PROCESSING

• Drying


  Removal of a liquid (moisture content) from a material by the application of heat and accomplished by the transfer of a liquid from a surface into an unsaturated vapor phase
• Drying processes


  • Air and contact drying under atmospheric pressure
  • Vacuum drying
  • Freeze drying
• Air and contact drying

  • Heat is transferred through the foodstuff either from heated air or from heated surfaces
  • The water vapor is removed with the air
• Vacuum drying


  • Evaporation of water occurs more readily at lower pressures than at higher ones
  • Heat transfer is generally by conduction, sometimes by radiation
• Freeze drying


  • The water vapor is sublimed off frozen food
  • The food structure is better maintained under these conditions
  • Suitable temperatures and pressures must be established in the dryer to ensure that sublimation occurs

  
• Drying


  A complex operation involving transient transfer of heat and mass transfer
• Sensible and latent heat


  Must be transferred to the food to cause the water to evaporate
• Physical changes


  May occur including shrinkage, puffing, crystallization and glass transitions
• Chemical or biochemical reactions


  May occur, leading to changes in color, texture, odor, or other properties of the solid product
• Drying curve


  Plots the drying rate versus drying time or moisture contents

  
• Stages of drying


  • Transient early stage, during which the product is heating up (transient period)
  • Constant rate period, in which moisture is comparatively easy to remove
  • Falling rate period, in which moisture is bound or held within the solid matrix

  
• Constant rate period


  • The surface of the product is very wet and the water activity is equal to one
  • The water is being evaporated effectively as a free water surface
  • The rate of removal of water can then be related to the rate of heat transfer

  
• Falling rate period


  • The rate of drying is governed by the internal flow of liquid or vapor
  • First falling rate (C to D) when wetted spots in the surface continually diminish until the surface is dried
  • Second falling rate period (D to E) when the surface is completely dry and until the EMC is reached

  
• Mechanisms of moisture transport within the solid(CLVKSTHC)

  • Capillary flow
  • Liquid diffusion, if the wet solid is at a temperature below the boiling point of the liquid
  • Vapor diffusion, if the liquid vaporizes within material
  • Knudsen diffusion, if drying takes place at very low temperatures and pressures, example is in freeze drying
  • Surface diffusion (possible although not proven)
  • Thermal diffusion
  • Hydrostatic pressure differences, when internal vaporization rates exceed the rate of vapor transport through the solid to the surroundings
  • Combinations of the above mechanisms

  
• Moisture content of wet solids

  Expressed as kilograms of moisture associated with 1 kg of the moisture – free solid
• Total moisture content


  The total amount of liquid associated with a wet solid, including the free moisture content and the equilibrium moisture content
• Unbound water


  Water that exists as a liquid and exerts its full vapor pressure, it can be removed readily by evaporation
• Equilibrium moisture content


  The moisture content present in a solid under steady-state ambient conditions, its value changes with temperature, humidity and the nature of the solid

  
• Bound water


  Moisture that is adsorbed on surfaces of the solid or within its structure, preventing it from developing its full vapor pressure and from being easily removed by evaporation
• Theories and principles of drying

  • Equilibrium relationships
  • Rate relationships

  
• Equilibrium relationships


  Air of constant temperature and humidity is passed over the wet solid until an equilibrium is reached where the vapour pressure of the wet solids is equal to that of the surrounding atmosphere

  
• Rate relationships


  Heat is transferred to evaporate liquid and mass is transferred as a liquid or vapor within the solid and as a vapor from the surface
• Drying curve


  Represents the relationship between drying rate and drying time or moisture content, with a constant rate period and a falling rate period

  
• Constant-rate period


  Represents removal of unbound water from the product, where the surface is very wet and the water activity is equal to one
• Falling-rate period


  The rate of drying is governed by the internal flow of liquid or vapor, with a first falling rate when wetted spots diminish and a second falling rate when the surface is completely dry

  
• Unique features of drying

  • Product size may range from microns to tens of centimeters
  • Product porosity may range from zero to 99.9 percent
  • Drying times range from 0.25 sec to five months
  • Production capacities may range from 0.10 kg/h to 100 t/h
  • Product speeds range from zero to 2000 m/s
  • Drying temperatures range from below the triple point to above the critical point of the liquid
  • Operating pressure may range from fraction of a millibar to 25 atmospheres
  • Heat may be transferred continuously or intermittently by convection, conduction, radiation or electromagnetic field

  
• Stages of the drying process

  • Preheating period, with slowly increasing drying rate and very slight change in moisture content
  • Constant-rate period, with constant drying rate and removal of surface water
  • Falling-rate period, with declining drying rate and transfer of internal moisture to the surface
• Temperatures for drying paddy

  • Seeds: maximum of 43°C
  • for milling: During constant-rate period: >100°C
  • During falling-rate period: <55°C (depends on drying system)

  
• Drying rate


  Stated in percent moisture removed per hour, affected by temperature, relative humidity and air velocity

  
• Uniform drying


  Achieved by mixing grain, using low temperatures, and re-circulating batch dryers to reduce moisture gradients

  
• Tempering


  Temporary stopping of drying to allow moisture to equalize inside and between grains, then re-starting drying at a higher rate

  
• Mechanical drying methods


  • Heated-air drying
  • Low-temperature drying
• Heated-air drying


  • Drying air temp.: 43°C
  • Air velocity: 0.15-0.25 m/s
  • Airflow rate per t grain: >0.7m3/s
  • Power requirement: 1.5-2.5 kW/t grain
  • Layer depth: <40cm
  • Drying time: 6-12 hrs
  • Initial MC: up to 30%+

  
• Low-temperature drying


  • Drying air temp.: ΔT=0-6°K
  • Air velocity: 0.1 m/s
  • Airflow rate per t grain: >0.05-0.4 m3/s
  • Power requirement: 0.05-0.15 kW/t grain
  • Layer depth: <2m
  • Drying time: days to weeks
  • Initial MC: 18% (28%)

  
• Sun drying


  Advantages: free energy, low capital investment

  Disadvantages: weather risk, difficult temperature control, high qualitative and quantitative losses, high labor requirement
• Drying equipment


  • IRRI Flat-bed dryer
  • Vietnamese Low-cost dryer
  • Vietnamese 4t Flat Bed Dryer
  • Reversible Airflow Flat Bed Dryer
  • Low-cost seed dryer
  • Re-circulating batch dryer
  • Agridry seed dryer

  
• Components of a flat bed dryer

  • Drying bin/Bed – holds the grain
  • Furnace/Burner – provide drying air to remove water
  • Fan/Blower – create pressure and air flow
  • Conveyors – for loading and unloading
  • Moisture meter – monitor drying process
  • Dust separator – clean exhaust air
• Re-circulating batch dryers


  • Cross flow - grain moves downwards, air across
  • Mixed flow - air flows from inlet to outlet ducts, better mixing