Physico-chemical properties, oxidative stability and non-enzymatic browning in marine phospholipid emulsions and their use in food applications

Abstract

Marine phospholipids (PL) contain a high level of eicosapentaenoic acids (EPA) and docosahexaenoic acids (DHA), which have documented beneficial effect on human health. In addition, marine PL are more advantageous than crude or refined fish oils. Marine PL are more resistant to oxidation, provide better bioavailability and ability to form liposomes. All these unique properties of marine PL make them an attractive choice as ingredients for food fortification. Nowadays, a wide range of food products fortified with n-3 triglycerides (TAG) are available worldwide. However, the feasibility of using marine PL for food fortification has not been explored. The main objective of the present Ph.D. study was to explore the feasibility of using marine PL for food fortification. The secondary objective was to study the physical and oxidative stability of marine PL emulsions while identifying the important factors affecting their stability. Marine PL contain a high level of phosphatidycholine (PC), which has amphiphilic properties. Therefore it is feasible to prepare marine PL emulsions without addition of other emulsifiers. Emulsions containing solely marine PL with a high physical stability could be prepared by using 2-10 % marine PL. The high physical stability of these emulsions was most likely due to the coexistence of micelles, liposomes and emulsified oil droplets. However, there was a requirement for at least 3 % of marine PL (equivalent to 0.8 - 1.3 % of PC depending on the marine PL sources) to avoid phase separation and to form physically stable emulsions containing both marine PL and fish oil. Emulsions with high oxidative stability could be prepared by using marine PL of high quality with a high content of PL, cholesterol, antioxidants and a low content of prooxidants such as transition metals and initial hydroperoxides. In addition, the presence of other antioxidative compounds such as residues of free amino acids and pyrroles (formed via nonenzymatic browning reactions) in marine PL most likely have improved the oxidative stability of marine PL emulsions. In addition, hydrolysis of PL in marine PL emulsions was minimal at pH 7. In general, both physical and oxidative stability of marine PL emulsions varied in relation to the chemical composition of the marine PL used for emulsion preparation. Therefore, marine PL were purified through acetone precipitation in order to eliminate the effect of other factors such as the content of TAG, antioxidant or other minor components on lipid oxidation in marine PL. The oxidative stability of emulsions prepared from different levels of purified marine PL was investigated. Results obtained seem to suggest that the oxidative stability of purified marine PL emulsions was greatly improved by the addition of α-tocopherol. Non-enzymatic browning reactions were observed in marine PL emulsions through the a) measurements of Strecker degradation (SD) products of amino acid residues, and b) measurements of hydrophobic and hydrophilic pyrroles (which are pyrrolisation products of phosphatidylethanolamine (PE) and amino acids), respectively. Several mechanisms were proposed for non-enzymatic browning reactions in marine PL. It is speculated that these reactions might have occurred in marine PL mainly during their manufacturing process due to the interactions between lipid oxidation products with the primary amine groups from PE and residues of amino acids/protein that are present in marine PL. In addition, the content of pyrroles, SD products and the degree of browning in marine PL might be influenced by chemical compositions of marine PL and their manufacturing processes. In order to further investigate if the presence of pyrroles or degradation products of amino acids have any influence on oxidative stability of marine PL, liposomal dispersions were prepared from pure PC and PE compounds and purified marine PL with and without addition of amino acids. The obtained result from this model study confirmed the proposed mechanisms of non-enzymatic browning reactions in marine PL. The presence of PE and amino acids led to formation of pyrroles, generation of SD products and decreases in both browning development and lipid oxidation in liposomal dispersions. The low lipid oxidation in dispersions containing amino acids might be attributed to the antioxidative properties of pyrroles or amino acids. In addition, it is speculated that PE and amino acids pyrrolisation or oxypolymerisation of lipid oxidation products in marine PL might be the cause of browning development. Incorporation of marine PL into fermented milk product adversely affected the oxidative stability and sensory quality of fortified products despite the use of a low percentage of marine PL in combination with fish oil for fortification. This unexpected result was mainly due to the quality of current marine PL that was used for emulsion preparation and food application. In addition, the oxidative stability and sensory quality of marine PL fortified products varied depending on the quality and source of marine PL used for fortification. Although the attempts to incorporate marine PL into food system did not produce the expected results, the findings from the present Ph.D. study provide food industries and academia with new insights into the oxidative stability of marine PL and further inspirations for improving the quality of current marine PL.