Ambient Air Drying of English Walnuts

Abstract

ABSTRACT MOST English walnuts in California are dried on farm in batch dehydration systems. The drying season lasts from September through the end of October. Heated air, at temperatures up to 43C, is forced through the fixed bed dryers. Temperatures above 43C will cause oil in the nut meat to become rancid (Woodroof, 1978). The initial moisture content of the nuts ranges from 15 to 50% (dry basis), and they are required to be dried to 8.7%. Due to increasing energy costs and scarcity of fuel supplies, research into natural air drying of walnuts was started at the Agricultural Engineering Department at the University of California in 1977 (Thompson et al., 1978). Initial field trials showed the feasibility of drying walnuts with ambient air. The drying times were longer, but there were no signs of deterioration of nut quality. A second phase of the project has been to modify a grain drying simulation computer program to model walnut drying. The program modified was the Michigan State University fixed bed grain drying program (Bakker-Arkema et al., 1974). This paper describes the modifications that were made, and compares results predicted by the model to field drying tests. The model was used to simulate ambient air drying at three walnut growing locations in California.