F.2.1 Describe the difference in the structure of a saturated and unsaturated fatty acid. F.2.2 Predict the degree of crystallization and melting point of fats and oils from their structure and explain the relevance of this.
F2: FATS & OILS
Saturated Fatty Acids Generally found in animal, palm, cocoa and
coconut and the foods made from them. Carboxylic acids General formula CnH2n+1 COOH Single carbon-carbon bonds. Tetrahedral shape (109.5°) around each C atom on the chain. Trigonal Planar (120°) shape around C atom of carboxylic acid group. Range from 10-30 C atoms long. Insoluble non-polar hydrocarbon chain and soluble polar head. Overall non-polar and insoluble in water, since the length of the non-polar chain out weighs polar head so they can’t form hydrogen bonds with water. Van der Waals intermolecular forces between the molecules.
Unsaturated Fatty Acids Generally found in plant and fish and the foods made from them. Carboxylic acids One or more C=C double bonds. Classified according to the predominant type of unsaturation present. Mono-unsaturated fat – contains fatty acids with 1 C=C double bond. E.g. olive, canola
and peanut oils. Poly-unsaturated fat – contains fatty acids with 2 or more C=C double bonds. E.g. safflower, sunflower, corn, fish oils). Insoluble in water (for same reasons as a saturated fatty acid) Van der Waals intermolecular forces between the molecules.
Melting Point of Fats
Melting point is defined at the
temperature at which a solid turns to a liquid. Melting point depends on length of fatty acid chain and amount of unsaturation. In saturated fats, as the length of the fatty acid chain increases the number of atoms and number of electrons increases. The strength of the van der Waals forces between the fat molecules increases, increasing the melting point.
The double bond in unsaturated fatty
acids produces a ‘kink’ in the chain which means that the fat molecules can not pack closely together. As the number of C=C double bonds increases (amount of unsaturation increases) the amount of kinking increases and the distance between the fat molecules increases. This decreases the strength of the van der Waals forces between the molecules, decreasing the melting point.
Solid-Liquid Nature of Fats Saturated Fats Generally crystalline solids at room temperature. Straight chained fatty acids can pack close to one another forming stronger van der Waals forces between the molecules. Unsaturated Fats Generally liquids at room temperature. The ‘kink’ in the chain caused by the C=C bonds prevents the molecules from packing close to one another.
Activity 1: Lauric (mpt 44°C), palmitic (mpt 63°C) and stearic (70°C) acids are the most common fatty acids found in saturated fats. 1. Explain whether fats made from these fatty acids would be solids or liquid at room temperature (25°C). 2. Give a reason for their increasing melting points. Oleic (mpt 16°C), linoloeic (mpt -5°C) and Linolenic (-11°C) acids are the most common fatty acids found in unsaturated fats. 1. Explain whether they would be solids or liquid at room temperature. 2. Why are their melting points are lower than those of saturated fatty acids? 3. These fatty acids all have 18 C atoms. Deduce why their melting points differ.
Activity 2: Explain why lard is a solid and room temperature and
avocado oil a liquid.
Cocoa Butter
Lipid obtained from the seed (bean) of the
cocao fruit and used to make chocolate. On the lipid the outside fatty acids are stearic acid, C17H35COOH and/or palmitic (both saturated) and the middle fatty acid is oleic (mono-unsaturated). Very stable, contains natural antioxidants (flavonoid molecules) that prevent rancidity and give it a storage life of two to five years. Chocolate contains: cocoa butter (referred to in % cocoa solids), other fats (palm oil, animal - cream), sugar, lecithin (acts as an emulsifier), flavoring etc. The smooth lubricating texture of cocoa butter make it a popular ingredient in cosmetics and skin care products, such as soaps and moisturizing lotion.
Fats and Oils and Cooking Fats and oils are chosen for cooking and food making
based on their melting points. Cocoa butter is used to make chocolate because it has a narrow range melting point of 34-38°C slightly below body temperature and so it is a solid at room temperature (25°C) but melts in the mouth. Also used to give baked foods a melt in the mouth feel. E.g. chocolate cake, and brownies. Butter (saturated animal fat) is commonly used for baked goods like cakes and cookies because it is made of a mix of saturated fats enabling it to melt in the range of temperatures needed in baking. Mono and polyunsaturated oils are sometimes used as a butter substitute in some baking to reduce the amount of saturated fat. Since butter influences the flavor, texture and shape substituting unsaturated oils will change the final product.
Cis and Trans Fats C=C double bond prevents free
rotation of H atoms, allowing for only two possible positions on the C of the C=C double bond. Cis isomer – H atoms on the same side of the double bond. Trans isomer – H atoms on opposite sides of the C=C double bond. Called geometric isomers because the molecules have the same molecular and structural formula but different spatial arrangement of atoms attached to the C of the double bond.
