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Questions and Answers
Drying as a method of preserving food is a relatively modern practice.
Drying as a method of preserving food is a relatively modern practice.
False
Microorganisms that cause food spoilage cannot grow in the absence of sufficient moisture.
Microorganisms that cause food spoilage cannot grow in the absence of sufficient moisture.
True
Bound water refers to water molecules that are loosely associated with food components.
Bound water refers to water molecules that are loosely associated with food components.
False
Movement of water or water vapor is a crucial factor in the drying process.
Movement of water or water vapor is a crucial factor in the drying process.
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In vacuum drying, evaporation of water occurs more readily at higher pressures.
In vacuum drying, evaporation of water occurs more readily at higher pressures.
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Freeze drying maintains the food structure better due to the sublimation of water vapor from frozen food.
Freeze drying maintains the food structure better due to the sublimation of water vapor from frozen food.
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Air and contact drying involves heat transfer through heated air or surfaces.
Air and contact drying involves heat transfer through heated air or surfaces.
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Surface moisture contributes to the microbial stability of food.
Surface moisture contributes to the microbial stability of food.
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Pure water can only exist in two states: solid and liquid.
Pure water can only exist in two states: solid and liquid.
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Sublimation occurs when the vapor pressure of a product is less than the vapor pressure of the ambient air.
Sublimation occurs when the vapor pressure of a product is less than the vapor pressure of the ambient air.
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The equilibrium moisture content signifies that a material is gaining moisture.
The equilibrium moisture content signifies that a material is gaining moisture.
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During the constant-rate period of drying, the drying rate remains steady and primarily removes surface water.
During the constant-rate period of drying, the drying rate remains steady and primarily removes surface water.
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Latent heat of vaporization refers to the energy required to vaporize water from a solid state.
Latent heat of vaporization refers to the energy required to vaporize water from a solid state.
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When the vapor pressure of the product equals the vapor pressure of the ambient air, the system is in an equilibrium state.
When the vapor pressure of the product equals the vapor pressure of the ambient air, the system is in an equilibrium state.
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Heat must be supplied to vaporize water, and the amount required varies with temperature.
Heat must be supplied to vaporize water, and the amount required varies with temperature.
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In the falling-rate period, the drying rate increases as internal moisture is transferred to the surface.
In the falling-rate period, the drying rate increases as internal moisture is transferred to the surface.
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The latent heat of vaporization of water at 100°C is 2257 kJ kg-1.
The latent heat of vaporization of water at 100°C is 2257 kJ kg-1.
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The initial temperature of the food in the drying process is 100°C.
The initial temperature of the food in the drying process is 100°C.
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In air drying, the heat transfer rate is represented by the equation q = hsA(Ta - Ts).
In air drying, the heat transfer rate is represented by the equation q = hsA(Ta - Ts).
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The energy requirement per kilogram of water removed is not important in drying processes.
The energy requirement per kilogram of water removed is not important in drying processes.
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The dryer efficiency formula h = (T1 - T2)/(T1 - Ta) applies specifically to tray dryers.
The dryer efficiency formula h = (T1 - T2)/(T1 - Ta) applies specifically to tray dryers.
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In roller dryers, heat transfer occurs by conduction from the drum to the foodstuff.
In roller dryers, heat transfer occurs by conduction from the drum to the foodstuff.
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The specific heat capacity of water is 4.186 kJ kg-1 °C-1.
The specific heat capacity of water is 4.186 kJ kg-1 °C-1.
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Tray dryers heat food primarily using radiation from heated surfaces.
Tray dryers heat food primarily using radiation from heated surfaces.
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Tray dryers primarily use water to remove moist vapours.
Tray dryers primarily use water to remove moist vapours.
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Tunnel dryers may involve trays that move through a heat-applying tunnel.
Tunnel dryers may involve trays that move through a heat-applying tunnel.
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Drum drying is considered a form of conduction drying.
Drum drying is considered a form of conduction drying.
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In fluidized bed dryers, food particles are held down by gravity while being dried.
In fluidized bed dryers, food particles are held down by gravity while being dried.
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Spray dryers are suitable for materials that can withstand long exposure to heat.
Spray dryers are suitable for materials that can withstand long exposure to heat.
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Pneumatic dryers rely on high velocity and turbulence in the air stream to maintain particle suspension.
Pneumatic dryers rely on high velocity and turbulence in the air stream to maintain particle suspension.
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Rotary dryers operate with material contained in a vertical cylinder.
Rotary dryers operate with material contained in a vertical cylinder.
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In the spray drying process, heated air and solids can only move in parallel flow.
In the spray drying process, heated air and solids can only move in parallel flow.
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In trough dryers, air is blown through a conveyor belt made from mesh that contains the materials to be dried.
In trough dryers, air is blown through a conveyor belt made from mesh that contains the materials to be dried.
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Bin dryers operate by blowing cold air downwards through the material from a solid bottom.
Bin dryers operate by blowing cold air downwards through the material from a solid bottom.
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Belt dryers typically use a stationary belt while the food is transported by air.
Belt dryers typically use a stationary belt while the food is transported by air.
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Batch vacuum dryers rely primarily on convection for heat transfer.
Batch vacuum dryers rely primarily on convection for heat transfer.
