Lipids I
Lipids II
 Elmhurst College
Hydrogenation Micelle Anabolic Steroids  Chemistry Department
Olestra Detergents and Surfactants Birth Control  Virtual ChemBook

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Hydrogenation of Unsaturated Fats
Trans Fat

Saturated vs Unsaturated Fats and Health:

It has long been recognized that saturated fats tend to increase the blood level of the "bad" LDL cholesterol.
Monounsaturated (one double bond) and polyunsaturated fats (two or more double bonds) found primarily in vegetable oils tend to lower "bad" LDL cholesterol. An elevated LDL-C increases the risk of developing coronary heart disease.

Way back in the 1950s it was recognized that vegetable oils could be substituted for animal fats such as in butter, by making a product we know as margarine. But how do you make an oil into a solid? Recall that vegetable oils which contain more unsaturated fatty acids are liquids while saturated fatty acids are solids. See Fatty Acids.

Hydrogenation Reaction:

Unsaturated fatty acids may be converted to saturated fatty acids by the relatively simple hydrogenation reaction. Recall that the addition of hydrogen to an alkene (unsaturated) results in an alkane (saturated).

A simple hydrogenation reaction is:

H2C=CH2 + H2 ---> CH3CH3
alkene plus hydrogen yields an alkane

The hydrogenation of a oleic fatty acid is shown in the graphic on the left.

QUES. Write the hydrogenation reaction for linoleic acid to hydrogenate all of the double bonds. What is the new name for this fatty acid? Hint: count carbons.

Quiz: After the hydrogenation of an unsaturated fatty acid, would it exist at room temperature as a liquid or solid?  

Making Margarine:

Vegetable oils are commonly referred to as "polyunsaturated". This simply means that there are several double bonds present. Vegetable oils may be converted from liquids to solids by the hydrogenation reaction. Margarines and shortenings are "hardened" in this way to make them solid or semi-solids.

Vegetable oils which have been partially hydrogenated, are now partially saturated so the melting point increases to the point where a solid is present at room temperature. The degree of hydrogenation of unsaturated oils controls the final consistency of the product.

What has happened to the healthfulness of the product which has been converted from unsaturated to saturated fats? There is still less saturated fat in a margarine when compared to butter.

 Quiz: Compare a "hard" type margarine vs. a "soft" margarine.
Which has the most double bonds?  
 Which is the most saturated?

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Trans Fat:

When naturally occurring unsaturated fatty acids are altered by partial hydrogenation, they are converted to saturated fatty acids, which have the effect of straightening the chains and changing the physical properties.

Also during partial hydrogenation, some of the unsaturated fatty acids, which are normally found as the cis isomer about the double bonds, are changed to a trans double bond and remain unsaturated. Trans fatty acids of the same length and weight as the original cis fatty acids, still have the same number of carbons, hydrogens, and oxygens but they are now shaped in a more linear form, as opposed to the bent forms of the cis isomers. See graphic on the left.

See the definitions of cis-trans isomers.

Although the trans fatty acids are chemically "monounsaturated" or "polyunsaturated" they are considered so different from the cis monounsaturated or polyunsaturated fatty acids that they can not be legally designated as unsaturated for purposes of labeling. Most of the trans fatty acids (although chemically still unsaturated) produced by the partial hydrogenation process are now classified in the same category as saturated fats.

Trans fat has both the benefits and drawbacks of a saturated fat. On the plus side, it has a longer shelf life than regular vegetable fat and is solid at room temperature. The major negative is that trans fat tends to raise "bad" LDL- cholesterol and lower "good" HDL-cholesterol, although not as much as saturated fat. Trans fat is found in margarine, baked goods such as doughnuts and Danish pastry, deep-fried foods like fried chicken and French-fried potatoes, snack chips, imitation cheese, and confectionary fats.