Podcast
Questions and Answers
What does EMC stand for in the context of hygroscopic materials?
What does EMC stand for in the context of hygroscopic materials?
- External Moisture Condition
- Estimated Moisture Capacity
- Equilibrium Moisture Content (correct)
- Evaporative Moisture Control
Which factor does NOT influence the equilibrium moisture content of a hygroscopic material?
Which factor does NOT influence the equilibrium moisture content of a hygroscopic material?
- Type of material (correct)
- Constant humidity
- Water vapor pressure of the surrounding air
- Temperature of the air
What is one of the main drawbacks of sundrying food?
What is one of the main drawbacks of sundrying food?
- It reduces the moisture content efficiently.
- It is a quick process that prevents spoilage.
- It requires minimal space for operation.
- It is less effective in preserving food compared to modern methods. (correct)
Which theory explains the process of moisture migration during drying?
Which theory explains the process of moisture migration during drying?
Which statement describes a benefit of drying food?
Which statement describes a benefit of drying food?
How does an increase in dry-bulb temperature affect the relative humidity of air?
How does an increase in dry-bulb temperature affect the relative humidity of air?
What is a key factor that affects the state of water?
What is a key factor that affects the state of water?
What is the recommended air velocity range for drying food products?
What is the recommended air velocity range for drying food products?
What is the desired outcome of drying food?
What is the desired outcome of drying food?
What does the removal of water during drying primarily prevent?
What does the removal of water during drying primarily prevent?
Which drying system typically encourages faster drying rates?
Which drying system typically encourages faster drying rates?
How does temperature affect the drying process according to the vapor pressure theory?
How does temperature affect the drying process according to the vapor pressure theory?
What factor is diminished during the falling rate period of drying?
What factor is diminished during the falling rate period of drying?
What type of product support is commonly used in drying chambers for faster drying?
What type of product support is commonly used in drying chambers for faster drying?
What is NOT a method of removing water during the drying process?
What is NOT a method of removing water during the drying process?
How does the thickness of the product layer on drying trays influence drying rates?
How does the thickness of the product layer on drying trays influence drying rates?
What is one disadvantage of using a cross-flow drying system?
What is one disadvantage of using a cross-flow drying system?
What advantage does suspending food materials on skewers provide in the drying process?
What advantage does suspending food materials on skewers provide in the drying process?
In the sorption isotherm concept, which of the following substances tends to lower the water activity of solutions?
In the sorption isotherm concept, which of the following substances tends to lower the water activity of solutions?
During which stage of drying does the food material reach its critical moisture level?
During which stage of drying does the food material reach its critical moisture level?
What is the gas constant 'R' valued at for calculations within the given context?
What is the gas constant 'R' valued at for calculations within the given context?
What happens to food material at point E during the drying process?
What happens to food material at point E during the drying process?
What does the term 'equilibrium moisture content' (EMC) signify in the drying process?
What does the term 'equilibrium moisture content' (EMC) signify in the drying process?
Which variable in the equation is influenced by the adsorption conditions and relative humidity?
Which variable in the equation is influenced by the adsorption conditions and relative humidity?
What does the symbol hs represent in the heat transfer equation?
What does the symbol hs represent in the heat transfer equation?
Which factor marks the termination of the constant drying rate period (CRP)?
Which factor marks the termination of the constant drying rate period (CRP)?
What describes the relationship between moisture diffusion and the falling rate period (FRP)?
What describes the relationship between moisture diffusion and the falling rate period (FRP)?
In the equation for water vapor mass flux, what does the variable L represent?
In the equation for water vapor mass flux, what does the variable L represent?
Which assumption is NOT correct regarding the effective diffusion coefficient (De) in the context provided?
Which assumption is NOT correct regarding the effective diffusion coefficient (De) in the context provided?
What is indicated by the 'dry patch' area at the end of the constant drying rate period?
What is indicated by the 'dry patch' area at the end of the constant drying rate period?
Which condition is assumed when using the Fickian model for moisture movement?
Which condition is assumed when using the Fickian model for moisture movement?
What does the variable D represent in the context of moisture diffusion?
What does the variable D represent in the context of moisture diffusion?
