Evaluation of quality and thermodynamic properties of walnuts (Juglans regia L.) at different ripening levels

Walnuts at different ripening levels may have different nutritional and sensory properties, and their quality and oxidative stability may be altered. In this study, walnuts with cracked and with fully opened husks were hand-picked from trees and dried. Walnut oil and kernels were analyzed for their fatty acid, tocopherol and volatile profile by chromatographic techniques (GC-FID, UPLC-FLD, and GC-IMS, respectively). In addition, quality parameters (acidity, peroxide value, conjugated dienes) and thermodynamic properties using differential scanning calorimetry were evaluated. Walnuts with open husks had a higher concentration of polyunsaturated fatty acids (specifically linoleic acid), tocopherols, and linoleic acid-derived volatiles. No differences were registered in terms of free acidity. However, tocopherol content was negatively correlated with peroxide value and K232 value, and positively with the onset temperature related to the oxidative stability of the oil. The results could suggest that walnuts should be harvested when husks are fully open to achieve high-quality production.