Characterization of Lipid- and Protein Co-oxidation Mechanisms in Oleogels using Kinetic Modelling and Multivariate Statistics
Lipid oxidation is one of the main causes of food spoilage. In addition, co-oxidation can lead to toxic products, which are even classified as carcinogenic. However, despite the lability of unsaturated fatty acids, they are important health promoting compounds according to the mediterranean diet and prevent for example cardiovascular diseases. Furthermore, ω3-fatty acids are essential compounds of our nutrition as they are precursors of hormones. In this context, the aim of this thesis was to investigate the interaction of proteins and lipids in oxidizing systems that lack typical lipid oxidation indicators of food monitoring. For this purpose, lipid-rich suspensions with low moisture were incubated with the addition of amino acids at moderately elevated temperatures related to room temperature to simulate the storage condition of a food, but not to change basic mechanisms by increased thermal energy. The suspensions provide insights into the relevance of interfaces for the interaction of primary lipid oxidation products, their formation and degradation (Chapter 2). Whey protein oleogels were introduced as model systems to subsequently maximize the protein-lipid interface and to focus on protein-lipid co-oxidation. This is a relative new class of lipids, which is a high unsaturated fat alternative to saturated and conventional hardened fat, which are associated with disease promoting trans-fatty acids. The lipid oxidation in such protein-based oleogels, as far as the author knows, was characterized for the first time and it could be shown that especially characteristic volatile aldehydes are degraded in such systems (Chapter 3). Furthermore, the degradation reactions of the lipid oxidation products are accompanied by modifications of the protein, which were characterized in relation to lipid oxidation in oleogels (Chapter 4). These protein modifications were then further investigated by multivariate statistical methods to identify the underlying potential major mechanisms and to evaluate their overall contribution (Chapters 5 and 6). Lipid-protein co-oxidation is an interdependent relationship in which the protein can act as an oxidation initiator. On the other hand, lipids are known for their autocatalyzed formation of lipid hydroperoxides, which fragment into hydroxyl radicals and other radicals as well as other oxygen-containing products such as aldehydes. By regression of a differential reaction equation, a model was utilized in which the formation rate of lipid hydroperoxides (as conjugated dienes) in this system correlates with the addition of water. The formation of water droplets and thus the formation of additional interfaces plays an important role in the formation of dienes. In contrast, it has been shown that the degradation rate decreases slightly with the addition of water. During this degradation a large number of different secondary lipid oxidation products are formed, whereby aldehydes, such as hexanal, the most common lipid oxidation product, react with amino groups of the protein to form Schiff's bases and addition products corresponding to those of Michael reaction products. These compounds, some of which are brown polymers, could then be detected by fluorescence spectroscopy and protein carbonyl content. For the latter, it was concluded that these carbonyls, as well as those formed in the lipid phase, are subject to degradation reactions, which prevent the accumulation of carbonyls. In addition to the secondary lipid oxidation products, hydroxyl radicals among other radicals are also formed by the decomposition of lipid hydroperoxides. These highly reactive radicals are able to abstract hydrogen atoms from the amino acid residues as well as from the protein peptide backbone. The former is involved in the formation of dityrosine and N-formylkynurenine, which are typical oxidation products of tyrosine and tryptophan and can be detected by fluorescence spectroscopy. In contrast, when the peptide bond is cleaved, which is promoted by a lipophilic amino acid residue, additional protein carbonyls and primary amines are formed, which in turn counteracts the accumulation of secondary lipidaldehydes.