Dairy Chemistry And Biochemistry

MILK LIPIDS 81

varies; some can form lactones, e.g. the 4- and 5-hydroxy acids can form

y- and 8-lactones, respectively.

C ‘2

I

R

A 6-lactone

Lactones have strong flavours; traces of S-lactones are found in fresh milk
and contribute to the flavour of milk fat, but higher concentrations may
occur in dried milk or butter oil as a result of heating or prolonged storage
and may cause atypical flavours.
The fatty acids in the various polar lipids and cholesteryl esters are
long-chain, saturated or unsaturated acids, with little or no acids of less than
C,,:, (Table 3.7; for further details see Christie, 1995).

3.5 Synthesis of fatty acids in milk fat

In non-ruminants, blood glucose is the principal precursor of fatty acids in
milk fat; the glucose is converted to acetyl CoA in the mammary gland. In
ruminants, acetate and P-hydroxybutyrate, produced by micro-organisms in
the rumen and transported to the blood, are the principal precursors; in fact,
ruminant mammary tissue has little ‘ATP citrate lyase’ activity which is
required for fat synthesis from glucose. Blood glucose is low in ruminants
and is conserved for lactose synthesis. The differences in fatty acid precur-
sors are reflected in marked interspecies differences in milk fatty acid
profiles. Restriction of roughage in the diet of ruminants leads to sup-
pression of milk fat synthesis, possibly through a reduction in the available
concentration of acetate and P-hydroxybutyrate.
In all species, the principal precursor for fatty acid synthesis is acetyl
CoA, derived in non-ruminants from glucose and in ruminants from acetate
or oxidation of 8-hydroxybutyrate. Acetyl CoA is first converted, in the
cytoplasm, to malonyl CoA: