Naturally Occurring Molecules from plants and animals

14. Fats, Oils and Margarine

• Oils and Fats are an important way of storing chemical energy in living systems and are also a source of essential long-chain fatty acids.

• Most of them are esters of the tri-alcohol ('triol') glycerol (systematic name propane-1,2,3-triol, but that can wait until AS-A2 level).

• The carboxylic acids which combine with the glycerol are described as 'long-chain fatty acids'.

• The resulting ester is called a 'triester' or 'triglyceride' and they are major components in animal fat, vegetable oils, processed fats like margarine etc..

• The 'long-chain fatty acids' can be saturated, with no C=C double bonds, and so forming saturated oils or fats (1st diagram below of the triglyceride formed from palmitic acid).

• The 'long-chain fatty acids' can be unsaturated, with one or more C=C double bonds, and so forming unsaturated oils or fats (2nd diagram below of the triglyceride formed from oleic acid).

• Some sub-notes on Oil and Fat Structure: (health issues dealt with further down)

  • They have the same linkages as Terylene but with different units.

  • They are not as big as polymer molecules, but a lot bigger than a single petrol or sugar molecule.

  • There can be 1 to 3 different saturated or unsaturated fatty acid components, so lots of variation possible in structure of the oil or fat. The diagrams just assume three molecules of the same 'fatty' acid.

  • Monounsaturated fats have one C=C double bond in them, polyunsaturated fats usually have at least three C=C bonds in their molecular structure.

  • For the same molecular size in terms of carbon number, unsaturated fats have slightly lower intermolecular forces because the C=C double bond produces a kink in the carbon chain and they can't pack as closely together as the saturated molecules.

    • This gives unsaturated fats a lower melting point and so they tend to occur as e.g. vegetable oils rather than saturated low melting solids from meat and dairy products.

  • However, this means these unsaturated oils are not as conveniently 'spreadable' as 'butter'.

    • To overcome this problem, 'margarine' was invented. The vegetable oils are reacted with hydrogen gas using a nickel catalyst. Theses are called hydrogenated fats and have higher melting point so that they are a low melting solid at room temperature rather than the sticky vegetable oil you might use is cooking and salad dressings.

    • This reaction adds hydrogen atoms to the double bonds making a more saturated and more 'spreadable' higher melting soft solid fat that we call 'margarine'.

    • Saturated means no double bond and unsaturated means double bond in this context.

    • The reaction for any double bond is: >CH=CH< + H₂ ==> -CH₂-CH₂-, which is converting an unsaturated part of the molecule to a saturated structure.

    • BUT it does mean that it is more like animal fat now but various blendes have been developed to suit your dietary needs or desires!

    • Margarine and other 'spreadable' fats based on vegetable oils are quite a mixture of molecules known as an emulsion. A typical mixture might be

    • 14-21% saturated fats (triglycerides with almost no double bonds in the hydrocarbon chains)

    • 15-30% monounsaturates in which there is about one double bond per molecule.

    • 14-22% polyunsaturates which have more than one double bond per molecule.

      • In terms of melting points, the order is saturates > monounsaturates > polyunsaturates.

    • Sodium chloride and water ('salt' solution'), small amounts of protein and carbohydrate and whey or buttermilk are added to the fat/oil mixture.

    • To stop the salt solution separating out from the 'oily' fats an emulsifier is added, which keeps the aqueous salt solution dispersed in the fats or they would separate into two layers and affect the look and taste. Incidentally the emulsifiers may be mono- or di-glycerides of fatty acids, that is molecules like the vegetable oils but only 1 or 2 fatty acids attached to the glycerol rather than 3, which leaves 2 or 1 -OH hydroxy groups on the glyceride molecule. These molecules have the bifunctional structure because through the action of intermolecular forces they bind with both fats (via hydrocarbon chain, 'water hating' hydrophobic end of molecule) and bind with water too (via hydroxy group OH, the 'water loving' hydrophilic end of molecule) so holding the emulsion or dispersion together.

• Since fats and oils are important to our diet, there is the ever present danger of over-consumption (speaking as someone who loves chips and spicy crisps!). So there are health and social, as well as 'molecular' issues to address!

  • We need oils and fats as sources of important essential fatty acids.

  • We need both saturated and unsaturated fats or oils.

    • The main sources of saturated fats are from meat and dairy products e.g. 'dripping' and butter.

    • The main sources of unsaturated fats are plant oils e.g. olive oil.

  • It is recommended that we do not overdo the fat intake but we do need both saturated and unsaturated fats.

    • However, too much saturated fat raises cholesterol levels and is not too good for the heart.

SOAP

• 'Traditional' soap is a product of the hydrolysis of fats.

  • 'Soapy' soaps (not modern detergents) are the sodium salts of long chain fatty acids formed by heating fatty oils with sodium or potassium hydroxide to hydrolyse them. This is known as a saponification reaction and a typical equation is given above.

  • This reaction breaks the fat molecule down into one glycerol molecule (triol alcohol) and three sodium salts of the long chain carboxylic fatty acids.