a gas stream, e.g., air, applies the heat by convection and carries away the vapor as humidity. Other possibilities are vacuum drying, where heat is supplied by conduction or radiation (or microwaves), while the vapor thus produced is removed by the vacuum system. Another indirect technique is drum drying (used, for instance, for manufacturing potato flakes), where a heated surface is used to provide the energy, and aspirators draw the vapor outside the room. In contrast, the mechanical extraction of the solvent, e.g., water, by filtration or centrifugation, is not considered "drying" but rather "draining".

Drying mechanism

In some products having a relatively high initial moisture content, an initial linear reduction of the average product moisture content as a function of time may be observed for a limited time, often known as a "constant drying rate period". Usually, in this period, it is surface moisture outside individual particles that is being removed. The drying rate during this period is mostly dependent on the rate of heat transfer to the material being dried. Therefore, the maximum achievable drying rate is considered to be heat-transfer limited. If drying is continued, the slope of the curve, the drying rate, becomes less steep (falling rate period) and eventually tends to become nearly horizontal at very long times. The product moisture content is then constant at the "equilibrium moisture content", where it is, in practice, in equilibrium with the dehydrating medium. In the falling-rate period, water migration from the product interior to the surface is mostly by molecular diffusion, i,e. the water flux is proportional to the moisture content gradient. This means that water moves from zones with higher moisture content to zones with lower values, a phenomenon explained by the second law of thermodynamics. If water removal is considerable, the products usually undergo shrinkage and deformation, except in a well-designed freeze-drying process. The drying rate in the falling-rate period is controlled by the rate of removal of moisture or solvent from the interior of the solid being dried and is referred to as being "mass-transfer limited". This is widely noticed in hygroscopic products such as fruits and vegetables, where drying occurs in the falling rate period with the constant drying rate period said to be negligible. [2]

Methods of drying

In a typical phase diagram, the boundary between gas and liquid runs from the triple point to the critical point. Regular drying is the green arrow, while supercritical drying is the red arrow and freeze drying is the blue. Drying.svg
In a typical phase diagram, the boundary between gas and liquid runs from the triple point to the critical point. Regular drying is the green arrow, while supercritical drying is the red arrow and freeze drying is the blue.

The following are some general methods of drying:

Applications of drying

Drying of fish in Lofoten in the production of stockfish Stockfish in Lofoten.jpg
Drying of fish in Lofoten in the production of stockfish


Foods are dried to inhibit microbial development and quality decay. However, the extent of drying depends on product end-use. Cereals and oilseeds are dried after harvest to the moisture content that allows microbial stability during storage. Vegetables are blanched before drying to avoid rapid darkening, and drying is not only carried out to inhibit microbial growth, but also to avoid browning during storage. Concerning dried fruits, the reduction of moisture acts in combination with its acid and sugar contents to provide protection against microbial growth. Products such as milk powder must be dried to very low moisture contents in order to ensure flowability and avoid caking. This moisture is lower than that required to ensure inhibition to microbial development. Other products as crackers are dried beyond the microbial growth threshold to confer a crispy texture, which is liked by consumers.

Non-food products

Among non-food products, some of those that require considerable drying are wood (as part of timber processing), paper, flax, and washing powder. The first two, owing to their organic origins, may develop mold if insufficiently dried. Another benefit of drying is a reduction in volume and weight.

Sludges and fecal materials from sanitation processes

In the area of sanitation, drying of sewage sludge from sewage treatment plants, fecal sludge or feces collected in urine-diverting dry toilets (UDDT) is a common method to achieve pathogen kill, as pathogens can only tolerate a certain dryness level. In addition, drying is required as a process step if the excreta based materials are meant to be incinerated. [3]

See also


  1. Greensmith, M. (1998). Practical Dehydration. Woodhead Publishing, Ltd.
  2. Genskow, L.R.; Beimesch,W.E.; Hecht, J.P.; Kemp,I.C.; Langrish,T.; Schwartzbach, C.; Smith, (F).L. (2007). Chemical Engineers' Handbook. Mc Graw Hill Professional. pp. Chapter 12 (Evaporative Cooling and Solids Drying).
  3. A.S., Mujumdar (1998). Handbook of Industrial Drying. Boca Ratón: CRC Press.

