Rice moisture content should be reduced quickly after harvest

Rice is one of the world’s major grain foods and like many grains it is highly susceptible to spoilage when improperly stored.  A hygroscopic grain, it both absorbs and gives off moisture from ambient sources.  That’s why rice moisture content is important to quality control and why a moisture analyzer is an important instrument to check for moisture content of rice.

Harvesting, Drying and Packaging Rice

At harvesting rice has a moisture content of 20-25%.  Best practices call for reducing moisture content to 14% or less for rice grain and to 12% or less for rice seed.  Ideally drying will be completed within 24 hours of harvest.  If moisture content is too high risks include heat, mold development, insect infestation and discoloring. 

Several methods can be employed for drying ranging from open air in the sun to large commercial flat bed dryers. Rice processors will establish their own procedures for reducing moisture content to the recommended levels.  A moisture analyzer is an excellent quality control tool for checking final moisture content just prior to packaging.  The packaging should be such that the rice is protected to the extent possible from ambient air and humidity.

How to Use a Moisture Balance to Check Rice Quality

Moisture analyzers such as the Kern MLB 50-3N available from Tovatech consists of a halogen heater mounted on a precision analytical balance.  These benchtop instruments can be programmed to conduct moisture analysis based on government or industry standards and can yield in a relatively short time a highly accurate measure of moisture content in the material being tested, using very small samples.  Once the rice packager establishes a standard operating procedure for a moisture analysis it can be programmed into the moisture analyzer and quickly called up for periodic quality control measurements.

The results of the test can be used to either accept or modify the bulk drying procedures.

Rice harvesters and packagers should follow their industry or government standards when setting up a moisture analysis procedure.  This will enable quality control personnel to program the moisture analyzer so that analysis is accomplished quickly and efficiently.  Following is an example of conducting a moisture analysis on rice.* 

• Program the moisture balance.  As an illustration, you can set the drying program to standard, the drying temperature to 165⁰C and the switch-off criteria to less than 1mg in 120 seconds.  (This means that the halogen heater will shut off when the rate of weight loss on drying reaches that point.)
• Place a sample pan on the balance sample pan holder and tare the moisture balance.
• Carefully and evenly spread approximately 3g of prepared sample (see below) on the sample pan and close the moisture analyzer lid.  Testing will start immediately and cease when the shut off criterion has been satisfied.
• Testing time, temperature and final moisture content in percent will be displayed on the moisture analyzer LCD panel.
• Transfer the test data to a printer or PC for record keeping.

Sample Prep.  This should be done just prior to testing.

Prepare your sample by grinding a small amount of rice in a water-cooled mill.  Care should be taken that the sample does not heat, which is why a water-cooled mill is recommended.  

Moisture Analyzer Maintenance

Moisture balances are precision instruments.  Their positioning in your facility and their maintenance are very important to accuracy and they should be periodically recalibrated.  Follow the manufacturer’s instruction manual.

Contact the scientists at Tovatech for detailed advice on selecting, operating and maintaining a moisture analyzer.

*If you’d like to know more about using a moisture balance check out our post on how to set up a moisture analyzer .

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