High Temperature Walnut Drying

On farm walnut drying is often a major bottle neck during the start of walnut harvest. The recommended maximum drying air temperature for English walnuts is 110 of. This limit is based on tests by university personnel during the introduction of heated air drying (Batchelor, 1924) which demonstrated that long term exposure to temperatures higher that 110°F caused walnuts to turn rancid. This project investigated the use of high temperature (200 of -1200 oF)drying air temperatures for short time periods (on the order of minutes). Several reports have appeared in the literature that indicate high temperature-short time drying might be feasible for drying English walnuts. Tang, et al. (1982)used air temperatures up to 1000 of to flash dry macadamia nuts form 18% to 6% in under 3 minutes. Lowe, et a1.(1961) used moderately high temperatures (250 oF)to dry low moisture (8%) inshell English walnuts without apparent damage. We conducted over 50 experiments (consuming some 340 pounds of walnuts) to investigate drying of English walnuts using air temperatures in the moderate (200°F -350°F) and high (1200°F)ranges. Tests using moderately high temperature drying air usually cracked the shells of the walnuts before any significant moisture could be removed. Several experiments were performed using moderately high temperature air for a short time (less that 4 minutes) followed by drying at 110 of. This predrying reduced the initial moisture by about 4%; however, the total drying time to 8% moisture was the same ~ a control group without predrying. A series of high temperature (1200 oF) flash drying experiments demonstrated that this is not a feasible technique for English walnuts. Flash drying for times greater than 5 minutes proved to be destructive as the the kernels and shells turned offcolor and exhibited an off-flavor. For times less than five minutes, shell cracking was a significant problem and the amount of moisture removal was only on the order of 2% to 3%.