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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.

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