Rice Quality-99


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Project Leader and Principal UC Investigators

Randall Mutters, farm advisor, UC Cooperative Extension, Butte County

James Thompson, specialist, UC Cooperative Extension, Dept. of biological and Agricultural Engineering, UC Davis

Jim Ecker, postgraduate researcher UC Cooperative Extension, UC Davis

As market demand for premium quality rice increases, growers and handlers are becoming increasingly aware of the factors that enhance and determine rice quality. Chief among them are harvest moisture, drying techniques and storage conditions.  Researchers examined these three areas for the Japanese premium quality variety Akitakomachi in experiments last year.

Optimal Harvest Moisture

Scientists harvested Akitakomachi at three nominal moistures (26, 23 and 20 percent ) from a cooperating grower’s field.  Across a range of drying treatments, harvest at 26 percent moisture produced about a six percent reduction in germination compared to the other two harvest moistures.  Germination is a characteristic used by the Japanese to evaluate table rice.   panicles.jpg (14435 bytes)The unfavorable germination reduction was likely caused by a portion of the rice being immature at higher moisture content.   Although data collected in this study is insufficient to make a clear recommendation of optimum harvest moisture, preliminary findings suggest anything above 25 percent harvest moisture is too high for Akitakomachi.

Drying Techniques

In a laboratory dryer, researchers simulated two different drying regimens.  In the first, rice was dried to 13 percent moisture using constant air temperatures of 70, 85, 100, 115 and 130 degrees Fahrenheit.  In the second, rice was dried at a constant 115 degrees for 25 or 35 minute exposures or with 130 degree air for 17 or 25 minute exposures with four-hour tempering times between passes.  This procedure was repeated until 18 percent moisture was reached and then finished to 13 percent moisture with unheated air.  Each of the tests was done with rice harvested at 25.4 percent, 21.5 percent and 19.4 percent.

The 130 degree constant air temperature treatment reduced germination by about 2 to 4 percent over controls.  None of the other drying treatments caused any noticeable germination loss.  Further evaluation of head rice quality will likely show that the other more extreme drying conditions cause damage.

Storage Odors

Japanese literature indicates that rice quality can suffer if field moisture rice is held too long before drying.  Researchers constructed a constant-temperature incubator to test the effect of holding 23 or 25 percent moisture rice at 90 or 120 degrees Fahrenheit.  The incubator was filled with high moisture rice and monitored for off odors as the temperature rose.

Germination was not affected when rice was held at 90 degrees for up to 24 hours.   At 120 degrees, the germination dropped rapidly after four hours.   Off odors are apparent after rice is held for eight to 12 hours.

Akitakomachi rice at 26 percent moisture heated quite rapidly from 77 to 97 degrees Fahrenheit in 17 hours and continued to rise for two days.  A similar test with 23 percent moisture M-401 rice showed only about 10 degrees heating after two days.

Further Analysis

Researchers will have additional information on head rice quality, Japanese taste scores and off odors after this report is published.

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