Assessment of the influence of soybean oil minor components, and of certain added tocopherols and dietary amino compounds on the oil oxidation process under thermoxidative conditions and/or on its evolution during in vitro digestion

Abstract

In the present doctoral thesis three different topics have been addressed, these are:1- The influence of the minor compounds naturally present in commercial soybean oil on their evolution under accelerated storage conditions.It has been proved that in oils of the same botanical origin with very similar composition in acyl groups, their minor component profile has an influence on their evolution under accelerated storage conditions. This finding could contribute to establish criteria for the selection of vegetable oils with better oxidative stability among those from the same botanical source.2- The effect of adding alpha-tocopherol, gamma-tocopherol or L-lysine on the oxidative stability and the oxidation process of commercial soybean oil submitted to accelerated storage conditions.The effect of the addition of varying proportions of either alpha-tocopherol, gamma-tocopherol or L-lysine on the oxidation process of soybean oil under accelerated storage conditions has been studied in depth by means of 1H NMR. This technique provides a broad vision of the oxidation process since it allows one to monitor simultaneously the evolution of the oil acyl groups and the generation and further evolution of a wide variety of oxidation products, both primary and secondary. The differences in the mechanism of action of the studied tocopherols and of lysine have become clear, as well as their effect on soybean oil at different concentration levels. Although alpha-tocopherol exerts an overall prooxidant effect on soybean oil at most of the proportions tested, it could be said that, especially in the case of alpha-tocopherol, it is difficult to define the action of the studied tocopherols either as antioxidant or prooxidant, due to their complex effect on the rates of oil degradation and of hydroperoxide concentration increase, on the time needed to reach oil total polymerization and on the generation pace of secondary oxidation products. However, the addition of lysine clearly delays the oil oxidation process, extending its shelf-life. Furthermore, some of the modifications undergone by lysine as a consequence of the cooxidation process with soybean oil have also been identified by means of LC/MS.3- The study of the in vitro digestion process of commercial soybean oil and of the influence of the oil composition in minor components, of its initial oxidative status and of the presence of different proportions of ovalbumin on lipolysis and oxidation reactions.Within this section it has been shown that the minor components naturally present in soybean oil can slightly affect the occurrence of oxidation reactions taking place under in vitro digestion conditions. It has also been proved that the initial oxidative status of the oil not only negatively affects lipolysis but also can favour oxidation during digestion depending on the oil oxidation extent, all this reducing the bioaccessibility of lipid nutrients. Moreover, although not all the types of oxidation products initially present in the oils subjected to digestion evolve in the same way during this process, some potentially toxic ones remain in the digestates. Finally, while the presence of a low proportion of ovalbumin during the in vitro digestion of oxidized soybean oil does not significantly affect either lipolysis or oxidation or oxidation compound evolution, a high proportion of this protein can affect all these processes and contribute to both improve lipid digestion and reduce the concentration of some toxic compounds in the digestates