Infrared Pre-Drying and Dry-Dehulling of Walnuts for Improved Processing Efficiency And Product Quality

Abstract

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 min followed by hot air (HA) drying at 43°C for up to 24 h 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 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 on the quality of processed products compared with HA. The studied approaches have potential to improve processing efficiency and quality of dried walnuts.