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food chem 1 > 6 Autoxidation & Antioxidants > Flashcards

6 Autoxidation & Antioxidants Flashcards

1
Q

define autoxidation

A

spontaneous free radical rxn with oxygen. results in rancidity and loss in functionality and nutritional value. extensive autoxidation can lead to toxicity.

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2
Q

what is rancidity?

A

describes a wide variety of undesired flavors/odors associated with oxidized fats and oils

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3
Q

what are the 3 steps of autoxidation?

A

1) initiation
2) propagation
3) termination

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4
Q

what happens during the initiation step of the autoxidation mech?

A
  • H is abstracted from FA chain
  • O2 must be present
  • homolytic cleavage of H from R
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5
Q

what are the methods of H abstraction?

A
  • hv (light/ionizing radiation)
  • thermal energy (heat)
  • presence of metallic cations (Cu2+, Fe2+)
  • enzymatic catalysis (lipoxygenase)
  • reactive singlet oxygen (chlorophyll, myoglobin)
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6
Q

what happens during the propagation step of the autoxidation mech?

A
  • R and H radicals mingle to make RH, H2, and R-R molecules

- an R radical can attack O2 to make a PEROXYRADICAL

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7
Q

how does a peroxyradical prefer to terminate?

A

by abstracting a H from another FA

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8
Q

what happens during the termination step of the autoxidation mech?

A
  • ROO radical reacts with ROO radical or RH radical

- forms ROOR + O2 or ROOH respectively

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9
Q

what’s the general formula for a hydroperoxide?

A

ROOH

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10
Q

statistically, what is the predominant result of our FAs being in the presence of O2?

A

accumulation of hydroperoxides over time

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11
Q

what happens when the propagation reaction is more frequent than the termination reaction?

A

you get a net accumulation of hydroperoxides

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12
Q

what to hydroperoxides do?

A

they act as precursors of aldehydes, alcohols, ketones, and short chain fatty acids

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13
Q

how are rancid compounds formed?

A

2 possible reactions:

1) monomolecular; [ROOH] low
2) bimolecular; [ROOH] high

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14
Q

what happens during the monomolecular reaction of ROOH?

A
  • ROOH undergoes homolytic cleavage to RO and OH radicals
  • RO* = alkoxy radical
  • OH* = hydroxy radical
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15
Q

what happens during the biomlecular reaction of ROOH?

A

2 ROOH –> RO* + ROO* + H2O

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16
Q

what is PV?

A

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)
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17
Q

what is the slow initial accumulation of hydroperoxides called?

A

induction period

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18
Q

what is the end of the induction period signaled by?

A

rapid rise in PV (decomposition of peroxides outpaces their formation)

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19
Q

what is the TBA test?

A

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
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20
Q

what is the AV test?

A
  • Anisidine value - reaction of p-anisidine with aldehydes

- adduct measured in UV

21
Q

what are the secondary oxidation tests?

A
  • TBA
  • AV
  • carbonyl number
22
Q

what are the relative rates of oxidation for the C18 carbons?

A

18:0 (x1), 18:1 (x100), 18:2 (1200), 18:3 (2500)

23
Q

what factors influence the rate of oxidation of FAs?

A
  • heat
  • radiant energy
  • enzymes
  • divalent metal ions
  • singlet oxygen
24
Q

are unconjugated FAs more or less liable to be oxidized?

A

more, because they have a methylene group for resonance structures to revolve around

25
Q

why is linolenic acid more readily oxidized?

A

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.

26
Q

why is TBA and AV tests more useful in the detection of rapid compound derived from linolenic FAs?

A

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

27
Q

what is reversion?

A

rapid onset of rancidity before an apparent rise in peroxide values are detected

28
Q

what oils are susceptible to reversion?

A

soybean and fish

29
Q

what happens as a result of the formation of peroxides?

A
  • increased entropy

- increased polarity

30
Q

what do antioxidants do?

A
  • lengthens the induction period
  • interrupts the propagation step of autoxidation
  • stabilizes ROOHs
  • competes for binding with O2
  • removes oxygen
31
Q

what properties make a good antioxidant?

A
  • 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
32
Q 
expand:
BHA
BHT
TBHQ
PG
A 
BHA = butylated hydroxyanisole
BHT = butylated hydroxytoluene
TBHQ = Tert-butyl hydroquinone
PG = propyl gallate
33
Q

are pure antioxidants (natural) or are antioxidant mixtures (synthetic) more effective?

A

mixtures (synthetic)

34
Q

What is TENOX a mixture of?

A

20% BHA, 6% propyl gallate, 4% citric acid

35
Q

what is the antioxidant legal limit?

A

<0.02% or 200 ppm

36
Q

what happens when you have a large concentration of antioxidant?

A

it becomes a pro-radical/pro-oxidant.

37
Q

what conditions break down PG, BHT, and BHA?

A

PG: break down in frying fats with high pH

BHA & BHT: lost via volatilization as fat is heated

38
Q

the vapor pressure of BHA & BHT is (high/low)?

A

low, but high enough to migrate into food

39
Q

what other compounds can limit oxidation? (but technically not antioxidants bc they don’t mess with the radicals)

A

chelators, ascorbic acid, EDTA, and phospholipids

40
Q

what compounds catalyze oxidation?

A

metal cations - they shorten the induction period

41
Q

how much metal cation is needed to speed up the rxn?

A

trace (ppm) amounts

42
Q

what are chelators considered?

A

synergistic compounds/synergists

43
Q

what do metal ions do?

A

allow the formation of singlet oxygen (more reactive); can attack lipids directly

44
Q

what is the significance of lipoxygenase (lipoxidase)?

A
  • catalyzes direct oxidation of PUFAs which have a cis, cis-1,4-pentadiene group
  • found in most PUFAs
45
Q

what’s the purpose of blanching?

A

deactivate lipoxygase to prevent oxidation of produce

46
Q

what happens to pigments like beta carotene or chlorophyll as a result of autoxidation?

A

bleaching

47
Q

how do we reduce the enzyme activity in oilseed crush?

A

heat to inactivate enzymes

48
Q

what is responsible for the beany flavor of soy milk?

A

lipoxygenase

food chem 1 (12 decks)

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