Processing of Horticultural Crops (1+2)

Types of Unit Operations (Contd..)

8. COOLING
Cooling is the subtraction of heat energy which is added during processing. The cooling may be done to the degree where food is chilled to refrigerated temperature. The milk is cooled by passing them in thin layers through heat exchangers or through coolers (cold water or refrigerants are pumped). Fruits such as apple slice, berries, and cherries are frozen. Thawing will be done by the unit operations of heating or disintegrating.

• Air blast freezers automatically freeze peas, beans and other vegetables, mushrooms individually.
• Freezing of canned or packaged foods may be done by direct immersion in refrigerants.

9. EVAPORATION
Evaporation in the food industry is used principally to concentrate foods by removal of water. All liquids boil at lower temperature under reduced pressure and are the key to modern evaporation. Vacuum evaporators and multi stage evaporators can easily remove water.

10. DRYING
Drying involves the removal of water with minimum damage to the food. Evaporators will concentrate the food 2-3 folds or more while driers take the foods very close to total dryness that is to 97-98% solids. Driers are used to prepare well known products like milk powder and instant coffee. Subdivision of a liquid is the basic principle behind the widely used spray driers. The liquid is atomized by a spray nozzle and at the same time the hot air is passed, which results into drying. Sun and solar drying, atmospheric dehydration including stationary or batch processes (kiln, tower, and cabinet driers) and continuous processes (tunnel, continuous belt, belt-trough, fluidized-bed, explosion puffing, foam-mat, spray, drum, and microwave-heated driers) are used. Vacuum shelf, vacuum belt, vacuum drum and freeze driers are the type of driers that can be used for drying and dehydration of products. Prior to drying, the fruits are pretreated in sulphur fumigation chambers by burning sulphur or are dipped in a solution of potassium metabisulphite.

11. FORMING
It is an important unit operation in the breakfast cereals and snack food industries. The characteristic shapes of the popular breakfast cereals are the result of pressure extrusion through dies, together with adherence operating conditions like pressure, temperature, dough consistency and other variables. The special kind of forming is known as extrusion cooking. Further examples of forming are shaping of butter, bars, pressing of cheese curd into various shapes, bread dough shapes and shaping of sausages.

12. PACKAGING
The packaging of food is necessarily required to protect the food from microbial contamination, dirt, dust, light, moisture and the losses. The foods are packaged in metal cans, glass, plastic bottles, paper and metallic films, pouches etc. Now a days the packaging of food products has emerged as an important industry and automatic packaging units are in great demand. The containers for packaging are automatically formed, filled and sealed by passing through machines. Such packages are easy to open and dispose off. The newer packaging systems have some advantages like saving of space in food plants, during transportation and marketing.
Thus, for preparation of any furnished product, different unit operations are used. Depending upon the availability of facilities, the processing can be carried out as a batch process, semi-continuous process or as an automatic continuous process.