Chapter 3
CHEMICAL MODIFICATION OFOILS AND FATS
From the fats and oils obtained from natural resources, the majority of them are useddirectly or just after refinement. While the others are used after modification by chemicalprocess. This chapter lists some typical modifications of fats and oils by chemical means.

3-1

Alkaline Hydrolysis

Figure 3-1-1: Alkaline hydrolysis (saponification) of oil to make soaps.
There are many methods for hydrolysis of triacylglycerol molecule. The most common
method is alkaline hydrolysis. Heating (around 100˚C) triacylglycerols with aqueous solution of sodium hydroxide results in glycerol and alkaline salt of fatty acid (i.e. soap) (Figure3-1-1). This is called saponification, and used for production of soap.

3-2

Hydrogenation

Number of double bonds in oils and fats affects physical property such as melting point,
crystallinity. Generally, double bonds reduce the oil’s melting point. Therefore, oils rich inunsaturated fatty acids are liquid, while ones with small amount of unsaturated fatty acids
are solid or semi-solid.
Hydrogenation is a process to add hydrogen atoms into double bonds of unsaturated fatty
acids (Figure 3-2-1). As the result of hydrogenation, liquid oil becomes solid or semi-solid. A
typical example of hydrogenation is in the process of margarine and shortening production.
Vegetable oil is hydrogenated with gaseous H2 in the presence of a metal catalyst (usuallynickel catalyst). If the hydrogenation is completely performed, all the double bounds are19

Figure 3-2-1: Hydrogenation.

converted to the saturated ones with the same carbon number. For example, completehydrogenation of linoleic acid (18:2 ω-6) generates stearic acid (18:0) (Figure 3-2-2).

Figure 3-2-2: Complete hydrogenation of linoleic acid, giving stearic acid.
Vegetable oil is too soft for margarine or shortening because it is liquid. Saturated
fat obtained by complete hydrogenation is too hard. Margarine requires something in themiddle, i.e. not too hard but not too soft. Margarine and shortening makers “partiallyhydrogenate” their product. They only add hydrogen atoms until the oil is at the desiredconsistency. During the hydrogenation process, hydrogen atoms are inserted in no particularorder. When they stop the incomplete hydrogenation process, unsaturated fatty acids arein varying stages of hydrogenation. Some molecules are mostly hydrogenated, while others
are not. And the double bonds have often shifted to unnatural positions, resulting in thegeneration of trans fatty acids or trans fat, which is thought to increase risk of coronary
heart disease.

Figure 3-2-3: Trans(or E) double bond structure of trans fatty acids.

In 2003, Food and Drug Administration (FDA) in USA issued a regulation requiring
manufacturers to list trans fat, on the Nutrition Facts panel of foods and some dietary
supplements (Figure 3-2-4). With this rule, consumers have more information to makehealthier food choices that could lower their consumption of trans fat as part of a hearthealthy diet.

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Figure 3-2-4: Nutrition Facts panel of a food indicating the trans fat content

3-3Transesterification

Figure 3-3-1: Transesterification.

Transesterificaton includes chemical reactions where an ester is reacted with alcohol(alcoholysis), acid (acidolysis), or another ester (intereseterification or ester exchange), togenerate a new ester (Figure 3-3-1).
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3-3-1

Alcoholysis

Figure 3-3-2: Methanolysis of oil for the production of fatty acid methylester.
When methanol is used in alcoholysis, the reaction is called methanolysis, in which fatty
acid methyl ester and glycerol are generated (Figure 3-3-2). Fatty acid methyl esters can beused as an alternative fuel for diesel engine (biodiesel). Biodiesel is a substitute or extender
for traditional petroleum diesel. It can be used in conventional diesel engines, and the useof biodiesel is advantageous for reducing emission of CO2 , CO, SO2 and particle materials.
Biodiesel is now spreading world wide. For example, in France, all the diesel fuel marketed
contains 5% of biodiesel.
Another example of alcoholysis is glycerolysis, where triacylglycerol is reacted with glycerol (alcohol) in the presence of alkaline catalyst to form partial glyceride such as monoacylglycerol. A mixture of triacylglycerol and glycerol are heated at 200-250˚C in the presenceof sodium hydroxide. The resulting main product is monoacylglycerol, which is used as foodemulsifier after purification by molecular distillation.

Figure 3-3-3: Glycerolysis. In practical process, the product is a mixture of mono- and diacylglycerols.

3-3-2Acidolysis

An industrial application of acidolysis is production of long chain fatty acid vinylesters fromvinylacetate (this is obtained by chemical reaction of ethylene and acetic acid) and fatty
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Figure 3-3-4: Production of fatty acid vinyl ester by acidolysis.

acids. Long chain fatty acid vinyl esters are industrial raw material for making plastics(Figure 3-3-4).

3-3-3

Interesterification

Interesterification of oil changes its molecular composition. Oils or fats are mixture ofvarious triacylglycerol molecules having different fatty acids and positional distribution.
Treating oils and fats with sodium methoxide as a catalyst at 80 ˚C causes intermolecule
ester exchange, changing the molecular composition, while leaving the fatty acid composition
unchanged. As a result, the oil changes its physical properties such as melting point andconsistency.
An important application of interesterification is improvement of natural lard (pig fat).
Natural lard tends to form a rough crystal, which is difficult to handle, during storage. Thisis because 64% of palmitic acid is attached to 2nd position of triacylglycerol molecules.
Randomizing the positional distribution of fatty acids of natural lard by interesterification
improves its physical property, making it a smooth “rearranged lard”.
Another example of interesterification is in the field of margarine production. Interesterification of soybean oil and completely hydrogenated soybean oil provides a material
for margarine. This rearranged oil has an advantage that it does not contain trans fatty
acid, because it is not made through partial hydrogenation.

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Figure 3-3-5: Interesterification of triacylglycerol molecules. Fatty
acid distribution is randomized, resulting in the changeof the physical property of the oil.

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