Fish products and Byproducts Technology

Unit 3- Salting and Drying

3.2.1. Factors effecting drying
What are all the factors affecting the constant rate drying period?
• Air velocity
• Temperature
• Product surface area
• Relative humidity and
• Product thickness
Air velocity
The higher the air velocity, the greater is the drying rate. The moisture to be carried away should move through a stagnant surface layer of air by diffusion. Once through this layer, the moisture is transferred to the outer layer by convection. The higher the air velocity, the thinner is the surface layer, and greater will be the evaporation of moisture.
Relative Humidity
Humidity influences the drying rate by limiting the amount of water the air can absorb. If the air is saturated, then the fish cannot dry at all. If the RH is less than 100% the air can absorb moisture and drying will take place until the air gets saturated.
Temperature
The amount of heat transferred to the product is proportional to the difference in temperature between the air and the product. High temperature differences increase heat transfer and drying rate. Warm air holds more moisture than cold air. If the temperature of the air is increased without the addition of water, the relative humidity falls. Lower RH favours more rapid evaporation and higher drying rates.
Surface area
The more the surface area, the greater is the surface available for drying. More area is exposed; faster will be the drying rate. Hence larger fishes are split open to increase the surface area.
Product thickness
Increased product thickness result in longer drying period, than in the falling rate period. Thinner fishes tend to dry faster than thicker ones mainly because of the increased surface area to volume ratio for thinner fishes.