Prooxidant effect of polyphenols on the oxidative stability of almonds and walnuts after thermal treatments

SALCEDO, C.L.; NAZARENO M.A.

Sitges, Barcelona

Conferencia; 5th International Conference on Polyphenols and Health; 2011

International Conference on Polyphenols and Health (ICPH)

Walnuts and almonds are nutritionally valuable foods, mainly due to their high lipid and antioxidant compounds contents. Nut lipid fraction is mainly constituted by unsaturated fatty acids, which are readily prone to oxidation reactions and as a consequence, being rather unstable. On the other hand, both nuts are also rich in phenolic compounds, which are concentrated in their brown skins. Their antioxidant ability has been previously assessed in different systems. Possible effects of brown skin phenolic compounds on the oxidative status of ground walnuts and almonds after short term thermal treatments were analysed in the food matrix. Oxidation advance was also evaluated in a model system, in order to mimic the different food microenvironments. Peeled and unpeeled nuts were ground and heated at different temperatures for short time lengths. Secondary oxidation products were analyzed by TBARS method and volatile emission induced by heating by SPME-GC.  To explain the oxidation mechanism pathways and specific interactions, a water-in-oil model system was designed to study the thermal induced oxidation of these nuts using linoleic acid as oxidisable substrate in a reverse micelle system formed by AOT/isooctane/water. Primary oxidation products were analysed in terms of formation as conjugated diene hydroperoxides. Phenolic compounds were incorporated in the reverse micelle system as aqueous extracts. Unpeeled ground nuts showed higher MDA equivalents than peeled ones at room temperature, demonstrating a brown skin antioxidative effect; on the contrary, at 70°C and 100°C, unpeeled nuts showed significantly higher MDA equivalents than peeled ones. In the same trend, hydroperoxide levels were higher at 70 and 100°C in the model system, when aliquots of phenolic compounds extracts were added to the micelle systems, than those without phenolic compounds extracts. High levels of both primary and secondary oxidation product compounds in presence of brown skins, indicate a prooxidant effect in the heated samples. Secondary oxidation product contents were higher in walnuts than in almonds, which is consistent with the walnut oil fraction richer in oxidisable fatty acids than the almond one.