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What is the major difference between a double-drum drier and a single-drum drier in terms of structure?
What is the major difference between a double-drum drier and a single-drum drier in terms of structure?
Double-drum drier consists of two drums situated close together and rotating towards each other.
Which type of material is suitable for a vacuum-drum dryer?
Which type of material is suitable for a vacuum-drum dryer?
The vacuum-drum dryer equipment is enclosed within a vacuum tight chamber to reduce pressure, allowing the use of __________ temperatures.
The vacuum-drum dryer equipment is enclosed within a vacuum tight chamber to reduce pressure, allowing the use of __________ temperatures.
low
What is the principle behind food preservation by dehydration?
What is the principle behind food preservation by dehydration?
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Which range of water activity (aw) is typical for low moisture foods (LMF)?
Which range of water activity (aw) is typical for low moisture foods (LMF)?
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Temperature control in drying should be at an optimum level to evaporate _______ from food.
Temperature control in drying should be at an optimum level to evaporate _______ from food.
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Fungistatic substances like sorbic acid are used to preserve intermediate moisture foods (IMF).
Fungistatic substances like sorbic acid are used to preserve intermediate moisture foods (IMF).
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Match the drying equipment with suitable factors affecting their selection:
Match the drying equipment with suitable factors affecting their selection:
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What is the purpose of the knife located ½ to ¾ of a revolution from the point of feed application in the double drum dryer?
What is the purpose of the knife located ½ to ¾ of a revolution from the point of feed application in the double drum dryer?
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What distinguishes the double-drum dryer from the single-drum dryer?
What distinguishes the double-drum dryer from the single-drum dryer?
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The vacuum-drum dryer is suitable for materials that are not heat-sensitive.
The vacuum-drum dryer is suitable for materials that are not heat-sensitive.
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In what type of conditions can the vacuum drum dryer be operated?
In what type of conditions can the vacuum drum dryer be operated?
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What is the main principle of food preservation by dehydration?
What is the main principle of food preservation by dehydration?
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List two advantages of dehydrated foods.
List two advantages of dehydrated foods.
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Dehydrated foods typically maintain all of their original nutritional value.
Dehydrated foods typically maintain all of their original nutritional value.
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The objectives of drying are to reduce the water content in the product to less than __% or achieve a water activity (aw) between 0.00 to 0.60.
The objectives of drying are to reduce the water content in the product to less than __% or achieve a water activity (aw) between 0.00 to 0.60.
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Match the following factors with their effects on the drying rate:
Match the following factors with their effects on the drying rate:
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Study Notes
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
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Description
Understand the concept of preservation through dehydration, including high and intermediate moisture foods, their water activity ranges, and examples.
