Food Chemistry

186 3 Lipids

Fig. 3.14.Arrangement of polar acyl lipids in aqueous
medium.∞Polar lipid tails;≈hydrophobic lipid tails

Fig. 3.15.Fluid mosaic model of a biological mem-
brane. The protein (P) is not fixed but is mobile in the
phospholipid phase

The favored structure for most phospho- and
glycolipids in water is a bimolecular arrange-
ment, rather than a micelle. Two model systems
can exist for such bimolecular arrangements.
The first is a lipid vesicle, known as a liposome,
the core of which is an aqueous compartment
surrounded by a lipid bilayer, and the second is
a planar, bilayer membrane. The latter, together
with the micellar model, is presented in Fig. 3.14.
Globular proteins, often including enzymes,
are found in animal cell membranes and are
well embedded or inserted into the bimolecular
layer. Some of these so-called integral membrane
proteins protrude through both sides of the mem-
brane (fluid mosaic model, Fig. 3.15). Although
integral proteins interact extensively with the
hydrophobic acyl tails of membrane lipids they
are mobile within the lipid membrane.

3.6 Diol Lipids, Higher Alcohols, Waxes and Cutin

3.6.1 DiolLipids.............................................

The diol lipids which occur in both plant and
animal tissues are minor lipid constituents. The
diol content is about 1% of the content of gly-
cerol. Exceptions are sea stars, sea urchins and

mollusks, the lipids of which in summer contain

25–40% diol lipids. This proportion decreases

sharply in winter and spring. Neutral and polar

lipids derived from ethylene glycol, propane-(1,2

and 1,3)-diol and butane-(1,3; 1,4- and 2,3)-diol

have been identified in the diol lipid fraction. Sev-

eral of those isolated from corn oil have the fol-

lowing structures:

(3.44)

(3.45)

In a glycodiol lipid one hydroxyl group of

ethylenediol is esterified with a fatty acid.

Diol lipids with structures analogous to phos-

phatidyl choline or plasmalogen have also been

identified.

3.6.2 Higher Alcohols and Derivatives

3.6.2.1 Waxes.................................................

Higher alcohols occur either free or bound in

plant and animal tissues. Free higher alcohols are

abundant in fish oil and include:

Cetyl alcohol C 16 H 33 OH

Stearyl alcohol C 18 H 37 OH

Oleyl alcohol C 18 H 35 OH

Waxes are important derivatives of higher alco-

hols. They are higher alcohols esterified with

long-chain fatty acids. Plant waxes are usually

found on leaves or seeds. Thus, cabbage leaf

wax consists of the primary alcohols C 12 and

C 18 –C 28 esterified with palmitic acid and other

acids. The dominant components are stearyl

and ceryl alcohol (C 26 H 53 OH). In addition to

primary alcohols, esters of secondary alcohols,

e. g., esters of nonacosane-15-ol, are present:

(3.46)