Demonstration and Commercial Implementation of Energy-Efficient Drying for Walnuts

An energy efficient drying system for walnuts using infrared (IR) as the heating source was demonstrated to reduce drying times and energy use in walnut drying. A pilot scale infrared walnut dryer with a capacity of 1 ton per hour was built in collaboration with Wizard Manufacturing Inc., and tested at Emerald Farms, Maxwell, California, in 2015. The drying test was performed after sorting the walnuts into two moisture groups of low- and high-moisture walnuts. The high moisture walnuts were pre-dried in the IR walnut dryer by heating the walnuts to the kernel temperature of 104-122°F (40-50°C) in the pilot-scale IR dryer. The IR pre-dried walnuts were conveyed to the hot-air drying bins and dried using hot air at 109°F (43°C) to a moisture content of 8 percent. The nearby hot-air drying bins were filled with walnuts without IR pre-drying and dried to 8 percent MC for comparison. The IR-preheated walnuts were dried faster and saved about 12 percent of energy used for drying.

A commercial scale IR walnut dryer with a capacity of 10-15 tons per hour (tph) was designed, built, tested and demonstrated. IR pre-drying of walnuts for 4 minutes prior to hot-air drying reduced the hot-air drying time by 13.6 percent to 26.5 percent when compared with walnuts that were not heated by IR and saved 9.96 percent to 19.94 percent of energy used in walnut drying. The IR pre-dried walnuts had the better quality and longer shelf life as those of walnuts dried by hot air alone.