Drying and Dehydration

• For dehydration and drying, water activity is preferred to be used than moisture content since there are foods with high moisture content but low water activity like the case of jams and jellies
• Mechanical pressing or concentration of liquid is not included in drying. Food is subjected to heat, which removes the moisture away from the food
• Primary Technological Objectives of Drying/Dehydration

  • Lengthen the shelf life of food
  • Reduction in weight and volume
  • Convenience for storage, packaging, transport, and use
  • Creation of new products with desirable features such as a different flavor, crispiness, chewiness
• Inactivating enzymes prior to drying and dehydration, particularly on fruits and vegetables, is highly recommended as there are enzymes that may still be active even at low water activity
• Learning Outcomes

  1. Comprehend with the terms associated with drying and dehydration

  2. Demonstrate understanding on the principles of drying and dehydration as preservation technique

  3. Show familiarization with different processes of drying and dehydration

  4. Explain possible causes of the problems with dried foods

  5. Appreciate drying and dehydration as a preservation technique
• Additional Primary Technological Objectives of Drying/Dehydration

  • Dried foods require less storage space than other methods of food preservation
  • Dried foods weigh much less than the equivalent amount of canned products; and
  • They can be preserved without the addition of sugar or any other preserving medium
  • Dried foods are lightweight and easy to transport and store
• Dehydration


  The general term applied to all methods of removing moisture in food, whether by the heat of the sun or by artificially produced heat by other means
• Water Activity

  The amount of water contained in the food material. It defines the amount of water that can be used for the growth and reproduction of microorganisms and water for biological reactions. A water activity of 1.0 indicates pure water and 0 indicates absence of water molecules. The addition of solutes always lowers the water activity
• Drying


  The reduction of available moisture in food to a level that will no longer be conducive for the growth and multiplication of spoilage microorganisms
• Artificial Drying


  Drying by artificially-produced heat under controlled conditions of temperature, humidity, and air flow within a chamber
• Water Activity levels

  • High water activity (0.8 and above) indicates “wet/moist” system
  • Low water activity (0.7 or less) indicates “dry” system
  • Water activity at 0.2, all chemical reactions requiring water stops; 0.6 – no microbial growth occurs; 0.3 – 0.5 – loss of crispness and brittleness, lumping may occur
• Principles of Drying/Dehydration

  The different methods drying/dehydration employs any of the following principles: Exposing the food to warm air, Exposing food to circulating air
• Heating the food under vacuum

  Increases water loss at low temperature while retaining the natural characteristics of the food. More efficient in conserving volatile substances in foods such as color, flavor, and nutrients
• Solar drying


  Uses equipment to collect the sun’s rays in a unit designed to ventilate moisture. Temperature in the unit is usually 20-30 °C higher than in open sunlight. Applicable in countries with high outdoor temperatures
• Limitations of sun drying

  • Relies on weather conditions
  • Can only dry up to 15-25%
  • Requires a lot of space
  • Food being dried is exposed to contamination and losses
• Osmotic dehydration


  Food is soaked in very high concentrated salt or sugar solutions or solvents to leach out the liquid within the cell of the food. Process is repeated until the final moisture reduction level is attained
• Exposing food to circulating air

  Circulating air brings a continuous supply of fresh air that comes in contact with the food. As warm air circulates around the food, water is removed from the food
• Dehydration using artificial heat

  Food is exposed to hot air that moves across it. Heat is controlled for shorter drying time
• Sun drying


  Most common method of drying. Oldest and simplest way of water removal. Food is exposed to solar energy. Economical for small-scale processing
• Exposing food to warm air

  Warm air has the capacity to hold more moisture compared to cold air. When food is subjected to warm air, water is removed from the food
• Freeze drying

  1. The process of dehydrating frozen foods under vacuum so the moisture content changes directly from a solid to a gaseous state without having undergone through the liquid state (the process is called sublimation)

  2. With the process of sublimation, there is better retention of flavor, color and nutrients
• Factors affecting the length of drying time
• Factors affecting the drying rate

  • Temperature. The best temperature for drying is 50-60°C
  • Relative humidity of the air
  • Time. The more moisture to be removed, the longer drying time would be required
  • Velocity of the air. The faster is air velocity, the faster is the drying rate
• Osmotic dehydration

  1. Food is soaked in very high concentrated salt or sugar solutions or solvents to leach out the liquid within the cell of the food

  2. Process is repeated until the final moisture reduction level is attained

  3. Osmotic dehydration is usually done prior to dehydration using warm or circulating air

  4. Example of this is the preparation of candied fruits

  5. Fruits are soaked in sugar where sugar replaces the water in the fruit

  6. After osmotic dehydration, the fruits are dried using cabinet dehydration

  7. Resulting dried products are sweet
• Factors affecting the drying rate
• Frying

  1. Drying by frying is applicable to very thin or small pieces of food and extruded puffed products

  2. It is important to monitor frying temperature and proportion of oil to food being fried to avoid large inflow of oil into the food

  3. Deep-fat frying of foods usually is carried out under atmospheric conditions at temperatures near 190°C

  4. Conventional frying may use the frying pan or the electric deep fat drying
• Defects of dehydrated/dried foods


  • Pronounced shrinkage of solids
  • Case hardening
  • Difficulty of rehydration
  • Browning or scorching
  • Souring
  • Molds growth
• Factors affecting the length of drying time

  • Food with higher water activity take more time to dry
  • Type of food
  • Size of food
• Vacuum frying

  Vacuum fryer