Soap, for example, was a very early human invention and possibly the first such innovation to be the result of a chemical reaction. In the ancient world, soap was made by first boiling rainwater with ashes from burnt wood to produce lye: a very basic, or alkaline, solution high pH see our Acids and Bases: An Introduction module. Next, this solution was combined with animal fat or vegetable oil and cooked over a low fire for many hours until the mixture changed into a gel.
The fundamental procedure of this chemical reaction, now called saponification , is still used today to make soap. The first steps toward understanding lipids were taken in the early s by a young French scientist named Michel Chevreul Chevreul began his career in the laboratory of Louis Vauquelin, where his role was to use various solvents such as water, alcohol , and ether to separate the colored dye pigments from natural products like vegetable oils, waxes, tree gums, and resins.
At the end of each experiment , Chevreul would wash out the glassware using a lot of soap. While conducting his research , Chevreul observed that if he accidentally left soapy water in some glassware and it evaporated overnight, salt crystals would be left behind. He was confused by this because he had added only water or another solvent and soap to the glassware.
It raised the question: Where was the salt coming from? Through deductive reasoning, Chevreul realized it must be the result of the soap. When he learned how soap was made by mixing animal or vegetable fat with alkali water, though, he was still confused because there was no salt in that process either. Intrigued and persistent, Chevreul went on to study the process of soap-making in his own laboratory. As he made various kinds of soap, he observed that as oils react with the alkali water, they turn from a translucent liquid into a thick, milky pudding, which gradually hardens.
At the time, he knew that oils and fats contain large amounts of carbon and hydrogen and only small amounts of oxygen. He hypothesized that the reaction with the alkali solution , which had a high pH and thus a higher concentration of hydroxide ions OH - , was somehow adding oxygen atoms to the structure of the fats to change them from pure hydrocarbons to molecules with some salt-like properties.
This was an excellent hypothesis because it would explain two different phenomena at the same time. First, it explained the salt crystals left when soapy water dries. Second, it explained why soap is soluble in both water and oil.
The hydrocarbons from the fat would still be oil-soluble, but their new salt-like properties, coming from the added oxygen atoms , would allow them to be soluble in water, a property that all salts have.
Although it took him most of his career to do it, Chevreul demonstrated that his hypothesis was correct. He did this by performing painstaking chemical analyses of various fats, oils, and the soaps that are produced when alkali is added to them. Chevreul discovered that, during saponification , some of the hydroxide OH - ions from the alkali solution are indeed added to the hydrocarbons from the fats.
When this happens, some chemical bonds in the fat molecules are broken, releasing long-tailed fatty acids Figure 2. Many of the names of common fatty acids that we use today were given to these molecules by Chevreul Cistola et al. The reason that hydrocarbon tails from fats are not soluble in water is because almost all of the bonds are symmetrical and thus nonpolar. However, when the hydroxide ions break the ester group in fat molecules during saponification , a charged and polar group is created — a carboxylic acid group — which is very soluble in water.
These fatty acids have a very special structure. They have long chains of nonpolar bonds , which makes them easily dissolvable in oil and grease; but they also have a polar charged group at one end, which makes them easily dissolvable in water. Thus, these molecules have a dual nature — they are both water-soluble hydrophilic, "loves water" and oil-soluble lipophilic, "loves fat". The word for this is amphiphilic , which means "loves both. What Chevreul and others showed was that an alkali solution breaks up the fat molecules and two parts are released: glycerol and fatty acids.
We now know the complete structure of the fat molecule Figure 3. During the process of saponification , the hydroxide ions in the alkali solution "attack" the ester group and thus release the fatty acid chains from the glycerol backbone. Chevreul was able to figure this out by analyzing the chemical composition of the fats before the reaction , and then repeating the analysis with the fatty acids that resulted.
He did this again and again with different kinds of fats, which made slightly different kinds of soaps. The result was the common theme that fats are made of glycerol and fatty acids.
Animals and plants use fats and oils to store energy. As a general rule, fats come from animals and oils come from plants. Because of slight differences in structure, fats are solid at room temperature and oils are liquid at room temperature. However, both fats and oils are called triglycerides because they have three fatty acid chains attached to a glycerol molecule , as shown in Figure 3.
The carbon-hydrogen bonds abbreviated C-H found in the long tails of fatty acids are high-energy bonds. Thus, triglycerides make excellent storage forms of energy because they pack many high-energy C-H bonds into a compact structure of three tightly packed fatty acid tails. For this reason, dietary fats and oils are considered "calorie dense. Animals, particularly carnivores, are drawn to high-fat foods for their high caloric content.
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Jensen Ed. New York: Academic Press. Kartha, A. To connect these molecules to foods students are familiar with, the major lipid component in several fatty foods comes in the form of triglycerides — a complex lipid where 3 fatty acid molecules carboxylic acid are esterified to a molecule of glycerol alcohol.
While the glycerol portion of the triglyceride remains constant, the fatty acid portion of the triglyceride can vary widely. As mentioned above, fatty acids are carboxylic acids that contain long, unbranched hydrocarbon chains.
These chains can vary in length, typically containing between 12 and 20 carbon atoms. Furthermore, fatty acids can be saturated ie no carbon-carbon bonds or unsaturated contains at least 1 carbon-carbon bond. Examples of a saturated and unsaturated fatty acid are shown below. As you can see from the structural schematics above, the presence of a double bond in the palmitoleic acid dramatically alters the structure of the fatty acid, which, in turn, dramatically alters its melting point.
Because of the linear nature of the saturated fatty acid, these molecules can pack more tightly in the solid phase, leading to an increased number of dispersion interactions and hence the requirement of more energy to break them. Alternatively, the kink resulting from the double bond in unsaturated fatty acids prevents efficient packing, decreasing the overall strength of the dispersion forces holding the fat molecules together.
The general rule of thumb is that saturated fatty acids have a higher melting point than unsaturated fatty acids.
Triglycerides containing unsaturated fatty acids are liquid at room temperature whereas triglycerides containing saturated fatty acids are solid at room temperature. This is the difference between an oil and a fat. Also affecting the melting point is the length of the hydrocarbon chain — longer chains have a higher melting point than shorter chains. This is again related to the strength of the dispersion forces that longer chains afford. Naturally occurring oils and fats are usually made up of a mixture of triglycerides, meaning that each glycerol backbone can contain up to 3 different fatty acid molecules.
However, there are trends. For instance, triglycerides from animal fats typically have a higher percentage of saturated fatty acids compared to triglycerides extracted from plants oils. As such, they also have different impacts on human health.
When speaking of the double bonds in naturally occurring unsaturated fatty acids, we are usually referring to a cis double bond formation. Cis bonds are formed in nature as opposed to trans bonds because the enzymes responsible for desaturation reactions — reactions that transform a saturated carbon-carbon bond into an unsaturated carbon-carbon bond — operate in a way that only results in cis bond formation.
If enough autooxidation occurs, the oil will go rancid — this is why food oils tend to have a much shorter shelf life than fats saturated fatty acids are more stable. To address this issue, and prevent food waste, food scientists implemented a method to partially hydrogenate unsaturated oil mixtures until a desired texture was obtained. Compounds isolated from body tissues are classified as lipids if they are more soluble in organic solvents, such as dichloromethane, than in water.
Hence, the lipid category includes not only fats and oils, which are esters of the trihydroxy alcohol glycerol and fatty acids, but also compounds derived from phosphoric acid, carbohydrates, amino alcohols and steroids. They may be saturated or unsaturated.
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