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food chem 1 > 7 Autoxidation ctd > Flashcards

7 Autoxidation ctd Flashcards

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1
Q

what is it called when you break a bond into a radical and a hydrogen?

A

homolytic cleavage

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2
Q

what are the factors that influence autoxidation (11)

A
  • energy (heat/light)
  • catalyst
  • double bonds
  • enzymes
  • chemical oxidants
  • oxygen content/type of oxygen
  • antioxidants
  • phospholipids
  • FAs
  • mono/diglycerides
  • polymers
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3
Q

what are the possible products of termination?

A
  • dimers (R R)
  • peroxide + O2 (ROO ROO)
  • ether (RO R)
  • peroxide (ROO R)
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4
Q

what are the products of autoxidation of linoleic acid?

A

pentane, decadi-something (determines shelf life of chips)

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5
Q

how many types of hydroperoxides can linoleic acid produce?

A

4, 2 for each bond

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6
Q

what happens during hydroperoxide decomposition?

A

the FA is cleaved, potentially on either bond around the ROOH, resulting in 4 possible products

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7
Q

what’s the difference between singlet and triplet oxygen?

A

in singlet, the electrons are pointing in opposite directions. this configuration makes it extremely reactive

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8
Q

what generates singlet oxygen?

A
  • pigment (e.g. chlorophyll, riboflavin, hematoporphyrines)
  • dyes (methylene blue, roe bengal, eosin, crystal violet, acridian orange)
  • enzymes that form superoxides
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9
Q

does processing extra virgin olive oil prolong its shelf life?

A

yes - by removing the chlorophyll

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10
Q

when you excite a photosensitizers and oxygen is present, what happens?

A

it transfers its electron energy to oxygen

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11
Q

what is the wavelength range for light absorbance in photosensitizers?

A

380-900 nm (visual spectrum)

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12
Q

the most effective photosensitizers have the ______ _______ ________ of a long-lived triplet state

A

high quantum yield

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13
Q

what happens when light is absorbed by a photosensitizer?

A

excited singlet state occurs

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14
Q

how is a triplet state achieved in a photosensitizer

A

singlet state goes through intesystem crossing (ICS)

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15
Q

how is a singlet oxygen formed?

A

triplets state of the photosynthesizer is transferred to a triplet oxygen forming a singlet state, which is extremely reactive to form ROOH

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16
Q

how do we know that death is basically inevitable?

A

there’s too many ways to create singlet oxygen lollll

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17
Q

how does singlet oxygen add itself to a FA?

A

CYCLOADDITION it adds directly - it doesn’t need to wait for a radical

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18
Q

what is the byproduct of interest from oil oxidation (specifically reversion)?

A

2-(2-pentyl)-furan

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19
Q

describe the steps of singlet oxygen oxidation to 2-(2-pentyl)-furan

A
  • singlet oxygen directly adds to a double bond on 9 position
  • O-O bond is unstable and breaks off leaving two O radicals
  • there is cleavage between C9 and C10 and the O radicals contribute their electrons to their respective Cs to form double bonds and therefore aldehydes
  • the aldehyde still containing double bonds can have O2 added again
  • this time the the C-O bond is broken, leaving a carbon radical and a peroxi which can pick up a hydrogen to form a hydroperoxide
  • the hydroperoxide breaks to form a OH radical, leaving behind the O radical. now there are 2 radicals present on your FA
  • radicals rearrange so that CH-O carbon’s hydrogen shifts to the radical carbon to make a methylene and the
  • the electron on the O radical moves to form a double bond to the carbon its attached to, leaving you with a molecule with both a ketone and aldehyde functional group
  • molecule undergoes resonance so double bonds exist on the aliphatic chain, shifting the Hs on the methylene groups to the Os to make two OH groups
  • one of the Hs and one of the OHs is lost in the form of H2O to form your furan
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20
Q

what does lipoxygenase do?

A
  • destroys FAs
  • free radicals produced damage other compounds (vits/proteins)
  • develops hay-like flavor
  • can be used to generate certain desired properties in perfumes
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21
Q

true or false: lipoxygenases are selective

A

true. they will only react with unconjugated double bonds separated by a methylene group that are in cis-cis conformation

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22
Q

what is the substrate for lipoxygenase?

A

cis,cis-1,4-pentadiene

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23
Q

the methylene group of the substrate is on what omega position?

A

8

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24
Q

the enzyme is _____specific

A

stereo (requires cis,cis)

25
Q

describe the mechanism of lipoxygenase oxidation

A
  • enzyme abstracts H from omega-8 position producing a free radical
  • FA free radical isomerizes to produce a conjugated db
  • O2 reacts with the free radical at omega 6 resulting in a peroxy free radical
  • H is tacked onto the free radical to form hydroperoxide
26
Q

true or false: it takes more energy to remove a H from an antioxidant than a fatty acid

A

false - it takes less energy (300-500 mV) as opposed to the FA’s 1000 mV. this makes it react faster than the FA.

