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Food Processing Operations, (Inhibit microbial growth and enzyme activity,…
Food Processing Operations
Preservation Operations
Dehydration / Drying
Drying
Drying vs. Evaporation:
Drying: Small amount of liquid removed
Importance: Very important in food processing
Evaporation: Large amount of liquid removed
Definition
Removal of liquid from a solid using heat
Process
Dehydration
Irradiation
Chilling
Pasteurisation
Freezing
Pasteurisation
Use of Heat
Removes water
Heat changes food qualities:
Nutritional
Sensory
major processing cost
Heat extends shelf life:
Destroys enzymes and microbes
Dehydration
Controlled application of heat to remove water
Extend shelf life
Reduce water activity
Inhibit microbial growth and enzyme activity
Does not kill microbes and enzymes
Insufficient temperature for inactivation
Effects:
Prevents microbial growth and reproduction
Reduces water activity, weight, and bulk
Preserves the product
Pre-drying processes:
Blanching to inactivate browning enzymes
Dicing to increase surface area
Dehydration (cont.)
Negative effects:
Reduces eating quality and nutritional value
Examples of dried foods:
Coffee, milk, raisins, fruits, pasta, flours, beans, nuts, cereals, tea, spices
Factors affecting drying rate:
Processing conditions
Nature of the food
Drying using heated air
Factors controlling moisture removal:
Water vapor in air
Air temperature
Airflow rate
Water vapor measurement:
Absolute humidity
Relative humidity (RH)
Psychrometry: Study of air-water vapor systems
Mechanism of drying
Drying curves:
Constant temperature and humidity
Heat supplied by convection
Factors affecting drying
Food composition and structure
Amount of food in dryer
Moisture Measurements
Methods:
Loss on drying (LOD)
Moisture Content (MC)
LOD:
Wet weight basis
Formula: %LOD = (Weight of water / Total wet weight) x 100
Moisture Measurements (cont.)
LOD determination:
Moisture balance with heat source and scale
Dry sample to constant weight
MC:
Dry weight basis
Formula: %MC = (Weight of water / Dry weight) x 100
Moisture Measurements (cont.)
Calculation example:
10g solid (wet) -> 4g solid (dry)
LOD = 60%
MC = 150%
LOD range: ~0 to 100%
MC range: ~0 to infinity
Importance: Determine moisture to be removed for dryer selection
Process Control Parameters
Factors:
Load size
Temperature
Relative humidity
Airflow rate
Drying time
Types of Drying
Sun drying:
Slow, uncontrolled
High nutrient loss, risk of contamination
Used for grains, fruits, spices
Hot air drying:
More efficient, controlled
Lower nutrient loss
More expensive
Used for vegetables, pasta, fruits
Drum drying:
More efficient than hot air
Lower nutrient loss
Used for pastes and slurries
Spray drying:
Low nutrient loss
More expensive
Good control, efficient
Used for liquids (milk, tea, coffee)
Puff drying:
Uses heating systems and pressure drop
Freeze drying:
Best nutrient quality, product quality
Most expensive
Good control
Used for coffee, camping foods, etc.
Types of Driers
Hot-air driers:
Bin driers: Large containers with mesh base, low air velocity
Cabinet driers: Insulated cabinet with trays, air blown over trays
Tray Drier: Air flows over shelves with trays of wet material
Tunnel driers: Hot air blown over product, concurrent or counter-current flow
Conveyor driers: Continuous belt, air flow changes direction
Fluidised bed drying: Warm air lifts food, suitable for small items
Drum Dryer: Rotating heated drum, liquid dried in thin film
Freeze-drying:
Combines freezing and drying
Removes water through sublimation
Advantages:
Minimal chemical decomposition
Light and porous product
No concentration of solution
Minimal oxidation
Disadvantages:
Requires special packaging
Slow and expensive
Chilling
Definition: Reducing food temperature to 1-8°C
Purpose:
Reduce biochemical and microbial changes
Extend shelf life
Advantages:
Minimal changes to sensory and nutritional properties
Convenient, easy to prepare, high quality
Often combined with other preservation methods
Freezing
Definition: Reducing food temperature below freezing point to form ice crystals
Purpose:
Lower water activity
Preserve food through low temperature and reduced water activity
Often involves pre-treatment by blanching
Inhibit microbial growth and enzyme activity
Limitations:
Dryer design