Moisture-Dependent Color Characteristics of Walnuts

Abstract

Characterizing the shell color of walnuts based on their moisture content at harvest can provide valuable information for performing walnut sorting before drying. The objective of this study was to investigate the relationship between the color characteristics of the shell of walnuts and their moisture content. Measurements were carried out for three walnut varieties, Tulare, Howard, and Chandler. The walnuts were sorted into two categories, namely with and without hulls at harvest. The Commission International de l'Esclairage's L*, a*, and b* color indices were measured to quantify the shell color after hull removal, and total color difference, hue angle (h*), and chroma (C*) values were calculated. The results indicated that the moisture content of walnuts with hulls at harvest was much higher than that of walnuts without hulls. The L* and ΔE values are highly correlated with moisture content and had different trends for walnuts with and without hulls. There was no clear relationship between other color characteristics and moisture content, although the a* values of walnuts with hulls were higher than those of walnuts without hulls. The results obtained revealed the potential of using the relationship between color indices of L* or ΔE and walnut shell moisture content to sort walnuts before drying, which is essential to avoid over-drying, increase drying capacity, reduce energy use, and obtain high quality walnut products.