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
Peroxide value defined as ?
-milli-equivalents of peroxide oxygen per 1000 g of fat
Trend of peroxide value over time? (5)
- first: stable
- then slowly rises –> monomolecular reaction = induction period
- end of induction period = rapid rise in PV –> point at which bimolecular reaction comes into play
- reaches max
- then [hydroperoxide] decreases bc they’re destroyed faster then being made (bimolecular + making short molecules)
one can obtain low peroxide value when fat is rancid –> solution?
- PV correlated with sensory data/taste panel to provide one with indicator of acceptability
What test looks at secondary breakdown products? vs primary reaction products?
- primary: peroxide value
- secondary: thiobarbituric acid test (TBA)
TBA test measures what? on what assumption?
- malonaldehyde
- assumes that quantities of malonaldehyde produced are directly proportional to degree of oxidation –> this measure can correlate to flavor defects found in fats and oils
How is malonaldehyde measured? (3 ways)
- reacts with 2 molecules of thiobarbituric acid –> produces red complex which can be assessed spetrophotometrically
- OR distilled off directly and reacted with TBA
- OR quantified directly by High pressure liquid chromatography
do we need sensory evaluation and taste panel when TBA test?
yes! because malonaldehyde formation is a function of the lipid makeup
2 methods to evaluate secondary oxidation products (apart from TBA test)
- anisidine value (AV)
- carbonyl number
rate of autoxidation is a function of __________________ of FA in lipid system
degree of unsaturation
relative rates of autoxidation:
1x, 100x, 1200x, 2500x
C18:0, C18:1, C18:2, C18:3
which is associated with which?
1x, 100x, 1200x, 2500x
C18:0, C18:1, C18:2, C18:3
- C18:3 autoxidation is 2500x faster than stearic acid
Why are double bonds more susceptible to abstraction of Hydrogen? (2 ish) consequence?
because the lifetime of free radical formed is extended significantly
- electrons from double bonds can resonate and stabilize FR structure
- more lifetime = more time for O2 to attack
can autoxidation form isomers?
yes! because of resonance, number of isomeric hydroperoxides (positional or cis/trans) depends on which FR form exists at time of oxygen attacks
unconjugated double bonds have (more/less?) labile methylene group btw double bonds once resonance is initiated
- how many possible positional FR forms can exist?
more!
- 6 positional FR forms! + their corresponding cis/trans isomers
Once the free radical has reacted with molecular oxygen, the linolenic hydroperoxide are very ___________- and will undergo almost immediate ______________
- unstable
- decomposition
linolenic acid or FA having high degrees of unsaturation rapidly form low molecular weight aldehydes, ketones, alcohols and carboxylic acids, even before ?
detectable peroxide levels develop
high __________ and rapid ____________ of polyunsaturated FA explains phenomena of ______________ where there is rapid onset of ____________ without an apparent rise in peroxide value
- reactivity
- decomposition
- reversion
- rancidity
reversion = _________ ___________ reaction, which does not allow ____________ to linger
- reversion vs rancidity time frame?
- accelerated oxidation reaction
- hydroperoxides
- reversion = minutes
- rancidity = months
Singlet oxygen-promoted hydroperoxide formation? what happens?
with sunlight/UV/radiation/metals, electrons of O2 either go one in each orbital, or both in same orbital –> = singlet oxygen –> short lived: don’t wait to radical to form, O2 adds directly on double bond to form hydroperoxide
is the double bond lost during autoxidation?
NO!
autoxidation –> lipid system increasing/decreasing disorder by incorporating oxygen and becoming more polar
increasing
is autoxidation only limited to fats and oils?
no! also in products containing fat and even low fat
Factors affecting autoxidation (10)
- energy in form of heat and light
- catalysts (metal)
- enzymes
- chemical oxidants
- oxygen content and types of oxygen (singlet or triplet)
- natural antioxidants
- phospholipids
- FA
- mono/diglycerides
- polymers
(last 4 allows emulsions –> can accelerate autoxidation)
singlet vs triplet oxygen –> which one is excited and which one is ground state?
