UC Davis

Walnut kernels are rich in polyunsaturated fatty acids. These confer numerous health benefits, but are readily oxidized, leading to the development of off-flavors. Current commercial methods of reducing lipid oxidation in walnut add complication and expense to storage and distribution practices. To develop shelf-stable walnut cultivars, a better understanding of the influence of fatty acid and antioxidant content on lipid oxidation is necessary. In this study, kernels from a diverse population were measured for lipid content and oxidative stability using TD-NMR and Rancimat instruments, respectively. Genome-wide association studies (GWAS) were performed on oxidative stability and oil content data. Thirty-four trees were screened for lipid, tocopherol, and phenol profiles to determine which chemicals are associated with oxidative stability. More phenotypic and genetic variation was observed for oxidative stability than for oil content, and the two traits had a moderate negative correlation (r = -0.36). GWAS identified significant associations for lipid content and oxidative stability on chromosome 10. A supplemental file contains raw phenotypic data and best linear unbiased predictions for the two traits, parental information for the phenotyped taxa, and temperature data for the 2019-2021 growing seasons. Fatty acid, tocopherol, and phenol profiles in walnut also varied significantly. However, significant correlations with oxidative stability were found only for oleic acid, linolenic acid, moisture, and total phenols as measured by the Colin-Ciocalteu method. The associations suggest that breeding for high-oleic, low-linolenic cultivars would be the best method of developing a cultivar with a long shelf life. Unfortunately, this may sacrifice some of the health-promoting benefits of walnut consumption. Further study is needed to resolve whether increasing tocopherol and phenol content in walnut kernels can improve stability.