Food oils and fats : technology, utilization, and nutrition

305 Pages·1995·27.806 MB·English

by  Harry W Lawson

Chapter 2
The Basic Chemistry of Oils
and Fats
The carbon atom is the basic element in food chemistry, induding
oils and fats. Carbon atoms, with a valence of 4, may bond together
with other carbon atoms to form molecules with long chains. Fur
thermore, carbon's ability to form bonds or react with other ele
ments such as hydrogen, oxygen, iodine, nitrogen, and phosphorus
is fundamental to understanding the chemistry of oils and fats.
Basically, oils and fats are mixtures of triglycerides. This is the
manner in which they are composed naturally.
H
I
H-C-OH
I
H-C-OH
I
H-C-OH
I
H
Glycerol
The glycerol molecule has three carbon atoms, together with five
hydrogen atoms and three OH or hydroxyl groups. It should be
H. Lawson, Food Oils and Fats 3
© Springer Science+Business Media Dordrecht 1995
4 The Basic Chemistry of Oils and Fats
noted that there are four bonds or linkages to each of the three car
bon atoms. When three fatty acids are combined with one glycerol
molecule, we have a triglyceride.
H H 0
I I II
H - C - Fatty acid H-C-O-C-R]
I I
H - C - Fatty acid or I 0
I I II
H - C - Fatty acid H-C-O-C-R2
I I
H I 0
I II
H-C-O-C-R3
I
H
Triglyceride
"R" is the method of abbreviating portions of long-chain radicals
such as those of fatty acids. When all of the fatty acids in a triglyc
eride are identical, it is called a simple triglyceride. However, the
much more common forms are the mixed triglycerides in which two
or three different fatty acids are present in the molecules.
H 0 H 0
I II I II
H-C-O-C-R] H-C-O-C-R]
o o
II II
H-C-O-C-R] H-C-O-C-R2
I
I 0 o
I II II
H-C-O-C-R] H-C-O-C-R3
I I
H H
Simple triglyceride Mixed Triglyceride
If, in the above illustrations, we designate R] to be stearic acid,
R2 palmitic acid, and R3 oleic acid; then the triglyceride on the left
The Basic Chemistry of Oils and Fats 5
would be simply tristearin. The mixed triglyceride on the right would
be stearopalmitolein.
If only two fatty acids are attached to a specific glycerol molecule,
we have a diglyceride; if only one fatty acid is attached, the mole
cule is a monoglyceride. Each carbon atom continues to have four
linkages.
H H
I I
H-C-OH H-C -OH
I
o
H-C -OH
II
o
H-C- 0 -C-R
I II
I 0 H-C - O-C-R
I II I
H-C- 0 -C-R H
Diglyceride Monoglyceride
Diglycerides can be further described as a 1,2-diglyceride or a 1,3-
diglyceride, depending on the position of fatty acids on the glycerol
molecule.
H H
I
0 I 0
II I II
H-C- 0 -C-R H-C- 0 -C-R
I
0 H-C-OH
II I
H-C- 0 -C-R I 0
I I II
H-C-OH H-C- 0 -C-R
I I
H H
1,2-Diglyceride 1,3-Diglyceride
Monoglycerides may be either 1 (or alpha)-monoglyceride or 2 (or
beta )-monoglyceride.
6 The Basic Chemistry of Oils and Fats
H H
I I
o
I H-C-OH
I II
o o
H-C- -C-R
I II
H-C-OH H-C- 0 C-R
I
I
H-C-OH H-C-OH
I I
H H
1 (or alpha)-monoglyceride 2 (or beta)-monoglyceride
Mono and diglycerides are important as emulsifiers in food prod
ucts. Their preparation and use will be discussed more fully later in
this book. Monoglycerides and diglycerides are also formed in the
intestinal tract as the result of the normal digestion of triglycerides.
In addition, they occur naturally in minor amounts in both vege
table oils and animal fats.
Any fatty acid not linked to a glycerol or some other molecule in
an oil or fat is referred to as a "free fatty acid." The major com
ponent of all fats and oils is triglycerides, representing well over
95% of the weight of most food fats in the form in which they are
consumed (1).
