Milling Appraisal - 87
 

 

 

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Project Leader and Principal UC Investigators

Jim Thompson, Agricultural engineer, UC Cooperative Extension

 

Preliminary results indicate that moisture content of appraisal samples at the time of milling and rice cultivar both have significant effect on head and total rice fractions of the sample.

Samples evaluated after one week and one month of storage showed that head rice and total rice fractions increase as moisture content of the sample decreases. The effect is greatest for long-grain rice, less for medium-grain rice, and very small for short-grain rice. A small-scale test to see if these moisture-related differences appeared with rice that had an overall low head yield indicates that the effects may be smaller.

Storage time appears to have no significant effect on milling appraisal.

Tests using either an open-ended vacuum probe or multi-chambered probe for sampling loads of rice suggest that variability caused by truck sampling technique is not large. U.S. Department of Agriculture and California Department of Food and Agriculture results have a relatively small variability. Standard sample drying procedures slightly reduce head rice results but do not affect total results.

Lab and truck sampling variability is random. If enough samples are taken from a lot of rice, the average of the samples will be close to the quality of the lot. However, the effects of sample moisture and sample drying method are not random and can cause the appraisal results to differ considerably from the actual quality of a lot of rice. Probably the best way to deal with these potentials for error is to insure that appraisal samples are properly dried and that their moisture content is equal to the moisture content of the lot they represent.

 
MOISTURE AFFECTS HEAD RICE

Since growers are paid on the basis of the quality of their rice, results of state milling appraisals are of considerable interest. When unexplained variability started turning up in those appraisals several years ago, the warehouses that do most of the sample drying contacted UC Davis Agricultural Engineer James F. Thompson to investigate.

Jim Thompson, UC Davis Cooperative Extension agricultural engineer, examines factors affecting milling appraisals.

Thompson was commissioned by CCRRF to determine the effect of the following factors on head and total rice milling appraisals: moisture content of appraisal sample, rice cultivar, length of time sample is stored, and appraisal test error. What Thompson's research team discovered may very well change the way the industry stores rice.

Sampling error at the USDA-CDFA laboratory in Sacramento was found to be well within the bounds of statistical reason. And contrary to popular belief, the length of time a sample is stored or "tempered" was not a factor. "Everyone had always assumed that the longer the rice was held the better," Thompson said. "But we found storage time does not affect appraisal quality."

Thompson's team discovered that moisture content at the time of sampling and rice variety were the key factors affecting the production of both total and head rice, particularly the latter. Thompson anticipates the industry will consider prescribing a tight range of milling moisture contents, especially for long grain, so the growers can optimize their milling yields.

 

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