In vitro gastrointestinal digestion of oil-in-water emulsions : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology at Massey University, Auckland, New Zealand

Abstract

Oil-in-water (O/W) emulsions are widely used as a dispersion system for oil or fat or as a delivery system for lipophilic bioactive compounds in aqueous food products. There is a growing interest among food scientists in understanding the digestion behaviour of O/W emulsions when they are ingested and pass through the gastrointestinal (GI) tract. In recent years, a number of researches have been carried out to investigate the lipid digestion of emulsions using in-vitro models such as simulated gastric fluid (SGF) and simulated intestinal fluid (SIF) that mimic the biological conditions of human bodies because of the complexity of in vivo study. However, most studies have been conducted to study the effect of gastric or intestinal digestion using SGF or SIF, and the studies on the effect of sequential digestion of emulsions first in SGF and then in SIF have been very limited. The objective of this study was therefore to investigate the effect of in vitro digestion of emulsions sequentially in SGF and SIF on the physicochemical properties and lipolysis of emulsions. In this study, sodium caseinate, whey protein isolate (WPI) and Tween 20 were used as emulsifiers to prepare O/W emulsions (20% soy oil and 1% emulsifier). The mean particle size and particle size distribution, zeta potential and microstructure of freshly prepared emulsions were initially measured, and the changes in the physicochemical properties of emulsions occurring during digestion in SGF or SIF and sequentially in SGF and SIF were analysed. The hydrolysis of fatty acids from emulsified lipid core was also determined during digestion in SIF after gastric digestion. In acidic simulated gastric conditions (pH 1.6 and 3.2 mg/mL pepsin), sodium caseinate-stabilized emulsions showed extensive flocculation with some coalescence, resulting in change in the size and microstructure of the emulsions. In contrast, the emulsions stabilized with WPI or Tween 20 showed no pronounced changes over time during 2 hrs of gastric digestion. In simulated intestinal conditions (pH 7.5, bile salts and pancreatin), a massive coalescence by pancreatic lipase took place in both sodium caseinate and WPI-stabilized emulsions, leading to a pronounced increase and change in the droplet size and microstructures, whereas Tween 20- stabilized emulsions were relatively stable with much less droplet coalescence and size increase. After sequential digestion in SGF and SIF, protein-stabilized O/W emulsions showed more extensive aggregation and coalescence of droplets in comparison with their digestion in SIF only without gastric digestion, whereas Tween 20-stabilized emulsions were relatively stable with only some extent of coalescence after 2 hrs of its sequential digestion in SIF after SGF. The amounts of free fatty acids released in SIF after gastric digestion were similar between three types of emulsions and were not affected significantly by the gastric digestion prior to the intestinal digestion. The overall results indicated that the digestion behaviour of emulsions was affected by types of emulsifiers, and that the sequential digestion of emulsions in SGF and SIF resulted in more pronounced changes in the emulsion particle size and microstructure compared to the digestion in SGF or SIF. However, the rate of lipid digestion was not affected by the sequential digestion. The results of this study provide a significant insight into the effect of sequential gastric and intestinal digestion on the size and properties of emulsion systems and its effect being different depending on type of emulsifiers used to stabilise oil droplets.