UA Researcher Examines Microwave Rice Drying – Arkansas On-line
Microwave technology may offer a faster drying system for rice than traditional hot air drying systems, said Griffiths Atungulu, associate professor of food processing and post-harvest systems engineering at the University of Arkansas Agricultural Systems Department.
Atungulu is a co-principal investigator with AMTek Microwaves, a Cedar Rapids, Iowa company, on a $ 100,000 Innovation Research Fellowship from the US Department of Agriculture’s National Institute of Food and Agriculture.
Raw rice is ideally harvested when the crop moisture content is 19-21%, Atungulu said. Before grinding, processors must reduce the moisture content to a target of around 12.5%.
Traditional rice drying systems use natural air-in-bin or crossflow column dryers. Drying in the container can take many days to dry rice. Rice processors commonly use cross-flow column dryers that pass heated air over a column through which raw rice falls. Dryers can speed the process but still take a long time, Atungulu said.
Crossflow column dryers typically require multiple passes of the rice through the column, separated by tempering periods, which keep the rice at a warm temperature. It typically takes about three passes through the cross-flow dryer to reduce the rice to 12.5% moisture content.
Because tempering often takes place overnight, the drying process often takes two days or more, Atungulu said.
It also affects rice yield, the percentage of grains that are at least three-quarters of their original length after milling, Atungulu said. Prolonged exposure to heated air can crack the physical structure of the rice kernel, making it brittle. Milling the rice then causes some of the fissured kernels to be broken, reducing the head rice yield. Other factors, including environmental conditions and rice genetics, contribute to rice yield. In the US, the average rice yield is 55-58% of the total rice volume.
“The yield could be lower depending on the prevailing environmental conditions during harvest,” said Atungulu.
Using an AMTek microwave dryer the size of a commercial restaurant oven, Atungulu developed a method for drying rice to the target moisture content of 12.5% in a single pass under laboratory conditions, he said.
His goal was to develop a one-pass drying method where head rice yield was at or above the national average. He looked for a microwave drying method that would not affect the color or taste of the rice, or increase the rancidity that can appear in the bran layer.
“We didn’t want to change anything that would affect consumer acceptance of rice products,” he said.
AMTek offers a large microwave drying oven that will allow Atungulu to take its rice drying process to a commercial scale in one pass.
“This will be a proof-of-concept study,” he said, “based on extensive preliminary research.”
In years of preliminary research in collaboration with AMTek and several rice processing companies, Atungulu has shown that a microwave frequency of 915 megahertz – most household microwaves operate at a maximum of 2.45 MHz – can dry rice in one pass without putting a strain on the head. It also met the requirement of not affecting the color or taste desired by the consumer.
Atungulu has two goals in the proof of concept.
“First we have to please the rice processors,” he said. “We want to show that microwave drying cuts the time it takes to dry rice while reducing cracks to improve head rice yield.”
Atungulus aims to improve the national average from 55-58% rice yield to at least 65%.
“That would increase the value of rice by $ 145 million annually,” he said.
A microwave drying system also requires less machine floor space, which saves space, Atungulu said.
“Second, we have to keep consumers happy,” he said. “That means we have to preserve the taste, texture, color and cooking quality.”
While doing industry-level research, Atungulu said he will work to optimize the system to meet these industry and consumer needs.
His research to date has identified 915 MHz as an efficient frequency to achieve his goals. But on a large scale, Atungulu said, this may not be the ideal frequency for all rice varieties.
“Some frequencies may not fully penetrate some strains,” he said. “We may also need to adjust how the microwave energy is emitted. We may need to design some components to control how the energy diffuses into the rice.
“These are the things we have to play around with to find the optimal design and control for commercial microwave rice drying,” Atungulu said.
Existing crossflow, multi-pass column dryers may be less efficient than microwaves, but Atungulu states they have a proven track record. They are also durable and will keep working for decades with regular maintenance.
Rice processors will not be easily convinced to convert their drying systems. A key goal of the proof-of-concept phase of his research is to use conclusive data to show the economic advantages of microwave rice drying.
“We understand the feasibility of microwave drying,” said Atungulu. “We also want to be able to articulate the benefits of the system.”
Even with convincing data, Atungulu expects the conversion won’t happen overnight.
“It’s more likely to happen gradually,” he said. Given the longevity of existing rice dryers, this can take a significant amount of time.
For more information on the Department of Agriculture’s research, visit the Arkansas Agricultural Experiment Station website: https://aaes.uark.edu. Follow the agency on Twitter at @ArkAgResearch and Instagram at ArkAgResearch.
Fred Miller is in the Agriculture Systems Department at the University of Arkansas.