Hygroscopic Materials Quiz

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