Food Structure for Nutrition - Food Colloids: Self-Assembly and Material Science - page 10

casein subunits. Thus, from this criterion of structure, casein would appear to
be unusually susceptible to rapid hydrolysis by intestinal proteases. In vitro
studies of protein digestibility have confirmed 41 that caseins are highly suscep-
tible to proteolysis. This would logically lead to rapid digestion and absorption
of the amino acids of casein proteins. Nonetheless, the casein proteins evolved
with a quite remarkably distinct additional property. Assembled as multi-
subunit complexes termed casein micelles, the caseins are actually large protein
aggregates that, while soluble, are dispersible only in water, thanks to an
exterior of predominantly hydrophilic k-casein subunits. When the casein
micelle encounters the mammalian infant stomach, one of the most ingenious
events in nutrition occurs - the enzyme chymosin, expressed in the stomach of
the infant, and with an acidic activity range, hydrolyses a single peptide bond
on k-casein. This reaction splits off the hydrophilic peptide of k-casein (glyco-
macropeptide), which contains the surface-stabilizing element of the entire
casein micelle. As a result, the balance of casein proteins within the stomach
rapidly aggregates into a large, insoluble curd. This association process has the
net effect of slowing the release, digestion, and absorption of casein's amino
acids. Thus, mammalian casein has, in fact, a unique structure and structuring
process that, at least in part, slows the delivery of milk protein.
1.6.3 Milk and Lipid Bioavailability
The digestion and absorption of lipids has been well studied from a wide variety
of sources, including milk. There is a considerable challenge to the process
considering the inherent insolubility of most lipids; thus, the conspicuous
success of fat digestion by animals has been studied extensively. Although
normally quite successful, several genetic and physiological defects in the
intestine can lead to failures in fat absorption, termed steatorrhea. The di-
culties that are presented by absorbing fat are the likely reason that fat is the
most common form of intestinal malabsorption. Invariably, this clinical con-
dition is highly deleterious, because of both the failure to absorb essential fatty
acids and fat-soluble vitamins, and the loss of the substantial caloric density of
dietary lipids. Therefore, there is considerable medical importance to ensuring
that fat is in a highly absorbable form in milk.
Given the basic requirement to produce a fat globule for export into milk,
there has existed in place, within the ancestors of mammals, some genetic
motifs that were ostensibly 'available' to evolution of a fat-globule delivery
system. The synthesis of lipoproteins was a well-established biological process
in early animal evolution, with highly effective transport of intact fat globules
being critical to the physiology of reptiles, amphibia, and fish. 42-44 These
phospholipid-bound and apoprotein-targeted lipoproteins deliver triglycerides
from the liver through the blood to the surface of various peripheral tissues in
these organisms, and lipoprotein lipase activity on the endothelial surfaces is
capable of very rapidly transferring the lipid contents to the targeted tissues. 45
Therefore, producing lipoproteins as fat globules in milk was an available
alternative.