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Food Chemistry 1 > 7. Autoxidation and antioxidants > Flashcards

7. Autoxidation and antioxidants Flashcards

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1
Q

What is autoxidation?
- 2 characteristics (starting with s)
- overall result is the development of (2)

A
  • a free radical reaction of unsaturated FA with molecular oxygen
  • spontaneous and self-sustaining reaction
  • rancidity + spoilage of edible fats and oils
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2
Q

What is rancidity?

A

wide variety of undesirable off-flavors and odors associated with oxidized fats and oils

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3
Q

extensive autoxidation leads to losses in _________ and a decrease in ____________ _______ –> the oil may eventually become ________

A
  • functionality
  • nutritional value
  • toxic
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4
Q

do free radicals need O2 to form?

A

no!

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5
Q

3 steps of autoxidation reaction

A
  1. initiation –> formation of an alkyl free radical
  2. propagation –> catalytic increase of radicals –> chain reaction of alkyl free radicals, molecular oxygen and peroxy free radicals
  3. termination –> reaction stops: 2 radicals meet and annihilate each other –> formation of non-radical products
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6
Q

Initiation of autoxidation requires 2 conditions:

A
  • a hydrogen must be abstracted (leave) from FA chain
  • molecular oxygen must be present
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7
Q

abstraction of hydrogen from FA chain can be induced by (5)

A
  • 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)
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8
Q

abstraction of H from FA chain represents _________ _________ of a covalent bond –> products formed are ____ __________

A
  • homolytic cleavage
  • free radicals
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9
Q

free radicals are extremely reactive species –> survive only for ?

A

nanoseconds/picoseconds (10^-9 to 10^-12)

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10
Q

in the absence of O2, what do free radicals due (during initiation)?

A

they quickly recombine to terminate = minimal change to FA

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11
Q

Reaction of the propagation step? forms what?

A

R° + O2 –> ROO°
- forms a peroxy radical

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12
Q

key to self-propagating nature of autoxidation reaction?
- net result?

A
  • 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
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13
Q

Why is it called AUTOxidation?

A

because it can self-propagate

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14
Q

Termination step?

A
  • 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
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15
Q

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

A
  • low
  • accumulation of hydroperoxide
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16
Q

hydroperoxide vs peroxide

A

hydroperoxide: ROOH
peroxide: ROOR

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17
Q

What is responsible for off flavors and odors?

A

NOT hydroperoxides –> hydroperoxides = precursors of low molecular weight aldehydes, alcohols, ketones and short chain FA –> they are responsible for off-flavors

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18
Q

2 basic hydroperoxide breakdown mechanisms
- occur simultaneously? which one predominates when?

A

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)

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19
Q

What is the monomlecular breakdown of a hydroperoxide?

A

ROOH –> RO° + OH°
- RO° = alkoxy radical
- OH° = hydroxy radical
+ also decomposes to short-chain aldehydes, ketones

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20
Q

What is the bimolecular breakdown of a hydroperoxide?

A

2 ROOH –> RO° + ROO° + H2O
- RO° = alkoxy radical
- ROO° = peroxy radical

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21
Q

Which compounds are propagators of the reaction?
Which compounds result in breakdown products/off flavors?

A
  • peroxy radicals
  • alkoxy radicals
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22
Q

What is peroxide value used for? (2)

A
  • common test to evaluate oxidative status of oil
  • measures only hydroperoxides, which are primary oxidation products
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23
Q

reaction of hydroperoxide with potassium iodine?
- in what kind of environment?

A

2 KI + 2 ROOH –> I2 + 2 KOH + 2 ROH
- in acidic environment (H+)

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24
Q

how is iodine released from hydroperoxide reaction with KI measured?

A
  • by titration with standardized sodium thiosulfate
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25
Q

Peroxide value defined as ?

A

-milli-equivalents of peroxide oxygen per 1000 g of fat

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26
Q

Trend of peroxide value over time? (5)

A
  • 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)
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27
Q

one can obtain low peroxide value when fat is rancid –> solution?

A
  • PV correlated with sensory data/taste panel to provide one with indicator of acceptability
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28
Q

What test looks at secondary breakdown products? vs primary reaction products?

A
  • primary: peroxide value
  • secondary: thiobarbituric acid test (TBA)
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29
Q

TBA test measures what? on what assumption?

A
  • 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
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30
Q

How is malonaldehyde measured? (3 ways)

A
  • 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
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31
Q

do we need sensory evaluation and taste panel when TBA test?

A

yes! because malonaldehyde formation is a function of the lipid makeup

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32
Q

2 methods to evaluate secondary oxidation products (apart from TBA test)

A
  • anisidine value (AV)
  • carbonyl number
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33
Q

rate of autoxidation is a function of __________________ of FA in lipid system

A

degree of unsaturation

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34
Q

relative rates of autoxidation:
1x, 100x, 1200x, 2500x
C18:0, C18:1, C18:2, C18:3
which is associated with which?

