Walnut paste: oxidative stability and effect of grape skin extract addition.

Walnut paste, obtained by roasting and grinding of kernels, was characterized and supplemented with encapsulated grape skin extract aiming to evaluate its potential effect on oxidative stability and/or antioxidant capacity. Based on the oxidation induction period in screening trials 5000 ppm (w/w) extract addition was selected as effective in inhibiting oxidation processes. Walnut paste with and without 5000 ppm grape skin extract were maintained for 15 days at 60 °C, simulating 2 year storage at 20 °C, based on an estimated activation energy of 80,327 kJ/mol for walnut lipid oxidation. Monitoring of data from peroxides, conjugated dienes and trienes, total phenolics, ABTS, ORAC, FRAP, and tocopherols values showed the deterioration of walnut paste started at the end of the observed period, even remaining below the threshold of unacceptability. Moreover, 5000 ppm extract addition did not prove to enhance oxidative stability nor antioxidant properties of the walnut paste. In the future, specific parameters of oxidation kinetics and antioxidant activity in the advanced phase of storage could be investigated.