COMBINED RADIO FREQUENCY AND HOT AIR TREATMENTS FOR INSECT CONTROL AND DRYING OF IN-SHELL WALNUTS

Abstract

Insect control and drying process are two important postharvest treatments of walnuts. We investigated the possibility of using combined 27 MHz radio frequency (RF) and short-time hot air treatments on in-shell walnuts to replace the current chemical fumigation against infesting pests, and prolonged hot air method for drying. Different heating rates from 0 to 21.3°C/min within walnut kernels were linearly correlated with the coupled RF power. The walnut kernel temperature distributions at 5 locations both for one layer and three layers were obtained at the end of the RF treatments that would result in adequate control of most insects pests. The kernel temperature differences among the 5 locations were 53.2±1.7 and 52.6±0.6°C for one layer and three layers when subjected the combined RF and hot air treatments. Hot air treatments improved heat uniformity in walnuts especially during holding period. The kernel temperature heated to 53°C by 3-min RF heating alone with 5 min holding provided 100% mortality of third-instar codling moths. RF for internal heating with hot air for surface heating could reduce the walnut moisture content from 10.0% after washing to less than 7% of the required level for storage. Therefore, combined RF and hot air treatments can be effectively used for insect control and drying of in-shell walnuts.