Maintaining Rice Quality
After Harvest-2000



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Project Leader and Principal UC Investigators

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

James Thompson, extension specialist, Dept. of Biological & Agricutural Engineering, UC Davis


This project, in its second year, continued to focus on the factors affecting rice quality after harvest. Objectives include:
  • Determine the effects of time, temperature and grain moisture on quality loss of temporarily stored, field moisture rice
  • Determine whether rice can be dried in a single step from high moisture to below- threshold moisture for safe, temporary storage
  • Test aeration of wet rice as a means of preventing off-odor development
  • Evaluate harvest at lower moistures

33Chart.jpg (67584 bytes)Off-odors studied

In laboratory tests, freshly harvested 500-pound batches of rice were held in insulated metal boxes for 48 hours to observe microbial activity. Seven batches of Akitakomachi and six batches of M-202 with moistures ranging from 20 to 28 percent were tested.

Previous research indicated that off-odors in high-moisture grain are caused by a range of bacteria, yeasts, fungi and other organisms. A hand-held sensor was used to detect the presence of ethanol, an indicator of off-odor development. Rates of ethanol rise were particularly high at moistures over the 26 percent threshold. Little odor developed if rice was held at less than a 22-24 percent moisture range. Akitakomachi proved to be much more susceptible to off-odor development at higher moisture levels than M-202. This could be a particular problem for this variety because it is usually harvested at high moistures to minimize grain fissuring.

34Trailer.jpg (127848 bytes)Researchers also monitored a full 12-ton trailer load of 28.2 percent M-202 paddy rice. Low oxygen levels near the bottom of the trailer increased the activity of anaerobic microbes, producing ethanol levels two to five times higher than at the top of the load. (See illustration.)

One-pass drying looks promising

Drying rice quickly to a safe holding moisture can minimize off-odor development. Lab tests demonstrated that rice could be dried from 25 percent to 19 percent moisture in a single pass without significant quality loss. The one disadvantage of single pass drying is that the time in the dryer increases by about 50 percent. This test needs to be conducted in a commercial dryer before this type of drying can be recommended.

Ventilating wet rice

A hopper-bottomed trailer was fitted with a two-horsepower fan that directed outside air to a perforated air plenum in the trailer. Rice at 28.2 percent moisture filled the trailer and was monitored for 48 hours. Within five to six hours the rice was cooled from a harvest temperature of 86 degrees to 65 degrees Fahrenheit. Ventilation resulted in no signs of off-odor development. Aeration may have reduced off-odors by simply diluting them or ventilation may have made the trailer environment less conducive to microbial growth. Bottom line is aerating wet rice reduces ethanol concentration in rice and may reduce off-odor development.

Harvest at lower moistures

Traditional recommendations for optimum harvest moisture have been based on head rice yield. For typical California medium grains, such as M-202, highest head yields are obtained at paddy moistures of 22 to 26 percent. Yet high moisture rice costs more to dry and may have slightly reduced total yields than lower moisture paddy.

Researchers analyzed two years worth of receiving records from California medium grain rice delivered to Farmer's Rice Cooperative to evaluate the effect of harvest moisture on head and total yields and net grower return (value of dried rice less drying and handling costs). In 1999 the best returns were obtained at 23 percent moisture and in the 2000 season the optimum harvest moisture was in the ranged of 19 to 21 percent. The results of the net return analysis and of the off-odor development tests suggest that growers receive greatest net return from their rice and the industry reduces the likelihood of off-odor development by not harvesting California medium grain rice above 22-24 percent moisture.

Akitakomachi, because of its different quality characteristics, may need to be harvested at moistures that are high enough to allow rapid microbial growth. After harvest it must be quickly dried to safe moistures or aerated to prevent off-odor development.

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