Site pages
Current course
Participants
General
Topic 1
Topic 2
Topic 3
Topic 4
Topic 5
Topic 6
Topic 7
Topic 8
Topic 9
Topic 10
Topic 11
Topic 12
Topic 13
Topic 14
Topic 15
Topic 16
Topic 17
Topic 18
Topic 19
Topic 20
Topic 21
Topic 22
Topic 23
Topic 24
Topic 25
Topic 26
Topic 27
Topic 28
Topic 29
Topic 30
Topic 31
Topic 32
Topic 33
Topic 34
Topic 35
Topic 36
Topic 37
Topic 38
Topic 39
Topic 40
Topic 41
Topic 42
Topic 43
Topic 44
Topic 45
Topic 46
Topic 47
Topic 48
Topic 49
Topic 50
Topic 51
Introduction
Introduction Drying refers to the method of removal of moisture content from the food to a level at which the activities of food spoilage and food poisoning micro-organism are inhibited. Drying under open sun is probably the oldest method of food preservation used for agricultural crops including food grains, oilseeds as well as fruits and vegetables. Most of the fruits and vegetables contain enough moisture to permit the activity of enzymes and micro-organisms for spoilage and drying is necessary to reduce the water activity. Therefore, reduction in water activity of the food is the main principle of preservation by drying. Two commonly used terms for dried foods are low moisture foods and intermediate moisture foods (IMF). During drying, a single layer of fruit and vegetables, either whole or sliced after primary pretreatments is spread on trays which are placed inside the dehydrator or in the open sun for drying. In mechanical dehydrator, the initial temperature is generally kept at 43oC which is then gradually increased to 60-66oC for vegetables and 66-71oC for fruits.
Advantages of drying
Drying v/s dehydration: Drying and dehydration are inter-related terms and are used interchangeably to describe the unit operations involved in removal of water by evaporation or sublimation. Drying generally refers to the method of removal of moisture from the food under natural condition such as sunlight and wind such as open sun drying, shade drying etc. Whereas, dehydration refers to a process of removal of moisture by application of artificial heat under controlled conditions of temperatures, humidity and air flow Mechanism of drying: Drying or dehydration involves the simultaneous application of heat and removal of water from the food. When hot air is blown over a wet food, the water vapour diffuses through a boundary film of air surrounding the food and is carried away by the moving air. A water vapour pressure gradient is established from the moist interior of the food to the dry air which provides the driving force for water removal from the food. The boundary film acts as a barrier to both heat transfer and water vapour removal during drying. The moving air velocity determines the thickness of the boundary film. Water vapour leaves the surface of the food and increase the humidity of the surrounding air, to cause reduction in the water vapour pressure gradient which reduces the rate of drying. Therefore, the moving air should be faster to reduce the thickness of boundary film and hence to achieve faster rate of drying. Hence, for drying of moist horticultural produce the parameters taken into consideration are moderately high dry bulb temperature, low relative humidity and high air velocity. Drying curve: For each and every product, there is a representative curve that describes the drying characteristics for that product at specific temperature, velocity and pressure conditions. This curve is referred to as the drying curve for a specific product. Fig 10.1 shows a typical drying curve. Drying occurs in three different periods or phases like the first phase or initial period, the second phase or constant rate period and the third phase or falling rate period. Advantage of dehydration over open sun drying:
|
Last modified: Wednesday, 7 March 2012, 5:20 AM