ABHISHEK GOYALB.TECH FOOD TECHNOLOGY

9999921744

INTRODUCTION:

Drying occurs by effecting vaporization of the liquid by supplying heat to the wet feedstock. Heat may be supplied by convection (direct dryers), by conduction (contact or indirect dryers).Drying is used to remove water from foods for two reasons: to prevent (or inhibit) micro-organisms and hence preserve the food and to reduce the weight and bulk of food for cheaper transport and storage. When carried out correctly, the nutritional quality, colour, flavor and texture of rehydrated foods are slightly less than fresh food but, for most people, this has only minor nutritional significance as dried foods form one component in the diet. For effective drying, air should be hot, dry and moving. These factors are inter-related and it is important that each factor is correct (for example, cold moving air or hot, wet moving air is unsatisfactory). The dryness of air is termed 'humidity' - the lower the humidity, the drier the air.

Drying methods and processes can be classified in several different ways:1. It can be classified as batch, where the material is inserted into the drying equipment

and drying proceeds for a given period of time, or as continuous, where the material is continuously added to the dryer and dried material continuously removed.

2. Drying processes can also be categorized according to the physical conditions used to add heat and remove water vapour: (a) in the first category, heat is added by direct contact with heated air at atmospheric pressure, and the water vapour formed is removed by the air; (b) in vacuum drying, the evaporation of water proceeds more rapidly at low pressures, and the heat is added indirectly by contact with a metal wall or by radiation (low temperatures can also be used under vacuum for certain materials that may discolor or decompose at higher temperatures).

Dryers, which expose the solids to a hot gas, are called adiabatic or direct dryers; those in which heat is transferred from an external medium are known as non-adiabatic or indirect dryers. Dryers heated by dielectric, radiant, or microwave energy are also non-adiabatic. Some units combine adiabatic and non-adiabatic drying; they are known as direct-indirect dryers.Most commercial dryers are insulated to reduce heat losses, and they recirculate hot air to save energy. Many designs have energy-saving devices, which recover heat from the exhaust air or automatically control the air humidity.

DRYING CURVE:

This curve is referred to as the drying curve for a specific product. It shows a typical drying curve. Variations in the curve will occur principally in rate relative to carrier velocity and temperature.

Drying occurs in three different periods, or phases, which can be clearly defined.The first phase, or initial period, is where sensible heat is transferred to the product and the contained moisture. The second phase, or constant rate period, is when the free moisture persists on the surfaces and the rate of evaporation alters very little as the moisture content reduces. The third phase, or falling rate period, is the phase during which migration of moisture from the inner interstices of each particle to the outer surface becomes the limiting factor that reduces the drying rate.

TYPES OF AIR FLOW:

1.2. T

TRAY OR CABINET DRYERS:

In these types of dryers, the food is spread out, generally quite thinly, on trays in which the drying takes place. Heating may be by an air current sweeping across the trays, by conduction from heated trays or heated shelves on which the trays lie, or by radiation from heated surfaces. Most tray dryers are heated by air, which also removes the moist vapours.

Principle:The product to be dried is placed in thin layers on trays stacked in a column within the dryer chamber. Heated air is circulated vertically through the column with a circulating fan. Fresh air is brought into the cabinet and moist air is exhausted by using a dehumidistat to control an exhaust fan and air intake shutters. A perforated plywood floor is used to uniformly distribute the air within the dryer. The dryer trays are tight-fitting in the cabinet to prevent air from bypassing the material to be dried.

Application: This dryer can be utilized for drying many different crops. They commonly are used to dry fruit and vegetable pieces, and depending upon the food and the desired final moisture, drying time may be of the order of 10 or even 20 hr.

3. FREEZE DRYERS

Freeze-drying (also known as lyophilization or cryodesiccation) is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. The material is held on shelves or belts in a chamber that is under high vacuum. In most cases, the food is frozen before being loaded into the dryer. Heat is transferred to the food by conduction or radiation and the vapour is removed by vacuum pump and then condensed

PRINCIPLE:Freeze-drying works by freezing the material and then reducing the surrounding pressure and adding enough heat to allow the frozen water in the material to sublime directly from the solid phase to the gas phase. The scientific principle in freeze-drying is sublimation, the conversion of a solid (ice) directly into its gaseous form (water vapour). A typical freeze-drying machine consists of three major components - a freeze-drying chamber, a freezing coil connected to a refrigerator compressor, and a vacuum pump. First the temperatures are

lowered in the freeze-drying chamber and the food item is frozen solid. This way the water content of the food is converted to solid ice.

APPLICATION:Products most often freeze-dried include:

• Instant coffee, Vegetables for dried soup mixes, Mushrooms, Herbs, Spices, Cheese, Shrimp, Fruits for ready-to-eat breakfast cereals

Advantages• Little thermal damage, Good retention of volatile flavors, Good vitamin retention, Rapid product rehydration, Little product shrinkage, Long product storage life—if suitably packed, Good retention of biological activity (with use of cryoprotectants)

Disadvantages• High drying cost, Damage to certain products by initial freezing, Rapid deterioration unless products are packed and maintained at low humidity, Product friability (crumbles easily), Pre-treatment sometimes necessary (e.g., with carrots) to avoid color loss.

