Processing of Horticultural Crops (1+2)

Types of Heated Surface driers

Types of Heated Surface driers
1). Vacuum drum drier: Vacuum drum drier is the same except that they operate under a vacuum and heat transfer is by conduction or by radiation. The rollers are enclosed in a large cabinet which is evacuated. Vacuum drum driers are used for drying of potato flakes, dry soup and fruit juices.

2). Drum drier (Roller drier): In drum drier, slowly rotating hollow steel drums are heated internally by pressurized steam to 120-170⁰C. A thin layer of food is uniformly placed over the outer surface of drum either by dipping, spraying, or spreading by feed rollers. Before the drum completes one revolution (within 20 seconds to 3 minutes) the dried food is scrapped off by a ‘doctor’ blade which contacts the drum surface uniformly along its length. Driers may have single drum, or double drum (twin drums). Drum driers are used for drying of milk powder.

Specific features of drum driers include:

• High drying rates with high energy efficiencies and their suitability for slurries where particles are too large for spraying.
• Heat damage to sensitive foods and high capital cost are the major limitations of such driers.

3). Vacuum shelf drier
Vacuum shelf driers consist of hollow shelves in a vacuum chamber. Food is placed in thin layers on flat metal trays which are carefully made to ensure good contact with the shelves. A partial vacuum of 1-70 torr (1.3 kPa to 93.3 kPa) is drawn in the chamber and steam or hot water is passed through the shelves to dry the food. They are used to produce puff dried foods.
Advantages

• It gives rapid drying and limited heat damage
• Suitable for heat sensitive foods.

Limitations: Relatively high capital and operating costs and low production rates.

4). Explosion puff drying: It involves partially drying of foods to moderate moisture content and then sealing it into a pressure chamber. The pressure and temperature in the chamber are increased and instantly released. The rapid loss of pressure causes the food to expand and develop a fine porous structure. It allows faster final drying and rapid rehydration. Sensory and nutritional qualities are well retained in this type of drying.

5). Freeze drying: Freeze drying is the sublimation/removal of water content from frozen food. The dehydration occurs under a vacuum, with the food product solidly frozen during the process. In the process of freeze drying, the food is first frozen at -18⁰C in the lower chamber of a freeze drier and then the frozen material is dried initially at 30⁰C for 24 hrs then at 20⁰C until complete drying under high vacuum (0.1 mm Hg) in the upper chamber. The final product is highly hygroscopic, excellent in taste and flavour. Mango pulp, concentrates, passion fruit juice and guava pulp are dehydrated by this method and give freeze-dried powders of excellent quality for taste, flavour and reconstitution property.
Freeze dryer utilizes the principle that under high vacuum (27-133 Pa pressure) frozen water can be removed from a food and collected without going through a liquid phase. Because the material remains frozen, no heat damage occurs. The material is directly dried by sublimation of ice without passing through intermediate liquid phase.

Osmotic dehydration
The fruit which are highly acidic and have sensitive aroma can be dried by using osmotic dehydration. In this method the fruits after preliminary treatment are placed in hypertonic solution of 70⁰B sugar syrup and kept for 4 hrs to overnight. During this period, the water oozes out in syrup due to osmosis. About 50% of moisture from the fruit is removed in can process. The fruits are then drained from the syrup, rinsed and further dried in hot air drier to desired moisture content. During osmotic drying, acid from the fruit oozes out in the syrup while some sugar enters in the fruit thus the final product attains the required sugar acid balance. Apricots, grapes, apple etc. can be dried by using osmotic drying.

Packaging and storage of dried products
After foods are dried, cool them completely. Then packaging is done in clean moisture-vapor-resistant containers. Glass jars, metal cans or freezer containers are good storage containers, if they have tight-fitting lids. Fruit that has been sulfured or sulfited should not touch metal. Place the fruit in a plastic bag before storing it in a metal can. Dried food should be stored in a cool, dry, dark place. Most dried fruits can be stored for 1 year at 60°F, 6 months at 80°F. Dried vegetables have about half the shelf-life of fruits. Polypropylene (PP), laminate of metallised polyester (PET), low density polyethylene (LDPE) is suitable for a shelf-life of at least 6 months.

Different types of packaging materials used are:

1. Rigid containers like metal cans and plastic containers, which are air-tight and light proof and check the entry of moisture and oxygen. They are easy to handle during transportation.
2. Semi-rigid packs, like line carton and bag-in-box, maintains the freshness of the food product till it is opened. An ideal laminate is made up of layers of paper/low density polyethylene (LDPE)/Al-foil/ which ensures the shelf life required.
3. Flexible pouches can be handled and opened easily.