7 Autoxidation ctd Flashcards
what is it called when you break a bond into a radical and a hydrogen?
homolytic cleavage
what are the factors that influence autoxidation (11)
- energy (heat/light)
- catalyst
- double bonds
- enzymes
- chemical oxidants
- oxygen content/type of oxygen
- antioxidants
- phospholipids
- FAs
- mono/diglycerides
- polymers
what are the possible products of termination?
- dimers (R R)
- peroxide + O2 (ROO ROO)
- ether (RO R)
- peroxide (ROO R)
what are the products of autoxidation of linoleic acid?
pentane, decadi-something (determines shelf life of chips)
how many types of hydroperoxides can linoleic acid produce?
4, 2 for each bond
what happens during hydroperoxide decomposition?
the FA is cleaved, potentially on either bond around the ROOH, resulting in 4 possible products
what’s the difference between singlet and triplet oxygen?
in singlet, the electrons are pointing in opposite directions. this configuration makes it extremely reactive
what generates singlet oxygen?
- pigment (e.g. chlorophyll, riboflavin, hematoporphyrines)
- dyes (methylene blue, roe bengal, eosin, crystal violet, acridian orange)
- enzymes that form superoxides
does processing extra virgin olive oil prolong its shelf life?
yes - by removing the chlorophyll
when you excite a photosensitizers and oxygen is present, what happens?
it transfers its electron energy to oxygen
what is the wavelength range for light absorbance in photosensitizers?
380-900 nm (visual spectrum)
the most effective photosensitizers have the ______ _______ ________ of a long-lived triplet state
high quantum yield
what happens when light is absorbed by a photosensitizer?
excited singlet state occurs
how is a triplet state achieved in a photosensitizer
singlet state goes through intesystem crossing (ICS)
how is a singlet oxygen formed?
triplets state of the photosynthesizer is transferred to a triplet oxygen forming a singlet state, which is extremely reactive to form ROOH
how do we know that death is basically inevitable?
there’s too many ways to create singlet oxygen lollll
how does singlet oxygen add itself to a FA?
CYCLOADDITION it adds directly - it doesn’t need to wait for a radical
what is the byproduct of interest from oil oxidation (specifically reversion)?
2-(2-pentyl)-furan
describe the steps of singlet oxygen oxidation to 2-(2-pentyl)-furan
- 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
what does lipoxygenase do?
- destroys FAs
- free radicals produced damage other compounds (vits/proteins)
- develops hay-like flavor
- can be used to generate certain desired properties in perfumes
true or false: lipoxygenases are selective
true. they will only react with unconjugated double bonds separated by a methylene group that are in cis-cis conformation
what is the substrate for lipoxygenase?
cis,cis-1,4-pentadiene
the methylene group of the substrate is on what omega position?
8
the enzyme is _____specific
stereo (requires cis,cis)
describe the mechanism of lipoxygenase oxidation
- 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
true or false: it takes more energy to remove a H from an antioxidant than a fatty acid
false - it takes less energy (300-500 mV) as opposed to the FA’s 1000 mV. this makes it react faster than the FA.
true or false: the radical formed by an antioxidant is relatively stable
true - this is due to the possibility of resonance due to the phenol
what compounds result in rapid deterioration of fat?
- 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
how do you eliminate this reaction?
- 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)
superoxides are precursors of
singlet oxygen. this is why destroying superoxides you limit the amount of singlet oxygen being formed
describe the superoxide dismutase reaction
- 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
what is superoxide?
anionic O2
describe the beta carotene reaction
singlet O2 + singlet beta carotene –> triplet O2 + triplet bet carotene
what are the scavenging antioxidants?
- vit C
- tocopherol
- quercetin
- anthocyanin
what do scavenging antioxidants do?
break free radical chain by donating H to free radicals
why be wary of transition metal ions?
ex) iron can be reduced by our FA to form a radical
- also activates singlet oxygen
the ____ the _____ potential in mV the _____ the species affinity for electrons. hence the _____ its tendency to be _____.
greater; reduction; greater; greater; reduced
better antioxidants have (higher/lower) reduction potential
lower
why does ascorbate have such a low reducing potential?
it has 4 oxygens that can pass around a radical
how is propagation of free radical chain rxns minimized?
donation of H from antioxidants and metal chelating agents
what do antioxidants do?
- delay initiation by donating H to radicals
- terminate reaction radicals with their own radical
what do antioxidants do the the induction period?
delay
why is ascorbic acid a worse antioxidant for a cereal box?
it’s solid and not volatile compared to BHT. it also doesn’t have as long of an induction period
name some chelators
- phosphoric acid
- citric acid
- ascorbic acid
- EDTA
expand EDTA
ethylene-diamine-tetra-acetate
what do chelators do?
bind to metal to make them inaccessible to react with lipids
how can synergy occur?
- phenolic antioxidants
- metal chelating agents
what is synergism?
whe mixtures of antioxidants produce a more pronounced activity than the sum of the activities of the individual antioxidants separatey
what factors effect antioxidant efficiency?
- (low) activation energy
- (high) oxidation potential
- (low) reduction potential
- stability of pH
- high oil solubility
what’s the permitted level of antioxidant according to the MIA and PIA?
200 ppm (mg/kg)
ideal antioxidant characteristics:
- 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
what are the natural antioxidants?
- tocopherols
- nordihydroguaretic acid (NDGA)
- sesamol
- gossypol
what are the synthetic antioxidants?
- BHA
- BHT
- TBHQ
- PG
how is flavor quality evaluated?
- sensory evaluation (trained ppl, controlled conditions)
- peroxide value (amount of peroxide)
- benzidine test (measures aldehydes –> yellow)
- TBA test (2 malonaldehydes –> red)
expand SPME
solid phase microextension
the higher the GC, the _____ the sensory value
lower
what happens when you fry a potato chip?
- water is lost from potato the the oil, leading to hydrolysis to free fatty acids
- oxidation to hydroperoxides from aeration
- dehydration of product
what would your potato chip taste like if you did it under atmospheric nitrogen?
- 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