4. SPRAY DRYERS

In a spray dryer, liquid or fine solid material in a slurry is sprayed in the form of a fine droplet dispersion into a current of heated air. Air and solids may move in parallel or counterflow. Drying occurs very rapidly, so that this process is very useful for materials that are damaged by exposure to heat for any appreciable length of time. The dryer body is large so that the particles can settle, as they dry, without touching the walls on which they might otherwise stick. Commercial dryers can be very large of the order of 10 m diameter and 20 m high.

PRINCIPLE: Spray drying involves evaporation of moisture from an atomized feed by mixing the spray and the drying medium. The drying medium is typically air. The drying proceeds until the desired moisture content is reached in the sprayed particles and the product is then separated from the air. The mixture being sprayed can be a solvent, emulsion, suspension or dispersion.

APPLICATION:Spray drying is a very widely applied, technical method used to dry aqueous or organic solutions, emulsions etc., in industrial chemistry and food industry. Dry milk powder, detergents and dyes are just a few spray dried products currently available. Spray drying can be used to preserve food or simply as a quick drying method. It also provides the advantage of weight and volume reduction. It is the transformation of feed from a fluid state into a dried particulate form by spraying the feed into a hot drying medium.

Salient features of Spray dryers are as follows. Solutions, suspensions, slurries and pastes, which can be pumped, can be dried on

spray dryers. The advantage of spray dryer is rapid and non-contact drying. Much higher initial temperature of drying medium can be used. High evaporation

rates and thermal efficiencies are achieved. It can be quickly started and shut down. It is capable of handling volatile or inflammable

solvents in a closed cycle.

5. FLUIDIZED BED DRYERS

They provide an effective method of drying relatively free flowing particles with a reasonably narrow particle size distribution. In general, fluid bed dryers operate on a through-the-bed flow pattern with the gas passing through the product perpendicular to the direction of travel. The dry product is discharged from the same section.Fluid bed drying offers important advantages over other methods of drying particulate materials. Particle fluidization gives easy material transport, high rates of heat exchange at high thermal efficiency while preventing overheating of individual particles.

PRINCIPLE:

In a fluidized bed dryer, the food material is maintained suspended against gravity in an upward-flowing air stream. There may also be a horizontal air flow helping to convey the food through the dryer. Heat is transferred from the air to the food material, mostly by convection.

APPLICATION:

Fluid bed drying is suited for powders, granules, agglomerates, and pellets with an average particle size normally between 50 and 5,000 microns. Very fine, light powders or highly elongated particles may require vibration for successful fluid bed drying.

6. ROTARY DRYERS:

A slightly inclined rotating metal cylinder is fitted internally with flights to cause the food to cascade through a stream of hot air as it moves through the dryer. Airflow may be parallel or counter-current. The agitation of the food and the large area of food exposed to the air produce high drying rates and a uniformity dried product.

PRINCIPLE:The foodstuff is contained in a horizontal inclined cylinder through which it travels, being heated either by air flow through the cylinder, or by conduction of heat from the cylinder walls. In some cases, the cylinder rotates and in others the cylinder is stationary and a paddle or screw rotates within the cylinder conveying the material through.

APPLICATION:The method is especially suitable for foods that tend to mat or stick together in belt or tray dryers. However, the damage caused by impact and abrasions in the drier restrict this method to relatively few foods (e.g. sugar crystals and cocoa beans).

7. PUFF DRYERS:

In general, puff-drying may be defined as the formation of a highly expanded, sponge-like structure of dried material from a thin film of liquid concentrate,under conditions of high vacuum and low temperature. The product would be expected to disperse rapidly because of its large surface area per unit weight and to possess natural flavor because of the high-vacuum, low-temperature drying conditions employed.

APPLICATIONPuff-drying has been successfully applied to heat-sensitive citrus juices and other food products in both batch and continuous processes As applied to whole milk, drying at low temperatures would also be expected to inhibit protein destabilization, and dehydrating at low oxygen concentrations at low temperature would preclude atmospheric oxidation.

8. TUNNEL DRYERS

These may be regarded as developments of the tray dryer, in which the trays on trolleys move through a tunnel where the heat is applied and the vapours removed. In most cases, air is used in tunnel drying and the material can move through the dryer either parallel or counter current to the air flow. Sometimes the dryers are compartmented, and cross-flow may also be used.

9. PNEUMATIC DRYERS

In a pneumatic dryer, the solid food particles are conveyed rapidly in an air stream, the velocity and turbulence of the stream maintaining the particles in suspension. Heated air accomplishes the drying and often some form of classifying device is included in the equipment. In the classifier, the dried material is separated, the dry material passes out as product and the moist remainder is recirculated for further drying.