A

1x, 100x, 1200x, 2500x
C18:0, C18:1, C18:2, C18:3
- C18:3 autoxidation is 2500x faster than stearic acid

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35
Q

Why are double bonds more susceptible to abstraction of Hydrogen? (2 ish) consequence?

A

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

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36
Q

can autoxidation form isomers?

A

yes! because of resonance, number of isomeric hydroperoxides (positional or cis/trans) depends on which FR form exists at time of oxygen attacks

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37
Q

unconjugated double bonds have (more/less?) labile methylene group btw double bonds once resonance is initiated
- how many possible positional FR forms can exist?

A

more!
- 6 positional FR forms! + their corresponding cis/trans isomers

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38
Q

Once the free radical has reacted with molecular oxygen, the linolenic hydroperoxide are very ___________- and will undergo almost immediate ______________

A
  • unstable
  • decomposition
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39
Q

linolenic acid or FA having high degrees of unsaturation rapidly form low molecular weight aldehydes, ketones, alcohols and carboxylic acids, even before ?

A

detectable peroxide levels develop

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40
Q

high __________ and rapid ____________ of polyunsaturated FA explains phenomena of ______________ where there is rapid onset of ____________ without an apparent rise in peroxide value

A
  • reactivity
  • decomposition
  • reversion
  • rancidity
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41
Q

reversion = _________ ___________ reaction, which does not allow ____________ to linger
- reversion vs rancidity time frame?

A
  • accelerated oxidation reaction
  • hydroperoxides
  • reversion = minutes
  • rancidity = months
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42
Q

Singlet oxygen-promoted hydroperoxide formation? what happens?

A

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

43
Q

is the double bond lost during autoxidation?

A

NO!

44
Q

autoxidation –> lipid system increasing/decreasing disorder by incorporating oxygen and becoming more polar

A

increasing

45
Q

is autoxidation only limited to fats and oils?

A

no! also in products containing fat and even low fat

46
Q

Factors affecting autoxidation (10)

A
  • 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)
47
Q

singlet vs triplet oxygen –> which one is excited and which one is ground state?

A
  • singlet = excited
  • triplet = ground state
48
Q

singlet or triplet:
- diradical nature
- no unpaired electrons = electron rich

A
  • diradical nature –> triplet
  • no unpaired e- –> singlet
49
Q

molecular orbital for triplet vs singlet oxygen

A

triplet: e- in separate antibonding orbitals
singlet: e- in the same antibonding orbital

50
Q

singlet or triplet reacts more quickly?

A

singlet!

51
Q

how to generate singlet oxygen? Mainly using ? But also …

A

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

52
Q

photosensitizers absorb light in what range?

A

380 - 900 nm

53
Q

emission of absorbed light occurs by both ___________ (rapid) and ______________ (delayed), reflecting 2 separate electronically excited state

A
  • fluorescence
  • phosphorescence
54
Q

how does a singlet state oxygen get excited?
- long lifetime?
- how does it decay (2 ways)?

A
  • 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
55
Q

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

A
  • slow rate –> varies depending on sensitizer
  • ground-state (triplet) oxygen –> to produce singlet oxygen
56
Q

how does chlorophyll form a singlet oxygen (review diagram too)?

A
  • 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
57
Q

what is reversion flavor in soybean oil?

A

development of characteristic beany and grassy flavor in soybean oil prior to development of rancidity

58
Q

what is the end product of breakdown of hydroperoxides formed by reaction of linolenic acid with singlet oxygen? (in soybean oil)

A

2-pentenylfuran

59
Q

What are antioxidants?

A

natural or synthetic chemical compounds that can delay the start or slow the rate of lipid oxidation in food systems

60
Q

can authoxidation be completely stopped?

A

No! only slowed down

61
Q

what is the common structure between synthetic antioxidants BHA, BHT, TBHQ, PG?

A

aromatic ring + OH

62
Q

true antioxidants have property of interrupting or retarding ______________ step of autoxidation process

A

propagation

63
Q

what happens during the propagation step/what does the peroxy radical want to do?
- how can a true antioxidant stop that?

A

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

64
Q

How does the antioxidant free radical differ from FA free radical?

A

antioxidant has poor tendancy to react with molecular oxygen

65
Q

Antioxidant free radical prefers combining with ___________________ to form _________ species

A

other free radicals (not oxygen) to form neutral species

66
Q

what is the induction period?
- how can it be lengthened?

A
  • time before monomolecular reactions start to occur
  • lengthened by antioxidants’ activity!
67
Q

why are vegetable oils generally less susceptible to autoxidation than animal fats? (think of antioxidants)

A

because vitamin E is not naturally present in animal fats

68
Q

are some antioxidants toxic?

A

yes! like gossypol found in cottonseed oil

69
Q

antioxidants are usually used as ___________ why?

A
  • combinations
  • because a mixture tends to be more effective than a single antioxidant by itself
70
Q

typical commercial antioxidant product is TENOX BHA –> mixture of 3 + carrier?

A
  • 20% BHA
  • 6% propyl gallate
  • 4% citric acid
  • propylene glycol as carrier
71
Q

what is the antioxidant legal limit? based on what?

