6 Autoxidation & Antioxidants Flashcards
define autoxidation
spontaneous free radical rxn with oxygen. results in rancidity and loss in functionality and nutritional value. extensive autoxidation can lead to toxicity.
what is rancidity?
describes a wide variety of undesired flavors/odors associated with oxidized fats and oils
what are the 3 steps of autoxidation?
1) initiation
2) propagation
3) termination
what happens during the initiation step of the autoxidation mech?
- H is abstracted from FA chain
- O2 must be present
- homolytic cleavage of H from R
what are the methods of H abstraction?
- hv (light/ionizing radiation)
- thermal energy (heat)
- presence of metallic cations (Cu2+, Fe2+)
- enzymatic catalysis (lipoxygenase)
- reactive singlet oxygen (chlorophyll, myoglobin)
what happens during the propagation step of the autoxidation mech?
- R and H radicals mingle to make RH, H2, and R-R molecules
- an R radical can attack O2 to make a PEROXYRADICAL
how does a peroxyradical prefer to terminate?
by abstracting a H from another FA
what happens during the termination step of the autoxidation mech?
- ROO radical reacts with ROO radical or RH radical
- forms ROOR + O2 or ROOH respectively
what’s the general formula for a hydroperoxide?
ROOH
statistically, what is the predominant result of our FAs being in the presence of O2?
accumulation of hydroperoxides over time
what happens when the propagation reaction is more frequent than the termination reaction?
you get a net accumulation of hydroperoxides
what to hydroperoxides do?
they act as precursors of aldehydes, alcohols, ketones, and short chain fatty acids
how are rancid compounds formed?
2 possible reactions:
1) monomolecular; [ROOH] low
2) bimolecular; [ROOH] high
what happens during the monomolecular reaction of ROOH?
- ROOH undergoes homolytic cleavage to RO and OH radicals
- RO* = alkoxy radical
- OH* = hydroxy radical
what happens during the biomlecular reaction of ROOH?
2 ROOH –> RO* + ROO* + H2O
what is PV?
peroxide value
- measures hydroperoxides (primary reaction products)
- measured BY the amount of I2 released by titrating with sodium thiosulfate
- expressed as (mEQ ROO)/(1 kg fat)
what is the slow initial accumulation of hydroperoxides called?
induction period
what is the end of the induction period signaled by?
rapid rise in PV (decomposition of peroxides outpaces their formation)
what is the TBA test?
TBA = thiobarbituric acid
- measures secondary breakdown products (e.g. malonaldehyde)
- assumes monoaldehyde produced is directly proportional to degree of oxidation
- 2 TBA + fat –> red complex, assessed with spectrophotometry
- monaldehyde often comes attached to proteins which must be treated with acid to release it
what is the AV test?
- Anisidine value - reaction of p-anisidine with aldehydes
- adduct measured in UV
what are the secondary oxidation tests?
- TBA
- AV
- carbonyl number
what are the relative rates of oxidation for the C18 carbons?
18:0 (x1), 18:1 (x100), 18:2 (1200), 18:3 (2500)
what factors influence the rate of oxidation of FAs?
- heat
- radiant energy
- enzymes
- divalent metal ions
- singlet oxygen
are unconjugated FAs more or less liable to be oxidized?
more, because they have a methylene group for resonance structures to revolve around
why is linolenic acid more readily oxidized?
it can be converted to a mixed bonding system which results in an active methylene group, which is slightly more potent than just normally unconjugated FAs.
why is TBA and AV tests more useful in the detection of rapid compound derived from linolenic FAs?
linolenic acids form the rancid compounds too rapidly before detectable peroxides can develop, so it is good to use TBA and AV because they detect these secondary breakdown products
what is reversion?
rapid onset of rancidity before an apparent rise in peroxide values are detected
what oils are susceptible to reversion?
soybean and fish
what happens as a result of the formation of peroxides?
- increased entropy
- increased polarity
what do antioxidants do?
- lengthens the induction period
- interrupts the propagation step of autoxidation
- stabilizes ROOHs
- competes for binding with O2
- removes oxygen
what properties make a good antioxidant?
- the antioxidant must have an H that is more readily donated than the FA
- the radical formed by the antioxidant must have a POOR TENDENCY to react with oxygen (already be relatively stable on its own)
- often contains a phenol
expand: BHA BHT TBHQ PG
BHA = butylated hydroxyanisole BHT = butylated hydroxytoluene TBHQ = Tert-butyl hydroquinone PG = propyl gallate
are pure antioxidants (natural) or are antioxidant mixtures (synthetic) more effective?
mixtures (synthetic)
What is TENOX a mixture of?
20% BHA, 6% propyl gallate, 4% citric acid
what is the antioxidant legal limit?
<0.02% or 200 ppm
what happens when you have a large concentration of antioxidant?
it becomes a pro-radical/pro-oxidant.
what conditions break down PG, BHT, and BHA?
PG: break down in frying fats with high pH
BHA & BHT: lost via volatilization as fat is heated
the vapor pressure of BHA & BHT is (high/low)?
low, but high enough to migrate into food
what other compounds can limit oxidation? (but technically not antioxidants bc they don’t mess with the radicals)
chelators, ascorbic acid, EDTA, and phospholipids
what compounds catalyze oxidation?
metal cations - they shorten the induction period
how much metal cation is needed to speed up the rxn?
trace (ppm) amounts
what are chelators considered?
synergistic compounds/synergists
what do metal ions do?
allow the formation of singlet oxygen (more reactive); can attack lipids directly
what is the significance of lipoxygenase (lipoxidase)?
- catalyzes direct oxidation of PUFAs which have a cis, cis-1,4-pentadiene group
- found in most PUFAs
what’s the purpose of blanching?
deactivate lipoxygase to prevent oxidation of produce
what happens to pigments like beta carotene or chlorophyll as a result of autoxidation?
bleaching
how do we reduce the enzyme activity in oilseed crush?
heat to inactivate enzymes
what is responsible for the beany flavor of soy milk?
lipoxygenase