What does the variable 'k' represent in the equation for moisture independent diffusivity?
What does the variable 'k' represent in the equation for moisture independent diffusivity?
Which theory is NOT mentioned regarding moisture movement in foods?
Which theory is NOT mentioned regarding moisture movement in foods?
How does relative humidity affect the rate of drying?
How does relative humidity affect the rate of drying?
What factors can lead to case hardening during the drying process?
What factors can lead to case hardening during the drying process?
In the Sherwood equation, what variable does 'l' represent?
In the Sherwood equation, what variable does 'l' represent?
What is the primary role of relative humidity in the drying process?
What is the primary role of relative humidity in the drying process?
Which of the following options is associated with the linear portion of the X versus t plot to calculate D?
Which of the following options is associated with the linear portion of the X versus t plot to calculate D?
What does the variable 'We' represent in the diffusivity equation?
What does the variable 'We' represent in the diffusivity equation?
Flashcards
Equilibrium Moisture Content (EMC)
Equilibrium Moisture Content (EMC)
The constant moisture content a hygroscopic material reaches when exposed to unchanging air temperature and humidity.
Sorption Isotherm
Sorption Isotherm
A graph showing the relationship between relative humidity and the equilibrium moisture content (EMC) of a product at a constant temperature.
What does the Henderson equation model?
What does the Henderson equation model?
The Henderson equation models the relationship between water activity (Aw) and equilibrium moisture content (M) in a hygroscopic material.
What does the Halsey equation model?
What does the Halsey equation model?
Signup and view all the flashcards
B.E.T. Equation
B.E.T. Equation
Signup and view all the flashcards
Sundrying
Sundrying
Signup and view all the flashcards
Food Drying
Food Drying
Signup and view all the flashcards
Why is food drying a good preservation method?
Why is food drying a good preservation method?
Signup and view all the flashcards
Vapor Pressure Theory
Vapor Pressure Theory
Signup and view all the flashcards
Drying Rate
Drying Rate
Signup and view all the flashcards
States of Water
States of Water
Signup and view all the flashcards
Factors Affecting Drying Rate
Factors Affecting Drying Rate
Signup and view all the flashcards
Drying's Impact on Food
Drying's Impact on Food
Signup and view all the flashcards
Water Activity (Aw)
Water Activity (Aw)
Signup and view all the flashcards
Monolayer Value (M1)
Monolayer Value (M1)
Signup and view all the flashcards
Heat of Adsorption (Q1)
Heat of Adsorption (Q1)
Signup and view all the flashcards
Constant Drying Rate Period (CRP)
Constant Drying Rate Period (CRP)
Signup and view all the flashcards
Critical Moisture Content (CM)
Critical Moisture Content (CM)
Signup and view all the flashcards
Falling Rate Period (FRP)
Falling Rate Period (FRP)
Signup and view all the flashcards
Moisture Independent Diffusivity
Moisture Independent Diffusivity
Signup and view all the flashcards
Sherwood Equation
Sherwood Equation
Signup and view all the flashcards
Moisture Dependent Diffusivity
Moisture Dependent Diffusivity
Signup and view all the flashcards
Liquid Diffusion Theory
Liquid Diffusion Theory
Signup and view all the flashcards
Capillary Theory
Capillary Theory
Signup and view all the flashcards
Evaporation-Condensation Theory
Evaporation-Condensation Theory
Signup and view all the flashcards
Relative Humidity (RH)
Relative Humidity (RH)
Signup and view all the flashcards
Case Hardening
Case Hardening
Signup and view all the flashcards
Dry Bulb Temperature
Dry Bulb Temperature
Signup and view all the flashcards
Dry Bulb Temperature & Drying
Dry Bulb Temperature & Drying
Signup and view all the flashcards
Air Velocity & Drying
Air Velocity & Drying
Signup and view all the flashcards
Through-flow System
Through-flow System
Signup and view all the flashcards
Cross-flow System
Cross-flow System
Signup and view all the flashcards
Perforated Trays & Drying
Perforated Trays & Drying
Signup and view all the flashcards
Thermal Conductivity & Drying
Thermal Conductivity & Drying
Signup and view all the flashcards
Product Thickness & Drying
Product Thickness & Drying
Signup and view all the flashcards
What is the equation for the heat flux?
