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Title: Infrared pre-drying and dry-dehulling of walnuts for improved processing efficiency and product quality

item ATUNGULU, GRIFFITHS - University Of California
item HUI EAN, TEH - University Of California
item WANG, TIANXIN - University Of California
item FU, RUIPENG - University Of California
item WANG, XIAOTUO - University Of California
item KHIR, RAGAB - Suez Canal University
item Pan, Zhongli

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/31/2013
Publication Date: 10/31/2013
Citation: Atungulu, G., Hui Ean, T., Wang, T., Fu, R., Wang, X., Khir, R., Pan, Z. 2013. Infrared pre-drying and dry-dehulling of walnuts for improved processing efficiency and product quality. Applied Engineering in Agriculture. 29(6):961-971.

Interpretive Summary: There are several major problems related to processing cost, energy use, and product quality in the current postharvest processing of walnuts. Walnuts with hull are transported from orchards to drying site for dehulling and then the hulls must be transported away for disposal, which uses a significant amount of energy. Also, existing dryers commingle all nuts regardless of the large difference in moisture content (MC) of the nuts entering the drying process which results in over drying of the dryer nuts to ensure the wet nuts achieve the safe storage moisture of 8%. The walnut industry has an urgent need to develop new and cost effective walnut dehulling and drying methods which can improve energy efficiency, reduce negative environment footprint and provide high quality processed products. The study revealed that there are differences in nut size characteristics including axial dimensions and geometric mean diameters for nuts with and without hulls. Therefore, it is feasible to separate walnuts based on size characteristics, which could effectively enable practical separation of walnuts with and without hulls for in-field dehulling. During IR pre-drying of the high moisture nuts, the temperature of the meat at center of the nut remained considerably below 43°C in the first 150 s which provided time to drive out significant amount of moisture and also retained product quality. Based on overall product appearance and shelf life studies, IR partial drying time of 3 min is recommended before low temperature air drying of walnuts. Overall, the studied new approaches hold great promises in improving processing efficiency without affecting quality of walnuts.

Technical Abstract: The walnut industry is faced with an urgent need to improve post-harvest processing efficiency, particularly drying and dehulling operations. This research investigated the feasibility of dry-dehulling and infrared (IR) pre-drying of walnuts for improved processing efficiency and dried product quality. Freshly harvested walnuts (ethephon and non-ethephon treated) with whole and partly-attached hulls were dehulled using a test device to determine dry-dehulling time and frequency. The physical dimensions of walnuts without, with partly-attached, and with whole hulls were determined. In-shell walnuts of high (43%, w.b.) and low (18%, w.b.) moisture were pre-dried with IR for 2, 3 and 4 minutes followed by hot air (HA) drying at 43°C for up to 24 hours and effects on drying rate and product quality were studied. Based on results, walnuts with whole and partly-attached hulls could be dry-dehulled to achieve over 90% dehulled nut in 45 s and 15 s, respectively. Ethephon treatment had no significant influence on dry-dehulling of walnuts with partly-attached hulls. Contrarily, Ethephon treated walnuts with whole hulls had dehulled nut percentage higher than untreated ones. Walnuts without, with partly-attached and with whole hulls could be separated based on axial dimensions. IR pre-drying improved walnut drying rate. Up to 7% moisture reduction for high moisture nuts was achievable in 240 s of IR pre-drying with nut center temperature relatively below 43°C in the first 150 s. IR pre-drying for 180 s followed by HA drying had no effect in the quality of processed products compared with HA. The studied approaches have potential to improve processing efficiency and quality of dried walnuts.