Fats & Oils

1. AL Chemistry Fats & Oils • Structure & Properties • Hydrolysis of Fats & Oils • Iodine Value • Hardening of Vegetable Oil • Hydrolytic & Oxidative Rancidity p. 1

2. AL Chemistry Structure & Properties (1) Fats and oils = glycerol + fatty acids Propane-1,2,3-triol if the fatty acid part has many C=C bonds unsaturated p. 2

3. AL Chemistry Structure & Properties (2) forms fats (or oils) with glycerol p. 3

4. p. 4

5. p. 5

6. AL Chemistry Structure & Properties (3) • Unsaturated Oils usually have a lower melting point and exist as a liquid. • Hydrogenation of some of the C=C bonds converts to solid fats. • e.g. Margarine p. 6

7. AL Chemistry soap! Hydrolysis of Fats & Oils • Fats and oils are hydrolysed into carboxylic acid and glycerol in human body (acidic medium). • In lab, hydrolysis can be carried out in alkaline medium more effectively, to give carboxylate and glycerol. (saponification) p. 7

8. AL Chemistry Iodine Value (1) ► To measure the degree of unsaturation of a fat or vegetable oil. ► It is defined as the number of grams of iodine that reacts with 100 grams of fats/oils. ► Vegetable has a high iodine value than fats. p. 8

9. Iodine Value (2) # higher iodine value  higher degree of unsaturation p. 9

10. Chemical Names and Descriptions of some Common Fatty Acids Common Name CarbonAtoms DoubleBonds Scientific Name Sources Oleic Acid 18 1  9-octadecenoic acid olive oil Linoleic Acid 18 2  9,12-octadecadienoic acid corn oil g-Linolenic Acid (GLA)  18 3  6,9,12-octadecatrienoic acid borage oil Gadoleic Acid 20 1  9-eicosenoic acid fish oil EPA 20 5  5,8,11,14,17-eicosapentaenoic acid fish oil DHA 22 6  4,7,10,13,16,19-docosahexaenoic acid  fish oil Examples of Unsaturated Fatty Acid: p. 10

11. AL Chemistry Hardening of Vegetable Oil • Animal fats ---- usually saturated regular packing higher melting point, therefore solid state at rm temp. But the vegetable oil should not be completely hydrogenated, otherwise, the solid will be too hard! catalytic (Ni) hydrogenation • Vegetable oil ---- usually unsaturated irregular packing (due to presence of cis-isomers) lower melting point, therefore liquid state at rm temp. p. 11

12. AL Chemistry Disadvantage of Animal Fats? They contain larger amount of “Cholesterol”. p. 12

13. AL Chemistry Hydrolytic and OxidativeRancidity (酸敗) of Fats & Oils: Hydrolytic and Oxidative Reactions of triglyceride molecules give out unpleasant odour……. due to volatile and foul smelling RCHO & Fatty Acids Hydrolytic Rancidity occurs due to the presence of moisture in the oils. p. 13

14. AL Chemistry Oxidative Rancidity (1) : Oxidative Rancidity occurs under exposure to air (oxygen) • Very common for those with high iodine value► high degree of unsaturation --- more susceptible to oxidation! free radical mechanism!! p. 14

15. AL Chemistry “Autoxidation” Oxidative Rancidity (2) : volatile aldehyde,ketones …. R-O-O-H R-O-O [hydroperoxide] [hydroperoxide free radical] p. 15

16. AL Chemistry Oxidative Rancidity (3) : Oxidative Rancidity can be prevented by using antioxidants (BHA or BHT) R-O-O R-O-O-H i.e. the hydroperoxide free radical is “killed” by the H atom given by BHT  chain reaction stops. p. 16

17. AL Chemistry Oxidative Rancidity (4) : If the triglyceride molecules do not contain any unsaturation carboxylic acid chain …..  no hydroperoxide will be generated,and therefore oxidative rancidity does not occur. p. 17

18. AL Chemistry HKALE …. • 1997: Vegetable oil (iodine value, hydrogenation and saponification) • 1998: Animal Fats and Vegetable oil (iodine value, hardening, saponification and rancidity) • 2001: Vegetable oil [unsaturated oil] (principle of anti-rancidity by BHT) p. 18