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

Abstract

Backgrounds/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.