DRYING AND DEHYDRATION

• Drying


  Removal of water from food and agricultural products to produce a solid product
• Dehydration


  Removal of water from food and agricultural products to produce a solid product
• Drying and dehydration are often used interchangeably, especially when referring to food products
• Only the word drying is commonly used when referring to processing of non-food products
• Applications of drying/dehydration


  • On-farm drying of grain, fruits, and vegetables
  • Large scale commercial drying of fruits, vegetables, snack food products, milk products, coffee, and other products
• Evaporation


  Removal of some water from a liquid product to produce a more concentrated liquid
• Applications of evaporation

  • Concentration of milk, fruit juices, and syrup products
• Most evaporation systems are large-scale commercial operations, although small-scale farm operations still exist for production of maple, sorghum, and sugar cane syrups
• Wet basis (m)


  Method of expressing moisture content where the total mass of the product, water plus dry matter, is the reference
• Dry basis (M)


  Method of expressing moisture content where the mass of dry matter in the product is the reference
• Wet basis moisture content is always lower than dry basis moisture content
• For high moisture materials like fruits and vegetables, moisture is often given as "mass of water per unit mass of dry product"
• Measuring moisture content


  1. Drying sample in oven

  2. Using microwave drying ovens

  3. Chemical analysis
• Equilibrium moisture content (EMC or Me)


  Moisture content that a material reaches when held at a fixed temperature and relative humidity, where there is no net exchange of moisture between the material and the air
• Equations for equilibrium moisture content


  • Halsey equation
  • Henderson equation
  • Modified Henderson equation
  • Modified Halsey equation
• No single equation is suitable for all products, but most can be represented by one of several available equations
• Equilibrium relative humidity (ERH) is the relative humidity for equilibrium between air and a specific product at a given temperature
• Water activity (aw)


  Indicator of water availability in a product to support degradation activities such as microbial action
• Water activity is the equilibrium relative humidity (ERH) in decimal form for a product at a given temperature and moisture content
• Perishability


  Not directly related to moisture content, varies greatly among products with the same moisture content. A better indicator is the availability of water in the product to support degradation activities such as microbial action
• Water activity (aw)


  The equilibrium relative humidity (ERH) in decimal form for a product at a given temperature and moisture content
• Actual moisture content increases rapidly with aw at the higher values of water activity
• Drying


  1. Moisture must move from the interior of a material to the surface

  2. Surface water must be evaporated into the air
• Factors affecting drying rate


  • Nature of the material
  • Size, shape, and arrangement of the pieces
  • Wet-bulb depression or relative humidity or partial pressure of water vapor in the air
  • Air temperature
  • Air velocity
• Case hardening can occur if the initial stage of drying occurs at low relative humidity and high temperature, resulting in a hardened layer on the surface that slows subsequent drying
• Spray drying


  • Droplets should be small and uniform in size
  • Special procedures for atomization and powder collection

  
• Vacuum drying


  • Permits drying at lower temperatures, important for products that may suffer flavor changes at higher temperatures
• Freeze drying


  • Removes moisture from a frozen product without thawing, requires low temperature and pressure
• Heated air drying apple slices in a counter flow dryer

  • Water removed = 178 kg/h
  • Entering air flow rate = 7.7 m3/min
• Calculating output rate of dried product

  1. Knowing moisture content and dry matter

  2. Calculate water removed

  3. Calculate water removed per unit mass of product
• Water removed from apple slices is 178 kg H2O/h
• Determining air properties using psychrometric chart

  1. Find wet-bulb temperature from exit air conditions

  2. Follow constant wet-bulb line to inlet air temperature

  3. Determine air properties at inlet and exit

  
• Inlet air properties: tdb = 50°C, twb = 23.7°C, φ = -, v = 0.925 m3/kg DA, W = 7.7 g H2O/kg DA
• Exit air properties: tdb = 25°C, twb = 23.7°C, φ = 90%, W = 18 g H2O/kg DA
• Calculating output with different moisture content


  1. Same input parameters

  2. Solve for water in "dried" slices

  3. Calculate moisture removed
• Moisture removed = 177.78 kg H2O/h
• Evaporation


  Removal of water to produce a more concentrated liquid
• Open (atmospheric) evaporators


  • Higher operating temperatures
  • Higher energy cost compared to vacuum evaporators
• Vacuum evaporators


  • Lower boiling point
  • Multiple effect systems use vapor from one evaporator to provide energy for the next
• Analyzing open pan evaporator for sorghum syrup


  1. Calculate production rate of syrup

  2. Calculate mass of water removed per unit mass of syrup

  3. Calculate energy required for evaporation

  4. Calculate steam flow required