2009_Book_FoodChemistry

164 3 Lipids
Table 3.9. Taste of unsaturated fatty acids emulsified in
water
Compound Threshold Quality
(mmol/1)
Oleic acid 9–12 bitter, burning,
pungent
Elaidic acid 22 slightly burning
Linoleic acid 4–6 bitter, burning,
pungent
Linolelaidic acid 11–15 bitter, burning,
scratchy
γ-Linolenic
acid 3–6 bitter, burning,
pungent
α-Linolenic acid 0.6–1.2 bitter, burning,
pungent, like fresh
walnut
Arachidonic acid 6–8 bitter, repugnant
off-taste
acids, with an even number of carbon atoms, in
which the carbonyl group is located on C-5 to C-
13: One of 47 identified compounds of this substance
class has the following structure:
(3.2)
Furan Fatty Acids. These occur in fish liver oil in
a range of 1–6% and up to 25% in some freshwater
fish. Furan fatty acids are also part of the
minor constituents of some plant oils and butter
(Table 3.10). They are also present in fruits
(lemon, strawberry), vegetables (cabbage, potato)
and mushrooms (champignons).
Two of these acids have the following formulas
3.2.1.3 Substituted Fatty Acids
Hydroxy Fatty Acids. Ricinoleic acid is the best
known of the straight-chain hydroxy fatty acids.
Its structure is 12-OH, 18:1 (9). It is an optically
active acid with a D(+)-configuration:
(3.1)
Ricinoleic acid is the main acid of castor bean oil,
comprising up to 90% of the total acids. Hence, it
can serve as an indicator for the presence of this
oil in edible oil blends.
D-2-Hydroxy saturated 16:0 to 25:0 fatty
acids with both even and odd numbers of
carbons in their chains occur in lipids in
green leaves of a great number of vegetables.
γ- or ∂ -Lactones are obtained from
4- and 5-hydroxycarboxylic acids (C 8 to C 16 )
by the elimination of water. ∂ -Lactones
have been found in milk fat and fruits.
They are very active aroma components
(cf. 5.3.2.3).
Oxo Fatty Acids. Natural oxo (or keto) acids are
less common than hydroxy acids. About 1% of
milk fat consists of saturated (C 10 –C 24 ) oxo fatty
(3.3)
Photooxidation (cf. 3.7.2.1.4) of these acids can
deteriorate especially the quality of soybean oil.
Substituted fatty acids are also derived by autoxidation
or enzymatic peroxidation of unsaturated
fatty acids, which will be dealt with in more detail
in 3.7.2.3 and 3.7.2.4.1.
Table 3.10. Examples for the occurrence of furan fatty
acids I and II
Oil
Concentration (mg/kg)
I a II a
Soya oil 120–170 130–230
Wheat germ oil 100–130 105–150
Rapeseed oil 6–16 7–20
Corn oil 8–11 9–13
Butter 13–139 24–208
Leaves of the tea shrub b 50 713
Green tea b 4 80–100
Black tea b 10 159
Spinach b 86 733
a I: 10,13-epoxy-11,12-dimethyloctadeca-10,12-
dienoic acid.
II: 12,15-epoxy-13,14-dimethyleicosa-12,14-dienoic
acid (Formula 3.3).
b Values based on dry weight.