3.2 Color of walnut butter
Color was usually an unappreciated physical parameter (Jose Moyano et al., 2010). However, the study by Gambaro showed that color could influence the preferences of consumers (Gambaro et al., 2014). In the color analysis, L* represented lightness and darkness (black and white), a* represented red-green, and b* represented yellow-blue. Table 1 represented the color difference results of different walnut butter. There were significant differences (p<0.05 ) in the L*, a*, and b* between the FLWB and commercial walnut butter. MCT-WB had slightly higher L* than SS1 and SS2. The L*, a* and b* of DG-WB and CLA-WB were lower than those of commercial walnut butter. As a result, the color of the sauces showed different degrees of yellow color for commercial walnut butter and FLWB. The b* of FLWB was significantly lower than the commercial walnut butter. The oil in traditional walnut butter was mainly derived from the walnut oil in the walnut kernels, and its color was mainly influenced by the beta-carotene contained in the walnut oil (Misawa, 2009). The carotenoid content in walnuts ranged from 0.08-0.49 mg/kg (Ozrenk et al., 2012), which was resulting in a higher yellow value of walnut oil (Martínez et al., 2008). As a result, walnut oil had a more yellow color due to its highest carotenoid content, while other oils appear lighter yellow or even colorless. Walnuts could lose the carotenoids after the extraction of walnut oil, which caused that the defatted walnut meal lost the original color of walnuts and took on a grayish-yellow color. This could also affect the color of walnut butter which could appeared light-yellow or grayish-yellow. It may also be the reason why commercial walnut butter containing walnut oil is more yellowish than walnut butter prepared with FL. Tables 2S also showed differences in the color of the four oils. There is a correlation between the prepared walnut butter and FL, which may account for the differences in the color of the walnut butter.