Drying is defined as the regulated application of heat to eliminate water from foods by evaporation, yielding solid solids. It differs from evaporation in that it produces concentrated liquid products. The primary goal of drying is to increase the shelf-life of foods by decreasing their in-water activity. In the absence of sufficient water, microorganisms that cause food deterioration and decay, as well as many enzymes that induce undesirable changes in the chemical makeup of food, are unable to grow, multiply, or operate.
Food drying is the method of food preservation in which food is dried (dehydrated or desiccated). Drying inhibits the growth of bacteria, yeasts and mould through the removal of water. Water is traditionally removed through evaporation (air drying, sun drying, smoking or wind drying), although today electric food dehydrators or freeze – drying can be used to speed the drying process and ensure more consistent results.
2.Field of application
Dairy products (milk, whey, creamers), coffee, coffee surrogates, tea, flavouring, powdered drinks, processed cereal-based foods, potatoes, starch derivatives, sugar beetroot pulp, fruits, vegetables and spices are common applications for drying processes. The elimination of water from wet germinated grain is used in the manufacturing of malt, a process known as kilning. The drying step is critical to the malting process and is required to get the proper colour and flavour.
The drying process is categorized in 3 parts:
· Air and contact drying under atmospheric pressure
· Freeze drying
In air drying, Heat is delivered via the foodstuff through air and contact drying, either from hot surfaces. At lower pressures, vacuum happens more easily than at higher pressures.
In freeze drying, Water vapour is released from frozen food during freeze drying. Under these conditions, the food structure is better preserved. To ensure sublimation, appropriate temperatures and pressures must be established in the dryer.
4.Types of drying equipment
4.1. Spray dryer
The material to be dried is suspended in air during spray drying, which converts the liquid into a fog-like mist (atomised), offering a vast surface area. In a drying chamber, the atomised liquid is exposed to a flow of heated air. Air and solids can move in parallel or in opposite directions. Because the moisture evaporates quickly, this method is especially effective for materials that have been harmed by prolonged contact to heat. The solids are recovered in the form of a powder composed of fine, hollow, spherical particles. Steam, direct gas-fired air heaters or indirect heaters powered by gas, liquid or solid fuels can all be used to heat the drying air. Generally, the exhaust air is routed through cyclones and/or filters as part of the process to recover particulate particulates (dust) carried over in the exhaust air. The recovered substance is reintroduced into the product. Generally, the exhaust air is routed through cyclones and/or filters as part of the process to recover particulate particulates (dust) carried over in the exhaust air. The recovered substance is reintroduced into the product.
4.1.1. Spray drying parameters
The drier body is large so that the particles can settle as they dry without coming into contact with the walls, which would otherwise attach to them. Commercial dryers can be very big, measuring up to 10m in diameter and 20m in height. Air inlet temperatures of up to around 250°C or even higher (depending on the type of product) are employed, however, evaporation causes the air temperature to rapidly drop to about 95°C (air output temperature). The temperature of the product will be 20 to 30°C lower than the temperature of the air exit.
4.2. Roller dryer
The roller drying process works by applying a thin film of material to the flat surface of a continually spinning, steam-heated metal drum. A fixed knife placed opposite the point of application of the liquid material continuously scrapes off the dry material's layer. The dryer is made out of a single or two drums with or without "satellite" rollers. Depending on the product, the applied steam pressure in the drums might range from 4 to 8bar. Roller drying is used to dry milk, starch, and potato flakes, among other things. Conduction drying can be applied to drum drying.
4.3. Tunnel dryer
These are advancements of the tray dryer, in which trays on trolleys go through a tunnel where heat is provided and vapours are evaporated. Tunnel drying is usually done with air, and the material can go through the dryer either parallel or countercurrent to the air flow. The dryers are sometimes compartmented, and cross-flow may be used.
4.4. Fluidized bed dryer
The food item in a fluidized bed dryer is kept suspended against gravity in an upward-flowing air stream. A horizontal air flow may also assist in transporting the food through the drier.
Convection is the primary mode of heat transfer from the air to the food substance. With especially difficult-to-fluidize materials, a vibrating motion of the drier itself is employed to help fluidization; this is known as a vibro-fluidizer, which is on springs. The fluidized solid particles then act similarly to liquids, allowing them to be transported. Air velocities vary with particle size and density, but are typically in the 0.3-0.75 m/s range. They can be used for drying as well as chilling.
4.5. Rotary dryer
Rotary dryers: The food is confined in a horizontal inclined cylinder through which it passes, and it is heated either by air flow through the cylinder or by heat conduction from the cylinder walls. In some circumstances, the cylinder rotates, while in others, the cylinder remains fixed while a paddle or screw revolves within it.
4.6. Pneumatic dryer
In pneumatic drier, solid food particles are swiftly transported in an air stream, with the velocity and turbulence of the stream keeping the particles suspended. The drying is accomplished with heated air, and a classification mechanism is frequently integrated in the equipment. The dried material is sorted in the classifier, the dry material is discharged as product, and the damp remnant is recirculated for further drying.
Techniques for reducing moisture content and concentrating food products. Reduced water content may inhibit the growth of microorganisms, hence preventing food deterioration. Reduced water content may help save on transportation costs, etc. The essay concludes that the employment of drying techniques in the food sector is an important method.