Milk lipid complexation and interaction with food ingredients: Digestibility and absorption: A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Science at Lincoln University

Milk phospholipids and non-polar lipids can form conjugations with antioxidants and starches, in the matrices of phytosomes and milk lipid-starch complexes, respectively. In addition to complexation, milk lipids also interact with starches in food matrices, leading to physiochemical changes of associated foods. Milk lipids possess neutral colour, taste, odour, natural, clean-label, and non-allergic. However, there has been little research on interactions or complexes between milk lipids and other food components when milk lipid was used as an ingredients in different food matrices. This thesis focuses on the extraction of bovine milk phospholipids from dairy products, purification of phospholipids, and preparation of vitamin C and E phytosomes in comparison with liposomes, followed by bovine milk phospholipid/triacylglycerol dispersions. Subsequently, non-polar lipids were used to prepare milk lipid-starch complexes in two kinds of food matrices: firstly bread and secondly starch gels. In the final research chapter, starch gel-stabilized milk fats were investigated in terms of lipid digestibility. The phytosomes were made by food-grade ethanol evaporation, and were verified by FTIR, DSC, UV and CI. In vitro models were used to measure the phospholipid digestibility and cellular uptake. Amylose-lipid complexes were prepared by thermal methods. The complexes were then verified by spectroscopy analysis. Starch and milk fat digestibility was determined by in vitro assays and simulated by a multi-step reaction model. The results on milk phospholipids showed that the polar heads of milk phospholipids in phytosomes interacted with hydroxyl groups of ascorbic acid resulting in the shifting of major bonds in the phosphatidyl residues in phospholipids, and therefore, milk phospholipid-based phytosomes had greater encapsulation efficiency and in vitro digestion stability than liposomes. Additionally, in opposition to triacylglycerol, milk phospholipids showed greater lipid digestibility and exhibited antioxidant activity due to differences in molecular and hydrocolloid structure. Therefore, milk phospholipids have potential application in fortification of foods including infant formulas. In contrast to milk phospholipid-based complexes, milk fats can interact and complex with corn, rice and wheat starches in food matrices. When milk fats were hydrolysed by fungal lipase, they formed starch-fatty acid complexes during baking, delaying bread firming rate and extending shelf-life due to reduced re-crystallisation of amylopectin. Hence, lipase treatment of milk fats offers a feasible way to improve both textural and physiological properties of bread. Also, milk fats-amylose conjugation can be produced by cooking at 95˚C, providing a practical method to lower the glycaemic response of starchy foods, reducing starch digestibility by 19% in corn starch, 17% in wheat starch, and 25% in rice starch. In addition to conjugation with milk fats, starch particles can also be used to stabilize milk fat emulsions. In vitro digestion showed that the lipolysis reaction speed and extent of starch gel-stabilized milk fats were two – three-fold that of milk fat dispersion, confirming the calculation by the multi-step enzymatic reaction model. This indicates the possibility to regulate lipid digestibility by designing starch-based matrices. Beginning with bovine buttermilk, membrane filtration was proposed to manufacture enriched milk phospholipid products (11 – 20 g polar lipids/100 g products), with the current, best available process efficiency. Supercritical fluid extraction was an effective, food-compatible method to produce high purity (65 – 90%) milk phospholipid products. Overall, this thesis demonstrates the structure-property-functionality relationship of milk phospholipids/lipids in several food matrices. This thesis provided novel approaches to the further use of milk lipids as functional food ingredients, such as vesicles (Chapter 4), antioxidants for infant formulas (Chapter 5), bakery product textural improvers (Chapter 6), glycaemic index reducers of starchy foods (Chapter 7), and lipophilic compound carriers using starch-stabilized milk lipid matrices (Chapter 8). Last research chapter (Chapter 9) offered practical knowledge to develop new product and processes using milk phospholipids. In future research, besides milk phospholipid neuro-functionality, milk lipid interactions with proteins and phenolic compounds should provide interesting research questions to explore.