2. Introduction
• Dehydration is the oldest method of food preservation
• Drying facilities range from simple sun or hot air driers to high
capacity, sophisticated spray drying or freeze- drying installations.
• Reducing the water activity below the threshold of relevant
microbial activity
3. Objectives
• Minimal chemical and biochemical degradation reactions
• Selective removal of water over other salts and volatile flavor and aroma substances
• Maintenance of product structure (for a structured food)
• Control of density
• Rapid and simple rehydration or redispersion
• Storage stability: less refrigeration and packaging requirements
• Desired color
• Lack of contamination or adulteration
• Minimal product loss
4. Rapid rate of water removal (high capacity per unit amount
of drying equipment)
Inexpensive energy source (if phase change is involved)
Inexpensive regeneration of mass separating agents
Minimal solids handling problems
Facility of continuous operation
Noncomplex apparatus (reliable and minimal labor
requirement)
Minimal environmental impact
5. Theory of drying
• Dehydration is usually described as a simultaneous heat and mass
transfer operation.
• Sensible and latent heat must be transferred to the food to cause the
water to evaporate.
• Placing the food in a current of heated air is the most widely used
method of supplying heat.
• The heat is transferred by convection from the air to the surface of the
food and by conduction within the food
6. Drying curves
• Drying curve usually plots the drying rate versus drying time or moisture contents
• Three major stages of drying can be observed in the drying curve:
1. Transient early stage, during which the product is heating up (transient period)
2. Constant or first period, in which moisture is comparatively easy to remove (constant rate period)
3. Falling or second period, in which moisture is bound or held within the solid matrix (falling rate period)
7. Typical drying curves (water content versus drying time): (a) with a lag period, (b) without a lag period.
8. Typical drying rate curves: (a) drying rate versus drying time, (b) drying rate versus water
content.
