OPTIMIZING WALNUT SHELL MECHANICAL PROPERTIES FOR EFFICIENT SHELLCRACKING USING HEAT TREATMENTS: A COMPARATIVE STUDY OF RADIO FREQUENCY AND HOT AIR HEATING METHODS

The mechanical properties of walnuts play an influential role in the process of walnut shell-cracking. To determine the optimal mechanical properties of walnuts, the effect of moisture content (MC) on the mechanical properties of walnuts was investigated. The results showed that Rupture force (F), Rupture displacement (D), and Rupture energy (E) of walnuts decreased proportionally with a reduction in the MC. To select an optimal pre-treatment for enhancing the mechanical properties of walnuts prior to shell-cracking, the effects of radio frequency (RF) and hot air (HA) heating treatment to change the mechanical properties of the walnuts were examined. The results indicated that the heating treatments of walnuts could lead to a brittle and easily breakable shell, with the F decreasing from 231.99±34.31 N to 174.73±24.89 N, the D decreasing from 1.68±0.18 mm to 1.36±0.13 mm, and the E decreasing from 207.31±44.29 mJ to 119.47±25.99 mJ. The mechanical properties of walnut shells are optimized to the best condition with the application of either a 2-minute RF treatment, a 3-minute RF treatment, or an 8-minute HA treatment. Notably, RF heating is significantly more time-efficient compared to the HA treatment. Quality evaluation indicated that there were no significant (p > 0.05) changes in color values, hardness, and brittleness between the heat-treated walnut kernels and the untreated walnut kernels. Overall, the results obtained from this study demonstrate that RF heating treatment is an effective method for optimizing the mechanical properties of walnuts prior to shell-cracking, and the results may provide guidance for the design and improvement of walnuts shell-cracking processes.