What is the equation for the heat flux?
Signup and view all the flashcards
What is the relationship between mass flux and heat flux during constant drying rate?
What is the relationship between mass flux and heat flux during constant drying rate?
Signup and view all the flashcards
What marks the end of the CRP?
What marks the end of the CRP?
Signup and view all the flashcards
What happens in the falling rate period (FRP)?
What happens in the falling rate period (FRP)?
Signup and view all the flashcards
What equation describes the Fickian model?
What equation describes the Fickian model?
Signup and view all the flashcards
What are the assumptions for the Fickian model?
What are the assumptions for the Fickian model?
Signup and view all the flashcards
What is the diffusion coefficient (D)?
What is the diffusion coefficient (D)?
Signup and view all the flashcards
What is the 'moisture independent diffusivity' assumption?
What is the 'moisture independent diffusivity' assumption?
Signup and view all the flashcards
Study Notes
Food Process Engineering - Drying
- Course: ABEN 4510
- Topic: Drying (Theories and Principles)
- Instructor: May Alisbo-Cabral
Learning Outcomes
- Explain the theories and principles of drying
- Discuss the trends in drying of food materials
Topic Outline
- Theories and principles of drying
- Trends in drying food materials
Introduction - Sundrying
- Sundrying is the oldest method of dehydration, conducted by humans,
- However, it has problems such as susceptibility to contamination, spoilage, and requires a large operational area that is unreliable due to unpredictable weather.
Introduction - Food Drying
- Food drying is a method of food preservation where food is dehydrated or desiccated.
- It's one of the oldest methods of food preservation by mankind.
- Drying inhibits the growth of bacteria, yeasts, and mold by removing water.
- Traditional methods of removing water include air drying, sun drying, smoking, and wind drying.
Drying
- Moisture removal creates an unfavorable environment for yeast, molds, and bacteria.
- Drying foods often leads to changes in chemical, physical, and biological characteristics.
Theory of Drying
- Moisture migration from wet solids to surrounding media is supported by the vapor pressure theory.
- As the temperature of a product increases, while moisture remains constant, water vapor pressure increases, too.
- The rate of mass transfer is directly proportional to the vapor pressure difference between the product surface and absorbing medium.
States of Water
- Pure water can exist as a solid, liquid, or vapor.
- The state of water depends on temperature and pressure conditions.
1. Equilibrium Moisture Content
- A hygroscopic material, under constant air temperature and humidity, will reach a constant equilibrium moisture content (EMC).
- EMC depends on the temperature and water vapor pressure of the surrounding air.
Sorption Isotherm Models
- Henderson Equation: Aw = 1 - exp(-KT M')
- Aw: water activity
- K, n: empirical constants
- T: absolute temperature
- M: equilibrium moisture
- Halsey Equation: Aw = exp(-(K1/[T_p])M^K2)
- Aw: water activity
- K1, K2: empirical constants
- Tp: pore radius
- T: absolute temperature
- M: equilibrium moisture
- M1: monolayer moisture
2. Drying Stages
- Point A: Initial condition of the food material at the start of drying.
- Point B: Food adjusts to the drying conditions.
- B-C (Constant Rate Period [CRP]): A constant rate of drying, if the food possesses a lot of free moisture.
- C (Critical Moisture [CM]): Food reaches its critical moisture content.
- C-D (Falling Rate Period [FRP]): The drying rate decreases due to the decrease in the effective area of evaporation and increase in the "dry patch" area.
- Point E: The food reaches a constant weight at the equilibrium moisture content (EMC).
Drying Rate Against Drying Time
- Drying data is plotted to show drying rate against time.
- Different drying stages are linked to the physical structure of the material.
- Liquid phase associated with the solid components of the product.
Classification of Food Materials
- 1. Liquid solutions and gels (milk, fruit juices)
- 2. Capillary-porous rigid materials (grains)
- 3. Capillary-porous colloidal solids (fish, meat)
Constant Drying Rate Period
- Moisture at the surface of a material is in the state of free surface water.
- The rate of evaporation depends on conditions of the drying air.
