Effect of continuous and intermittent drying on water mobility of fresh walnuts (Juglans regia L.): A LF-NMR study

In this study, water mobility and oil qualities (color indexes, peroxide value, acid value, fatty acids) between continuous and intermittent hot air drying of fresh walnut were investigated based on LF-NMR technique and chemical assay. The results showed that intermittent drying of fresh walnut was comparable or superior to continuous drying, in view of drying rate, moisture ratio, energy saving, oil quality (L*, a*, b*, peroxide value, acid value, and fatty acids). LF-NMR relaxometry could be used for observing the moisture mobility and distribution of fresh walnut during tempering process. The R2 values of moisture prediction model were 0.9808 and 0.9702 for calibration and validation, respectively, exhibiting fine linear relationship between the T21 signals and moisture level. Meanwhile, principal component analysis showed that five geographical origins of dried walnut could be discriminated based on LF-NMR relaxometry. Thus, intermittent drying with tempering process was recommended in practice for fresh walnut drying, and LF-NMR technique could be used for moisture prediction and origin discrimination of dried walnuts.