Dairy Chemistry And Biochemistry

MILK LIPIDS 85

from P-hydroxybutyrate and acetate mainly via two other pathways not

involving malonyl CoA.

In the mammary gland, essentially 100% of C,8:o, C,,:, and c. 50% of C,,

are derived from blood lipids (chylomicrons, free triglycerides, free fatty

acids, cholesteryl esters). The blood lipids are hydrolysed by lipoprotein

lipase which is present in the alveolar blood capillaries, the activity of which

increases eightfold on initiation of lactation. The resulting monoglycerides,

free fatty acids and some glycerol are transported across the basal cell

membrane and re-incorporated into triglycerides inside the mammary cell

(Figure 3.10).

In blood, lipids exist as lipoprotein particles, the main function of which

is to transport lipids to and from various tissues and organs of the body.

There is considerable interest in blood lipoproteins from the viewpoint of

human health, especially obesity and cardiovascular diseases. Lipoproteins

are classified into four groups on the basis of density, which is essentially a

function of their triglyceride content, i.e. chylomicrons, very low density

lipoprotein particles (VLDL), low density lipoprotein (LDL) particles and

high density lipoprotein (HDL) particles, containing c. 98, 90, 77 and 45%

total lipid, respectively (Figure 3.1 1).

Lipoproteins, especially chylomicrons, are at an elevated level in the

blood after eating, especially after high-fat meals, and give blood serum a

milky appearance. They are also elevated during or after tension (so-called

Chylomicron

VIDL

HDL

LDL

0 Proteins Triacylglycerols Cholesterol Phospholipids

Figure 3.11 Composition (%) of human serum lipoproteins; VLDL, very low density lipo-
proteins; LDL, low density lipoproteins; HDL, high density lipoproteins.