Short Cold Storage as a Sustainable Postharvest Handling Method for Natural Enrichment in Antioxidants of Fresh and Dried Walnut Kernels—Cultivar Effect

Fresh (raw, non-dried) walnuts (kernel moisture > 17%) have unique sensory and nutritional attributes but a narrow time availability due to their rapid deterioration during storage. In the present study, the storage (1 °C, 90% RH) potential of fresh walnuts for 20 and 40 days was assessed in relation to cultivar (Chandler, Hartley, Ioli) and the form of exposure to storage (shelled or in-shell). The effect of low-temperature exposure (at 1 °C for 0, 10 and 20 days) before nut drying was also examined. Fresh walnuts from different cultivars showed diverse quality (size, color) and physiological (respiration, weight loss) traits. Using a very low storage temperature (1 °C) was feasible to store fresh walnuts marginally up to 40 days without losing the ‘fresh’ character. The form of in-shell storage compared with shelled ones helped to determine the retention of kernel moisture and had a mild protective role in the prevention of kernel browning. The storage of fresh walnuts at 1 °C resulted in increased total phenolics (TP, by 26% in average) and antioxidant capacity (by 46%, in average) of the kernels, supporting the improvement of nutritional value due to low temperatures. The dried kernels after this short cold storage showed increased TP levels by 35–40% in comparison with conventional dried ones. Therefore, the 10 d cold exposure could be proposed as a sustainable step for incorporation in the regular postharvest handling chain for the natural enrichment of fresh and dried kernels in antioxidants.