The release of lipids from dairy food matrices under in vitro gastrointestinal digestive conditions : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Manawatū, Palmerston North, New Zealand

Abstract

Figures are re-used with the publishers' permission.The milk fat globule membrane (MFGM) is a trilayer phospholipid and protein interface that separates the triacylglycerides from the serum phase. Consumption of MFGM phospholipids decreases serum cholesterol in adults with high risk for hypercholesterolemia and modulates serum cholesterol in healthy adults. The biochemical mechanism behind this putative effect is currently unknown. The large majority (~80%) of the cholesterol in the milk fat globule is localized in the MFGM in complexation with sphingomyelin to form liquid-ordered (Lo) domains. The nutritional and digestive implications of consuming Lo domains have been scarcely considered. Considering sphingomyelin strongly complexes with cholesterol, the MFGM is rich in sphingomyelin, and the consumption of MFGM material has shown to modulate serum cholesterol, there is a likely link between the presence of Lo domains and the nutritional benefit of consuming MFGM material. Model bilayer systems, phospholipid liposomes/vesicles, were created from phospholipids derived from natural sources: soy phosphatidylcholine (SPC), porcine brain phosphatidylcholine (BPC), egg sphingomyelin (ESM), milk sphingomyelin (MSM), milk fat globule membrane (MFGM) phospholipids extracted from bovine beta-serum, and ovine cholesterol. The structural and thermotropic properties of the vesicles were investigated under in vitro gastrointestinal conditions. First, a screening test was performed on MSM/cholesterol multilamellar vesicles (MLVs). Instruments were capable of detecting shifts in the vesicles from solid-ordered (So) and liquid-disordered (Ld) to liquid-ordered at 2 × the MSM concentration found in raw milk. Incubation with physiologically relevant concentrations of bovine bile demonstrated 3:2 mol/mol MSM/cholesterol MLVs were capable of resisting detergent-induced solubilization by bile salts. The SPC/cholesterol, BPC/cholesterol, and MFGM/cholesterol phospholipid vesicles were disrupted. At the endpoint of in vitro gastrointestinal digestion, 3:2 mol/mol MSM/cholesterol MLVs also experienced micellization – however there was no statistically significant difference in the total bilayer order of the vesicles, implying a degree of structure possibly remained. Further investigation on the detergent solubilization of MFGM phospholipid vesicles by bile salts revealed that although the vesicle was disrupted, the mixed micellar size growth was minimal. That is, there was a lack of phospholipid exchange between vesicle and micelle. These findings set the stage for further research into the bioactive potential of milk phospholipids