- singlet = excited
- triplet = ground state
singlet or triplet:
- diradical nature
- no unpaired electrons = electron rich
- diradical nature –> triplet
- no unpaired e- –> singlet
molecular orbital for triplet vs singlet oxygen
triplet: e- in separate antibonding orbitals
singlet: e- in the same antibonding orbital
singlet or triplet reacts more quickly?
singlet!
how to generate singlet oxygen? Mainly using ? But also …
using photosensitizers
- dyes: methylene blue, rose bengal, eosin, crystal violet, acridine orange
- pigments (all hemes that contain metal): chlorophyll, hemaoporphyrins, riboflavins
also: (big schéma slide 19)
- enzymes
- hydroperoxide + O2
- superoxides
- ozonies
- endoperoxides
photosensitizers absorb light in what range?
380 - 900 nm
emission of absorbed light occurs by both ___________ (rapid) and ______________ (delayed), reflecting 2 separate electronically excited state
- fluorescence
- phosphorescence
how does a singlet state oxygen get excited?
- long lifetime?
- how does it decay (2 ways)?
- when light is absorbed
- excited state has a short lifetime –> decaying by fluorescence to ground state and by nonradiative intersystem crossing (ISC) to triplet excited state
How long does it take fortriplet excited state oxygen to decay to ground state?
during its lifetime, triplet excited state can react with _____________ to produce ________ oxygen
- slow rate –> varies depending on sensitizer
- ground-state (triplet) oxygen –> to produce singlet oxygen
how does chlorophyll form a singlet oxygen (review diagram too)?
- ground state chlorophyll –> excited by light –> goes to excited state 1
- then, either decays through fluorescence OR jumps to inter-system crossing –> excited state 2/3 (?)
- then excited state 2/3 either decays through phosphorescence OR transfers energy to triplet O2 to form singlet oxygen
what is reversion flavor in soybean oil?
development of characteristic beany and grassy flavor in soybean oil prior to development of rancidity
what is the end product of breakdown of hydroperoxides formed by reaction of linolenic acid with singlet oxygen? (in soybean oil)
2-pentenylfuran
What are antioxidants?
natural or synthetic chemical compounds that can delay the start or slow the rate of lipid oxidation in food systems
can authoxidation be completely stopped?
No! only slowed down
what is the common structure between synthetic antioxidants BHA, BHT, TBHQ, PG?
aromatic ring + OH
true antioxidants have property of interrupting or retarding ______________ step of autoxidation process
propagation
what happens during the propagation step/what does the peroxy radical want to do?
- how can a true antioxidant stop that?
peroxy radical (ROO°) aims to eliminate its free radical state by abstracting a hydrogen from another FA
- a true antioxidant donates a H to the peroxy radical more readily than a FA does
How does the antioxidant free radical differ from FA free radical?
antioxidant has poor tendancy to react with molecular oxygen
Antioxidant free radical prefers combining with ___________________ to form _________ species
other free radicals (not oxygen) to form neutral species
what is the induction period?
- how can it be lengthened?
- time before monomolecular reactions start to occur
- lengthened by antioxidants’ activity!
why are vegetable oils generally less susceptible to autoxidation than animal fats? (think of antioxidants)
because vitamin E is not naturally present in animal fats
are some antioxidants toxic?
yes! like gossypol found in cottonseed oil
antioxidants are usually used as ___________ why?
- combinations
- because a mixture tends to be more effective than a single antioxidant by itself
typical commercial antioxidant product is TENOX BHA –> mixture of 3 + carrier?
- 20% BHA
- 6% propyl gallate
- 4% citric acid
- propylene glycol as carrier
what is the antioxidant legal limit? based on what?
< 0.02% (200 ppm)
- based on fat content of product
what happens if too much antioxidant?
antioxidant can become a pro-oxidant
antioxidants acts like __________ agent, (slowing down/accelerating) rate of change
buffering
- slowing down
how is antioxidant free radical stabilized? long enough for what?
- resonance structures! from all double bonds on the ring
- lasts long enough to meet another free radical
what does E° mean? (reduction potential)
ability to pull an electron
are synthetic antioxidants stable under normal conditions?
yes! generally quite stable
Propyl gallate breaks down in ______ _____ or in systems with high ___ –> can form a (color?) complex with ferrous ions –> discoloring fat like lard
- frying fats
- high pH
- blue complex
BHA and BHT are slowly lost via ___________ as fat is heated
volatilization
vapor pressures of BHA and BHT are quite (high/low?) but (high/low) enough to allow migration of antioxidant impregnated into ____________ material to migrate to the ________
- technique used for what food?
low, but high enough
- packaging materials
- food
- dry cereals (can’t spray them with antioxidant)
metal ions are powerful catalysts for lipid oxidation –> they assist in catalyzing what?
breakdown of hydroperoxides to form alkoxy and peroxy radicals
metals ions (lengthen or shorten) induction people and (increase/decrease) overall rate of reaction
- a big amount is needed to have a significant change to reaction rate?
