Self-Assembly in Food – A New Way to Make Nutritious Products - Food Colloids: Self-Assembly and Material Science - page 23

Bile salts are found at concentrations above the critical micellar concentration
(CMC) in the small intestine during digestion. The pure pancreatic lipase is
inhibited in vitro by the physiological concentrations of bile salts. 40 Colipase
counteracts this effect, facilitating the adsorption of lipase to bile salt-covered
lipid-water interfaces. Lipolysis is a very fast process: it is almost complete on
moving from the duodenum into the jejenum where mucosal lipid uptake takes
place. Phospholipase A2 hydrolyzes the sn-2 position of a variety of phospho-
lipids producing lyso-phospholipids. This enzyme has an absolute requirement
for Ca 21 which binds in a 1:1 stoichiometry to both substrate and enzyme. 41 It
does not act on sphingolipids.
2.4 Liquid Crystalline Phase Formation during
Digestion
Hofmann and Borgstro¨ m 42-46 were the first to show the existence of mixed
micelles in human intestinal lipid digestion products. They separated the lipid
digestion products by ultracentrifugation into three parts: (a) an oil phase
containing triglycerides, diglycerides and minor amounts of fatty acids; (b) an
aqueous mixed micellar phase consisting of bile salts, sn-2 monoglycerides and
dissociated fatty acids; and (c) a precipitated pellet. Patton et al. 47,48 showed by
light microscopy that, during in vitro digestion of emulsion droplets, a lamellar
liquid crystalline phase is formed containing calcium and ionized fatty acids,
followed by the production of a viscous isotropic phase composed of monoglyc-
erides and protonated fatty acids. Similar structural observations were obtained
by Holt et al. 49 and Rigler et al. 50 Some examples are shown in Figure 2. The
latter group identified lamellar and vesicular lipolytic product phases by freeze-
fracture transmission electron microscopy (TEM). Using detergent removal
techniques such as dialysis and dilution in model biles made of phospholipids
or monoglycerides and native bile, the presence of micelle-vesicle transitions was
demonstrated 52-54 by dynamic light-scattering and freeze-fracture TEM. All
these later studies have revealed that the original mixed micelle model of
Hofmann and Borgstro¨ m is an oversimplification: some other phases do play
a role in lipid digestion.
Lindstro¨ m and co-workers 55 studied in more detail the in vitro transforma-
tion of aqueous self-assembled lipid phases into dilute micellar bile salt solu-
tions. They showed that the cubic phase is easily solubilized into bile micelles.
The reversed hexagonal phase is more difficult to solubilize, and the lamellar
phase can be dispersed into liposomes even in the absence of bile salts. Their
X-ray data indicate that a monoolein + oleic acid mixture of molar ratio of 1:2
at pH ¼ 6.5 (the composition and conditions of intestinal contents) forms an
inverse micellar L2-phase upon swelling with water. It can be suggested that the
L2 phase (W/O microemulsion) plays an important role in increasing the
efficiency of lipolysis. As cholesterol and phospholipids constitute only minor
fractions in the digestion of a fatty meal, they assumed 55 that these components
do not significantly influence the observed phase behaviour. Staggers et al. 28