Rice: Research Shows Promise for Microwave Drying Systems

Former graduate student Shantae Wilson of Jamaica and Dr. Griffiths Atungulu of Kenya investigate the use of a microwave unit to shorten the time needed to dry rough rice and improve quality. Photo: Fred Miller, University of Arkansas

Microwave technology may offer a faster drying system for rice than conventional heated-air drying systems, said Griffiths Atungulu, associate professor of food processing and post-harvest system engineering for the University of Arkansas System Division of Agriculture.

Griffiths is co-principle investigator with AMTek Microwaves, a Cedar Rapids, Iowa, company, on a $100,000 Small Business Innovation Research grant from the U.S. Department of Agriculture’s National Institute of Food and Agriculture.

Drying rice

Rough rice is reaped, ideally, at a harvest moisture content of 19-21 percent, Atungulu said. Before milling, processors must reduce the moisture content to a target of about 12.5 percent.

Conventional rice drying systems use natural air in-bin or crossflow column dryers. In-bin drying can require many days to dry rice. Rice processors commonly use crossflow column dryers, which direct heated air across a column through which rough rice falls. Dryers can speed up the process but still take considerable time, Atungulu said.

Crossflow column dryers usually require multiple passes of the rice through the column separated by tempering periods, which maintain the rice at a warm temperature. It commonly takes about three passes through the crossflow dryer to reduce the rice to 12.5 percent moisture content.

Because tempering often takes place overnight, the drying process often takes two days or more to complete, Atungulu said.

It also impacts head rice yield, the percentage of kernels that are at least three-quarters of their original length after milling’ Atungulu said. The extended exposure to heated air can cause fissures in rice kernels’ physical structure, making them brittle.

Milling the rice then results in breaking some of the fissured kernels, reducing the head rice yield. Other factors, including environmental conditions and rice genetics, contribute to head rice yield. In the U.S., head rice yield averages 55-58 percent of the total rice volume.

“The yield could be lower, depending on prevailing environmental conditions during harvest,” Atungulu said.

Using an AMTek microwave dryer about the size of a commercial restaurant oven, Atungulu has developed a method of drying rice to the target moisture content of 12.5 percent in a single pass under laboratory conditions, he said.

His goal was to develop a one-pass drying method that maintained head rice yield at or above the national average. He looked for a microwave drying method that did not adversely affect rice color or flavor or increase rancidity, which can occur in the bran layer. “We didn’t want to change anything that would affect consumer acceptance of rice products,” he said.

Scaling up

AMTek is providing a large microwave drying oven that will allow Atungulu to advance his single-pass rice drying process to a commercial scale. “This will be a proof-of-concept study,” he said, “based on extensive preliminary research.”

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In years of preliminary research, collaborating with AMTek and several rice processing companies, Atungulu showed that a microwave frequency of 915 Megahertz — most home microwaves operate at a maximum of 2.45 MHz — could dry rice in a single pass with less impact on head rice yield. It also met the requirement of not affecting consumer-desired color or flavor.

In the proof of concept, Atungulu has two goals. “First, we have to satisfy the rice processors,” he said. “We want to demonstrate that microwave drying reduces the time required for drying rice while reducing fissuring to improve head rice yield.”

Atungulu’s target is to improve the national average of 55-58 percent head rice yield to at least 65 percent. “That would translate to a $145 million increase in rice value annually,” he said.

Also, a microwave drying system requires a smaller machinery footprint, saving space, Atungulu said.

“Second, we have to satisfy consumers,” he said. “That means we have to preserve flavor, texture, color and cooking quality.”

During the industry-scale research, Atungulu said, he will be working on optimizing the system to meet those industry and consumer requirements.

His research so far has identified 915 MHz as an efficient frequency to meet his goals. But on a large scale, Atungulu said, that may not be the ideal frequency for all rice varieties.

“Some frequencies may not penetrate fully into some varieties,” he said. “We may also have to adjust how the microwave energy is delivered. Perhaps some components will have to be designed to control how the energy diffuses into the rice.

“These are the things we’ll have to play around with to find the optimal design and control for commercial microwave rice drying,” Atungulu said.

Commercialization

Existing multiple-pass crossflow column dryers may be less efficient than microwaves, but Atungulu says they have a proven track record. They are also durable, continuing to work for decades with regular upkeep.

Rice processors will not be easily convinced to convert their drying systems. An essential goal of the proof-of-concept stage of his research, Atungulu said, is to demonstrate, with conclusive data, the economic benefits of microwave rice drying.

“We understand the feasibility of microwave drying,” Atungulu said. “We also want to be able to articulate the merits of the system.”

Even with convincing data, Atungulu expects conversion will not happen overnight. “It’s more likely to happen in stages,” he said. Given the durability of existing rice dryers, that may take considerable time.




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