UACES Facebook Death by Midnight: Nighttime temperatures affect rice post-harvest quality
skip to main content

Death by Midnight: Nighttime temperatures affect rice post-harvest quality


By Fred Miller, Division of Agriculture Communications

Fast Facts:

  • Inconsistent milling quality led to decade-long research effort.
  • High nighttime air temperatures reduce head rice yield and other rice quality attributes.
  • Resulting high chalk content leads to broken kernels and higher processing temperatures.


It was a dark and stormy night on the Grand Prairie. Summer evenings fall like a sauna in Arkansas rice country. Hot. Steamy. Dripping with trouble. And all across the Delta, rice kernels were dropping like mosquitos sparking off bug zappers.

This mystery was too big for the average flat foot. It was a job for detectives with whiplash wits and crackshot credentials. Scientists, see? Men and women who know what makes a rice plant tick and aren’t afraid to get their hands dirty.

— • —

FAYETTEVILLE, Ark. — For many years, rice companies experienced unexplained variability in the processing quality of Mid-South rice. Head rice yield — the number of kernels that remain intact during milling — would vary from one year to the next.

In addition, processors found that in some years, rice had to be cooked at higher temperatures or for longer times during the manufacture of food products. This slowed production and drove up energy costs. 

Rice milling quality research
Research assistant Kashoua Thor of Siloam Springs mills rice samples to remove the bran layer. (U of A System Division of Agriculture photo by Fred Miller.) 

The inconsistencies occurred in almost all rice varieties grown in Arkansas and neighboring states. Variation was also noted between different regions.

Researchers at the University of Arkansas System Division of Agriculture’s Rice Processing Program set out to find answers.

“The Rice Processing Program employs basic and applied research to benefit the rice processing industry,” said Terry Siebenmorgen, program director and University Professor of food science.

Siebenmorgen said the research team investigates a wide range of topics from kernel developmet to the consumer. Their expertise includes plant physiology, carbohydrate chemistry, process engineering, plant pathology and sensory analysis.

“When you look at post harvest processing, you often have to go back to the field to see how management practices and environmental influences affect processing quality and consumer satisfaction,” he said.

— • —

The scene of the crime was the rice mill. But there was more to this caper than we bargained for. Things weren’t adding up. To crack this case we needed to get a slant on where it began. Something this foul hides in the sewer. And we were going to lift the manhole cover.

— • —

“We began by quantifying physical and chemical properties of kernels throughout harvest,” Siebenmorgen said.

The causes were suspected to be in the agricultural timeline. They investigated environmental influences, management practices, physiology of the plants. At one point, Siebenmorgen said they looked hard at variability in grain moisture content at harvest.

The investigation continued for a decade. Siebenmorgen said they began to see clues.

“Milling and processing quality in cooler years was higher than in hotter years,” Siebenmorgen said. “That got us thinking about the effects of high temperatures.”

They began collecting data on temperatures and correlating them with processing quality trends. Nighttime temperatures in particular seemed to be having an impact.

Contrasting years began to paint the picture. A cooler than normal summer in 2004 produced uniformly high rice processing quality. Subsequent warmer years coincided with poorer quality.

Then Arkansas had a historically hot summer in 2010. Data from earlier years projected that the high temperatures in 2010 would cause widespread reductions in rice processing quality.

“Rice quality was severely hurt in 2010 and it was a disastrous year for the industry,” Siebenmorgen said. “But it expanded our data set and helped us pin down the problem.”

Division rice physiologist Paul Counce had mapped out the growth cycle of rice plants in exacting detail. When his research with recorded nighttime temperatures was correlated with statistical data developed by Andy Mouromostakis, professor of agricultural statistics, the problem began to be defined.

Texas based RiceTec gave division scientists access to their Phytotrons — essentially huge growth chambers in which environmental variables, including nighttime air temperatures, could be controlled. The company also gave Nora Cooper, a master’s degree student in food science, an internship that allowed her to run the needed tests.

— • —

The pieces of the puzzle were falling into place. The button man thought he’d made a clean sneak. But he tipped his mitt. We put the screws on and he began to sing.

— • —

The Phytotron test results clearly showed a dramatic impact from nighttime air temperatures during kernel development on milling yields and other properties, Siebenmorgen said. This early research laid the foundation for continuing field studies.

High temperatures during kernel development disrupt the formation of starch in the kernel. Starch becomes less densely packed in the seed, resulting in a chalky texture with an undesirable appearance and weaker kernels that tend to break during milling. The chalky structure also changes the way rice cooks.

Siebenmorgen said farmers sometimes could adjust management practices to try and beat the heat — planting earlier or using earlier-maturing varieties. But weather’s the culprit and no one can control it.

“I think the ultimate answer will be in improved varieties,” Siebenmorgen said. “We do see different degrees of heat tolerance between varieties.”

Division scientists Ken Korth and Andy Pereira are working on finding the genetic triggers that control kernel filling and response to high nighttime temperatures.

— • —

This guy was tough. Tough as a nickel steak. But I don’t like to see cheap hoods messing with a sweet kid. We didn’t start this thing, but it was up to us to finish it. No one plays us for a sucker and gets away with it.

— • —

Siebenmorgen said the Division of Agriculture’s relationship with rice processing companies was a valuable asset during their investigation and continues to be very important to all the program’s research endeavors.

“Not only do they support us with funding and equipment,” he said, “ but they also provide us important end-user feedback.”

The annual Rice Processing Program Industry Alliance meetings bring division scientists together with industry leaders to compare notes on important research issues, exchange information and share resources.

“This meeting is a signature event for the Rice Processing Program,” Siebenmorgen said.

Media Contact: Mary Hightower
Dir. of Communication Services
U of A Division of Agriculture
Cooperative Extension Service
(501) 671-2126

Related Links