7. Autoxidation and antioxidants Flashcards
What is autoxidation?
- 2 characteristics (starting with s)
- overall result is the development of (2)
- a free radical reaction of unsaturated FA with molecular oxygen
- spontaneous and self-sustaining reaction
- rancidity + spoilage of edible fats and oils
What is rancidity?
wide variety of undesirable off-flavors and odors associated with oxidized fats and oils
extensive autoxidation leads to losses in _________ and a decrease in ____________ _______ –> the oil may eventually become ________
- functionality
- nutritional value
- toxic
do free radicals need O2 to form?
no!
3 steps of autoxidation reaction
- initiation –> formation of an alkyl free radical
- propagation –> catalytic increase of radicals –> chain reaction of alkyl free radicals, molecular oxygen and peroxy free radicals
- termination –> reaction stops: 2 radicals meet and annihilate each other –> formation of non-radical products
Initiation of autoxidation requires 2 conditions:
- a hydrogen must be abstracted (leave) from FA chain
- molecular oxygen must be present
abstraction of hydrogen from FA chain can be induced by (5)
- light or ionizing radiation
- thermal energy (heat)
- presence of metallic cations (Cu2+, Fe2+)
- enzymatic catalysis (lipoxygenase)
- reactive singlet oxygen produced by photosensitizers such as chlorophyll or myoglobin
(OR combinations of above)
abstraction of H from FA chain represents _________ _________ of a covalent bond –> products formed are ____ __________
- homolytic cleavage
- free radicals
free radicals are extremely reactive species –> survive only for ?
nanoseconds/picoseconds (10^-9 to 10^-12)
in the absence of O2, what do free radicals due (during initiation)?
they quickly recombine to terminate = minimal change to FA
Reaction of the propagation step? forms what?
R° + O2 –> ROO°
- forms a peroxy radical
key to self-propagating nature of autoxidation reaction?
- net result?
- the peroxy radical has a propensity for terminating its free radical state by abstracting a H from another FA
- ROO° + RH –> ROOH + R°
- formation of a hydroperoxide and a new FA radical
Why is it called AUTOxidation?
because it can self-propagate
Termination step?
- peroxy radical reacts with any other free radical in vicinity –> when 2 FR react –> an unreactive species is formed and FR chain reaction terminates/is interrupted
probability that peroxy radical encounters another free radical before it can abstract a H from another intact FA is relatively _______ –> net effect is a accumulation/degradation of ___________________ with time
- low
- accumulation of hydroperoxide
hydroperoxide vs peroxide
hydroperoxide: ROOH
peroxide: ROOR
What is responsible for off flavors and odors?
NOT hydroperoxides –> hydroperoxides = precursors of low molecular weight aldehydes, alcohols, ketones and short chain FA –> they are responsible for off-flavors
2 basic hydroperoxide breakdown mechanisms
- occur simultaneously? which one predominates when?
monomolecular and bimolecular reaction
- yes but mono predominates when [ROOH] is low and bimolecular predominates when [ROOH] is relatively high (bc 2 are likely to meet)
What is the monomlecular breakdown of a hydroperoxide?
ROOH –> RO° + OH°
- RO° = alkoxy radical
- OH° = hydroxy radical
+ also decomposes to short-chain aldehydes, ketones
What is the bimolecular breakdown of a hydroperoxide?
2 ROOH –> RO° + ROO° + H2O
- RO° = alkoxy radical
- ROO° = peroxy radical
Which compounds are propagators of the reaction?
Which compounds result in breakdown products/off flavors?
- peroxy radicals
- alkoxy radicals
What is peroxide value used for? (2)
- common test to evaluate oxidative status of oil
- measures only hydroperoxides, which are primary oxidation products
reaction of hydroperoxide with potassium iodine?
- in what kind of environment?
2 KI + 2 ROOH –> I2 + 2 KOH + 2 ROH
- in acidic environment (H+)
how is iodine released from hydroperoxide reaction with KI measured?
- by titration with standardized sodium thiosulfate