Variations in Fats and Oils
All fats have this much in common -- they have three fatty acid groups bonded to glycerol using three ester bonds. However, they vary from one to another in terms of how long the carbon chains are and how many (if any) double bonds there are, which affects how many hydrogen atoms are attached to them. Some consist completely of single bonds between the carbon atom but some have double bonds reducing the number of hydrogens that are present.
Sometimes fats are discussed as though they contain fatty acids, rather than containing ester functional groups. Part of the reason for this is that when these compounds are broken down to be used (metabolized), the fatty acid group is regenerated. Another reason is that the variations that occur from one fat to another occur in the part that came from the fatty acid.
Fats and Oils
Sometimes it is important to distinguish between fats and oils, or more completely, animal fats and vegetable oils. The distinction between these rests on what kinds of fatty acids went into the formation of that fat. Here (and also in Example 7 in your workbook) we have the structural formulas for four such fatty acids -- stearic acid, oleic acid, linoleic acid and linolenic acid. Note that all four have 18 carbon atoms.
Saturated and Unsaturated Fats
Let's take a closer look at the structural differences between these molecules. The difference has to do with the absence or presence of carbon-to-carbon double bonds.
The difference between these molecules is this. Stearic acid has all single bonds. It is saturated and any fat made just from stearic acid will be a saturated fat. The other three have at least one C-to-C double bond. They are unsaturated and any fat (or oil) made from them will be unsaturated. Fats made from linoleic acid and/or linolenic acid have the added distinction of being polyunsaturated fats or oils because they have more than one double bond.
Linolenic acid is also called an omega-3 fatty acid. Let me explain what that term means. You should recall that alpha amino acids have an amino group on the first available carbon atom next to the acid group. Just as alpha is the first letter of the Greek alphabet, omega is the last letter of the Greek alphabet. So the omega carbon atom is the last carbon in the chain, furthest away from the acid group. The omega-3 carbon is the third from the last carbon. Note that linolenic acid has a double on the third from the last carbon atom.
Cis- and Trans-Fatty Acids
It is possible to convert back and forth between saturated and unsaturated fats and oils, and commercially this is done to achieve a certain texture. The fats tend to be solid at room temperature, and the oils tend to be liquids at room temperature. For some functions the liquid is more desirable, such as a cooking oil. For other functions, such as spreading margarine, the solid is more desirable, unless it is too solid. So food processing companies have found ways to partially hydrogenate vegetable oils to make them more solidified. It's also possible to dehydrogenate these solids to put double bonds back in. When this is done, however, the product is not quite the same. In naturally occurring vegetable oils, the double bonds occur at particular locations have a cis- orientation. When they are prepared synthetically or chemically, those cis- orientations are not necessarily regenerated. Instead, they might be trans- orientations. Nor will the locations necessarily be the same and therefore the oils are not quite the same.
Fats tend to be solid at room temperature, and oils tend to be liquids at room temperature. The size and molecular weights of these molecules can be a factor in this property, but so is the kink in the molecule caused by the cis orientation of the doulble bond. That kink keeps the molecules from easily settling into the orderly arrangement that is needed for solids. When looking at the structures below, keep in mind that a fat (or oil) molecule will have three of these fatty acids bonded to a glycerol.
By the way, the shapes that can be achieved by some of these polyunsaturated fatty acids allows them to be synthesized into particularly useful molecules, such as cetain hormones.
You have probably also heard of Olestra, which can be used as a substitute for fat. It differs from fats in that it is not a triester or triglyceride. That is, it is not made from three fatty acids bonded to glycerol (glycerine) by three ester bonds. Instead, it is made from eight fatty acids bonded to sucrose by eight ester bonds. (When you study the structure of sucrose in Lesson 7, you will see why sucrose has eight hydroxyl groups that can form eight ester bonds.) In a sense, it could be called an "octo-ester" or an "octosucroside" if you wanted to use terms like that. Because of the similarity in structure to fats (fatty acid esters), it has some similar functions (taste, texture, etc.). Because of differences in structure (eight ester bonds to sucrose instead of three to glycerol), enzymes that specialize in hydrolyzing fats do not hydrolyze Olestra. That results in the advantage that it does not break down and enter the bloodstream and the rest of the body. That also results in the disadvantage that it remains in the digestive tract altering the digestive process to some degree. The relative merits and problems associated with those perceived advantages and disadvantages are at the heart of the controversy over the use of Olestra.
E-mail instructor: Sue Eggling
Clackamas Community College