Impact of grape skin extract addition on walnut paste stability

Source of polyunsaturated fatty acids, phenolic compounds and tocopherols, walnuts are widely consumed thanks to their nutritional and sensory profile. However, their functional properties are poorly exploited at industrial level due to the high susceptibility to oxidation. Walnut paste, specifically obtained by roasting and grinding of kernels, is used in various food products, from confectionery, to bakery, to meat industry and more. In order to delay the rancidity development, antioxidants can be added. Nowadays there is a great interest for obtaining and exploiting antioxidant from natural sources. Among them, winemaking by-products are rich in bioactive compounds that can be recovered to obtain high value products to be recycled into the food chain. In particular grape skins are a source of antioxidant phenolic compounds well known for their potential health beneficial effects. In the present study, walnut paste was characterized and supplemented with grape skin extract aiming to improve oxidative stability and antioxidant capacity. Based on the Oxitest induction period, 5000 ppm (w/w) extract addition were selected as effective in inhibiting oxidation processes. Therefore, an accelerated shelf life test was performed by keeping walnut paste with and without 5000 ppm extract for 15 days at 60°C, in order to simulate 2 year storage at 20°C. Data from monitoring of peroxides values, conjugated dienes and trienes, total phenols, ABTS, ORAC, FRAP, and tocopherols showed the deterioration in quality of walnut paste itself started only at the end of the observation period, even though remaining below the threshold of unacceptability. Moreover, 5000 ppm extract addition did not prove to enhance oxidative stability and/or antioxidant properties of the walnut paste during the accelerated shelf life test. The obtained results represent an important starting point for further studies on oxidation kinetics of walnut-based products and on potential health benefit of grape skin extract addition.