Self-Assembly in Food – A New Way to Make Nutritious Products - Food Colloids: Self-Assembly and Material Science - page 22

triglycerides. 26,27 Fats supply more than 40% of the calories ingested in the
Western diet. The majority of triglycerides contain saturated and unsaturated
long-chain fatty acids (length > C 14 ). The two unsaturated long-chain fatty
acids, oleic and linoleic, account for 34% and 19%, respectively, of all the
edible oils and fats produced industrially. The process of fat digestion and
absorption is very efficient (95%) and there is no feedback regulation to reduce
fat assimilation. In order to become absorbable, lipids undergo a number of
biochemical and colloidal transformations during fat digestion. For a detailed
description of this topic, the interested reader is referred to various literature
reviews. 26-30 In summary, lipid digestion is a highly complex sequential process
involving:
(i) enzymatic hydrolysis of triglycerides into partially ionized fatty acids
and monoglycerides (ratio 2:1) catalysed by lipolytic enzymes;
(ii) emulsification of bulk fat;
(iii) micellar solubilization due to association of monoglycerides and fatty
acids with themselves and with endogenous, biliary colloidal compo-
nents such as bile salts, phosphatidylcholine and cholesterol; and
(iv) transport of digestion products through an unstirred, mucine-rich water
layer into the enterocytes.
2.3 Lipases: Transformation of Lipids into Amphiphiles
The digestion of triglycerides in humans is done by gastric and pancreatic
lipases, hydrolysing the sn-1 and sn-3 ester bonds of triglycerides, and produc-
ing sn-2 monoglycerides. Enzymatic hydrolysis of lipids can only be achieved at
the interface between lipids and the surrounding aqueous medium. Lipid
hydrolysis begins in the stomach with the action of gastric lipase, a product
of the chief cells of the gastric mucosa. Around 30% of the total triglycerides
may be digested by gastric lipases in the stomach. 31 Only the short-chain fatty
acids can be absorbed into the bloodstream.
As it passes by peristaltic movements through the pylorus, the stomach digest
undergoes shear forces sufficient for emulsification of bulk fats. The potential
emulsifying agents are monoglycerides, phospholipids, polysaccharides and
peptidic fragments originating from protein digestion. The presence of these
surfactants can lead to a rather low interfacial tension and 'spontaneous
emulsification'. Whereas pure bile salts are poor emulsifiers of fat, when present
together with biliary lipids they facilitate fat emulsification. 32-34 The digestion
of dietary fat emulsions in humans and rats has been studied by Armand et al. 35
They found that dietary lipids are emulsified in the duodenum as droplets of
size 1-50 mm in humans 36 and 14-33 mm in rats. 37 These values are much larger
than those that were reported 50 years earlier by Frazer et al. 38
Pancreatic juice contains pancreatic lipase, phospholipase A2, colipase and
carboxylic ester hydrolase (which needs bile salts for its activity). Enzymatic
hydrolysis of lipids with pancreatic lipases can only occur at the oil-water
interface in the presence of a ternary lipase-colipase-bile salt micelle complex. 39