434 Part IV: Milk
- Pancreatic and many other lipases are specific
for the fatty acids at the sn-1 and sn-3 posi-
tions. Therefore, C4:0 to C8:0 are released rapid-
ly from milk fat; these are water soluble and
are readily absorbed from the intestine.
Medium- and long-chain acids are absorbed
more effectively as 2-monoglycerides than as
fatty acids; this appears to be quite important
for the digestion of lipids by human infants,
who have a limited ability to digest lipids due
to the absence of bile salts. Infants metabolize
human milk fat more efficiently than bovine
milk fat, apparently due to the very high pro-
portion of C16:0esterified at sn-2 in the former.
The effect of transesterification on the
digestibility of milk fat by infants merits inves-
tigation.
Short-chain fatty acids (C4:0–C10:0) have a
strong aroma and flavor, and their release by
indigenous lipoprotein lipase and microbial
lipases cause off-flavors in milk and many
dairy products; this is referred to as hydrolytic
rancidity.
RHEOLOGICALPROPERTIES OFMILKFAT
The melting characteristics of ruminant milk fat are
such that at low temperatures (e.g., ex-refrigerator),
it contains a high proportion of solid fat and has
poor spreadability. The rheological properties of
milk lipids may be modified by fractional crystal-
lization; for example, one effective treatment in-
volves removing the middle melting point fraction
and blending high and low melting point fractions.
Fractional crystallization is expensive and is prac-
ticed in industry to only a limited extent; in particu-
lar, securing profitable outlets for the middle melt-
ing point fraction is a major economic problem.
Alternatively, the rheological properties of milk
fat may be modified by increasing the level of
PUFAs, by feeding cows with protected PUFA-rich
lipids, but this practice is also expensive. The melt-
ing characteristics of blends of milk fat and veg-
etable oils can be easily varied by changing the pro-
portions of the different fats and oils in the blend.
This procedure is economical and is widely prac-
ticed commercially; blending also increases the lev-
el of nutritionally desirable PUFAs. The rheological
properties of milk fat–based spreads can also be
improved by increasing the moisture content of the
product; obviously, this is economical and nutrition-
ally desirable in the sense that the caloric value is
reduced, but the resultant product is less microbio-
logically stable than butter.
The melting characteristics and rheological prop-
erties of milk fat can also be modified by inter-
and transesterification. Chemically catalyzed inter-
and transesterification are not permitted in the food
industry, but enzymatic catalysis may be acceptable.
Lipases capable of such modifications on a commer-
cial scale are available,but their use is rather limited.
Enzymatic transesterification allows modification
of the nutritional as well as the rheological proper-
ties of lipids. The nutritional and rheological pro-
perties of lipids can also be modified by the use of
a desaturase, which converts C18:0to C18:1(these
enzymes are a subject of ongoing research; see
hppt://bioinfo.pbi.nrc.ca/covello/r-fattyacid.html,
Meesapyodsuk et al. 2000). However, this type of
enzyme does not seem to be available commercially
yet.MILKFAT A S A NEMULSIONAn emulsion consists of two immiscible, mutually
insoluble liquids, usually referred to as oil and water,
in which one of the liquids is dispersed as small
droplets (globules, the dispersed phase) in the other
(the continuous phase). If the oil is the dispersed
phase, the emulsion is referred to as an oil-in-water
(o/w) emulsion; if water is the dispersed phase, the
emulsion is referred to as a water-in-oil (w/o) emul-
sion. The dispersed phase is usually, but not neces-
sarily, the phase present in the smaller amount. An
emulsion is prepared by dispersing one phase into
the other. Since the liquids are immiscible, they will
remain discrete and separate if they differ in density,
as is the case with lipids and water, the densities of
which are 0.9 and 1.0, respectively; the lipid glob-
ules will float to the surface and coalesce. Coales-
cence is prevented by adding a compound that
reduces the interfacial tension,, between the phases.
Compounds capable of doing this have an amph-
iphatic structure, that is, they have both hydrophilic
and hydrophobic regions (e.g., phospholipids, mon-
oglycerides, diglycerides, proteins, soaps, and num-
erous synthetic compounds), and are known as
emulsifiers or detergents. The emulsifier forms a
layer on the surface of the globules, with its hy-
drophobic region penetrating the oil phase and its
hydrophilic region in the aqueous phase. An emul-
sion thus stabilized will cream if left undisturbed,