Processing of Horticultural Crops

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 43^oC which is then gradually increased to 60-66^oC for vegetables and 66-71^oC for fruits.

• Dried foods or low moisture foods generally do not contain more than 25% moisture and have a water activity (aw) in the range of 0 to 0.6.
• Intermediate moisture (IMF) foods on the other hand contain 15 - 50% moisture with a water activity (aw) of 0.6 to 0.85.
• Drying under open sun is probably the oldest method of food preservation used for agricultural crops including food grains, oilseeds as well as fruit and vegetables.

Dried grapes (raisins and munacca), apricot, dates, Morchella (Guchhi), bael, peas, chillies, aonla, fig, etc are the commercial products prepared by drying. However, the temperatures used for drying are usually insufficient to cause killing of micro-organisms and enzymes. Therefore, any increase in moisture content during storage due to faulty packaging can spoil the product. Thus greater care is required for packaging and storage of dried products.

Advantages of drying

1. Helps in food preservation by reduction in water activity.
2. Reduction in weight and volume of the food (weight or volume of the dried product is reduced by 4 to 10 times from its initial fresh weight /volume).
3. Reduction in space requirement due to reduction in bulk and hence involves lower cost of packaging, storage and transport.
4. In comparison to other methods, drying is the cheap and simplest method of preservation.
5. Dried foods add variety to the diet and supply convenient ready to eat foods to the consumers.
6. Nutrient concentration is very high per unit weight of dried product.

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:

1. The dehydration is much more rapid, controlled and efficient than open sun drying.
2. Dehydration requires less space (floor area) as compared to sun drying.
3. Dehydration is more hygienic as compared to open sun drying.
4. Dehydration is not dependant on weather conditions while drying is not possible under cloudy weather or during rains.
5. The colour of dehydrated product remains uniform due to uniform drying temperature.