- shorten
- increase
- no! only trace amounts (ppm) required
what causes steric hindrance, eliminating catalytic role metal ions would otherwise play?
chelation of metal ions!
Are chelating agents true antioxidants? why or why not?
what are the commonly termed?
no! because they don’t interfere with primary autoxidation mechanism itself
- commonly termed synergistic agents or synergists
Examples of synergists (5)
- chelating agents
- ascorbic acids
- phosphates
- EDTA
- phospholipids
how do metal ions mix into products/foods? (2)
- from metallic processing material (Cu 2+ and Fe2+)
- from metallo-porphyrin (chlorophyll or myoglobin)
downside of chelating agents?
allow formation of more reactive singlet oxygen, which can attack lipids directly –> hence why antioxidants are still required
what does lipoxygenase catalyze?
- direct oxidation of polyunsaturated FA that have a cis-cis 1,4 pentadiene group
- direct formation of hydroperoxides
how to protect frozen/dehydrated vegetables from rapid autoxidation? what happens if no success?
- blanching! (+ sprayed with citric acid)
- produces hay-like/cardboard aroma
the __________ of many pigments (chlorophyll and beta-carotene) which are highly (sat/unsat) occurs as a result of ?
- bleaching
- unsat
- autoxidation initiated by lipoxygenase
how to reduce effects of lipoxygenase and lipase during oil crushing?
heat is applied!
5 preventive antioxidant examples
- superoxide dismutase
- catalase
- glutathione peroxidase
- single oxygen quenchers
- transition metal chelators (EDTA)
radical scavenging antioxidants break FR chain reaction by?
- 4 examples
- donating hydrogen to free radicals
- vitamin C + tocopherol + quercetin + anthocyanin
Which free radical has highest reduction potential (E°)? (order them from highest to lowest)
HOO°, RO°, HO°, ROO°, R°, antioxidants
HO° > RO° > HOO° > ROO° > R° > antioxidants
initiation of free radical formation can be delayed by use of (3)
- metal chelating agents
- singlet oxygen inhibitors
- peroxide stabilizers
propagation of free radicals chain reactions can be minimized by (2)
- donation of hydrogen from antioxidants
- metal chelating agents
major antioxidant used in foods are __________ or _________ _______ compounds with various ring substitutions
- have high or low activation energy to donate hydrogen?
- can they initiate another free radical? why or why not?
- resulting antioxidant free radical subject to oxidation?
- monohydroxy or polyhydroxy phenol
- low –> <600
- no because of stabilization of delocalization of radical electron
- no due to its stability
Natural or synthetic antioxidants?
- sesamol
- propyl gallate
- tertiary-butylhydroquinone (TBHQ)
- butylated hydroxyanisole (BHA)
- gossypol
- tocopherols
- Butylated hydroxytoluene (BHT)
- nordihydroguaretic acid (NDGA)
- sesamol –> N
- propyl gallate –> S
- tertiary-butylhydroquinone (TBHQ) –> S
- butylated hydroxyanisole (BHA) –> S
- gossypol –> N
- tocopherols –> N
- Butylated hydroxytoluene (BHT) –> S
- nordihydroguaretic acid (NDGA) –> N
how to metal chelators deactivate trace metals that are free or in salts of FA?
by formation of complex ions or coordination compounds
examples of metal chelators (4)
- phosphoric acid
- citric acid
- ascorbic acid
- ethylenediaminetetraacetic acid (EDTA)
what is synergism of antioxidants/how does it occur?
when mixtures of antioxidants produce a more pronounced activity than sum of activities of individual antioxidants when used separately
to have maximum efficiency, primary antioxidants are often used in combination with (2)
- other phenolic antioxidants
- various metal chelating agents
factors affecting efficiency of antioxidants:
- activation energy to donate H should be (high/low)
- oxidation potential should be (high/low)
- reduction potential should be (high/low)
- solubility in oil should be (high/low)
- stability to pH and processing
- low
- high
- low
- high
should antioxidants be fat soluble?
should they be absorbed by body?
- fat soluble!
- not absorbed by body, if possible
