6 Autoxidation & Antioxidants

Question 1

define autoxidation

Answer 1

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

Question 2

what is rancidity?

Answer 2

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

Question 3

what are the 3 steps of autoxidation?

Answer 3

1) initiation
2) propagation
3) termination

Question 4

what happens during the initiation step of the autoxidation mech?

Answer 4

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

Question 5

what are the methods of H abstraction?

Answer 5

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

Question 6

what happens during the propagation step of the autoxidation mech?

Answer 6

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

- an R radical can attack O2 to make a PEROXYRADICAL

Question 7

how does a peroxyradical prefer to terminate?

Answer 7

by abstracting a H from another FA

Question 8

what happens during the termination step of the autoxidation mech?

Answer 8

• ROO radical reacts with ROO radical or RH radical

- forms ROOR + O2 or ROOH respectively

Question 9

what’s the general formula for a hydroperoxide?

Answer 9

ROOH

Question 10

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

Answer 10

accumulation of hydroperoxides over time

Question 11

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

Answer 11

you get a net accumulation of hydroperoxides

Question 12

what to hydroperoxides do?

Answer 12

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

Question 13

how are rancid compounds formed?

Answer 13

2 possible reactions:

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

Question 14

what happens during the monomolecular reaction of ROOH?

Answer 14

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

Question 15

what happens during the biomlecular reaction of ROOH?

Answer 15

2 ROOH –> RO* + ROO* + H2O

Question 16

what is PV?

Answer 16

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)

Question 17

what is the slow initial accumulation of hydroperoxides called?

Answer 17

induction period

Question 18

what is the end of the induction period signaled by?

Answer 18

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

Question 19

what is the TBA test?

Answer 19

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

Question 20

what is the AV test?

Answer 20

• Anisidine value - reaction of p-anisidine with aldehydes

- adduct measured in UV

Question 21

what are the secondary oxidation tests?

Answer 21

• TBA
• AV
• carbonyl number

Question 22

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

Answer 22

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

Question 23

what factors influence the rate of oxidation of FAs?

Answer 23

• heat
• radiant energy
• enzymes
• divalent metal ions
• singlet oxygen

Question 24

are unconjugated FAs more or less liable to be oxidized?

Answer 24

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

Question 25

why is linolenic acid more readily oxidized?

Answer 25

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.

Question 26

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

Answer 26

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

Question 27

what is reversion?

Answer 27

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

Question 28

what oils are susceptible to reversion?

Answer 28

soybean and fish

Question 29

what happens as a result of the formation of peroxides?

Answer 29

- increased entropy | - increased polarity

Question 30

what do antioxidants do?

Answer 30

- lengthens the induction period - interrupts the propagation step of autoxidation - stabilizes ROOHs - competes for binding with O2 - removes oxygen

Question 31

what properties make a good antioxidant?

Answer 31

- 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

Question 32

``` expand: BHA BHT TBHQ PG ```

Answer 32

``` BHA = butylated hydroxyanisole BHT = butylated hydroxytoluene TBHQ = Tert-butyl hydroquinone PG = propyl gallate ```

Question 33

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

Answer 33

mixtures (synthetic)

Question 34

What is TENOX a mixture of?

Answer 34

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

Question 35

what is the antioxidant legal limit?

Answer 35

<0.02% or 200 ppm

Question 36

what happens when you have a large concentration of antioxidant?

Answer 36

it becomes a pro-radical/pro-oxidant.

Question 37

what conditions break down PG, BHT, and BHA?

Answer 37

PG: break down in frying fats with high pH | BHA & BHT: lost via volatilization as fat is heated

Question 38

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

Answer 38

low, but high enough to migrate into food

Question 39

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

Answer 39

chelators, ascorbic acid, EDTA, and phospholipids

Question 40

what compounds catalyze oxidation?

Answer 40

metal cations - they shorten the induction period

Question 41

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

Answer 41

trace (ppm) amounts

Question 42

what are chelators considered?

Answer 42

synergistic compounds/synergists

Question 43

what do metal ions do?

Answer 43

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

Question 44

what is the significance of lipoxygenase (lipoxidase)?

Answer 44

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

Question 45

what's the purpose of blanching?

Answer 45

deactivate lipoxygase to prevent oxidation of produce

Question 46

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

Answer 46

bleaching

Question 47

how do we reduce the enzyme activity in oilseed crush?

Answer 47

heat to inactivate enzymes

Question 48

what is responsible for the beany flavor of soy milk?

Answer 48

lipoxygenase