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
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Date
2012
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Massey University
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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.
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Keywords
Oil-in-water emulsions, Emulsions, Digestion, Gastrointestinal tract