Drying in food processing, Lecture notes of Food Science and Technology

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Drying

Presented by: Miss Kulsum Ibrahim

Learning Objectives:

The Students might be able to know about;

● Drying ● And types of industrial drier

Equipment Used in drying purpose:

1) Cabinet (Tray) Drier

2) Tunnel Drier

3) Conveyor (Belt) Drier

4) Bin Drier

5) Fluidised Bed Drier

6) Pneumatic (Flash) Drier

7) Rotary Drier

8) Vacuum Cabinet (Tray or Shelf) Drier

9) Double Cone Vacuum Drier

Cabinet (Tray) Drier

This is a multipurpose, batch-operated hot air drier. It consists of an insulated cabinet, equipped with a fan, an air heater and a space occupied by trays of food. It can vary in size from a bench-scale unit holding one or two small trays of food to a large unit taking stacks of large trays. The air may be directed by baffles to flow the across surface of the trays of food or through perforated trays and the layers of food, or both ways. The moist air is partly exhausted from the cabinet. Small cabinet dryers are used in laboratories, while larger units are used as industrial dryers, mainly for drying sliced or diced fruits and vegetables. A number of large cabinets may be used in parallel, to process relatively large quantities of food, up to 20,000/day of raw material.

Conveyor (Belt) Drier

In this type of drier the food material is conveyed through the drying tunnel on a perforated conveyor, made of perforated metal plates or wire or plastic mesh. The heated air usually flows through the belt and the layer of food, upward in the early stages of drying and downward in the later stages. The feed is applied to the belt in a layer 75–150 mm deep. The feed must consist of particles that form a porous bed allowing the air to flow through it. Conveyors are typically 2–3 m wide and up to 50 m long. The capacity of a conveyor drier is much less than that of a tunnel drier. The use of multistage conveyor drying is common. The product from the first conveyor is redistributed, in a thicker layer, on the second conveyor. This may be extended to three stages. In this way, the conveyor is used more efficiently, compared to a single-stage unit. On transfer of the particles from one stage to the next, new surfaces are exposed to the heated air, improving the uniformity of drying. The air temperature and velocity may be set to different levels in each stage. Thus, good control may be exercised over the drying, minimising heat damage to the product. However, even when using two or more stages, drying in this type of drier is relatively expensive. Diced vegetables, peas, sliced beans and grains are examples of foods dried in conveyor driers.

Bin Drier:

This is a through flow drier, mainly used to complete the drying of particulate material partially dried in a tunnel or conveyor drier. It takes the form of a vessel fitted with a perforated base. The partly dried product is loaded into the vessel to up to 2 m deep. Dry, but relatively cool air, percolates up through the bed slowly, completing the drying of the product over an extended period, up to 36 h. Some migration of moisture between the particles occurs in the bin. This improves the uniformity of moisture content in the product.

Reference

● H.-K. Shin, Energiesparende Konservierungsmethoden für Fleischerzeugnisse, abgeleitet von traditionellen Intermediate Moisture Foods, PhD Thesis, Universität Hohenheim, Stuttgart-Hohenheim, 1984, 115 pp. 2

● Driscoll, R. H. and Paterson, J. L. 1999. Packaging and food preservation. In Handbook of Food Preservation, M. S. Rahman (Ed.), Marcel Dekker, New York. p. 687.