Fats and oils
In the body, that ever-present adipose tissue known as body fat is a storage form of
energy that provides more than twice as much energy as carbohydrates. Fats also
provide protection and insulation for internal organs.
Fats and oils, also known as triglycerides, are triesters of glycerol and three fatty acids.
Most fats and oils are mixed triglycerides that contain two or three different fatty acids.
O
O
CH2
O H
+
HO
C
(CH2)16CH3
CH2
O
C
O
O
CH
O
H
+
HO
C
(CH2)16CH3
CH
O
C
O H
+
HO
C
(CH2)16CH3 + 3H2O
O
O
CH2
(CH2)16CH3
CH2
(CH2)16CH3
O
C
(CH2)16CH3
A fat is a triglyceride that is a solid at room temperature, such as fat in meat, whole milk,
butter and cheese. Most fats come from animal sources.
An oil is a triglyceride, and most are liquid at room temperature. The most commonly
used oils come from plant sources. Olive oil and peanut oil are called monosaturated
because they contain large amounts of oleic acid. Oils from corn, cottonseed, safflower,
and sunflower are polyunsaturated because they contain large amounts of fatty acids with
two or more double bonds. However, a few oils such as palm oil and coconut oil are solid
at room temperature because they consist mostly of saturated fatty acids.
H
H
H
C
C
C
C
COOH
H
Cis double bond
Trans double bond
COOH
cis-Oleic acid
trans-Oleic acid
The double bonds in most unsaturated fatty acids are cis, cause one or more bends in the
carbon chain. Humans and other mammals do not synthesize sufficient amounts of fatty
acids that have more than one double bond, such as linoleic and linolenic acid. These
fatty acids are called essential fatty acids because they must be provided by the diet.
The uniform structure of the carbon atom in saturated fatty acids allows the molecules of
saturated fatty acids to fit close together in a regular pattern. The close pattern of the
saturated fatty acids allows strong attractions to occur between the carbon chains. As a
result, a significant amount of energy and high temperatures are required to separate the
fatty acids and melt the fat. In unsaturated fatty acids, the cis double bonds cause the
carbon chain to bend, which gives the molecules an irregular shape. As a result, the
molecules of unsaturated fatty acid cannot stack together in a regular pattern, so fewer
interactions occur between carbon chains. Consequently, the melting points of
unsaturated fats are lower than those of saturated fats.
Omega – 3 Fatty Acids in Fish Oils
Over the past several decades, Americans have been changing their diets to include
more polyunsaturated fats and fewer saturated fats. This change is a response to
research that indicates that atherosclerosis and heart disease are associated with high
levels of saturated fats in the diet. However, this association does not seem to correct for
the Inuit people of Alaska, who have a high fat diet and high levels of blood cholesterol,
but a very low occurrence of atherosclerosis and heart attacks. The fats in the Inuit diet
are primarily from fish rather than from land animals, as in many other people’s diets.
Both fish and vegetable oils have high levels of polyunsaturated fats. The fatty acids
in vegetable oils are omega-6 acids, which means that the first double bond occurs at
carbon 6 counting from the methyl group. Two common omega-6 acids are linoleic acid
and arachidonic acid. However, the fatty acids in the fish oils are mostly the omega-3 type,
in which the first double bond occurs at the third carbon counting from the methyl group.
Three common omega-3 fatty acids in fish are linolenic acid, eicosapentaenoic acid
(EPA), and docosahexaenoic acid (DHA).
In atherosclerosis and heart disease, cholesterol forms plaques that adhere to the
walls of the blood vessels. Blood pressure rises as blood has to squeeze through a
smaller opening in the blood vessel. As more plaques form, there is also a possibility of
blood clots blocking the blood vessels and causing a heart attack. Omega-3 fatty acids
lower the tendency of blood platelets to stick together, thereby reducing the possibility of
blood clots. However, high levels of omega-3 fatty acids can increase bleeding time if the
ability of the platelets to form blood clots is reduced too much. It does seem that a diet
that includes fish such as salmon, tuna, and herring can provide higher amounts of the
omega-3 fatty acids, which help lessen the possibility of developing heart disease.
