2009_Book_FoodChemistry

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 chloroformmethanol
and poorly soluble in water-free
acetone. The pK s value of the phosphate group is
between 1 and 2. Phosphatidyl choline and phosphatidyl
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 phosphoric
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 separates
lecithin into ethanol-soluble and ethanolinsoluble
fractions.
Pure lecithin is a W/O emulsifier with a HLBvalue
(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 fractions a
Unfrac- Ethanol Ethanol
tionated soluble insoluble
fraction fraction
Phosphatidyl
ethanolamine 13–17 16.3 13.3
Phosphatidyl
choline 20–27 49 6.6
Phosphatidyl
inositol 9 1 15.2
a Values 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 produced
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 tetrasaccharide
bound in position 3 of glycerol. Galactose
is predominant as the sugar component among
plant glycerolipids. The glyceroglycolipids occurring
in wheat influence the baking properties
(cf. 15.2.5).
(3.33)
Monogalactosyl diacylglycerol (MGDG)
(1,2-diacyl-3-β-D-galactopyranosyl-L-glycerol)
Digalactosyl diacylglycerol (DGDG)
(3.34)