Principles of Dehydration
Dehydration involves the application of heat to vaporize water and some means of removing water vapor after its separation from the fruit/vegetable tissues. Hence it is a combined/simultaneous (heat and mass) transfer operation for which energy must be supplied.
Osmotic dehydration is a useful technique for the concentration of fruit and vegetables, realized
by placing the solid food, whole or in pieces, in sugar solution of high osmotic pressure. It give rise to two major simultaneous counter-current flow. A significant water flow out of the food into the solution and a transfer of solute from solution into the food
In osmotic dehydration the prepared fresh material is soaked in heavy sugar solution and then the material is dried. The syrup has protective effect on color, flavor and texture.
Drying removes water form the surface of the food by the combined effect of air flow, air temperature, and air humidity. When the moisture content is lowered below a certain level, microorganisms cannot grow and the produce is preserve.
There are some products that are partially dried and then deep fried to further remove moisture. This process produces a snack food.
Factors Affecting Drying
Surface area. Generally the fruit and vegetables to be dehydrated are cut into small pieces or thin layers to speed heat and mass transfer. Subdivision speeds drying for two reasons:
Large surface areas provide more surface in contact with the heating medium (air) and more surface from which moisture can escape;
Smaller particles or thinner layers reduce the distance heat must travel to the centre of the food and reduce the distance through which moisture in the centre of the food must travel to reach the surface and escape.
Temperature. The greater the temperature difference between the heating medium and the food the greater will be the rate of heat transfer into the food, which provides the driving force for moisture removal. When the heating medium is air, temperature plays a second important role.
Air velocity. Not only will heated air take up more moisture than cool air, but air in motion will be still more effective. Air in motion, that is, high velocity air, in addition to taking up moisture will sweep it away from the drying food’s surface, preventing the moisture from creating a saturated atmosphere which would slow down subsequent moisture removal. This is why clothes dry more rapidly on a windy day.
Vacuum. If food is placed in a heated vacuum chamber the moisture can be removed from the food at a lower temperature than without a vacuum. Alternatively, for a given temperature, with or without vacuum, the rate of water removal from the food will be greater in the vacuum. Lower drying temperatures and shorter drying times are especially important in the case of heat-sensitive foods.
There are three basic types of drying process:
• sun drying and solar drying;
• atmospheric drying including batch (kiln, tower and cabinet dryers) and continuous (tunnel, belt, belt-trough, fluidised bed, explosion puff, foam-mat, spray, drum and microwave);
• sub-atmospheric dehydration (vacuum shelf/belt/drum and freeze dryers).
Dehydration – General Procedures
Preparation of the fruit . To obtain maximum yields of top quality dried roduct, all fruit should be ripe and free from bruising. Any rotten or bruised fruit should be thrown away. For maximum profitability, the dryer should be loaded to maximum capacity as often as possible, therefore it is advisable to buy more fruit than is required.
Bananas have a low level of acidity and turn brown very rapidly after peeling and cutting. To prevent this, they should be immersed in water containing sodium metabisulphite (400 parts per million of sulphur dioxide) immediately after peeling.
Unpeeled fruits should be washed in a mild disinfectant solution made from one part of bleach to 50 parts of water. Care must be taken not to break the skin of the fruits as this will contaminate the flesh. Gloves and aprons must be worn to protect the workers hands and clothes. Ten litres of treated water will be sufficient for about 20kg fruit.
The wash water should be changed after this amount has been washed as it becomes contaminated by the fruit.
Soft fruit, such as berries and apricots, are delicate and should be handled carefully to avoid bruising. Washed fruits are carefully peeled to remove all the peel and any damaged parts of the flesh. Fruits are cut into slices of varying thickness depending on the type of fruit and the dryer. The following points are useful to consider: thick pieces dry at a slower rate than thinner ones; very thin pieces tend to stick to the drying trays and may be difficult to remove; thicker pieces may not dry fully in the centre and will not store well; packets of mixed thick and thin pieces do not look attractive.
Recommended slice thickness for various fruits:
Blanching or sugaring. This stage is optional, but some processors choose to soak fruits in a sugar syrup prior to drying. There are several benefits of including this process. There are also constraints to sugaring.
Fruit pieces are immersed in a concentrated sugar solution for up to 18 hours. They are rinsed in clean water to remove any excess syrup before drying.
Most vegetables and some fruits are blanched before drying to inhibit enzyme activity and to help preserve the colour. The material is cut into appropriate sized pieces and plunged into boiling water for up to 5 minutes. They should be blanched in small batches to ensure that each piece is properly heated through.
If too many pieces are put into the water at one time, the water temperature will drop and prolong the blanching time. After blanching for the required time, vegetables are rapidly cooled by plunging into cold (or iced) water.
Sulphuring. Sulphuring or sulphiting is an optional stage of processing. The main benefit of sulphuring is to preserve the fruit colour. Some consumers object to chemical preservatives and prefer naturally dried fruits.
Sulphur dioxide gas (SO2) is applied to the fruit pieces by placing them in a cabinet or tent in which sulphur is burned. The gas is absorbed by the fruit. For most fruits, 5-6g sulphur per kg food is adequate. The gas given off is toxic and corrosive. Therefore, sulphuring should be carried out in a well ventilated place, using appropriate equipment. See the information on sulphuring for more details. Sulphite can be included in the sugar syrup (as sodium or potassium metabisulphite).
Drying. Fruit pieces are arranged on mesh-bottom trays so that they are not touching or overlapping. The fruit should be loaded into the trays as soon as it is cut. This prevents the pieces from sticking together and allows the drying process to start as soon as possible.
The trays should be brushed clean to remove any old fruit pieces.
The trays should be loaded into the dryer as soon as they are ready. The dryer doors should be closed after each tray is loaded.
Direct sunlight should be avoided as this bleaches the colour and reduces the level of vitamins A and C. The drying temperature should be controlled to avoid over-heating and spoilage of the fruit. Most fruits are dried at about 60-70 deg C.
Packaging. Dried fruits should be packaged immediately after drying to prevent them absorbing moisture from the surrounding air. After drying, fruits can be packed in bulk in sealed moisture-proof polyethylene bags then packed into smaller packets at a later date.