Lineolenic acid, 18 carbon atoms
CH3CH2CH
1
2
CHCH2CH
CHCH2CH
CH(CH2)7COOH
3
CH3CH2(CH
CHCH2)5(CH2)2COOH
CH3CH2(CH
CHCH2)6CH2COOH
EPA, 20 carbon atoms
DHA, 22 carbon atoms
Olestra: A Fat Substitute
In 1968, food scientist designed an artificial fat called olestra as a source of nutrition for
premature babies. However, olestra could not be digested and was never used for that
purpose. Then scientists realized that olestra had the flavour and texture of a fat without
the calories.
Olestra is manufactured by obtaining the fatty acids from the fats in cottonseed or
soybean oils and bonding the fatty acids with the hydroxyl groups on sucrose. Chemically,
olestra is composed of six to eight long-chain fatty acids attached by ester links to a sugar
(sucrose) rather than to a glycerol molecule found in fats. This makes olestra a very large
molecule, which cannot be absorbed through the intestinal walls. The enzymes and
bacteria in the intestinal tract are unable to break down the olestra molecule and it travels
through the intestinal tract undigested.
In 1996 the Food and Drug Adminstration approved olestra for use in potato chips,
tortilla chips, crackers and fried snacks. In 1996, olestra snack products were test
marketed in parts of the United States. by 1997, there were reports of some adverse
reactions, including diarrhea, abdominal cramps, and anal leakage, indicating that olestra
may act as a laxative in some people. However, the manufacturers contend there is no
direct proof that olestra is the cause of those effects.
The large molecule of olestra also combines with fat-soluble vitamins (A, D, E, and K)
as well as the carotenoids from the foods we eat before they can be absorbed through the
intestinal wall. Cartotenoids are plant pigments in fruits and vegetables that protect
against cancer, heart disease, and macular degeneration, a form of blindness in the
elderly. The FDA now requires manufacturers to add the four vitamins, but not the
carotenoids, to olestra products. The label on an olestra product must state the following:
“This product contains olestra. Olestra may cause abdominal cramping and loose stools.
Olestra inhibits the absorption of some vitamins and other nutrients. Vitamins A, D, E and
K have been added.” Some snack foods made with olestra are now in supermarkets
nationwide. Since there are already low-fat snacks on the market, it remains to be seen
whether olestra will have any significant effect on reducing the problem of obesity.
The chemical reactions of triglycerides are the same as those for alkenes, carboxylic
acids and esters.
Iodine value
The iodine value is defined as the number of grams of iodine that reacts with 100 grams
of fats/oils. The higher the value is, the greater the degree of unsaturation in the fat or oil.
In general, vegetable oils have higher iodine value than animal fats.
Hydrogenation
The hydrogenation of unsaturated fats converts carbon-carbon double bonds to single
bonds. The hydrogen gas (under pressure) is bubbled through the heated oil in the
presence of a nickel catalyst. Control of the degree of hydrogenation gives the various
types of partially hydrogenated vegetable oils products on the markets.
O
CH2
O
C
O
(CH2)7CH=CH(CH2)7CH3
CH2
O
O
O
C
(CH2)7CH=CH(CH2)7CH3 + 3H2
CH
O
O
CH2
O
C
(CH2)16CH3
O
Ni
CH
C
C
(CH2)16CH3
O
(CH2)7CH=CH(CH2)7CH3
CH2
O
C
(CH2)16CH3
Hydrolytic rancidity
Hydrolytic rancidity occurs as a result of hydrolysis of glyceride molecules to
propane-1,2,3-triol and free carboxylic acids, and is brought about by the presence of
moisture in the oils. This may occur in the frying of chips in oils at high temperature. The
water released from the chips causes the hydrolysis of oil. This reaction is speeded up
in the presence of certain microorganisms or in the presence of some enzymes.
Oxidative rancidity
A fat or oil becomes rancid when its double bonds are oxidized in the presence of
oxygen and microorganisms. The products are short-chain fatty acids and aldehydes that
have very disagreeable odours. The reaction proceeds through a free radical mechanism
and is accelerated by trace metals, light and free radical initiators. Oxygen first attack the
carbon-carbon double bonds forming hydroperoxides which easily decomposes to form
hydroperoxide free radicals. These highly reactive radicals trigger a chain reaction which
breaks the fat or oil molecules into short-chain aldehydes, ketones and fatty acids.
Saponification
When a fat or oil is heated with a concentrated alkali such as sodium hydroxide,
saponification of the fat gives glycerol and the sodium salts of the fatty acids, which are
soaps.