7. Autoxidation and antioxidants
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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