Food Chemistry

180 3 Lipids

ble bond in the cis-configuration. Plasmalogens
of 1-O-(1-alkenyl)-2-O-acylglycerophospholipid
type occur in small amounts in animal muscle
tissue and also in milk fat. The enol-ether linkage,
unlike the ether bonds of a 1-O-alkylglycerol
(cf. 3.6.2), is readily hydrolyzed even by weak
acids.

Plasmalogen (3.32)

Phospholipids are sensitive to autoxidation since
they contain an abundance of linoleic acid. The
other acid commonly present is palmitic acid.
Phospholipids are soluble in chloroform-
methanol and poorly soluble in water-free
acetone. The pKsvalue of the phosphate group is
between 1 and 2. Phosphatidyl choline and phos-
phatidyl ethanolamine are zwitter-ions at pH 7.
Phospholipids can be hydrolyzed stepwise by
alcoholic KOH. Under mild conditions, only
the fatty acids are cleaved, whereas, with strong
alkalies, the base moiety is released. The bonds
between phosphoric acid and glycerol or phos-
phoric acid and inositol are stable to alkalies, but
are readily hydrolyzed by acids. Phosphatidyl
derivatives, together with triacylglycerols and
sterols, occur in the lipid fraction of lipoproteins

(cf. 3.5.1).

Lecithin. Lecithin plays a significant role as
a surface-active agent in the production of
emulsions. “Raw lecithin”, especially that of
soya and that isolated from egg yolk, is available
for use on a commercial scale. “Raw lecithins”
are complex mixtures of lipids with phosphatidyl
cholines, ethanolamines and inositols as main
components (Table 3.19).
The major phospholipids of raw soya lecithin are
given in Table 3.19. The manufacturer often sepa-
rates lecithin into ethanol-soluble and ethanol-
insoluble fractions.
Pure lecithin is a W/O emulsifier with a HLB-
value (cf. 8.15.2.3) of about 3. The HLB-value
rises to 8–11 by hydrolysis of lecithin to
lysophosphatidyl choline; an o/w emulsifier
is formed. Since the commercial lecithins are
complex mixtures of lipids their HBL-values
lie within a wide range. The ethanol-insoluble

Table 3.19.Composition of glycerol phospholipids in

soya “raw lecithin” and in the resulting fractionsa

Unfrac- Ethanol Ethanol

tionated soluble insoluble

fraction fraction

Phosphatidyl

ethanolamine 13–17 16. 313. 3

Phosphatidyl

choline 20–27 49 6. 6

Phosphatidyl

inositol 9 1 15. 2

aValues in weight%.

fraction (Table 3.19) is suitable for stabilization

of W/O emulsions and the ethanol-soluble

fraction for O/W emulsions. To increase the

HLB-value, “hydroxylated lecithins” are pro-

duced by hydroxylation of the unsaturated acyl

groups with hydrogen peroxide in the presence

of lactic-, citric- and tartaric acid.

3.4.1.2 Glyceroglycolipids

These lipids consist of 1,2-diacylglycerols and

a mono-, di- or, less frequently, tri- or tetrasac-

charide bound in position 3 of glycerol. Galactose

is predominant as the sugar component among

plant glycerolipids. The glyceroglycolipids oc-

curring in wheat influence the baking properties

(cf. 15.2.5).

(3.33)

Monogalactosyl diacylglycerol (MGDG)

(1,2-diacyl-3-β-D-galactopyranosyl-L-glycerol)

(3.34)

Digalactosyl diacylglycerol (DGDG)