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10lipoxid - II.C.ii-1 Reactions of Triglycerides...

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II.C.ii.-1 Reactions of Reactions of Triglycerides Triglycerides Hydrolysis triglycerides diglycerides, monoglycerides, glycerol + fatty acids Hydrogenation unsaturated lipids more saturated lipids trans isomers Oxidation H 2 O H 2
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II.C.ii.-2 Lipid Oxidation Lipid Oxidation Lipid oxidation occurs by a free radical chain reaction C O - O - C H H + H• C O - O - C H Represent as RH R• + H• contains an unpaired electron
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II.C.ii.-3 Phases of Oxidation Reaction Phases of Oxidation Reaction Three phases of reaction: Initiation RH R• + H• Propagation R• + O 2 ROO• ROO• + RH ROOH + R• Termination ROO• + ROO• stable products Chain reaction-- very rapid; self-perpetuating Slow Needs high levels of free radicals or low O 2 RH: lipid ROOH: hydroperoxide ROO•: peroxy free radicals + R•
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II.C.ii.-4 Initiation Mechanisms Initiation Mechanisms Photooxidation energy from light is captured create excited oxygen state ( singlet oxygen ) aided by sensitizer molecules Initiated by enzymes called lipoxygenases Inactivate by blanching (short heating to denature enzymes) Metal ions (e.g. Fe, Co, Cu) can also initiate reaction found naturally in foods, from metal equipment Prevent using chelating agents, e.g. EDTA or citric acid
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II.C.ii.-5 Photooxidation Photooxidation Initial generation of free radicals is slow lipids do not react with oxygen in its ground state ( triplet oxygen, 3 O 2 ) Initiated by singlet oxygen ( 1 O 2 ) metastable, excited energy state of O 2 two unpaired electrons in same orbital triplet oxygen ground state 2 electrons w/ same spin in 2 orbitals singlet oxygen excited state 2 electrons w/ different spin in 1 orbital
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II.C.ii.-6 Photooxidation Photooxidation Singlet oxygen more reactive than triplet oxygen RH + 1 O 2 ROOH RO• + •OH RO• provides free radical to start propagation Oxygen raised to excited state ( 1 O 2 ) by light energy called photooxidation promoted by pigments called sensitizers sens sens* sens* + 3 O 2 sens + 1 O 2 e.g. chlorophyll, riboflavin light
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II.C.ii.-7 Formation of Lipid Radical
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