A

< 0.02% (200 ppm)
- based on fat content of product

72
Q

what happens if too much antioxidant?

A

antioxidant can become a pro-oxidant

73
Q

antioxidants acts like __________ agent, (slowing down/accelerating) rate of change

A

buffering
- slowing down

74
Q

how is antioxidant free radical stabilized? long enough for what?

A
  • resonance structures! from all double bonds on the ring
  • lasts long enough to meet another free radical
75
Q

what does E° mean? (reduction potential)

A

ability to pull an electron

76
Q

are synthetic antioxidants stable under normal conditions?

A

yes! generally quite stable

77
Q

Propyl gallate breaks down in ______ _____ or in systems with high ___ –> can form a (color?) complex with ferrous ions –> discoloring fat like lard

A
  • frying fats
  • high pH
  • blue complex
78
Q

BHA and BHT are slowly lost via ___________ as fat is heated

A

volatilization

79
Q

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?

A

low, but high enough
- packaging materials
- food
- dry cereals (can’t spray them with antioxidant)

80
Q

metal ions are powerful catalysts for lipid oxidation –> they assist in catalyzing what?

A

breakdown of hydroperoxides to form alkoxy and peroxy radicals

81
Q

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?

A
  • shorten
  • increase
  • no! only trace amounts (ppm) required
82
Q

what causes steric hindrance, eliminating catalytic role metal ions would otherwise play?

A

chelation of metal ions!

83
Q

Are chelating agents true antioxidants? why or why not?
what are the commonly termed?

A

no! because they don’t interfere with primary autoxidation mechanism itself
- commonly termed synergistic agents or synergists

84
Q

Examples of synergists (5)

A
  • chelating agents
  • ascorbic acids
  • phosphates
  • EDTA
  • phospholipids
85
Q

how do metal ions mix into products/foods? (2)

A
  • from metallic processing material (Cu 2+ and Fe2+)
  • from metallo-porphyrin (chlorophyll or myoglobin)
86
Q

downside of chelating agents?

A

allow formation of more reactive singlet oxygen, which can attack lipids directly –> hence why antioxidants are still required

87
Q

what does lipoxygenase catalyze?

A
  • direct oxidation of polyunsaturated FA that have a cis-cis 1,4 pentadiene group
  • direct formation of hydroperoxides
88
Q

how to protect frozen/dehydrated vegetables from rapid autoxidation? what happens if no success?

A
  • blanching! (+ sprayed with citric acid)
  • produces hay-like/cardboard aroma
89
Q

the __________ of many pigments (chlorophyll and beta-carotene) which are highly (sat/unsat) occurs as a result of ?

A
  • bleaching
  • unsat
  • autoxidation initiated by lipoxygenase
90
Q

how to reduce effects of lipoxygenase and lipase during oil crushing?

A

heat is applied!

91
Q

5 preventive antioxidant examples

A
  • superoxide dismutase
  • catalase
  • glutathione peroxidase
  • single oxygen quenchers
  • transition metal chelators (EDTA)
92
Q

radical scavenging antioxidants break FR chain reaction by?
- 4 examples

A
  • donating hydrogen to free radicals
  • vitamin C + tocopherol + quercetin + anthocyanin
93
Q

Which free radical has highest reduction potential (E°)? (order them from highest to lowest)
HOO°, RO°, HO°, ROO°, R°, antioxidants

A

HO° > RO° > HOO° > ROO° > R° > antioxidants

94
Q

initiation of free radical formation can be delayed by use of (3)

A
  • metal chelating agents
  • singlet oxygen inhibitors
  • peroxide stabilizers
95
Q

propagation of free radicals chain reactions can be minimized by (2)

A
  • donation of hydrogen from antioxidants
  • metal chelating agents
96
Q

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?

A
  • monohydroxy or polyhydroxy phenol
  • low –> <600
  • no because of stabilization of delocalization of radical electron
  • no due to its stability
97
Q

Natural or synthetic antioxidants?
- sesamol
- propyl gallate
- tertiary-butylhydroquinone (TBHQ)
- butylated hydroxyanisole (BHA)
- gossypol
- tocopherols
- Butylated hydroxytoluene (BHT)
- nordihydroguaretic acid (NDGA)

A
  • 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
98
Q

how to metal chelators deactivate trace metals that are free or in salts of FA?

A

by formation of complex ions or coordination compounds

99
Q

examples of metal chelators (4)

A
  • phosphoric acid
  • citric acid
  • ascorbic acid
  • ethylenediaminetetraacetic acid (EDTA)
100
Q

what is synergism of antioxidants/how does it occur?

A

when mixtures of antioxidants produce a more pronounced activity than sum of activities of individual antioxidants when used separately

101
Q

to have maximum efficiency, primary antioxidants are often used in combination with (2)

A
  • other phenolic antioxidants
  • various metal chelating agents
102
Q

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

A
  • low
  • high
  • low
  • high
103
Q

should antioxidants be fat soluble?
should they be absorbed by body?

A
  • fat soluble!
  • not absorbed by body, if possible

Food Chemistry 1 (22 decks)

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