Agricultural Products: Moisture Content

• Determining moisture content is important in processing and storing Agricultural products

Moisture and Moisture Content Topic Breakdown:

• Focus on moisture in dried products.
• Focus on methods for determining moisture content.
• Focus on equilibrium moisture content (EMC), water activity, sorption isotherms, and glass transition.
• Importance is placed on understanding the effect of moisture in products
• Become familiar with moisture in dried products
• Learn to identify methods for determining moisture content

Moisture Content

• Indicates the amount of water in a material.
• Used as a conventional indicator to determine if a material is dry enough for safe storage or milling.
• Moisture content is usually expressed as a percentage.

Importance of Moisture

• Affects the overall quality of grains.
• In grains, moisture serves as an index for various processes. These processes include transport, drying, trading, harvest, storage, and milling
• Affects the commercial value of grains
• To gauge optimum milling recovery.

Moisture in Grains

• Grains are hygroscopic, and their moisture content varies with ambient temperature and relative humidity.
• Grains absorb water when exposed to air with low temperature while also having high relative humidity
• This absorption causes fissures or cracks in the grains, known as rewetting.
• Grains release water when exposed to air with high temperature but low relative humidity, which is known as drying.

Safe Paddy Moisture Content

• Harvesting: a moisture content of 20-25% is generally acceptable for manual harvesting
  • Shattering may occur, or there may be unfilled or green grains if the grain is to wet
• Threshing: a moisture content of 20-25% is suitable for mechanical threshing and <20% for hand threshing.
  • Incomplete threshing and damage to grain may occur when moisture content is too high during threshing
• Drying: final moisture should be 14% or lower
  • Spoilage and fungal damage/discoloration might occur in drying
• Storing: a moisture content of <14% is safe for grain storage,<12% is recommended for seed storage, and <9% for long-term preservation.
  • High moisture content in storage can lead to fungal, insect, or rat damage, as well as loss of vigor.
• Milling: a moisture content of 13-14% is necessary
  • Grain cracking and breakage can occur from overmilling

Equilibrium Moisture Content (EMC)

• EMC is the moisture level grains reach when air conditions remain constant over time.
• EMC represents the moisture content a grain will equilibrate

Moisture Content Determination

• Experienced buyers commonly assess paddy moisture by checking kernel hardness between their teeth.
• More precise methods are needed for procurement, storage, and milling.

Moisture Measurement:

• Direct measurement/Primary
  • Involves removing moisture and measuring weight loss
• Indirect measurement
  • Measures an intermediate variable related to moisture content indirectly

Equipment for Direct Measurement

• Oven
• Analytical balance
• Desiccator

Direct Methods for Measuring Moisture:

• Single/One Stage Methods
• Double Stage Methods
• Brown-Duvel Distillation Method

Single/One Stage Methods

• Grind 2-3g of sample
• Keep the sample for an hour in an oven at 130°C ± 2°C.
• Place the sample in a desiccator, then weigh after cooling.

Two Stage Methods

• Keep 25-30g of whole grain sample in the air oven at 130°C ± 2°C for 14-16 hours to reduce content down to approximately 13%.
• Follow single stage methods afterwards

Brown-Duvel Distillation Method

• A distillation method used to directly measure moisture volume in cc, this involves condensing moisture in a measuring cylinder by heating a mixture of 100g grain and 150 cc oil in flask at 200°C for 30-40 minutes
• Vaporized moisture is then condensed and measured in a graduated cylinder.
• The J. W. T. Duvel and Edgar Brown invented the Direct Method
• The patent number is 848616

Indirect Methods

• Rely on characteristics, like electrical properties/calibration
• Calibration is against the primary method of moisture determination.
• Electrical Resistance Method
• Dielectric Method

Ideal Moisture Meter Charactersitics:

• Accuracy of 0.5-1.0%
• Should be Convenient for use on bag or bulk grain
• Should be portable and robust, unaffected by dust and humidity
• Should be Cheap and reliable
• Simple to operate
• Rapid read out
• Unaffected by temperature

Electrical Resistance Meter Types

• Meters use electrical resistance to determine moisture.
• Electrical resistance varies with the distribution of moisture within the sample, density, compactness, and the device's aging.
• Calibration is against oven method determination
• Compression Cell type
  • Works with ground samples
  • Uses resistance measurements calibrated to moisture content percentages
  • Requires correction for temperature
• Probe type
  • Works with bag or bulk stocks
  • Resistance measurements use a probe to rapidly sample bags

Dielectric Method

• Material is placed between plate of an electrical condenser to measure capacitance, varies with moisture content of the material.

Characteristics of Probe Type

• Pressure and temperature needed
• Capacity to detect wet spots in the stock
• Surface wet sample may yield inaccurate results
• Less accurate than compression type
• Average readings are more representative than fewer, more accurate determinations

Examples of Electrical Resistance Type

• The universal moisture meter (the United States)
• Tag-Happenstall moisture meter (the United States)
• Kett moisture meter (Japan)
• These only need 30 seconds to measure

Examples of CAPACITANCE

• The Motomco moisture meter (United States)
• Burrows moisture recorder (the United States)
• Only takes a minute to measure
• Safe crop moisture testers do not damage the grain sample.

Other Secondary Methods

• Microwave absorption
• Sonic
• Ultrasonic methods
• Spectroscopic methods

Moisture Content Representations

• Are either wet basis or dry basis

Wet Basis Moisture Content (%MCwb)

• Measures moisture as a percentage of the total weight.
• Formula: %MCwb = (Wm / WT) * 100, where WT = total weight of the material or %MCwb = [Wm / (Wm + Wd)] * 100

Dry Basis Moisture Content (%MCdb)

• Measures moisture as a percentage of the dry matter weight.
• Formula: %MCdb = (Wm / Wd) * 100

Basis Conversion

• Dry to Wet: MCwb = [MCdb / (MCdb + 1)] x 100%
• Wet to Dry: MCdb = [MCwb / (1 - MCwb)] x 100%

Wet Basis to Dry Basis conversions

• Use this formula: W₁ (1 – MC₁) = W₂ (1 – MC₂)
• The formula can alternatively be: W₁ (100 – %MC₁) = W₂ (100 – %MC₂)

Formula Variables

• W₁ = initial weight of wet material
• W₂ = final weight of dried product (kg)
• MC₁ and MC₂ = initial and final moisture contents (wet basis, %)

Equilibrium Moisture Content

• EMC describes moisture content so grain eventually equilibrate to, given constant air conditions over time.
• Stable level, known as EMC is where pv (product) = pv (air)
• Equilibrium is defined as the point where moisture gained and lost are equal.
• EMC is MC (dry basis).
• Equilibrium RH (ERH) is RH at this stage.
• EMC allows researchers to predict gains/losses at specified temperature + humidity levels.
• High moisture can lead to high microbial activity and product metabolism
• Product and surrounding air temperature(T) and relative humidity (RH) are based on Equilibrium Moisture Content
• A relationship between EMC and ERH can be expressed through moisture sorption isotherms
• Many mathematical models have been developed