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1
Chemistry of Fatty Acids
Charlie Scrimgeour
Scottish Crop Research Institute
Dundee, Scotland
1. INTRODUCTION
Fatty acids, esterified to glycerol, are the main constituents of oils and fats. The
industrial exploitation of oils and fats, both for food and oleochemical products, is
based on chemical modification of both the carboxyl and unsaturated groups present
in fatty acids. Although the most reactive sites in fatty acids are the carboxyl group
and double bonds, methylenes adjacent to them are activated, increasing their
reactivity. Only rarely do saturated chains show reactivity. Carboxyl groups and
unsaturated centers usually react independently, but when in close proximity, both
may react through neighboring group participation. In enzymatic reactions, the
reactivity of the carboxyl group can be influenced by the presence of a nearby double
bond.
The industrial chemistry of oils and fats is a mature technology, with decades of
experience and refinement behind current practices. It is not, however, static. Envir-
onmental pressures demand cleaner processes, and there is a market for new pro-
ducts. Current developments are in three areas: ‘‘green’’ chemistry, using cleaner
processes, less energy, and renewable resources; enzyme catalyzed reactions,
used both as environmentally friendly processes and to produce tailor-made
products; and novel chemistry to functionalize the carbon chain, leading to new
Bailey’s Industrial Oil and Fat Products, Sixth Edition, Six Volume Set.
Edited by Fereidoon Shahidi. Copyright # 2005 John Wiley & Sons, Inc.
1
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CHEMISTRY OF FATTY ACIDS
compounds. Changing perceptions of what is nutritionally desirable in fat-based
products also drives changing technology; interesterification is more widely
used and may replace partial hydrogenation in the formulation of some modified
fats.
The coverage in this chapter is necessarily selective, focusing on aspects of fatty
acid and lipid chemistry relevant to the analysis and industrial exploitation of oils
and fats. The emphasis is on fatty acids and acylglycerols found in commodity oils
and the reactions used in the food and oleochemical industries. The practical appli-
cation of this chemistry is dealt with in detail in other chapters. Current areas
of research, either to improve existing processes or to develop new ones, are also
covered, a common theme being the use of chemical and enzyme catalysts. Com-
pounds of second-row transition metals rhodium and ruthenium and the oxides of
rhenium and tungsten have attracted particular interest as catalysts for diverse reac-
tions at double bonds. Recent interest in developing novel compounds by functio-
nalizing the fatty acid chain is also mentioned. To date, few of these developments
have found industrial use, but they suggest where future developments are likely.
A number of recent reviews and books cover and expand on topics discussed
here (1–10).
2. COMPOSITION AND STRUCTURE
2.1. Fatty Acids
Fatty acids are almost entirely straight chain aliphatic carboxylic acids. The broad-
est definition includes all chain lengths, but most natural fatty acids are C 4 to C 22 ,
with C 18 most common. Naturally occurring fatty acids share a common biosynth-
esis. The chain is built from two carbon units, and cis double bonds are inserted by
desaturase enzymes at specific positions relative to the carboxyl group. This results
in even-chain-length fatty acids with a characteristic pattern of methylene inter-
rupted cis double bonds. A large number of fatty acids varying in chain length
and unsaturation result from this pathway.
Systematic names for fatty acids are too cumbersome for general use, and
shorter alternatives are widely used. Two numbers separated by a colon give,
respectively, the chain length and number of double bonds: octadecenoic acid
with 18 carbons and 1 double bond is therefore 18:1. The position of double bonds
is indicated in a number of ways: explicitly, defining the position and configuration;
or locating double bonds relative to the methyl or carboxyl ends of the chain.
Double-bond position relative to the methyl end is shown as n-x or ox, where x
is the number of carbons from the methyl end. The n-system is now preferred,
but both are widely used. The position of the first double bond from the carboxyl
end is designated x. Common names (Table 1) may be historical, often conveying
no structural information, or abbreviations of systematic names. Alternative repre-
3
COMPOSITION AND STRUCTURE
TABLE 1. Fatty Acids in Commodity Oils and Fats. (a) Nomenclature and Structure.
