Initiation • Initiation of autoxidation occurs when hydrogen atom at α-methylene group in double bonds of unsaturated fatty acids is removed to form an alkyl radical (R●). RH R• + H•
Initiation • Initiation of autoxidation occurs when hydrogen atom at α-methylene group in double bonds of unsaturated fatty acids is removed to form an alkyl radical (R●). RH R• + H•
Initiation • Initial generation of free radicals is slow – Initiated by singlet oxygen (1O2) • metastable, excited energy state of O2 • 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
Initiation • Initial generation of free radicals is slow – Initiated by singlet oxygen (1O2) • metastable, excited energy state of O2 • 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
Production of 1O2 by Photochemical, Chemical, and Biological Systems 1O2 RCOO RCOO • • + (1) (12) ENZYMES O2 + 3SENSITIZER (2) H2O2 + OCl - H2O2 + O2- (3) + (4) OH O2- (5) (7) (8) (9) (10) O2- O2- + O2- O2- + + Y+ H2O2 HO2- OZONIDES ENDOPEROXIDES SENSITIZER H2O ClOH- + OH OHe- Y 2H+ H2O2 RC O + RCOH PRODUCTS PRODUCTS H2O + OH- (11) + • •
Production of 1O2 by Photochemical, Chemical, and Biological Systems 1O2 RCOO RCOO • • + (1) (12) ENZYMES O2 + 3SENSITIZER (2) H2O2 + OCl - H2O2 + O2- (3) + (4) OH O2- (5) (7) (8) (9) (10) O2- O2- + O2- O2- + + Y+ H2O2 HO2- OZONIDES ENDOPEROXIDES SENSITIZER H2O ClOH- + OH OHe- Y 2H+ H2O2 RC O + RCOH PRODUCTS PRODUCTS H2O + OH- (11) + • •
Initiation • Oxygen raised to excited state (1O2) by light energy – called photooxidation – promoted by pigments called sensitizers sens sens* sens* + 3O2 sens + 1O2 – e.g. chlorophyll, riboflavin light
Initiation • Oxygen raised to excited state (1O2) by light energy – called photooxidation – promoted by pigments called sensitizers sens sens* sens* + 3O2 sens + 1O2 – e.g. chlorophyll, riboflavin light
Initiation Mechanisms • 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 such as EDTA or citric acid
Initiation Mechanisms • 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 such as EDTA or citric acid