- The drying rate of a given piece of material is controlled by the surrounding medium.
- The drying rate is equivalent to the rate of moisture loss from a water-saturated surface of the same size/shape.
Constant Drying Rate Period - Heat Flux Calculation
- Heat flux (q) toward the evaporating surface, for constant wet-bulb process.
- q = h_s * A * (Tinf - T_0)
- q: heat flux
- h_s: surface heat transfer coefficient
- A: surface area
- Tinf: medium temperature
- T_0: surface temperature
- The calculation supplies the necessary heat of vaporization.
Constant Drying Rate Period - Mass Flux Calculation
- For a constant wet-bulb drying process, the mass flux may be associated with the heat flux.
- dM/dt = hsA(Tinf – To)/L, where L = latent heat of vaporization
Falling Rate Period
- Moisture diffusion within the product decreases.
- The rate of moisture diffusion is slowed since the product needs moisture to replenish the surface.
- Drying rate also decreases, and process enters the FRP.
Falling Rate Period (Theory)
- The period is greatly affected by the movement of moisture within the material.
- Liquid movement in a solid follows the Fickian model: dM / dt = D * ΔC
- dM/dt: rate of moisture change
- D: diffusion coefficient
- ΔC: concentration gradient
Moisture Independent Diffusivity (Theory)
- The Fickian equation can be analytically solved to determine moisture diffusivity when certain assumptions are met.
- Isotropic material
- Diffusion coefficient independent of concentration; it is time-invariant
- Unaffected by shrinkage
Moisture Independent Diffusivity (Mathematical)
-
W-We/Wo-We = (8/Ï€^2)k(exp(-(Ï€^2)DxDyDz/(4(a^2+b^2+c^2))
-
W= product weight at time t
-
We= Product equilibrium weight
-
Wo= Initial product weight
-
Dx,Dy,Dz= diffusion coefficients in the x, y, z-direction
-
a,b,c= block dimensions
-
K= constant
Factors Affecting Drying Rates - Relative Humidity
- Rate of water vapor migration from the drying front to the drying medium is highly dependent on relative humidity (RH).
- Increase in drying rate when RH decreases.
- RH between 30 to 60 % is commonly used.
- Low RH, promotes rapid initial drying but leads to crust formation.
Factors Affecting Drying Rates - Dry-bulb Temperature
- Higher dry-bulb temperature increases the vapor pressure at the drying front.
- This facilitates quicker drying compared to the influence of relative humidity.
- Temperatures between 30°C and 70°C are common for drying food products.
Factors Affecting Drying Rates - Air Velocity
- Air velocity between 1 to 4 m/s is recommended.
- Increased air velocity during the constant rate period enhances the drying process.
- Drying during the falling rate period is limited by moisture diffusion so increased air velocity is less contributing.
Factors Affecting Drying Rates - Type of Product Support
- Using perforated trays enhances drying within through-flow systems.
- Using perforated bottoms facilitates drying on both sides of the product.
- High thermal conductivity trays promote fast heat transfer for increased drying rate.
- Skewer support in the drying system provides greater surface area for the hot air to contact, leading to faster drying.
Factors Affecting Drying Rates - Pre-drying Treatments
- Pre-drying practices impact drying due to physico-chemical changes.
- Cutting food into smaller pieces and shapes increases surface area, speeding up the drying process.
- Practices like salting, osmotic dehydration, and blanching affect the rate of moisture removal.
Factors Affecting Drying Rates - Nature of Product
- Product composition significantly affects how moisture is held or bound in food.
- Solutes with high affinity for moisture (e.g., sugars, salts, proteins) increase water holding capacity and reduce the rate of drying.
- Soluble solids are carried to the surface by migrating moisture, potentially forming a crust that slows further drying.
References
- Carpio, 2000. E.V. Engineering for Food Technologist. UPLB Publishing Center.
- Singh R. P and Heldman, D.R.. 2009. Introduction to Food Engineering Fourth Edition. Elsevier Inc.
- Food Process Engineering and Technology. (2018). doi: 10.1016/B978-0-12-812018-7.00003-8
Studying That Suits You
Use AI to generate personalized quizzes and flashcards to suit your learning preferences.