2009_Book_FoodChemistry

162 3 Lipids
3.2.1.2 Unsaturated Fatty Acids
The unsaturated fatty acids, which dominate
lipids, contain one, two or three allyl groups in
their acyl residues (Table 3.7). Acids with isolated
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 distinctly
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) and ω9 (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 the ω6 group, while the C 20 −C 22
fatty acids with 5 and 6 double bonds, occurring
in fish lipids, belong to the ω3 group (cf. 13.1.4.5
and 14.3.1.2).
Linoleic acid can not be synthesized by the human
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
the ω3 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 unsaturated
fatty acids with regard to the position of
the double bond when counted from the carboxyl
end of the chain. Oleic, palmitoleic and myristoleic
acids belong to such a Δ9 family (cf. Table
3.7); the latter two fatty acids are minor constituents
in foods of animal or plant origin.
Unsaturated fatty acids with an unusual structure
are those with one trans-double bond and/or
conjugated double bonds (Table 3.7). They are
formed in low concentrations on biohydrogenation
in the stomach of ruminants and are consequently
found in meat and milk (cf. 10.1.2.3).
Such trans-unsaturated acids are formed as artifacts
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 process.
Conjugated linoleic acids (CLA) are of special
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 occurrence
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) predominates
(Table 3.8). Conjugated fatty acids with
diene, triene or tetraene systems also occur frequently
in several seed oils, but do not play a role
in human nutrition. Table 3.7 presents, as an example,
two naturally occurring acids with conjugated
triene systems which differ in the configuration
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 indicated
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 CLA a 18:2(c9,tr11)
(g/kg fat) (% of CLA a )
Milk 2–30 90
Butter 9.4–11.9 91
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 7.5 78
Yoghurt 5.1–9.0 82
Beef, roasted 3.1–9.9 60
Plant oils, marine oils 0.2–0.5 45
a CLA, conjugated linoleic acid.