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Freeze dryers usually freeze the food before it is placed in the dryer chamber.
Freeze dryers usually freeze the food before it is placed in the dryer chamber.
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Accelerated freeze drying improves heat transfer by inserting sheets of expanded metal between the foodstuffs and heated plates.
Accelerated freeze drying improves heat transfer by inserting sheets of expanded metal between the foodstuffs and heated plates.
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In vacuum dryers, the water vapor produced is typically left in the chamber to saturate the environment.
In vacuum dryers, the water vapor produced is typically left in the chamber to saturate the environment.
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Trough dryers are designed to expose fresh surfaces of the material to the hot air by continually turning over the material.
Trough dryers are designed to expose fresh surfaces of the material to the hot air by continually turning over the material.
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Study Notes
Central Luzon State University
- Located in Science City of Munoz, Nueva Ecija, Philippines
- Established in 1907
- Postal code: 3120
Food Process Engineering (ABEN 4510)
- Course taught by Melba Domes Denson
- Department of Agricultural and Biosystems Engineering
- College of Engineering
Basic Drying Theory
- Drying is a method of preserving food, practiced by primitive societies long before recorded history
- Drying prevents food spoilage and decay by eliminating water, which is necessary for microbial growth and enzyme activity
- Two key process-controlling factors in drying are the transfer of heat to provide latent heat of vaporization, and the movement of water or water vapor through the food material
Movement of Moisture During Drying
- Bound water: Water molecules tightly bound to other components within the food
- Free water/Surface moisture: Readily available water that influences food texture, microbial stability, and overall quality
Categories of Drying
-
Air and contact drying: Heat is transferred from heated air or surfaces, removing water vapor.
- Atmospheric pressure
-
Vacuum drying: Advantages to lower pressures, speeds up evaporation
- Heat transfer primarily by conduction, sometimes by radiation
-
Freeze drying: Sublimes water vapor from frozen food
- Maintains food structure effectively
Basic Drying Theory (cont'd)
- Pure water can exist in three states: solid, liquid, and vapor. The state at any specific time depends on temperature and pressure conditions
- Phase diagram can illustrate the temperature and pressure conditions affecting the states of water.
Principle of Drying
- Drying happens when the vapor pressure of the product (VP product) is greater than the vapor pressure of the surrounding environment (VP ambient).
- Adsorption of moisture happens when VP product is less than VP ambient.
- Product is in equilibrium when VP product equals VP ambient.
Psychrometry
- Relationship between temperature, humidity, and heat content involved in drying processes
- Various charts (e.g., enthalpy, constant relative humidity, wet-bulb lines, constant volume) represent these complex relationships.
- Charts show critical data such as absolute humidity, dry bulb temperature and wet bulb temperature
Equilibrium Moisture Content (EMC)
- EMC refers to the moisture content where a material is in equilibrium with its surrounding environment.
- No net gain or loss of moisture occurs at EMC
- EMC is determined by humidity and temperature in the surroundings
Drying Curve
- Describes the relationship between moisture content and drying time.
- Three distinct periods exist: preheating, constant rate, and falling rate
Heat Requirements for Vaporization
- Energy needed to vaporize water depends on temperature.
- Latent heat of vaporization (liquid) or latent heat of sublimation (solid) is the energy required
- Heat energy required can be calculated from steam tables.
Latent Heat and Saturation Temperature Data
- Table of absolute pressure, latent heat of vaporization, and saturation temperature for varying pressures
Sample Problem (Heat Energy in Drying)
- Problem calculating the heat energy required to dry a food product with a given initial and final moisture content at a specific temperature
- Data includes initial and final moisture percentage, initial and drying temperatures, latent heat of vaporization, specific heats of food and water.
Dryer Equipment
- Tray Dryers: Food is spread thinly on trays, drying occurs by air current sweeping trays or by conduction or radiation from hot surfaces.
- Tunnel Dryers: Modified tray dryers with moving trays through a tunnel, heat and vapor removal.
- Roller Dryers: Food is placed on a rotating drum, dries as it moves. Conduction is primary drying method.
- Fluidized Bed Dryers: Food material is maintained suspended against gravity in an upward flowing air stream; horizontal air flow helps convey food through. Convection is primary method.
- Spray Dryers: Liquid or solid material sprayed into heated air current. Drying rapid with horizontal particle movement preventing particle contact.
- Pneumatic Dryers: Solids moved in air stream that maintains suspension.
- Rotary Dryers: Food kept in horizontal cylinder, rotated, air flow through or contact.
- Trough Dryers: Food on trough conveyor belt. Air flow between bed of material on belt, continually turning over the material exposes dry surfaces.
- Bin Dryers: Food in bin with perforated bottom, warm air blown vertically. Air passes through, drying the food.
- Belt Dryers: Thin layer of food spread horizontally on a mesh or solid belt, air passes and or over the product.
- Vacuum Dryers: Similar to tray dryers, operate under vacuum, heat transfer by conduction or radiation. - Freeze Dryers: Material held under vacuum in chamber at low temp.
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Description
Test your knowledge on the methods of food preservation, specifically focusing on drying techniques. Explore concepts such as bound water, vacuum drying, freeze drying, and more. Learn how moisture levels impact food stability and microbial growth through this engaging quiz.
