Almost every child in America gets its first taste of fast food with a packet of fries: Mmmm…so good! As children we grew up with occasional treats of fried potatoes that were irresistible. When I first landed in the U.S.,I always bought a bag of potato chips to remind me of fried food in India.
Oil has a unique way of adding flavor to any food. Today let us look at the chemistry of oils. Lipids are biological chemicals that do not dissolve in water.(Ref.1) These are a group of fat soluble compounds found in the tissues of plants and animals. Fats and oils are a sub-set or subgroup of lipids and are basically storehouses of energy. But lipids play an important role in the formation of cellular membranes,cellular signaling, nutrient functions with relationships to vitamins A,D,E,and K as well as being storehouses of energy. Furthermore, lipids are complex molecules with carbon, hydrogen and oxygen as well as nitrogen and sulfur and other small constituents.(Ref.2)
Fats and oils share a common molecular structure; fats are solids at room temperature, oils are liquids. What is the molecular structure?
They are esters of tri-alcohols, i.e., three -OH groups,also called glycerols,hence fats and oils are also commonly known as glycerides. An ester has the H in the -OH group replaced by a carbon chain and since there are 3 -OH groups there could be three different carbon chains called R1, R2 and R3 respectively.(Ref.1) Remember that an ester in organic chemistry is like a salt in inorganic chemistry . A salt is the neutralization reaction between a base/alkali and an inorganic acid, whereas an ester is a similar kind of neutralization reaction between an alcohol and an organic acid. The more accurate name would be Triacylglycerols. Hydrolysis (or reaction with water) of fats leads to glycerols and fatty acids.(Ref.1)
Triglyceride molecules contain carbon and hydrogen molecules and usually six oxygen atoms in every molecule. This makes them highly reduced or unoxidized. They resemble the hydrocarbons in petroleum and are good sources of fuel. In fact the normal human body stores fats as fuel/ energy for several weeks’ survival. Plants too do the same thing and this allows them to deal with unpredictable sources of food supply.(Ref.1)
A triglyceride is called a fat if it is a solid at 25°C, if it remains a liquid at this temperature, it is termed an oil. The difference in melting point reflects the difference in the degree of unsaturation and number of carbon atoms in the original fatty acids that resulted in the triglyceride. (Ref.3)
Pure fats and oils are colorless,odorless and tasteless. They are lighter than water, their densities are usually around 0.8gm/cc. They are poor conductors of heat and electricity and hence are excellent insulators for the body, slowing the loss of heat through the skin.(Ref.3)
The double bonds in fats and oils can undergo hydrogenation as well as oxidation. The hydrogenation of oils is used commercially to produce margarine. Most fats are obtained from animal sources whereas most plant sources produce oils. Saturated fats/oils are derived from saturated fatty acids ( less or no double bonds) while unsaturated fats/oils are derived from unsaturated fatty acids ( more double bonds). The high consumption of saturated fats, along with high cholesterol leads to increased risks of heart disease..(Ref.3)
The study of lipids, fats and oils indicates how necessary they are in the function and maintenance of our body’s functions. However, too much of saturated fats and oils consumption today, especially because of the fast food industry and in the changes in our lives has led to all kinds of medical problems. We shall look closer at this topic next week.
Here is a small table to illustrate the predominance of saturated fatty acids in fats and more of the unsaturated fatty acids in oils:.(Ref.4)
Animal fats Saturated acid(%) Unsaturated Acids(%)
C10 &less C12 C14 C16 C18 C18(a)C18(b) C18(c)
Butter 15 2 11 13 9 27 4 1
Lard – – 1 27 15 48 6 2
Plant oils Saturated acid(%) Unsaturated acids(%)
Coconut – 50 18 8 2 6 1 –
Corn – – 1 10 3 50 34 –
Olive – – – 7 2 85 5 –
Safflower – – – 3 3 19 76 7
Activities for Middle School Teachers:
Let students make models of different fatty acids and triglycerides. Students need to research the reasons why unsaturated fats are not solids even though there are more double bonds here and the molecule ( fatty acid or corresponding triglyceride) may be dense and therefore could become a solid. The Van der Waals weak intermolecular forces are stronger when you have long chain straight molecules; this happens with saturated fat molecules. On the other hand, the presence of double bonds causes kinks in the structures making them non-planar. Students will find a relationship between melting points and straight-chain versus non-planar structures. Students can plot a graph and present results.(Ref.5)
Nuggets of Information:
When all three hydroxyl groups (-OH) groups in the glycerol molecule are esterified with the same fatty acid, the resulting glyceride is called a simple glyceride. In nature, you never see a simple glyceride, since the esterification in nature involves several different fatty acids. That is because naturally occurring acids are a mixture of several fatty acids.(Ref.3) No single formula can be written to represent naturally occurring fats and oils. They may be derived from several different fatty acids. Palmitic acid is the most abundant saturated fatty acid and oleic acid is the most abundant unsaturated fatty acid.(Ref.3)
Even though most fats are derived from animals, the exception is fish oil; triglycerides from fish are found as oils..(Ref.4)
Seeds contain a lot of oil/fats; the stored energy helps seedlings during germination. later, they have solar energy available for photosynthesis.(Ref.3)
Hydrolysis of fats and oils in the presence of a base produces soap and the process is called saponification.(Ref.3)
Lipids can be further classified as fats, oils, phospholipids, waxes, sterols .(Ref.4) Phospholipids are a class of lipids similar to triglycerlides , where the fatty acid from which it is derived contains a phosphorus atom; they are present in cell membranes. Sterols are a class of lipids with structure similar to cholesterol.(Ref.6) Cholesterol comes under the umbrella of steroids which have a carbon backbone with four fused ring-like strucures and a hydoxyl group (-OH). Waxes are esters of a long-chain fatty alcohol and a fatty acid. Many plants and fruits have a waxy coating to prevent water loss.(Ref.7)
Ear wax is composed of phospholipids and esters of cholesterol!(Ref.7)
References:
1.http://scifun.chem.wisc.edu/chemweek/pdf/Fats&Oils.pdf
2.http://www.differencebetween.info/difference-between-lipids-and-fats
3.http://chemwiki.ucdavis.edu/Textbook_Maps/General_Chemistry_Textbook_Maps/Map%3A_The_Basics_of_GOB_Chemistry_(Ball_et_al.)/17%3A_Lipids/17.2_Fats_and_Oils
4.http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/lipids.htm
6.https://quizlet.com/94654736/chapter-5-lipids-fats-phospholipids-and-sterols-flash-cards/
7.http://biology.about.com/od/molecularbiology/ss/lipids.htm
Tags: phospholipids, saponification, simple glyceride, sterols, waxes
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