Fatty acid
Common name
Formula
Chain length
4:0
butyric
CH 3 (CH 2 ) 2 CO 2 H
short
6:0
caproic
CH 3 (CH 2 ) 4 CO 2 H
short
8:0
caprylic
CH 3 (CH 2 ) 6 CO 2 H
short/medium
10:0
capric
CH 3 (CH 2 ) 8 CO 2 H
medium
12:0
lauric
CH 3 (CH 2 ) 10 CO 2 H
medium
14:0
myristic
CH 3 (CH 2 ) 12 CO 2 H
medium
16:0
palmitic
CH 3 (CH 2 ) 14 CO 2 H
18:0
stearic
CH 3 (CH 2 ) 16 CO 2 H
CH 3 (CH 2 ) 7 CH
18:1 9c
oleic
CH(CH 2 ) 7 CO 2 H
CH 3 (CH 2 ) 4 (CH
18:2 9c12c
linoleic
CHCH 2 ) 2 (CH 2 ) 6 CO 2 H
CH 3 CH 2 (CH
18:3 9c12c15c
a -linolenic
CHCH 2 ) 3 (CH 2 ) 6 CO 2 H
CH 3 (CH 2 ) 7 CH
22:1 13c
erucic
CH(CH 2 ) 11 CO 2 H
long
EPA
CH 3 CH 2 (CH
20:5 5c 8c11c14c17c
CHCH 2 ) 5 (CH 2 ) 2 CO 2 H
long
DHA
CH 3 CH 2 (CH
22:6 4c7c10c13c16c19c
CHCH 2 ) 6 CH 2 CO 2 H
long
Abbreviations of the systematic names eicosapentaenoic acid and docosahexaenoic acid.
sentations of linoleic acid (1) are 9Z,12Z-octadecadienoic acid; 18:2 9c12c; 18:2
n-6; 18:2 o6; 18:2 9,12; or CH 3 (CH 2 ) 4 CH
CHCH 2 CH
CH(CH 2 ) 7 COOH.
18
COOH
1
12
9
1
The terms cis and trans, abbreviated c and t, are used widely for double-bond
geometry; as with only two substituents, there is no ambiguity that requires the sys-
tematic Z/E convention. An expansive discussion of fatty acid and lipid nomencla-
ture and structure appears in Akoh and Min (1).
TABLE 1. (b) Occurrence.
Fatty Acid
Significant Sources
4:0
butter, dairy fats
6:0
(coconut, palm kernel)
8:0
(coconut, palm kernel)
10:0
(coconut, palm kernel)
12:0
coconut, palm kernel
14:0
coconut, palm kernel
16:0
cottonseed, palm
18:0
cocoa butter, tallow
18:1 9c
cottonseed, olive, palm, rape
18:2 9c12c
corn, sesame, soybean, sunflower
18:3 9c12c15c
linseed
20:1 13c
high erucic rape
20:5 5c8c11c14c17c
fish and animal fats
22:6 4c7c10c13c16c19c
fish and animal fats
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CHEMISTRY OF FATTY ACIDS
Over 1000 fatty acids are known, but 20 or less are encountered in significant
amounts in the oils and fats of commercial importance (Table 1). The most common
acids are C 16 and C 18 . Below this range, they are characterized as short or medium
chain and above it as long-chain acids.
Fatty acids with trans or non-methylene-interrupted unsaturation occur naturally
or are formed during processing; for example, vaccenic acid (18:1 11t) and the con-
jugated linoleic acid (CLA) rumenic acid (18:2 9t11c) are found in dairy fats.
Hydroxy, epoxy, cyclopropane, cyclopropene acetylenic, and methyl branched fatty
acids are known, but only ricinoleic acid (12(R)-hydroxy-9Z-octadecenoic acid) (2)
from castor oil is used for oleochemical production. Oils containing vernolic acid
(12(S),13(R)-epoxy-9Z-octadecenoic acid) (3) have potential for industrial use.
OH
COOH
2
O
H
H
COOH
3
Typical fatty acid composition of the most widely traded commodity oils is
shown in Table 2.
TABLE 2. Fatty Acid Content of the Major Commodity Oils (wt%).
