Naturally Occurring Molecules from plants and animals

14. Fats, Oils and Margarine

• Plant Oils - Uses

  • Many plants produce useful oils that can be extracted and converted into consumer products including processed foods.

  • Emulsions can be made and have a number of uses.

  • Vegetable oils can be hardened to make margarine.

  • Biodiesel fuel can be produced from vegetable oils.

  • Vegetable oils are an important source of energy and even vitamins like vitamin E in seed oils.

  • Vegetable oils contain essential fatty acids which are bodies need for certain metabolic processes.

• Plant Oils - extraction and processing into useful products

  • The fruits and seeds of some plants contain appreciable and economically viable quantities of oil.

    • eg olives, peanuts, walnuts, brazil nuts, rape seed, avocados etc.

  • The traditional way to extract the oil from plant material is to crush it between metal plates of a press, which literally squashes the oil out eg extracting olive oil from crushed olives.

  • You can also extract the oil from crushed plant material by using a centrifuge (a high spin 'barrel' with holes in the out surface), whose rapid rotation spins the oil out to out regions of the container from which the oil can be collected through the outer holes.

  • It is also possible to extract the plant oil with a solvent.

  • However, before any oil can be used it must be purified to remove impurities.

  • Plant oils can be highly refined by fractional distillation which removes any solvents, water and other dissolved impurities.

  • You can also use steam distillation to extract lavender oil or orange oil etc. from the crushed plant leaves or peel etc.

    • 

    • BUT you can do it with much simple apparatus eg boiling the plant material with water in a flask connected to a condenser and collection flask (see below).

      • 

• 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)

  • Oils and fats are quite long carbon chain molecules which can be ...

    • saturated, with no double carbon = carbon bonds, or

    • unsaturated, with one or more carbon = carbon double bonds in the three fatty acid parts of the molecule.

    • Unsaturated oils will decolourise bromine water, a simple test for unsaturation.

    • This is because bromine atoms can add across the double bonds like any alkene molecule.

    • Ignoring the rest of the molecule, the reaction in the unsaturated part of the oil/fat molecule is ...

      • C=C + Br-Br ==> Br-C-C-Br

      • The oil is colourless or pale yellow and the much brighter brown/orange colour of the bromine disappears and the saturated product is virtually colourless.

  • Although much shorter than polymer molecules, they have the same ester linkages as Terylene plastic, but with different units, food for thought!

  • 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 two  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. runny vegetable oils rather than saturated low melting fatty 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 in which the runny vegetable oil is 'hardened' by hydrogenation to produce a higher melting spreadable solid.

      • The melting point is still low, but not to low ie a margarine is a soft solid at room temperature and doesn't go too hard in the refrigerator..

    • The vegetable oils are reacted with hydrogen gas at 60^oC using a nickel catalyst (Ni).

    • These are called hydrogenated fats and have higher melting point than unsaturated vegetable oils, so they are a low melting solid at room temperature rather than the sticky-syrupy 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 == Ni ==> -CH₂-CH₂-

      • which is converting an unsaturated part of the molecule to a saturated structure.

      • This type of reaction is called hydrogenation - quite literally - addition of hydrogen.

    • 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!

    • The hydrogenated oils are used as spreads and general baking like cakes, bread and pastries.

    • Technically, margarine is only partially hydrogenated because fully saturated fats would be too hard and difficult to spread, but if a high % of the double bonds are hydrogenated, the texture of the margarine is a bit like butter and the 'buttery effect' appeals to many consumers.

    • Instead of butter, margarine and other partially hydrogenated vegetable oils are used in processed foods because they are cheaper and gives food products a longer shelf-life.

    • 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 together with an emulsifier.

    • 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 and at the same time stopping the formation of two layers.

      • See Aqueous solution chemistry (section on emulsions for more details)

• 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!

  • Vegetable oils are an important source of energy and even vitamins like vitamin E in seed oils.

    • Vegetable oils are high calorie high energy food.

  • Vegetable oils contain essential fatty acids which are bodies need for certain metabolic processes.

  • So we need both oils and fats as sources of important essential fatty acids and energy.

  • We need both saturated and unsaturated fats or oils.

    • Animal fats tend to be saturated molecules and vegetable oils tend to be unsaturated fatty molecules.

    • 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, walnut oil.

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

    • Whatever fat or oil you use in cooking - food preparation, you are significantly increasing your calorie intake from these energy rich molecules and it doesn't matter the oil/fat is polyunsaturated, partially hydrogenated or fully saturated.

    • In general unsaturated fats are more healthy to consume than saturated fats.

    • However, too much saturated fat raises cholesterol levels and is not too good for the heart - increased blood pressure and poor blood circulation from blocked arteries and heart disease can result from a diet high in saturated animal fats - but you do need some and eating saturated fats in moderation shouldn't be a problem.

  • Natural highly unsaturated vegetable oils like walnut oil, olive oil, sunflower oil etc. do tend to reduce cholesterol levels.

  • The consumption of trans fats increases the risk of coronary heart disease by raising levels of LDL cholesterol and lowering levels of 'good' HDL cholesterol.

  • However even partially hydrogenated vegetable oils contain 'trans-fats' which are not supposed to be good for you, because they tend to increase cholesterol levels, and therefore the risk of heart disease, so, eating lots of food containing margarine etc. is not good for you!

  • See also Aspects of diet, food additives and cooking chemistry

• SOAP what is it? How is it made?

• '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 (a triol alcohol) and three sodium salts of the long chain carboxylic fatty acids that formed part of the original ester.

Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier-foundation-level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder-higher-level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and 3 linked easy Oil Products gap-fill quiz worksheets

ALSO gap-fill ('word-fill') exercises originally written for ...

... AQA GCSE Science Useful products from crude oil AND Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science Worksheet gap-fill C1.1c Air pollutants etc ...

... Edexcel 360 GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes

Advanced Level Organic Chemistry revision notes

Revise KS4 Science GCSE/IGCSE/O level Chemistry Revision-Information Study Notes for revising for AQA GCSE Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science WJEC/CBAC GCSE science-chemistry CCEA/CEA GCSE science-chemistry (and courses equal to US grades 8, 9, 10)