《食品化学 Food Chemistry》课程教学课件（讲稿，英文版）Effects of Lipid Oxidation

Effects of Lipid Oxidation • Flavor Quality Loss Rancid flavor Changes of color and texture Consumer Acceptance Economic loss • Nutritional Quality Loss Essential Fatty Acids Vitamins • Health Risks Toxic Compounds Growth Retardation Heart Diseases

Effects of Lipid Oxidation • Flavor Quality Loss Rancid flavor Changes of color and texture Consumer Acceptance Economic loss • Nutritional Quality Loss Essential Fatty Acids Vitamins • Health Risks Toxic Compounds Growth Retardation Heart Diseases

Unsaturated fatty acid or triglyceride Free Radicals Oxidation of +Oxygen pigments,flavors, vitamins Hydroperoxides Breakdown Products Polymerization Insolubilization (Including rancid (Dark Color) of proteins off-flavor compounds) (Possibly toxic) such as ketones aldehydes alcohols hydrocarbons acids epoxides Fig.1 Lipld Oxidation Mechanism

General Considerations • Unsaturated fatty acids [RH] main reactant Energy for H removal (kcal/mole) H - CH 2 - CH2 - CH 3 H - CH = CH2 H - CH 2 - CH = CH 2 CH2 = CH - CH - CH = CH 2 H 100 103 85 65 → H on carbon next to double bond easier to remove

General Considerations • Unsaturated fatty acids [RH] main reactant Energy for H removal (kcal/mole) H - CH 2 - CH2 - CH 3 H - CH = CH2 H - CH 2 - CH = CH 2 CH2 = CH - CH - CH = CH 2 H 100 103 85 65 → H on carbon next to double bond easier to remove

General Considerations • Requires oxygen • Auto-oxidation or Enzyme catalyzed • Catalyzed by trace metals (fe, Cu, Co.) and Heme compounds • Catalyzed by light (UV) and irradiation

General Considerations • Requires oxygen • Auto-oxidation or Enzyme catalyzed • Catalyzed by trace metals (fe, Cu, Co.) and Heme compounds • Catalyzed by light (UV) and irradiation

Free Radical Mechanism

Free Radical Mechanism

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 Cl￾OH- + OH OH￾e- 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 Cl￾OH- + OH OH￾e- 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