27
Q

true or false: the radical formed by an antioxidant is relatively stable

A

true - this is due to the possibility of resonance due to the phenol

28
Q

what compounds result in rapid deterioration of fat?

A
  • hydroxy radicals (most reactive)
  • alcoxy radicals

they pull any H in sight to form radicals because they have a super high potential, which can form peroxides and continue propagation

29
Q

how do you eliminate this reaction?

A
  • reduce amount of singlet oxygen
  • take away methyl (which produces singlet oxygen)
  • use enzymes that can destroy reaction intermediates like singlet oxygen
  • have antioxidants
  • have enzymes that reduce singlet oxygen (glutathione)
30
Q

superoxides are precursors of

A

singlet oxygen. this is why destroying superoxides you limit the amount of singlet oxygen being formed

31
Q

describe the superoxide dismutase reaction

A
  • 2O2- + superoxide dismutase –> H2O2
  • H2O2 + catalase –> O2 + H2O

two molecules of glutathione oxidase are used to form a dimer, which is then reduced to reform glutathione. it’s a way of destroying superoxides

32
Q

what is superoxide?

A

anionic O2

33
Q

describe the beta carotene reaction

A

singlet O2 + singlet beta carotene –> triplet O2 + triplet bet carotene

34
Q

what are the scavenging antioxidants?

A
  • vit C
  • tocopherol
  • quercetin
  • anthocyanin
35
Q

what do scavenging antioxidants do?

A

break free radical chain by donating H to free radicals

36
Q

why be wary of transition metal ions?

A

ex) iron can be reduced by our FA to form a radical

- also activates singlet oxygen

37
Q

the ____ the _____ potential in mV the _____ the species affinity for electrons. hence the _____ its tendency to be _____.

A

greater; reduction; greater; greater; reduced

38
Q

better antioxidants have (higher/lower) reduction potential

A

lower

39
Q

why does ascorbate have such a low reducing potential?

A

it has 4 oxygens that can pass around a radical

40
Q

how is propagation of free radical chain rxns minimized?

A

donation of H from antioxidants and metal chelating agents

41
Q

what do antioxidants do?

A
  • delay initiation by donating H to radicals

- terminate reaction radicals with their own radical

42
Q

what do antioxidants do the the induction period?

A

delay

43
Q

why is ascorbic acid a worse antioxidant for a cereal box?

A

it’s solid and not volatile compared to BHT. it also doesn’t have as long of an induction period

44
Q

name some chelators

A
  • phosphoric acid
  • citric acid
  • ascorbic acid
  • EDTA
45
Q

expand EDTA

A

ethylene-diamine-tetra-acetate

46
Q

what do chelators do?

A

bind to metal to make them inaccessible to react with lipids

47
Q

how can synergy occur?

A
  • phenolic antioxidants

- metal chelating agents

48
Q

what is synergism?

A

whe mixtures of antioxidants produce a more pronounced activity than the sum of the activities of the individual antioxidants separatey

49
Q

what factors effect antioxidant efficiency?

A
  • (low) activation energy
  • (high) oxidation potential
  • (low) reduction potential
  • stability of pH
  • high oil solubility
50
Q

what’s the permitted level of antioxidant according to the MIA and PIA?

A

200 ppm (mg/kg)

51
Q

ideal antioxidant characteristics:

A
  • not physiologically harmful
  • doesn’t alter sensory properties of food
  • effective in low concentration
  • fat soluble
  • not destroyed during processing
  • not absorbable by body
  • economic
52
Q

what are the natural antioxidants?

A
  • tocopherols
  • nordihydroguaretic acid (NDGA)
  • sesamol
  • gossypol
53
Q

what are the synthetic antioxidants?

A
  • BHA
  • BHT
  • TBHQ
  • PG
54
Q

how is flavor quality evaluated?

A
  • sensory evaluation (trained ppl, controlled conditions)
  • peroxide value (amount of peroxide)
  • benzidine test (measures aldehydes –> yellow)
  • TBA test (2 malonaldehydes –> red)
55
Q

expand SPME

A

solid phase microextension

56
Q

the higher the GC, the _____ the sensory value

A

lower

57
Q

what happens when you fry a potato chip?

A
  • water is lost from potato the the oil, leading to hydrolysis to free fatty acids
  • oxidation to hydroperoxides from aeration
  • dehydration of product
58
Q

what would your potato chip taste like if you did it under atmospheric nitrogen?

A
  • no oxidation –> no aldehydes/ketones
  • you get a dried, bland potato chip. no flavor
  • though oxidation is bad for bulk oils, it’s very desirable during cooking to produce aroma and flavor

food chem 1 (12 decks)

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