BLBS102-c24 BLBS102-Simpson March 21, 2012 13:47 Trim: 276mm X 219mm Printer Name: Yet to Come
24 Chemistry and Biochemistry of Milk Constituents 449Table 24.1.Composition of Fatty Acids (mol% of the
Total) Esterified to Each Position of the
Triacyl-sn-Glycerols in Bovine or Human MilkCow Human
Fatty
Acid sn-1 sn-2 sn-3 sn-1 sn-2 sn-34:0 – – 35.4 – – –
6:0 – 0.9 12.9 – – –
8:0 1.4 0.7 3.6 – – –
10:0 1.9 3.0 6.2 0.2 0.2 1.1
12:0 4.9 6.2 0.6 1.3 2.1 5.6
14:0 9.7 17.5 6.4 3.2 7.3 6.9
16:0 34.0 32.3 5.4 16.1 58.2 5.5
16:1 2.8 3.6 1.4 3.6 4.7 7.6
18:0 10.3 9.5 1.2 15.0 3.3 1.8
18:1 30.0 18.9 23.1 46.1 12.7 50.4
18:2 1.7 3.5 2.3 11.0 7.3 15.0
18:3 – – – 0.4 0.6 1.7attention is being focused on the latter as an acceptable
means of modifying the harness of butter.- Pancreatic and many other lipases are specific for the fatty
acids at thesn-1 andsn-3 positions. Therefore, C4:0–C8:0
are released rapidly 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 limited ability to digest lipids due to the absence of
bile salts. Infants metabolise human milk fat more effi-
ciently than bovine milk fat, apparently due to the very
high proportion of C16:0esterified atsn-2 in the former.
The effect of trans-esterification on the digestibility of
milk fat by infants merits investigation.
Short-chain fatty acids (C4:0–C10:0) have a strong aroma and
flavour and their release by indigenous lipoprotein lipase (LPL)
and microbial lipases cause off-flavours in milk and many dairy
products, referred to as hydrolytic rancidity.Rheological Properties of Milk FatThe melting characteristics of ruminant milk fat are such that, at
low temperatures (e.g.,ex-refrigerator), it contains a high pro-
portion of solid fat and has poor spreadability. The rheological
properties of milk lipids may be modified by fractional crystalli-
sation, for example an effective treatment involves removing the
middle MP fraction and blending high- and low-MP fractions.
Fractional crystallisation is expensive and is practised in industry
to only a limited extent; in particular, securing profitable outlets
for the middle-MP fraction is a major economic problem.
Alternatively, the rheological properties of milk fat may be
modified by increasing the level of PUFAs through feeding cows
with protected PUFA-rich lipids, but this practise is also expen-
sive. The melting characteristics of blends of milk fat and veg-etable oils can be easily varied by changing the proportions of
the different fats and oils in the blend. This procedure is eco-
nomical and is widely practised commercially; blending also
increases the level of nutritionally desirable PUFAs. The rheo-
logical properties of milk fat-based spreads can also be improved
by increasing the moisture content of the product; obviously,
this is economical and nutritionally desirable in the sense that
the caloric value is reduced, but the resultant product is less
microbiologically stable than butter.
The melting characteristics and rheological properties of
milk fat can also be modified by inter- and trans-esterification.
Chemically-catalysed inter- and trans-esterification are not per-
mitted in the food industry but enzymatic catalysis may be ac-
ceptable. Lipases capable of such modifications on a commer-
cial scale are available but their use is rather limited. Enzymatic
trans-esterification allows modification of the nutritional as well
as the rheological properties of lipids. The nutritional and rheo-
logical properties of lipids can also be modified by the use of a
desaturase that converts C18:0to C18:1(these enzymes are a sub-
ject of ongoing research, see hppt://bioinfo.pbi.nrc.ca/covello/r-
fattyacid.html; and Meesapyodsuk et al. 2000). However, this
type of enzyme does not seem to be available commercially yet.Milk Fat as an EmulsionAn emulsion consists of two immiscible, mutually insoluble liq-
uids, 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 dis-
persed 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) emulsion. The dispersed phase is usu-
ally, but not necessarily, 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 density of which are 0.9 and 1.0, respectively; the
lipid globules will float to the surface and coalesce. Coales-
cence is prevented by adding a compound which reduces the
interfacial tension,γ, between the phases. Compounds capable
of doing this have an amphipathic structure, that is hydropho-
bic and hydrophilic regions, for example phospholipids, mono-
glycerides, diglycerides, proteins, soaps and numerous synthetic
compounds, and are known as emulsifiers or detergents. The
emulsifier forms a layer on the surface of the globules with its
hydrophobic region penetrating the oil phase and its hydrophilic
region in the aqueous phase. An emulsion thus stabilised will
cream if left undisturbed, but the globules remain discrete and
can be redispersed readily by gentle agitation.
In milk, the lipids exist as an O/W emulsion in which the glob-
ules range in size from approximately 0.1 to 20μm, with a mean
of 3–4μm. The mean size of the fat globules is higher in high-
fat milk than in low-fat milk, for example Jersey compared to
Friesian, and decreases with advancing lactation. Consequently,
the separation of fat from milk is less efficient in winter than
in summer, especially when milk production is seasonal, and it