Equilibrium Moisture Content

• Equilibrium Moisture Content (EMC)


  The amount of moisture a material contains when it has reached equilibrium with its environment
• Moisture Equilibration


  The process of hygroscopic materials to reach equilibrium with the ambient relative humidity of the air by absorbing or desorbing moisture
• Equilibrium Moisture Content (EMC)

  • Expressed as a percentage, the EMC describes what percentage of the material's mass is made up by water
  • EMC is determined by the inherent properties of the material and the temperature and relative humidity of the environment
• Only hygroscopic materials - organic materials that naturally contain water - are susceptible to moisture equilibration
• Moisture Exchange


  1. Hygroscopic materials are constantly exchanging moisture with the air in the form of water vapor

  2. The moisture is transferred in the form of water vapor by the process of diffusion
• Moisture Content (MC) not in equilibrium with Relative Humidity (RH)


  The material adjusts its MC to reach equilibrium

  
• Relative Humidity (RH) increases


  Moisture Content (EMC) of the materials also increases at constant temperature
• Air temperature increases while maintaining RH level


  EMC value decreases
• Factors affecting rate of moisture equilibration

  • Inherent properties (cellular structure), hygroscopic nature of the material, dimensional characteristics and temperature and VP gradient
• If air remains in contact with a stored product for a sufficiently long time, partial pressure of water vapor in the air reaches equilibrium with partial pressure of water vapor in the material
• Determining EMC


  1. Sample collection

  2. Exposure of the samples to different relative humidities (RH) at a given temperature T (until equilibrium is reached, respectively)

  3. Determination of moisture content
• Static Method


  Atmosphere surrounding the product is allowed to equilibrate without mechanical agitation of air or product
• Dynamic Method


  Air surrounding the product as the product itself is moved mechanically
• RH control using Saturated Salt Solutions


  1. NaCl

  2. Calcium Chloride
• RH control using acid solutions


  Sulfuric acid (H2SO4), nitric acid (HNO3), hydrochloric acid (HCL)
• Dynamic methods


  1. Air is bubbled through absorption towers containing acid /salt solution to achieve constant RH before passing through the product

  2. Air is mechanically moved around the product in an enclosed chamber and allowed to come to equilibrium; RH is then measured
• Equilibrium Moisture Content (EMC)


  The moisture content at which a food attains equilibrium with its surroundings when there is no exchange of water between the food and its surroundings
• EMC is a function of temperature, relative humidity, and the product
• EMC is reached when the partial pressure of water vapor in the air reaches equilibrium with the partial pressure of water vapor in the material
• Importance of EMC


  • Allows researchers to predict whether a product will lose or gain moisture at a certain temperature and relative humidity
  • High moisture content tends to favor microbial activity and product metabolism
• Implications of EMC for drying

  • Grain will eventually reach the moisture levels shown in the tables when exposed to the corresponding temperature and humidity levels for long periods of time
  • Drying time will depend on the airflow rate through grain, which in turn depends on the depth of grain in a bin
• Implications of EMC for storage

  • The air space between kernels in a bin will have the humidity indicated at the corresponding moisture and temperature
  • Mold growth is suppressed during storage when the environment is maintained at a relative humidity level of 65% or lower
• Methods for determining EMC

  1. Static method

  2. Dynamic method
• Static method


  The grain is allowed to come to equilibrium with the surrounding still air without any agitation
• Dynamic method


  The air surrounding the product as the product itself is moved mechanically
• RH control using saturated salt solutions


  • The water vapor concentration, and therefore the relative humidity over a salt solution is less than that over pure water
  • Salt solutions are more stable, less corrosive, less expensive compared to acid solutions
• Salts used for RH control

  • NaCl
  • Barium Chloride
  • Calcium Chloride
  • Lithium Chloride
• RH control using acid solutions

  • Uses acid solutions of varying concentration to obtain different RH levels
  • Examples: sulfuric acid (H2SO4), nitric acid (HNO3), hydrochloric acid (HCL)
• Research on the different equilibrium moisture of grains used for drying and storage