Food Chemistry

162 3 Lipids

3.2.1.2 UnsaturatedFattyAcids..................................

The unsaturated fatty acids, which dominate
lipids, contain one, two or three allyl groups in
their acyl residues (Table 3.7). Acids with iso-
lated double bonds (a methylene group inserted
between the two cis-double bonds) are usually
denoted as isolene-type or nonconjugated fatty
acids.
The structural relationship that exists among the
unsaturated nonconjugated fatty acids derived
from a common biosynthetic pathway is dis-

tinctly revealed when the double bond position is
determined by counting from the methyl end of
the chain (it should be emphasized that position
designation using this method of counting
requires the suffix “ω” or “n”). Acids with the
same methyl ends are then combined into groups.
Thus, three family groups exist:ω3 (linolenic
type),ω6 (linoleic type) andω9 (oleic acid type;
Table 3.7). Using this classification, the common
structural features abundantly found in C 18 fatty
acids (Table 3.2) are also found in less frequently
occurring fatty acids. Thus, erucic acid (20:1)
occurring only in the mustard family of seeds
(Brassicaceae, cf. 14.3.2.2.5), belongs to the
ω9 group, arachidonic acid (20:4), occurring
in meat, liver, lard and lipids of chicken eggs,
belongs to theω6 group, while the C 20 −C 22
fatty acids with 5 and 6 double bonds, occurring
in fish lipids, belong to theω3 group (cf. 13.1.4.5
and 14.3.1.2).
Linoleic acid can not be synthesized by the hu-
man body. This acid and other members of the
ω6 family are considered as essential fatty acids
required as building blocks for biologically active
membranes.α-Linolenic acid, which belongs to
theω3 family and which is synthesized only by
plants, also plays a nutritional role as an essential
fatty acid.
A formal relationship exists in some olefinic un-
saturated fatty acids with regard to the position of
the double bond when counted from the carboxyl
end of the chain. Oleic, palmitoleic and myris-
toleic acids belong to such aΔ9 family (cf. Ta-
ble 3.7); the latter two fatty acids are minor con-
stituents in foods of animal or plant origin.
Unsaturated fatty acids with an unusual struc-
ture are those with one trans-double bond and/or

conjugated double bonds (Table 3.7). They are

formed in low concentrations on biohydrogena-

tion in the stomach of ruminants and are con-

sequently found in meat and milk (cf. 10.1.2.3).

Such trans-unsaturated acids are formed as ar-

tifacts in the industrial processing of oil or fat

(heat treatment, oil hardening). Since trans-fatty

acids are undesirable, their content in German

margarines has been lowered from 8.5% (1994)

to 1.5% (1996) by improving the production pro-

cess. Conjugated linoleic acids (CLA) are of spe-

cial interest because they are attributed to have

an anticarcinogenic effect. In fact, C 18 fatty acids

with two double bonds which differ in position

and geometry belong to the group CLA. The oc-

currence of CLA in foods is shown in Table 3.8.

Up to nine isomers have been identified in lipids

and, apart from exceptions, 18:2 (9c, lltr) predom-

inates (Table 3.8). Conjugated fatty acids with

diene, triene or tetraene systems also occur fre-

quently in several seed oils, but do not play a role

in human nutrition. Table 3.7 presents, as an ex-

ample, two naturally occurring acids with con-

jugated triene systems which differ in the con-

figuration of one double bond at position 9 (cis,

trans).

Unsaturated fatty acids emulsified in water taste

bitter with a relatively low threshold value for

α-linolenic acid (Table 3.9). Thus an off-taste can

be present due to fatty acids liberated, as indi-

cated in Table 3.9, by the enzymatic hydrolysis of

unsaturated triacyl glycerides which are tasteless

in an aqueous emulsion.

Table 3.8.Conjugated linoleic acids in food

Food Total CLAa 18:2(c9,tr11)

(g/kg fat) (% of CLAa)

Milk 2–30 90

Butter 9 .4–11. 991

Cheese 0 .6–7. 1 17–90

Processed cheese 3 .2–8. 9 17–90

Ice cream 3 .8–4. 9 73–76

Sour cream 7. 578

Yoghurt 5 .1–9. 082

Beef, roasted 3 .1–9. 960

Plant oils, marine oils 0.2–0. 545

aCLA, conjugated linoleic acid.