Food Biochemistry and Food Processing (2 edition)

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24 Chemistry and Biochemistry of Milk Constituents 449

Table 24.1.Composition of Fatty Acids (mol% of the

Total) Esterified to Each Position of the

Triacyl-sn-Glycerols in Bovine or Human Milk

Cow Human

Fatty

Acid sn-1 sn-2 sn-3 sn-1 sn-2 sn-3

4:0 – – 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 1.7

attention is being focused on the latter as an acceptable

means of modifying the harness of butter.

2. 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 Fat

The 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 Emulsion

An 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