Walnut Processing

Walnut has high nutrition value and health benefits, including high content of natural antioxidants and unsaturated fatty acids. Research proved that it’s beneficial for human brain and cardiovascular health. Walnut drying is a necessary post-harvest processing to reduce moisture content and increase shelf life. California has a huge walnut industry consisting of 4000 growers and 63 processors. Nearly 686 thousand tons of walnuts are produced annually and contribute $1.2 billion to state’s economy. Development of efficient walnut drying is required since currently consumes 1.27 x 1015J natural gas and 5.94 x 1013J electricity for drying process.

Main findings:

  • Efficient and sustainable drying technology based on step-down temperature hot air heating was developed for freshly harvested walnuts. The new drying methods could reduce up to 40% drying time and 24% energy consumption compared to the currently applied drying methods in the industry. Meanwhile, the product quality and the shelf life of dried walnuts were better than the products dried by conventional practice. The new drying method has been demonstrated in pilot-scale and showed great potential in commercial scale applications.
  • A new sequential IR and hot air (SIRHA) drying method was developed for freshly harvested walnuts, and has been demonstrated in commercial-scale. Compared to conventional HA drying at 43oC, the commercial-scale SIRHA drying led to 13.6% to 26.5% reductions in the drying time, and 10% to 20% in the energy savings, depending on the IMC of walnuts. Meanwhile, the product quality and shelf life were similar to the products dried by conventional methods.
  • A sorting technology, namely, the ’air knife’, was developed to separate the freshly harvested walnuts with different initial moisture levels based on their terminal velocities. The sorting method was demonstrated to improve the moisture uniformity of walnuts, and reduce the energy consumption for the drying process.

Selected Publications:

Chen, C., C. Venkitasamy, W. Zhang, R. Khir, and Z. Pan. 2020. Effective Moisture Diffusivity and Drying Simulation of Walnuts under Hot Air. International Journal of Heat and Mass Transfer. https://doi.org/10.1016/j.ijheatmasstransfer.2019.119283

Chen, C., C. Venkitasamy, W. Zhang, L. Deng, X. Meng, and Z. Pan. 2020. Effect of step-down temperature drying on energy consumption and product quality of walnuts. Journal of Food Engineering.  https://doi.org/10.1016/j.jfoodeng.2020.110105

Chen, C., W. Zhang, C. Venkitasamy, R. Khir, T.H. McHugh, and Z. Pan. 2019.Walnut structure and its influence on the hydration and drying characteristics. Drying Technology. https://doi.org/10.1080/07373937.2019.1605610.

Wang, X., G.G. Atungulu, R. Khir, Z. Gao, Z. Pan, S.A. Wilson, G. Olatunde, and D. Slaughter. 2017. Sorting in-shell walnuts by using near infrared spectroscopy for improved drying efficiency and product quality.  International Agricultural Engineering Journal. 26:165-172.

Khir, R., Z. Pan, and G.G. Atungulu. 2014. Characterization of physical and aerodynamic properties of walnuts. Transactions of the ASABE. 57(1):53-61.

Khir, R., Z. Pan, G.G. Atungulu, J.F. Thompson, and X. Zheng. 2014. Moisture-dependent color characteristics of walnuts. International Journal of Food Properties. 17(4):877-890.

Atungulu, G.G., H. Teh, T. Wang, R. Fu, X. Wang, R. Khir, and Z. Pan. 2013. Infrared pre-drying and dry-dehulling of walnuts for improved processing efficiency and product quality. Applied Engineering in Agriculture. 29(6): 961-971.

Khir, R., Z. Pan, G.G. Atungulu, J.F. Thompson, and D. Shao. 2013. Size and moisture distribution characteristics of walnuts and their components. Food and Bioprocess Technology – An International Journal. 6(3):771-782.

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