Cis unsaturated fats have lower melting points than trans
fats since this configuration causes more “kinking” of the chain. The molecules are not as closely packed, decreasing the strength of the van der Waals forces between them. Cis isomer is found in unsaturated fats. Trans isomer only occurs in saturated fats and processed unsaturated fats (margarine). The most commonly used fats in commercial baked foods.
Fats and Health Saturated fats and Trans fats raise level of ‘bad’ LDL cholesterol. A high level of
cholesterol in the blood is a major risk factor for coronary heart disease (stroke, heart attack, blockage or arteries). Trans fats release less energy than cis fats. American Heart Association (AHA) recommends limiting foods high in saturated fat, and trans fat Saturated fat intake should not exceed 7 percent of total kilojoule/calorie intake each day. Trans fat intake should not exceed 1 percent of total each day. Omega-3 and Omega-6 are an essential fatty acids that needs to be consumed in the food we eat. Omega-3 fatty acids have been associated with reducing the risk of heart disease and reducing inflammation. Fatty fish like salmon and tuna are rich sources. Tofu (soybean), canola, walnut and flaxseed, and their oils contain alpha-linolenic acid, a fatty acid which is a precursor (is converted in the body) to omega-3 fatty acids.
Activity 3:
Omega-3 (linolenic acid) and Omega-6 (Linoleic acid) are essential fatty acids. 1. What is an essential fatty acid? 2. Using the diagram alongside deduce how the omega fatty acids are assigned the numbers 3 and 6. 3. Compare their two structures. 4. Write the condensed structural formula for alpha-linolenic acids. 5. State the health benefits of Omega-3 fatty acids.
Activity 4:
Using the fish oil tablet label alongside identify the: 1. Essential fatty acids 2. Antioxidant 3. Ingredient that can increase risk of heart disease. 4. Geometric isomers 5. Fat with C=C double bonds 6. Energy content
Stability of Fats Milk fats are chemically unstable and will react or become
rancid or ‘off’ with time. The molecules produced give rancid food their unpleasant taste, texture and appearance. Some rancid foods tastes and textures are desirable. E.g. flavor of blue cheese. The stability of a fat depends on its degree of saturation i.e. the number of C=C double bonds present. C=C double bonds are more reactive than C-C single bonds making saturated fats the most stable. As the amount of unsaturation increases the stability decreases.
Hydrogenation Adding hydrogen, H2(g) across the C=C
double bond in poly-unsaturated vegetable oils. Hydrogenation is usually partial – hydrogen is not added all the C=C double bonds. Conditions: 140-225°C, finely divided metal catalyst (Zn, Cu, Ni). Used industrially to produce margarine. Partially hydrogenated fats are found in margarine, baked goods such as doughnuts and Danish pastry, deep-fried foods like fried chicken and fries, snack chips, imitation cheese, and confectionary.
Advantages of Hydrogenation
Disadvantages of Hydrogenation
Changes liquid unsaturated
Decreases the amount of
oil to a solid/semi solid oil so it can be spread. Makes the fat more stable, decreasing the rate of oxidation, increasing the shelf life. Increases hardness. Can be used to control the feel of the fat.
unsaturation. Unsaturated fats are healthier – lowers risk of heart disease. Produces trans fatty acids which increase levels of bad/LDL cholesterol and are a poorer energy source.
Bibliography Brown, Catrin and Mike Ford. Standard Level Chemistry developed
specifically for the IB diploma. Heinemann Baccalaureate, 2008. Derry, Lanna, et al. Chemistry for use with the IB Diploma Options: Standard and Higher Levels. Melbourne: Pearson Heinemann, 2009. Neuss, Geoffrey. IB Diploma Programme Chemistry Course Companion. Oxford: Oxford University Press, 2007. —. IB Study Guides, Chemistry for the IB Diploma. Oxford: Oxford University Press, 2007. Organisation, International Baccalaureate. —. "Chemistry Data Booklet." International Baccalaureate Organisation, March 2007. —. "Chemistry Guide." International Baccalaureate Organisation, March 2007. —. "IB Chemistry Examination Papers ." Cardiff: International Baccalaureate Organisation, 1999-2009.
Good Links Good site on physical properties of saturated and unsaturated fatty acids. http://www.elmhurst.edu/~chm/vchembook/551fattyacids.html American Heart Association http://www.americanheart.org/presenter.jhtml?identifier=4582 NYT article on the trans fats debate http://www.nytimes.com/2007/03/07/dining/07tran.html Rancidity http://www.foodsci.uoguelph.ca/dairyedu/grading.html
