Looking for a more efficient drying process for just harvested rice, scientist Zhongli Pan decided walnuts could also benefit from an improved drying method.
Pan of USDA’s Agriculture Research Service (ARS) has been working on a drying method that uses infrared technology to reduce energy use in the drying process and improve nut quality.
The standard hot air drying process for walnuts takes more than 24 hours with the use of natural gas and electricity. Infrared, a form of intense light that’s felt but not seen, is a type of electromagnetic radiation much like radio waves, ultraviolet radiation, and microwaves.
Walnuts can have a broad range of moisture at harvest and the washing process adds to the shell moisture making a 24-hour drying period necessary.
Pan and his colleagues at ARS’ Western Regional Research Center were searching for a more efficient way to dry walnuts less likely to over dry the nuts and reduce kernel darkening.
“We had two major concerns,” Pan said. “The hours it took to dry and the difficulty in drying the moisture inside the shell.”
Pan said pre-drying with infrared takes less than three minutes which is fast and energy efficient. Pre-dried walnuts are sent to a regular hot air dryer to finish the drying process to about eight percent moisture content for optimum storage.
Pan said the new infrared drying method could be integrated into an existing drying facility without the need to build a new structure.
Radiation heating of infrared is the key for improving drying performance since it can heat wet walnuts quickly. To use this new drying technology, presorting is done to separate walnuts based on moisture content of individual walnuts with the high and low moisture nuts processed separately to avoid over drying of low moisture walnuts.
In research station experiments, the new process saved up to 25 percent of the natural gas and electricity used to dry walnuts. In addition, the infrared drying process removed moisture from the surface of walnuts; reducing the total drying time by 35 percent.
Over drying walnuts, Pan says, not only wastes energy, and under drying presents a food safety risk due to mold caused by excess moisture in storage. There is also concern that due to the walnut’s high level of unsaturated fatty acids that they can be susceptible to oxidation reaction at high temperatures. He says the new drying technology improved both energy efficiency and product quality.
This new technology, Pan notes, also may be practical for drying other tree nuts washed during processing.
Pan’s research was a collaboration with the University of California, Davis and was supported by a California Energy Commission grant of about $1.1 million, plus funds from Wizard Manufacturing of Chico.
A commercial-scale pre-drying unit using this technology was successfully demonstrated during the 2016 walnut harvest season at Emerald Farms in California.
The California Energy Commission reports that infrared technology in the drying process could reduce GHG emissions as the infrared emitters don’t produce NOx or greenhouse gases, and the technology will also reduce natural gas consumption.
Bill Stone of Drumheller Dryers in northern California is not using this technology, but says the process has possibilities in walnut drying. What dryers need is a quick way to get the moisture out of the nut without affecting kernel quality, he says. It’s a complex process, but he believes infrared technology has some potential in commercial drying.
In addition to the cost of the new equipment and installation, Drumheller says handling larger walnut volume would be important.