Improving walnuts' preservation by using walnut phenolic extracts as natural antioxidants through a walnut protein-based edible coating

Walnut kernels contain high amounts of polyunsaturated fatty acids that determine a limited shelf life on these nuts. The application of walnut phenolics as antioxidants through a walnut protein-based coating, obtained from walnut oil cake residue, can help to increase the shelf life of walnuts. The objective was to evaluate the preservative effect of walnut polyphenols included in a walnut-proteic edible coating on walnut kernels. Three treatments of walnuts coated with walnut flour were prepared: without the addition of antioxidants (control); with the addition of a walnut phenolic extract; and with the addition of butylated hydroxytoluene (BHT). On the last storage day, the sample with the addition of walnut phenolics presented a lower peroxide (3.64 meq 0₂ /kg oil) and anisidine value (1.11), conjugated diene (15.92), and hexanal content (19.67 × 10⁶ e.c.) than the control sample (6.23, 1.81, 24.65, and 122.37 × 10⁶ e.c., respectively). Also, on the last day, the control sample showed the highest deterioration of polyunsaturated fatty acids (from 74.83 to 71.08 g/100g), carotenoid (from 3.43 to 1.90 mg/kg), and γ-tocopherol content (from 349.66 to 298.42 mg/kg). In addition, this sample exhibited the highest oxidized (20.33) and the lowest walnut flavor intensity (64.67) on day 84. Regarding consumer acceptance, the phenolic-added sample displayed a greater flavor acceptance score. Walnut phenolics, implemented through a walnut protein-based coating, improve the preservation of walnuts. PRACTICAL APPLICATION: The combination of walnut-phenolic extracts and walnut-based edible coating applied on walnuts by food industries allows to prolong their shelf life, by preserving their nutritional, sensory, and quality properties. Considering the practical feasibility, the procedure used to prepare these products is simple and requires machineries already present in food industries. In addition, the utilization of this coating with walnut-phenolics exerts benefits like, the prevention of allergen cross-contamination in the chain of production, the utilization of an industry's residue, the replacement of synthetic antioxidants and, and the diminishment of the amount and thickness of plastic needed for walnuts' packaging.

Keywords: Juglans regia L.; conservation; oxidation; proteic coating.