Most umefined oils contain relatively high levels of free fatty acids.
A typical level for crude soybean oil is from 0.5 to 1.5%. Crude palm
oil may contain 3.0-5.0% free fatty acids. Refined oils and fats that
are ready for use in foods usually have a free fatty acid level of less
than 0.05%.
Some of the more common fatty acids found in naturally occur
ring oils and fats are butyric, lauric, palmitic, stearic, oleic, and lin
oleic. A l-lb can of shortening, for example, contains innumerable
fat molecules consisting of mixtures of the various fatty acids at
tached to the glycerol molecules. The relative number of these var
ious fatty acids and their particular placement on the glycerol mol
ecules determine the various characteristics of the oil or fat products.
The processing techniques employed also affect a product's physical
and performance characteristics. All oils and fats are built from a
relatively small number of fatty acids.
Some fats are solid at room temperature, whereas others are liq
uid. Those that are liquid at room temperature are referred to as
oils/liquid oils/fluid fats/liquid shortenings. It should be kept in
The Basic Chemistry of Oils and Fats 7
mind that fluid fats/liquid shortenings are included in this grouping
even though they contain a small amount of solids at room tem
perature (usually less than 10%). These liquids also may be referred
to as "unsaturated" fats. This does not mean that all of the fatty
acids in that particular product are unsaturated, but merely that there
is generally a high proportion of unsaturated fatty acids, sufficient
to render this specific product liquid. For example, soybean oil has
a preponderance of unsaturated fatty acids, making it a liquid,
whereas lard has a greater proportion of saturated fatty acids, mak
ing lard a solid at room temperature. Coconut oil is a somewhat
special case in that it is liquid to just about 78°F (26°C) in spite of
the fact that it contains 85-95% saturated fatty acids.
Chain length, or the number of carbon atoms in a fatty acid, also
has a great influence on whether a fat is solid or liquid. Most fatty
acids have from 4 to 22 carbon atoms, primarily in even numbers.
The products containing high proportions of the longer-chain fatty
acids (14-22 carbon atoms) are likely to be solid at room tempera
ture, whereas those containing more of the shorter-chain fatty acids
(4-12 carbon atoms) are likely to be liquid. Coconut oil is very high
in lauric acid, which contains 12 carbon atoms, and about 60-65%
of the fatty acids are of 14 carbon atoms or less. This is the reason
for its liquidity at relatively low temperatures. Therefore, the most
important factors that render a product solid or liquid are the av
erage fatty acid chain length and the amount of saturated in relation
to unsaturated fatty acids.
The processing techniques employed and the ultimate crystal
structure of the product also have an effect on the product's final
physical form. This will be discussed in a later chapter.
H H 0
I I II
R-C-C-C-OH
I I
H H
Saturated Fatty Acid
In the illustration of a saturated fatty acid, all of the carbon atoms
have four linkages to the other atoms, including the other carbon
atoms in the molecule. The "R" refers to the balance of the mole
cule. If this were an 18-carbon molecule, 15 carbon atoms would be
included in "R."
The carboxyl group is characteristic of all fatty acids. It is also the
portion of the fatty acid molecule that is attached to glycerol to form
8 The Basic Chemistry of Oils and Fats
the monoglyceride, diglyceride, or triglyceride. This carboxyl radical
is quite often written as COOH.
o
II
COOH or -C- OH
Carboxyl Group
Fatty acids are predominantly saturated and unsaturated straight
aliphatic chains with an even number of carbon atoms and a car
boxyl group as illustrated.
H H 0
I I II
R-C-C-C-OH
I I
H H
Aliphatic Carboxyl
Chain Group
There are chains in which there is a double bond between a pair
of carbon atoms in a molecule; then we have an unsaturated fatty
acid as shown. This double bond results in a more reactive linkage
between these two carbon atoms, which, in turn, results in an abil
ity to add hydrogen (or other elements) at the site of this double
bond. In this manner, an unsaturated fatty acid may react with hy
drogen to break this double bond and form a saturated fatty acid.
