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Item Studying the relationship between emulsion structure and lipid digestibility for infant milk : a thesis was present in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology, at Massey University, Palmerston North, New Zealand(Massey University, 2020) Deng, LeMilk, whether maternal or formulated, provides the sole source of nutrition to infants in the early stages of life, providing critical micronutrients, support for the immune function and primary dietary macronutrients including lipids. In healthy adults, lipids are primarily digested in the small intestine. However, for infants, the neonatal small intestine is not fully developed after birth, so the gastric environment plays a more significant role in milk fat digestion. Clinical studies have shown that maternal milk fat is digested more efficiently than lipids in infant formulae in infants under infant gastric conditions. Compositional differences, the structure of the oil droplets, and especially the interfacial composition may all play a crucial role in influencing lipid digestibility in the infant's stomach. In this thesis, the simulated gastric digestion of model emulsions and commercial infant formula was studied. The model emulsions comprised either a phospholipid or complexed protein-phospholipid interface while keeping all other facets of emulsion properties equivalent. Gastric digestion of these emulsions was carried out across variable pH conditions using an analogue gastric lipase, alone and in combination with pepsin with findings providing insights into the role of each enzyme and their combined effect on gastric lipolysis. The rate and extent of lipolysis were characterised, along with morphological changes to the structure of the oil droplets. Results showed that gastric lipolysis might be influenced by pH conditions in the gastric environment when lipase was present alone in the simulated gastric fluid. The inclusion of pepsin resulted in significant structural changes when emulsions were stabilised with protein, in terms of droplet aggregation, size and morphology. However, no significant differences in the extent of lipolysis were determined. Thus, while the protein interface of both model and formulated emulsions was not observed to be a barrier for gastric lipolysis. Proteolysis of protein stabilised emulsions may lead to very different structural outcomes during gastric digestion when compared to phospholipid stabilised emulsions. While the research within this thesis demonstrates how the gastric environment influences emulsion structure as a consequence of interfacial composition, any specific relationship between structure and relative rate of gastric lipolysis currently remains undetermined. This research also highlights some of the ongoing challenges in the use of in vitro models to provide mechanistic understanding and interpretation of findings from clinical studies.Item Interfacial aspects of in vitro lipolysis using tensiometry and SAXS : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Manawatu, New Zealand(Massey University, 2019) Wang, XuerunBackgrounds/Aims: Lipolysis is an interfacial process in the conversion of dietary triglycerides to free fatty acids, and is affected by the interfacial compositions of the oil droplet. The aim of the study was to analyze any effects of co-dependency of lipase and protease enzymes on the hydrolysis of oil droplets stabilized by interfacial protein, and to determine the transitions in the internal structure of oil droplets during in vitro lipolytic digestion. Methods: The changes of interfacial tension and dilatational rheology at the oil interface during gastric digestion was measured by pendent drop tensiometer, in which a droplet of olive oil was expressed in a 0.1% WPI solution at various pH levels (pH 3.5 and pH 4.5). Additionally, small angle x-ray spectrometry (SAXS) was used for identifying the differences of the self-assembled structure formed during gastrointestinal lipolysis of three different types of oils (coconut oil, olive oil and palm stearin). Results: The interfacial tension of oil droplets at pH 3.5 was found to be higher than at pH 4.5 which indicated the interface was destabilized by acids. The emulsifier acted as a barrier at the oil interface and protected it from lipolysis by gastric lipase. The extent of pepsinolysis was enhanced with low pH because of the protein degradation by acids. At pH 3.5, the interfacial tension of WPI coated oil droplet was raised 35% whereas only 19% increased at pH 4.5. In the presence of both pepsin and gastric lipase, the protein layer was first weaken by pepsin and then lipase acted at the oil interface. From the SAXS data, there was no liquid crystals formed at the oil droplet during gastric digestion. The results showed the transitions of internal structure of oil droplets after intestinal digestion depended on the pre-digestion under gastric condition and the source of oil phase. After gastric digestion, lamellar phase was the dominate structure whereas various sub phases were formed after the intestinal digestion without the pretreatment, including bicontinuous cubic and hexagonal phase. Conclusion: In vitro, the stability of oil droplet interface was affected by pH, emulsifier and interactions between enzymes. The study has proved gastric lipolysis and pepsionolysis were co-dependent at the oil interface. Gastric lipolysis was important for the following intestinal digestion as the self-assembled structure was weaken by gastric condition.