Preservation by Dehydration

Chapter 5: Preservation by Dehydration

High Moisture Foods (HMF)

• Water activity (aw) range: 0.99 to 0.90
• Examples: fresh milk, meat, fish, vegetables, fruits
• Easily spoiled by bacteria or fast-growing moulds unless refrigerated or heated (canned)

Intermediate Moisture Foods (IMF)

• Water activity (aw) range: 0.90 to 0.60
• Examples: Aged cheese, hard salami, heavily salted fish, jams, marshmallows, and dried fruits
• Preserved by adding salt and/or sugar and intense drying process
• Spoilage caused by zero-tolerant moulds or osmo-tolerant yeasts, and halo-tolerant bacteria (less frequent)
• Fungistatic substances like sorbic acid, sulphur dioxide, or smoke are used to preserve IMF products

Low Moisture Foods (LMF)

• Water activity (aw) range: 0.60 to 0.20
• Examples: Chocolate, confectioneries, honey, dried eggs, cocoa, cake mixes, powdered milk, dried cereals, and vegetables
• Dried to suitable (low) aw value, making it unsuitable for microbial growth

Dehydration Principle

• All living organisms need water to survive
• Drying of foods reduces moisture content and water activity, stopping spoilage microbes, enzymes, and undesirable biochemical reactions
• Objectives of drying:
  • Reduce water content in the product (water content < 25% or aw between 0.00 to 0.60)
  • Extend shelf life
  • Reduce enzyme activity and undesirable chemical reactions

Advantages of Dehydrated Foods

• Smaller and lighter end products, reducing handling, storage, and transport costs
• Maintain much of the original nutritional value
• Long shelf life, requiring no preservatives

Changes Due to Dehydration

• Size and shape changes due to shrinkage
• Colour changes due to removal of water, exposure to high temperature, and air
• Texture changes due to shrinkage
• Flavour changes due to loss of volatile flavor compounds and development of an undesirable cooked flavor
• Nutritional quality changes due to loss of water-soluble nutrients

Factors Affecting Drying Rate

• Speed of drying: need to be conducted rapidly but not too fast to avoid case hardening
• Temperature: need to control temperature at an optimum level to evaporate moisture
• Humidity and ventilation: need to have low humidity in air and adequate ventilation in the dryer
• Uniformity: need to dry food evenly by spreading thin layers, flipping pieces frequently, and shifting racks in the dryer

Dehydration Methods and Drying Equipment

• Fluidized bed dryer
• Sun drying (solar)
• Cabinet drying (hot air)
• Microwave drying
• Freeze drying
• Spray drying
• Drum drying
• Single-drum dryer
• Double-drum dryer
• Vacuum-drum dryer

Chapter 5: Preservation by Dehydration

Moisture in Food

• High Moisture Foods (HMF):
  • Range of aw = 0.99 to 0.90
  • Examples: fresh milk, meat, fish, vegetables, fruits
  • Easily spoiled by bacteria or fast-growing moulds unless refrigerated or heated (canned)
• Intermediate Moisture Foods (IMF):
  • Range of aw = 0.90 to 0.60
  • Examples: Aged cheese, hard salami, heavily salted fish, jams, marshmallows, and dried fruits
  • Preserved by adding salt and/or sugar and intense drying process
  • Spoilage caused by zero-tolerant moulds or osmo-tolerant yeasts, and halo-tolerant bacteria
  • Fungistatic substances like sorbic acid, sulphur dioxide, or smoke used to preserve IMF products
• Low Moisture Foods (LMF):
  • Range of aw = 0.60 to 0.20
  • Examples: Chocolate, confectioneries, honey, dried eggs, cocoa, cake mixes, powdered milk, dried cereals, and vegetables
  • Dried to a suitable (low) aw value, unsuitable for microbial growth

Dehydration Principle

• Living organisms need water to survive
• Drying of foods reduces moisture content and water activity (aw), stopping microbial growth, enzyme activity, and undesirable biochemical reactions
• Objectives of drying:
  • Reduce water content in the product (<25% or aw between 0.00 to 0.60)
  • Extend shelf life
  • Reduce enzyme activity and undesirable chemical reactions

Advantages of Dehydrated Foods

• Smaller and lighter end products, reducing handling, storage, and transport costs
• Maintain much of the original nutritional value
• Long shelf-life with no preservatives required

Changes Due to Dehydration

• Size and shape changes due to shrinkage
• Colour changes due to removal of water, exposure to high temperature, and air
• Texture changes due to shrinkage
• Flavour changes due to loss of volatile flavor compounds and development of an undesirable cooked flavor
• Nutritional quality changes due to loss of water-soluble nutrients

Factors Affecting the Drying Rate

• Speed of drying: need to be conducted rapidly, but not too fast to prevent case hardening
• Temperature: need to control at an optimum level to evaporate moisture
• Humidity and Ventilation: need low humidity and adequate ventilation to prevent moisture accumulation
• Uniformity: need to dry food evenly by spreading thin layers, flipping pieces frequently, and shifting racks in the dryer

Dehydration Methods and Drying Equipment

• Fluidized Bed Dryer
• Sun Drying (Solar)
• Cabinet Drying (Hot Air Drying)
• Microwave Drying
• Freeze Drying
• Spray Drying
• Drum Drying

Specific Drying Methods

• Sun Drying:
  • Oldest method of food preservation
  • Advantages: renewable energy source, simple, and uncomplicated
  • Disadvantages: only practical in hot, dry climates, requires long time, hard to control weather, pollution, and animals
• Indirect Solar Dryer:
  • Collects sun's rays using heat exchanger (solar collectors)
  • Even drying using hot air flows through or over the product
• Cabinet Drying:
  • Suitable for solid food materials
  • Advantages: efficient, simple, and suitable for industrial application
• Microwave Drying:
  • Uses high-frequency electromagnetic waves of radiant energy
  • Advantages: penetrating heat leads to uniform drying, selective absorption by liquid water
  • Disadvantages: complicated equipment, expensive
• Freeze Drying:
  • Food is frozen, then placed in a vacuum chamber under reduced pressure
  • Ice changes directly to water vapour (sublimation) and is carried away by circulating heated air
  • Advantages: low thermal damage, maintains nutritional values and vitamins, no cooked flavour
  • Disadvantages: high capital and operation costs, initial freezing can damage certain products
• Spray Drying:
  • Suitable for foods in the form of liquids or slurries
  • Advantages: rapid drying, uniform particle size
  • Used to produce non-fat dried milk and instant coffee
• Drum Drying:
  • Continuous contact dryer for drying products initially in slurries
  • Suitable for heat-sensitive products, used to produce potatoes, corn syrup, and baby foods