H H H
I I I
R-C=C-C-COOH
I
H
Unsaturated Fatty Acid (Monounsaturated or Monoenoic)
A polyunsaturated fatty acid has two or more points of unsatu
ration in a specific fatty acid molecule. Two double bonds make this
molecule more unstable, and it reacts with hydrogen, oxygen, and
other elements more readily than a monounsaturated fatty acid.
HHHHHH
I I I I I I
R-C=C-C-C=C-C-COOH
I I
H H
Polyunsaturated Fatty Acid (or Polyenoic)
The Basic Chemistry of Oils and Fats 9
Table 2.1. Important Fatty Acids
Melting
Point
Carbon Double Major Occurrence
Atoms Bonds of °C in Natural Oils and Fats
Butyric 4 0 18 -8 Butter
Lauric 12 0 111 44 Coconut oil
Myristic 14 0 129 54 Butter, coconut oil, palm oil
Palmitic 16 0 145 63 Palm oil, butter, and meat
fats such as chicken fat,
lard, tallow
Stearic 18 0 157 69 Tallow, cocoa butter, lard,
butter
Oleic 18 1 58 14 Olive, peanut, lard, palm,
tallow, corn, rapeseed,
canola
Linoleic 18 2 23 -5 Soybean, safflower,
sunflower, corn,
cottonseed
Linolenic 18 3 12 -11 Soybean, canola
Gadoleic 20 1 Some fish oils
Arachidonic 20 4 -40 -40 Lard, tallow
20 5 Some fish oils
Behenic 22 0 176 80 Peanut, rapeseed
Erucic 22 1 91 33 High erucic acid rapeseed
22 6 Some fish oils
A fatty acid with three points of unsaturation in its molecule reacts
very rapidly with hydrogen or oxygen.
Table 2.1 lists the names of the most important fatty acids occur
ring in nature, along with their chain lengths and numbers of dou
ble bonds (3). This table shows the fatty acids' common names, which
are sufficient for a general understanding of food oils and fats.
However, for a more complete understanding of fat structure, the
Geneva system of nomenclature shown in Table 2.2 should also be
used, as it provides a systematic means of naming these acids (3).
In the Geneva system, a Greek prefix is used to designate the
number of carbon atoms. For example, stearic acid, with 18 carbon
atoms, would have the prefix "octadec." Furthermore, for saturated
fatty acids, one uses the suffix "anoic." Therefore, the Geneva sys
tem name for stearic acid becomes octadecanoic acid. For unsatu
rated fatty acids, the suffix is modified according to the number of
10 The Basic Chemistry of Oils and Fats
Table 2.2. Saturated Acids
Common Geneva No. of Carbon
Name Name atoms Formula
Acetic Ethanoic 2 CH3COOH
Butyric Butanoic 4 C3H7COOH
Caproic Hexanoic 6 CSHllCOOH
Caprylic Octanoic 8 C7H1SCOOH
Capric Decanoic 10 C9H19COOH
Lauric Dodecanoic 12 CllH COOH
23
Myristic Tetradecanoic 14 C H27COOH
13
Palmitic Hexadecanoic 16 C1sH31COOH
Stearic Octadecanoic 18 C H3SCOOH
17
Arachidic Eicosanoic 20 C19H39COOH
Behenic Docosanoic 22 C21H43COOH
points of unsaturation. One double bond is described by the suffix
"enoic," two double bonds by "dienoic," and three double bonds
by "trienoic." Therefore, linolenic acid with 18 carbon atoms and 3
double bonds becomes octadecatrienoic acid. More complete lists of
fatty acids with their Geneva names and formulas are shown in Ta
bles 2.2 and 2.3 (3).
Also in the Geneva system of nomenclature, the carbons in a fatty
acid chain are numbered consecutively from the end of the chain,
the carbon of the carboxyl group being considered as number 1. By
convention, a specific double bond in a chain is identified by the
lower number of the two carbons that it joins. In oleic acid (octa
decenoic acid), for example, the double bond is between the ninth
Table 2.3. Unsaturated Acids
No. of No. of
Common Geneva Double Carbon
Name Name Bonds Atoms Formula
Myristoleic Tetradecenoic 1 14 C H2SCOOH
13
Palmitoleic Hexadecenoic 1 16 C1sH29COOH
Oleic Octadecenoic 1 18 C H33COOH
17
Linoleic Octadecadienoic 2 18 C H31COOH
17
Linolenic Octadecatrienoic 3 18 C H29COOH
17
Arachidonic Eicosatetraenoic 4 20 C19H31COOH
Erucic Docosenoic 1 22 C21H41COOH
The Basic Chemistry of Oils and Fats 11
and tenth carbon atoms. For linoleic acid (9,12-octadecadienoic acid),
the double bonds are between the 9 and 10 carbon atoms and the
12 and 13 carbon atoms.
