‘Every seed counts’: Study compares drying conditions for seed rice performance

FAYETTEVILLE, Ark. — Rice grown for seed rice was the focus of a yearlong investigation by the Arkansas Rice Processing Program using X-ray imaging to determine the impact of higher drying temperatures.

While quality defects like opacity and small cracks called fissures are key measures for rice grown to eat, rice saved for seed is evaluated on two critical factors: germination potential and seed vigor.

Germination potential measures what percentage of rice seed will germinate and sprout, according to Griffiths Atungulu, director of the Rice Processing Program for the Arkansas Agricultural Experiment Station. Seed vigor indicates how fast it will germinate and whether the seedling will develop into a healthy plant.

“Good seed quality directly influences crop success and is crucial in improving productivity levels,” said Atungulu, who is also a professor in the food science department for the experiment station and the Dale Bumpers College of Agricultural, Food and Life Sciences. “As we often say here, every seed counts.”

Samuel Olaoni, a Ph.D. student in the food science department under Atungulu’s advisement, sought to determine the optimum drying parameters for seed rice. For two long grain rice varieties, the research showed that moisture content at the time of harvest had more of an effect on both germination and seed vigor than hot-air drying temperatures.

His study, “Use of X-ray imaging to elucidate impacts of drying conditions and storage on seed rice germination and vigor,” was published in the journal Drying Technology in July. Atungulu and Kaushik Luthra, assistant professor of postharvest processing in the biological and agricultural engineering department, were co-authors.

The study calculated germination rates of about 95 percent with good seed vigor when harvested at 13 to 16 percent moisture followed by drying conditions of 122 degrees Fahrenheit and 35 percent relative humidity. Germination rates of up to 90 percent were also still seen when dried at 140 degrees, which was the high end for this study.

“The experiment gave us confidence that the higher temperature will work in the end,” Olaoni said. “And the higher temperature will allow the processors to dry it faster without affecting germination too much. Time and cost are very important for the industry to get it done as quickly as possible without damaging the seed.”

Holistic view

Seeds dried at 35 and 50 percent relative humidity had higher seed germination than those dried at 25 percent, although with a slight difference of about 2 percentage points. Olaoni said the difference may be due to the drying effect, since a lower relative humidity would induce more stress on the seed than a higher humidity.

While there have been many studies looking at the effect of temperature on seed viability and vigor, Olaoni noted that less research has been done on the impact of relative humidity as a factor alongside temperature, air velocity, kernel fissuring and storage duration.

The two long grain rice varieties used in the study were harvested at three moisture content levels: 21 to 24 percent, 18 to 20 percent, and 13 to 16 percent. The rice was then dried to 12.5 percent moisture at temperatures ranging from about 100 to 140 degrees Fahrenheit, with relative humidity levels of 25 to 50 percent, and air velocity levels of 0.66 to 1.65 meters per second. A scale in the dryer weighed the samples to determine the moisture content.

Rice growers commonly harvest rice at a moisture content of 18 to 20 percent. They store the rice in large bins and begin the drying process to further reduce moisture content, Atungulu said. The optimum storage level of 12.5 percent moisture content wards off mold growth and helps maintain the rice’s structural quality.

Out fissuring

X-ray imaging was used as a noninvasive tool to see through the outer hull and examine rice grains for fissures before and after drying.

Olaoni’s previous research, published by the American Society of Agricultural and Biological Engineers, has shown that fissuring can impact seed rice germination and vigor, depending on the severity and proximity to the embryo and the specific rice variety.

There was a weak correlation and lack of statistical significance between kernel fissuring and seed vigor in this study. Although seeds that fissured less had higher germination rates, the difference in germination rates between seeds with higher and lower percentages of fissuring was about 3.5 percent.

Seed storage

Seed assessment tests were conducted at three, six, nine and 12 months of storage at room temperature — about 68 degrees Fahrenheit — with relative humidity levels of 25, 35 and 50 percent. There were no significant differences in germination rates or seed vigor between storage times and relative humidity levels in this study. However, there was more of a general decline in seed vigor during storage for the rice that was dried at higher temperatures compared to lower temperatures.

Overall, the authors noted that it was critical to dry the seeds to a safe moisture content of 12.5 percent before storage to minimize loss due to oxidation, enzymatic activity and respiration during storage.

The experiment station is the research arm of the University of Arkansas System Division of Agriculture.

This study was supported in part by the U.S. Department of Agriculture’s National Institute of Food and Agriculture Hatch Act funding.

To learn more about the Division of Agriculture research, visit the Arkansas Agricultural Experiment Station website. Follow us on X at @ArkAgResearch, subscribe to the Food, Farms and Forests podcast and sign up for our monthly newsletter, the Arkansas Agricultural Research Report. To learn more about the Division of Agriculture, visit uada.edu. Follow us on X at @AgInArk. To learn about extension programs in Arkansas, contact your local Cooperative Extension Service agent or visit uaex.uada.edu.

About the Division of Agriculture

The University of Arkansas System Division of Agriculture’s mission is to strengthen agriculture, communities, and families by connecting trusted research to the adoption of best practices. Through the Agricultural Experiment Station and the Cooperative Extension Service, the Division of Agriculture conducts research and extension work within the nation’s historic land grant education system. 

The Division of Agriculture is one of 20 entities within the University of Arkansas System. It has offices in all 75 counties in Arkansas and faculty on three system campuses.  

Pursuant to 7 CFR § 15.3, the University of Arkansas System Division of Agriculture offers all its Extension and Research programs and services (including employment) without regard to race, color, sex, national origin, religion, age, disability, marital or veteran status, genetic information, sexual preference, pregnancy or any other legally protected status, and is an equal opportunity institution.

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Media Contact: John Lovett
U of A System Division of Agriculture
Arkansas Agricultural Experiment Station
(479) 763-5929
jlovett@uada.edu