2009_Book_FoodChemistry

186 3 Lipids
Fig. 3.14. Arrangement of polar acyl lipids in aqueous
medium. ∞ Polar lipid tails; ≈ hydrophobic lipid tails
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. Several
of those isolated from corn oil have the following
structures:
(3.44)
Fig. 3.15. Fluid mosaic model of a biological membrane.
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 arrangement,
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 membrane
(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 Diol Lipids
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 glycerol.
Exceptions are sea stars, sea urchins and
(3.45)
In a glycodiol lipid one hydroxyl group of
ethylenediol is esterified with a fatty acid.
Diol lipids with structures analogous to phosphatidyl
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
Stearyl alcohol
Oleyl alcohol
C 16 H 33 OH
C 18 H 37 OH
C 18 H 35 OH
Waxes are important derivatives of higher alcohols.
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)