Dairy Chemistry And Biochemistry

MILK LIPIDS 87

racing driver syndrome). Chylomicrons, which are formed in the intestinal

mucosa, are secreted into the lymph and enter the blood via the thoracic

duct. VLDL lipoproteins are synthesized in intestinal mucosa and liver.

LDL lipoproteins are formed at various sites, including mammary gland, by

removing of triglycerides from VLDL.

Since about 50% of c16:o and 100% of C,,:,, C,,:, and C18:2 are derived

from blood lipids, about 50% of the total fatty acids in ruminant milk fat

originate from the blood via diet or other organs.

In liver mitochondria, palmitic acid, as its CoA ester, is lengthened by

successive additions of acetyl CoA. There is also a liver microsomal enzyme

capable of elongating saturated and unsaturated fatty acids by addition of

acetyl CoA or malonyl CoA.

The principal monoenoic acids, oleic (C18:J and palmitoleic (Cl6:1), are

derived from blood lipids but about 30% of these acids are produced by

microsomal enzymes (in the endoplasmic reticulum) in the secretory cells by

desaturation of stearic and palmitic acids, respectively:

Stearyl CoA + NADPH + 0, - oleoyl CoA + NADP' + 2H,O

Shorter chain unsaturated acids (Clo:l to C14: ,) are probably also produced

by the same enzyme.

Linoleic (&) and linolenic (c1@3) acids cannot be synthesized by

mammals and must be supplied in the diet, i.e. they are essential fatty acids

(linoleic is the only true essential acid). These two polyenoic acids may then

be elongated and/or further desaturated by mechanisms similar to stearic +

oleic, to provide a full range of polyenoic acids. A summary of these

reactions is given in Figure 3.12a, b.

b-Hydroxy acids are produced by &oxidation of fatty acids and p-keto

acids may arise from incomplete syntheses or via P-oxidation.

desaturase

3.6 Structure of milk lipids

Glycerol for milk lipid synthesis is obtained in part from hydrolysed blood
lipids (free glycerol and monoglycerides), partly from glucose and a little
from free blood glycerol. Synthesis of triglycerides within the cell is catalysed
by enzymes located on the endoplasmic reticulum, as shown in Figure 3.13.
Esterification of fatty acids is not random: c,,-c16 are esterified
principally at the sn-2 position while C, and (26 are esterified principally at
the sn-3 position (Table 3.8). The concentrations of C, and C,, appear to
be rate-limiting because of the need to keep the lipid liquid at body
temperature. Some features of the structures are notable:

0 Butanoic and hexanoic acids are esterified almost entirely, and octanoic
and decanoic acids predominantly, at the sn-3 position.