Related Research Articles

Humidity amount of water vapor in the air

Humidity is the concentration of water vapour present in air. Water vapour, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. The amount of water vapour needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing water mass. The amount of water vapour contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 grams of water per cubic metre of air at 30 °C, but only 8 grams of water per cubic metre of air at 8 °C.

Spray drying method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas

Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas. This is the preferred method of drying of many thermally-sensitive materials such as foods and pharmaceuticals. A consistent particle size distribution is a reason for spray drying some industrial products such as catalysts. Air is the heated drying medium; however, if the liquid is a flammable solvent such as ethanol or the product is oxygen-sensitive then nitrogen is used.

Blanching (cooking) cooking process

Blanching is a cooking process wherein a food, usually a vegetable or fruit, is scalded in boiling water, removed after a brief, timed interval, and finally plunged into iced water or placed under cold running water to halt the cooking process. Blanching foods helps reduce quality loss over time. People often use blanching as a treatment prior to freezing, drying, or canning—heating vegetables or fruits to inactivate enzymes, modify texture, remove the peel, and wilt tissue. Blanching is also utilized to preserve color, flavor, and nutritional value. The process has three stages: preheating, blanching, and cooling. The most common blanching methods for vegetables/fruits are hot water and steam, while cooling is either done using cold water or cool air. Other benefits of blanching include removing pesticide residues and decreasing microbial load. Drawbacks to the blanching process can include leaching of water-soluble and heat sensitive nutrients and the production of effluent.

Dried fruit fruit from which the majority of the original water content has been removed

Dried fruit is fruit from which the majority of the original water content has been removed either naturally, through sun drying, or through the use of specialized dryers or dehydrators. Dried fruit has a long tradition of use dating back to the fourth millennium BC in Mesopotamia, and is prized because of its sweet taste, nutritive value, and long shelf life.

Outgassing is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation, as well as desorption, seepage from cracks or internal volumes, and gaseous products of slow chemical reactions. Boiling is generally thought of as a separate phenomenon from outgassing because it consists of a phase transition of a liquid into a vapor of the same substance.


Freeze drying, also known as lyophilisation or cryodesiccation, is a low temperature dehydration process that involves freezing the product, lowering pressure, then removing the ice by sublimation. This is in contrast to dehydration by most conventional methods that evaporate water using heat.

Dielectric heating Heating using radio waves

Dielectric heating, also known as electronic heating, radio frequency heating, and high-frequency heating, is the process in which a radio frequency (RF) alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is caused by molecular dipole rotation within the dielectric.

Onion powder dehydrated, ground onion commonly used as a seasoning

Onion powder is dehydrated, ground onion that is commonly used as a seasoning. It is a common ingredient in seasoned salt and spice mixes, such as beau monde seasoning. Some varieties are prepared using toasted onion. White, yellow and red onions may be used. Onion powder is a commercially prepared food product that has several culinary uses. Onion powder can also be homemade.

Vacuum evaporation

Vacuum evaporation is the process of causing the pressure in a liquid-filled container to be reduced below the vapor pressure of the liquid, causing the liquid to evaporate at a lower temperature than normal. Although the process can be applied to any type of liquid at any vapor pressure, it is generally used to describe the boiling of water by lowering the container's internal pressure below standard atmospheric pressure and causing the water to boil at room temperature.

Level sensors detect the level of liquids and other fluids and fluidized solids, including slurries, granular materials, and powders that exhibit an upper free surface. Substances that flow become essentially horizontal in their containers because of gravity whereas most bulk solids pile at an angle of repose to a peak. The substance to be measured can be inside a container or can be in its natural form. The level measurement can be either continuous or point values. Continuous level sensors measure level within a specified range and determine the exact amount of substance in a certain place, while point-level sensors only indicate whether the substance is above or below the sensing point. Generally the latter detect levels that are excessively high or low.

Wood drying also known as seasoning, which is the reduction of the moisture content of wood prior to its use

Wood drying reduces the moisture content of wood before its use. When the drying is done in a kiln, the product is known as kiln-dried timber or lumber, whereas air drying is the more traditional method.

Moisture sorption isotherm

At equilibrium, the relationship between water content and equilibrium relative humidity of a material can be displayed graphically by a curve, the so-called moisture sorption isotherm. For each humidity value, a sorption isotherm indicates the corresponding water content value at a given, constant temperature. If the composition or quality of the material changes, then its sorption behaviour also changes. Because of the complexity of sorption processes, the isotherms cannot be determined explicitly by calculation, but must be recorded experimentally for each product.

An evaporator is a device in a process used to turn the liquid form of a chemical substance such as water into its gaseous-form/vapor. The liquid is evaporated, or vaporized, into a gas form of the targeted substance in that process.

Instant soup

Instant soup is a type of soup designed for fast and simple preparation. Some are homemade, and some are mass-produced on an industrial scale and treated in various ways to preserve them. A wide variety of types, styles and flavors of instant soups exist. Commercial instant soups are usually dried or dehydrated, canned, or treated by freezing.

Dynamic vapor sorption (DVS) is a gravimetric technique that measures how quickly and how much of a solvent is absorbed by a sample: such as a dry powder absorbing water. It does this by varying the vapor concentration surrounding the sample and measuring the change in mass which this produces. Water vapor is most commonly used, but it is also possible to use a wide range of organic solvents.

Vacuum drying is the mass transfer operation in which the moisture present in a substance, usually a wet solid, is removed by means of creating a vacuum. In chemical processing industries like food processing, pharmacology, agriculture, and textiles, drying is an essential unit operation to remove moisture. Vacuum drying is generally used for the drying of substances which are hygroscopic and heat sensitive, and is based on the principle of creating a vacuum to decrease the chamber pressure below the vapor pressure of the water, causing it to boil. With the help of vacuum pumps, the pressure is reduced around the substance to be dried. This decreases the boiling point of water inside that product and thereby increases the rate of evaporation significantly. The result is a significantly increased drying rate of the product. The pressure maintained in vacuum drying is generally 0.03–0.06 atm and the boiling point of water is 25-30 °C. The vacuum drying process is a batch operation performed at reduced pressures and lower relative humidity compared to ambient pressure, enabling faster drying.

Pasta processing

Pasta processing is the process in which wheat semolina or flour is mixed with water and the dough is extruded to a specific shape, dried and packaged.

Instant tea Food

Instant tea is a powder in which water is added, in order to reconstitute it into a cup of tea. The earliest form of instant tea was developed in the United Kingdom in 1885. A patent was granted for a paste made of concentrated tea extract, sugar, and evaporated milk, which became tea when hot water was added. However, no notable developments were made until spray drying technology allowed for drying the tea concentrates at a temperature which did not damage the flavors of the product.

Intermediate moisture food

Intermediate moisture foods (IMF) are shelf-stable products that have water activities of 0.6-0.84, with a moisture content ranging from 15% - 40% and are edible without rehydration. These food products are below the minimum water activity for most bacteria (0.90), but are susceptible to yeast and mold growth. Historically, ancient civilizations would produce IMF using methods such as sun drying, roasting over fire and adding salt to preserve food for winter months or when preparing for travel. Currently, this form of processing is achieved by using one of four methods: partial drying, osmotic drying using a humectant, dry infusion and by formulation. A variety of products are classified as IMF such as dried fruits, sugar added commodities, marshmallows, and pie fillings.

Food powder is the most common format of food material in the market. Powders are particulate discrete solid particles of size ranging from nanometres to millimetres that generally flow freely when shaken or tilted. The bulk powder properties are the combined effect of particle properties by the conversion of food products in solid state into powdery form for ease of use, processing and keeping quality. Various terms are used to indicate the particulate solids in bulk, such as powder, granules, flour and dust, though all these materials can be treated under powder category. These common terminologies are based on the size or the source of the materials.


  1. "drying - definition of drying by the Free Online Dictionary, Thesaurus and Encyclopedia". Farlex. Retrieved 23 April 2011.
  2. Onwude, Daniel I.; Hashim, Norhashila; Janius, Rimfiel B.; Nawi, Nazmi Mat; Abdan, Khalina (2016-02-04). "Modeling the thin-layer drying of fruits and vegetables: A review" (PDF). Comprehensive Reviews in Food Science and Food Safety. 15 (3): 599–618. doi:10.1111/1541-4337.12196.
  3. Strande, L., Ronteltap, M., Brdjanovic, D. (eds.) (2014). Faecal Sludge Management (FSM) book - Systems Approach for Implementation and Operation Archived 2017-10-14 at the Wayback Machine . IWA Publishing, UK ( ISBN   9781780404738)