Maintaining Rice Quality After Harvest, 2001

Maintaining rice quality after harvest has become increasingly important to grower returns. In its third year, this project sought to:

  • Further analyze the effect of holding time and rice moisture content on off odors
  • Determine the amount of moisture that can be removed in drying without head rice loss
  • Study the effect of harvest moisture on head rice and grower returns
  • Examine the effectiveness of moisture meter calibration

Moisture Levels and Off Odors

Laboratory incubation tests were repeated to verify previous observations on variety, holding time and grain moisture on quality loss of temporarily stored, field moisture rice. A number of 500-pound batches of freshly harvested Akitakomachi and M-202 were tested at moistures ranging from 20 to 28 percent.

Off odors in high moisture grain are caused by a range of microbes, including bacteria, yeasts and fungi. A hand-held sensor was used to detect ethanol, an indicator of volatile organic compounds that cause off odors. Dried grain samples were also analyzed by gas chromatography and larger samples were taken for a taste panel evaluation. Analyses are being conducted by USDA’s Agricultural Research Service.

Results from the incubation studies were similar for both varieties. They show a threshold between 22 and 24 percent moisture, below which microbial activity and the potential for off-odor development are very low. This only applies to freshly harvested rice. As rice is dried below 17 percent, field microorganisms become inactive. Spoilage under storage conditions is associated with a different group of organisms than those found in the field. Microbes found in storage bins behave differently and require different environmental conditions for growth than field microorganisms. Without the data from the USDA analysis, researchers are not prepared to draw any conclusions about safe holding times for wet rice as it related to aspects of quality. The direct relationship between ethanol levels and eating quality, for instance, has yet to be established. Results of sensory evaluation in lab analysis and panel tasting will be made available at completion of the study.

Researchers also used battery-operated, hand-held detectors to measure carbon dioxide and ethanol concentrations in truckloads arriving at a rice drying facility in West Sacramento. Moisture levels and grain temperatures were also monitored. Elevated ethanol levels were only observed in morning truckloads from rice harvested the previous day, although off odors were noticeable in only one load. This work illustrates the quality advantage in shipping rice during day of harvest.

First Pass Drying

A laboratory study was conducted to determine the amount of moisture that could be removed from rice without causing head rice loss. M-202 samples were harvested at different times with four different moisture contents ranging from 17.3 percent to 26.3 percent. This rough rice was first dried with heated air by 2 to 10 percent. Samples were then tempered for two hours before being dried with room air to 13 percent moisture. Samples were milled at the California Department of Food and Agriculture Laboratory in West Sacramento. Drying rice to below the 18 to 20 percent range in a single pass would allow storage for short times without running the risk of developing off odors.

A similar test was conducted at a farm-scale column dryer. The unit had two temperature sections, with the top section at the higher temperature. The dryer was filled with a fresh batch of rice and both the heated air and discharge roll were turned on. No head rice quality was lost in the first test, although only 1.5 points of moisture was removed. The second batch was drier, at 20.5 percent moisture, and showed head rice loss when more than two points of moisture were removed.

Together the lab and field data suggest that high moisture rice can be subjected to high amounts of moisture loss during the first pass. It appears that four percentage points of moisture can safely be removed at moistures above 23 percent. However, the field tests demonstrate that drying times become quite long when large amounts of moisture are removed in a single pass. Daily drying capacity may be too greatly reduced by removing large amounts of moisture. At typical harvest moistures, four percentage points of moisture removal is adequate to prevent off-odors in tempering after first pass drying.

Harvest Moisture and Head Rice

Rice moisture at harvest has a demonstrated effect on rice quality. Low moisture is associated with low head yield. High moisture allows more immature kernels and slightly lower total rice yields. General recommendations are to harvest medium grain at about 24 percent moisture for optimum head rice yield. But previous studies have not included the effect of handling costs (up to $1 per hundredweight) on optimum harvest moisture, as well as higher drying costs for higher moisture rice.

Five years of receiving data for California medium grain were analyzed to determine optimal harvest moisture. The data verified that harvest at 24 percent moisture produces maximum head rice. However, total yield increased slightly at lower moistures and drying costs were lower for drier rice. If net grower return was considered rather than head yield, then the optimum harvest moisture is 21 percent. Furthermore, harvesting at moistures ranging from 18 to 24 percent does not cause more than a five cent per hundredweight variation in return. However, factors other than harvest moisture affect rice quality and crop value. Harvesting above 24 percent is not recommended because of the risk of off-odor development. Grower return for M-401 is more sensitive to harvest moisture; maximum returns are achieved between 22 and 24 percent moisture.

Another consideration is energy costs. If drying charges were increased by 50 percent, maximum return is obtained at 20 percent moisture and drops off rapidly above 22 percent. High energy costs will encourage growers to allow their crop to dry to lower moistures before they harvest. In previous years, the average medium grain harvest moisture was 20 to 21 percent, which is in the range of the optimum return. If drying costs increase by 50 percent, the industry average harvest moisture could drop by one to two points, resulting in a slight total yield increase and an average head rice loss of two to three points.

Project Leader and Principal Investigators

Randall “Cass” Mutters, farm advisor, UC Cooperative Extension Butte County

Jim Thompson, extension specialist, Department of Biological and Agricultural Engineering, UC Davis