Hygroscopic Materials Quiz
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Questions and Answers

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?

  • 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?

  • 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?

    <p>Vapor Pressure Theory</p> Signup and view all the answers

    Which statement describes a benefit of drying food?

    <p>It inhibits the growth of bacteria, yeasts, and molds.</p> Signup and view all the answers

    How does an increase in dry-bulb temperature affect the relative humidity of air?

    <p>It decreases relative humidity.</p> Signup and view all the answers

    What is a key factor that affects the state of water?

    <p>Temperature and pressure conditions</p> Signup and view all the answers

    What is the recommended air velocity range for drying food products?

    <p>1 to 4 m/s</p> Signup and view all the answers

    What is the desired outcome of drying food?

    <p>Achieve the dehydrated state quickly to minimize spoilage.</p> Signup and view all the answers

    What does the removal of water during drying primarily prevent?

    <p>Microbial growth</p> Signup and view all the answers

    Which drying system typically encourages faster drying rates?

    <p>Through-flow system</p> Signup and view all the answers

    How does temperature affect the drying process according to the vapor pressure theory?

    <p>Higher temperature increases vapor pressure, aiding drying.</p> Signup and view all the answers

    What factor is diminished during the falling rate period of drying?

    <p>Air velocity contribution</p> Signup and view all the answers

    What type of product support is commonly used in drying chambers for faster drying?

    <p>Perforated trays</p> Signup and view all the answers

    What is NOT a method of removing water during the drying process?

    <p>Spraying</p> Signup and view all the answers

    How does the thickness of the product layer on drying trays influence drying rates?

    <p>Thinner layers allow for faster moisture removal.</p> Signup and view all the answers

    What is one disadvantage of using a cross-flow drying system?

    <p>Slower drying rates compared to through-flow systems.</p> Signup and view all the answers

    What advantage does suspending food materials on skewers provide in the drying process?

    <p>Allows greater surface contact with hot air.</p> Signup and view all the answers

    In the sorption isotherm concept, which of the following substances tends to lower the water activity of solutions?

    <p>Salts</p> Signup and view all the answers

    During which stage of drying does the food material reach its critical moisture level?

    <p>Falling Rate Period</p> Signup and view all the answers

    What is the gas constant 'R' valued at for calculations within the given context?

    <p>8.3144 J/(mole·K)</p> Signup and view all the answers

    What happens to food material at point E during the drying process?

    <p>It attains constant weight.</p> Signup and view all the answers

    What does the term 'equilibrium moisture content' (EMC) signify in the drying process?

    <p>The stable moisture level at which no further drying occurs</p> Signup and view all the answers

    Which variable in the equation is influenced by the adsorption conditions and relative humidity?

    <p>Aw</p> Signup and view all the answers

    What does the symbol hs represent in the heat transfer equation?

    <p>Surface heat transfer coefficient</p> Signup and view all the answers

    Which factor marks the termination of the constant drying rate period (CRP)?

    <p>Critical moisture content</p> Signup and view all the answers

    What describes the relationship between moisture diffusion and the falling rate period (FRP)?

    <p>Moisture movement is the determining factor in the FRP</p> Signup and view all the answers

    In the equation for water vapor mass flux, what does the variable L represent?

    <p>Latent heat of vaporization</p> Signup and view all the answers

    Which assumption is NOT correct regarding the effective diffusion coefficient (De) in the context provided?

    <p>De changes with time</p> Signup and view all the answers

    What is indicated by the 'dry patch' area at the end of the constant drying rate period?

    <p>Decrease in effective surface area of evaporation</p> Signup and view all the answers

    Which condition is assumed when using the Fickian model for moisture movement?

    <p>Food material is isotropic</p> Signup and view all the answers

    What does the variable D represent in the context of moisture diffusion?

    <p>Diffusion coefficient</p> Signup and view all the answers

    What does the variable 'k' represent in the equation for moisture independent diffusivity?

    <p>A constant varying from 1 to 3</p> Signup and view all the answers

    Which theory is NOT mentioned regarding moisture movement in foods?

    <p>Thermal conductivity theory</p> Signup and view all the answers

    How does relative humidity affect the rate of drying?

    <p>Drying rate increases with decreasing humidity</p> Signup and view all the answers

    What factors can lead to case hardening during the drying process?

    <p>Extremely low relative humidity</p> Signup and view all the answers

    In the Sherwood equation, what variable does 'l' represent?

    <p>Slab half thickness</p> Signup and view all the answers

    What is the primary role of relative humidity in the drying process?

    <p>Influences water vapor migration to the drying medium</p> Signup and view all the answers

    Which of the following options is associated with the linear portion of the X versus t plot to calculate D?

    <p>Logarithmic plot of weight change</p> Signup and view all the answers

    What does the variable 'We' represent in the diffusivity equation?

    <p>Equilibrium weight of the product</p> Signup and view all the answers

    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/π^2)k(exp(-(π^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

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    Food Process Engineering Drying (Theories and Principles) PDF

    Description

    Test your knowledge on the concepts related to hygroscopic materials, including EMC, water activity, and sorption isotherm models. This quiz explores key equations and their variables essential for understanding moisture behavior in materials.

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