Another system of nomenclature in use for unsaturated fatty acids
is the "omega" or "n minus" classification. "Omega" or "n minus"
notation often is used by biochemists to designate sites of enzyme
reactivity or specificity. The terms "omega" or "n minus" refer to
the position of the double bond of the fatty acid closest to the methyl
end of the molecule. The methyl end is the furthest away from the
carboxyl end of the fatty acid molecule. Thus, oleic acid, which has
its double bond 9 carbons from the methyl end, is considered an
omega-9 (or an n-9) fatty acid. Similarly, linoleic acid, common in
vegetable oils, is an omega-6 (n-6) fatty acid because its second dou
ble bond is 6 carbons from the methyl end of the molecule (i.e.,
between carbons 12 and 13 from the carboxyl end). Eicosapentae
noic acid, found in many fish oils, is an omega-3 (n-3) fatty acid.
Alpha-linolenic acid, found in certain vegetable oils, is also an ome
ga-3 (n-3) fatty acid.
When two fatty acids are identical except for the location of the
double bond, they are referred to as positional isomers. Fatty acid
isomers are discussed at greater length in Chapter 3.
Because of the presence of double bonds, unsaturated fatty acids
are more reactive chemically than are saturated fatty acids. This
reactivity increases as the number of double bonds increases.
Although double bonds normally occur in a nonconjugated po
sition, they can occur in a conjugated position (alternating with a
single bond) as illustrated below:
H H H H H H H H H
I I I I I I I I I
-C=C-C=C- -C=C-C-C=C-
I
H
Conjugated Nonconjugated
With the bonds in a conjugated position, there is a further in
crease in certain types of chemical reactivity. For example, oils and
fats are much more subject to oxidation and polymerization when
the double bonds are in the conjugated position.
Tables 2.4 and 2.5 show the typical fatty acid composition of some
of the more important, commercially used food oils and fats (4).
These composition figures should be considered as typical but not
exact. In the case of vegetable oils, the actual values vary, depend-
12 The Basic Chemistry of Oils and Fats
Table 2.4. Fatty Acid Composition of Some Vegetable Oils
% % % %
Soybean Cottonseed % Palm Coconut Canola
Cs Caprylic 8
C Capric 6
lO
C Lauric Trace 49
12
C Myristic Trace 1 1 18
14
CI6 Palmitic 12 24 48 8 4
CIS Stearic 4 2 5 3 2
CI8:I Oleic 23 18 38 7 62
C18: Linoleic 53 53 9 2 22
2
C18: Linolenic 8 Trace Trace 10
3
ing on such factors as location of growth area, soil conditions, cli
mate, and growing conditions. Meat fats also vary considerably in
accordance with the type of animal, the type of feed, the activity of
the animal, and climatic conditions. Meat fats also contain small
portions of other minor fatty acids, which we have not included in
this discussion.
Fish oil/marine oil is a versatile product and finds many appli
cations in the food, feed, and technical industries of the world. The
largest use for fish oil is in its partially hydrogenated form in Europe
Table 2.5. Fatty Acid Composition of Some Meat Food Fats
% Beef
% Butterfat % Lard Tallow
C Butyric 4
4
C6 Caproic 3
C8 Caprylic 1
C Capric 2 Trace
lO
C Lauric 3 Trace
12
C Myristic 12 2 4
14
CI6 Palmitic 26 25 26
CIS Stearic 13 13 22
CI8:I Oleic 29 45 39
C18: Linoleic 3 10 3
2
C18:3 Linolenic 1 Trace 1
C Arachidic Trace Trace
20
C2O:4 Arachidonic Trace 1 1