16:0
18:1
18:2
18:3
Other
(wt%)
(wt%)
(wt%) (wt%)
[Fatty Acid (wt%)]
butter
28
14
1
1
4:0 (9); 6:0–12:0 (18); 14:0 (14) þ odd chain
and trans
castor
1
3
4
18:1(OH) (90)
coconut
9
6
2
8:0 (8); 10:0 (7); 12:0 (48); 14:0 (18)
corn
13
31
52
1
cottonseed
24
19
53
fish
14
22
1
16:1 n-7 (12); 20:1 n-9 (12); 22:1 n-11 (11);
20:5 n-3 (7); 22:6 n-3 (7)
groundnut
13
37
41
C 20 –C 24 (7)
(peanut)
lard
27
44
11
1
14:0 (2) 18:0 (11) þ long and odd chain
linseed
6
17
14
60
olive
10
78
7
palm
44
40
10
palm kernel
9
15
2
8:0 (3); 10:0 (4); 12:0 (49); 14:0 (16)
rape
4
56
26
10
sesame
9
38
45
18:0 (6)
soybean
11
22
53
8
sunflower
6
18
69
18:0 (6)
tallow
26
31
2
14:0 (6) 18:0 (31) þ long and odd chain
Typical midrange values shown; the balance are minor components. Data from (9).
Cod liver oil.
Low-erucic-acid rape, e.g., Canola.
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COMPOSITION AND STRUCTURE
Most commodity oils contain fatty acids with chain lengths between C 16 and
C 22 , with C 18 fatty acids dominating in most plant oils. Palm kernel and coconut,
sources of medium-chain fatty acids, are referred to as lauric oils. Animal fats have
a wider range of chain length, and high erucic varieties of rape are rich in this
C 22 monoene acid. Potential new oil crops with unusual unsaturation or additional
functionality are under development. Compilations of the fatty acid composition of
oils and fats (6, 9, 11, 12) and less-common fatty acids (13) are available.
The basic structure, a hydrophobic hydrocarbon chain with a hydrophilic polar
group at one end, endows fatty acids and their derivatives with distinctive proper-
ties, reflected in both their food and industrial use. Saturated fatty acids have a
straight hydrocarbon chain. A trans-double bond is accommodated with little
change in shape, but a cis bond introduces a pronounced bend in the chain (Fig. 1).
In the solid phase, fatty acids and related compounds pack with the hydrocarbon
chains aligned and, usually, the polar groups together. The details of the packing,
such as the unit cell angles and head-to-tail or head-to-head arrangement depend on
the fatty acid structure (Fig. 2).
The melting point increases with chain length and decreases with increased
unsaturation (Table 3). Among saturated acids, odd chain acids are lower melting
than adjacent even chain acids. The presence of cis-double bonds markedly lowers
the melting point, the bent chains packing less well. Trans-acids have melting
points much closer to those of the corresponding saturates. Polymorphism results
in two or more solid phases with different melting points. Methyl esters are lower
melting than fatty acids but follow similar trends.
Fatty acid salts and many polar derivatives of fatty acids are amphiphilic, pos-
sessing both hydrophobic and hydrophilic areas within the one molecule. These are
surface-active compounds that form monolayers at water/air and water/surface
interfaces and micelles in solution. Their surface-active properties are highly
dependent on the nature of the polar head group and, to a lesser extent, on the
length of the alkyl chain. Most oleochemical processes are modifications of the car-
boxyl group to produce specific surfactants.
TABLE 3. Melting Points of Some Fatty Acids and Methyl Esters
Illustrating the Effect of Chain Length and Unsaturation.
Melting Point ( C)
Melting Point ( C)
Fatty acid
Fatty Acid
16:0
62.9 (30.7)
17:0
61.3 (29.7)
18:0
70.1 (37.8)
18:1 9 c
16.3, 13.4
18:1 9 t
45
18:2 9 c 12 c
5
18:2 9 t 12 t
29
19:0
69.4 (38.5)
20:0
76.1 (46.4)
Values for methyl esters in parenthesis.
Data from (8) and (9).
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