Lipids lecture 7 + 8 Flashcards
what is autoxidation?
- spontaneous and self-sustaining reaction.
- It is a free radical reaction of unsaturated fatty acids with MOLECULAR OXYGEN: The overall result is the development of “rancidity”.
what is randicity?
undesirable off-flavors and odors
what does extensive autoxidation lead to?
losses in functionality and a decrease in nutritional value - the oil may eventually become toxic
overall result of autoxidation?
spoilage of edible fats and oils - usually become inedible.
3 steps of aotuoxidation rxn?
(1) Initiation
(2) Propagation
(3) Termination
2 actions in atuoxidation?
(1) A hydrogen must be abstracted from the fatty acid chain (2) Molecular oxygen must be present
Hydrogen can be abstracted from the fatty acid chain by…
• Light or ionizing radiation (h )
• Thermal energy (heat)
• Presence of metallic cations: Cu2+, Fe2+
• Enzymatic catalysis - i.e., lipoxygenase
• Reactive singlet oxygen produced by photosensitizers such as
chlorophyll or myoglobin
or combinations/permutations of the above
describe homolytic cleave of a covalent bond to produce free radicals
RH –> R. + H.
where R is rest of the FA
how long do free radicals survive for?
10^-9 to 10^-12
what happens to free radicals in absence of O2?
free radicals quickly recombine
H. + H. –> H-H (H2)
thus, minimal changes occur to FA
when does a propagating rxn start?
in presence of oxygen
O2 reacts w/ free acid free radical and forms a new PEROXY RADICAL
what is formed in a propagating rxn?
peroxy radical
what does the peroxy radical do? what is the result of this?
has preference for terminating its free radical state by taking a H from another FA
ROO. + RH –> ROOH + R.
Net result: formation of a hydroperoxide (ROOH) and a new fatty acid radical (R.), which in turn can again react with molecular oxygen. = becomes self-propagating = atuoxidation
describe termination step
when 2 free radicals react with each other = neutral/unreactive species is formed = free radical chain rxn terminates
describe accumulation of hydroperoxides
when propagation reaction is more frequent than the termination reaction
(many peroxy radicals being foromed = accumulation)
what are primary oxidation products?
hydroperoxides (R-O-O-H) and peroxides (R-O-O-R).
hydroperoxides vs peroxides?
hydroperoxides (R-O-O-H)
peroxides (R-O-O-R).
flavor/odor of hydroperoxides?
not source of flavors/odors, but are their precursors
what are compounds responsible for off flavors?
low-molecular- weight aldehydes, alcohols, ketones and short-chain fatty acids.
stability of hydroperoxides?
not chemically stable
2 hydroperoxide breakdown mechanisms?
when does which one predominate?
(1) Monomolecular reaction
(2) Bimolecular reaction
Both occur simultaneously, but one predominates early on in the reaction, the other later on.
The monomolecular mechanism predominates while the hydroperoxide concentration is relatively low.
The bimolecular reaction predominates when the hydroperoxide concentration becomes relatively high.
describe the monomolecular rxn
ROOH –> RO. + OH.
formation of an ALKOXY radical and a HYDROXY radical (has more reactivity than peroxyradicals. has short life span)
decomposition reactions of the alkoxy radical?
- aldehyde generation
- alcohol generation
- formation of ketone
describe aldehyde generation
a alkoxy radical decomposition reaction
the unstable alkoxy radical steals an electron from the R2 portion, providing the electron to form the double bond.
thus, an aldehyde is formed with the release of a free radical (R2.) = perpetuating the autoxidation
aldehyde is not always stable = may be oxidized or reduced
describe alcohol generation
The alkoxy radical steals a hydrogen from R3H to form the alcohol = free radical is formed to carry on the rxn
describe formation of a ketone
H is abstracted from radical chain
rxn is a termination step
describe bimolecular rxn
reaction takes place at higher hydroperoxide concentrations
forms RO. + ROO. + H2O (alkoxy and peroxy radicals)
what do bimolecular vs monomolecular rxns form?
mono: alkoxy and hydroxy
bi: alkoxy and peroxy
what do peroxy radicals prefer to abstract a H from?
FA
what can decomposition of unstable alkoxy radicals lead to?
chain scission and the formation of aldehydes, ketones, alcohols, or carboxylic acids.
describe breakdown of hydroperoxide?
first in monomolecular rxn (alkoxy radical) = decomposes to short chain deldehydes and ketones and more hydroperoxides
what are propagators of the autoxidation rxn?
result of rate of rxn?
formation of peroxy
net result: rxn starts slow and increases exponentially
what is PV?
peroxide value: milli-equivalents (mEq) of peroxide oxygen per 1000 grams of fat.
text to evalueate oxidative status of an oil
measures the hydroperoxides which are considered primary oxidation products
what are hydroperoxides considered?
primary oxidation products
what do hydroperoxides release in acidic environments?
how is amount of I2 released measured?
releases I2 from KI
I2 is measured by titration with standardized sodium thiosulfate.
what is the induction period in accumulation of hydroperoxides?
monomolecular rxn
slow initial accumulation of ROOH
what rxn is at the rapid rise in the PV?
bimolecular rxn
what causes decrease in PV after propagation step?
more decomp of hydroperoxides than formation
what does metamorphosis of aldehydes and ketones cause?
flavor and odors = causes lipids to be organoleptically unacceptable
limits of the PV test?
rancid and fats that have just began the reaction will both have low PV
is there off flavor in the induction period?
no b/c ROOH are mostly being formed, not broken down (off flavor occurs during breakdown)
what is the TBA test?
thiobarbituric acid test
looks at one of the many secondary breakdown products of the autoxidation process ie malonaldehyde.
asummes that quantity of malonaldehyde produced directly proportional to the degree of oxidation which can be correlated to flavor defects
reactants and products of the TBA test?
malonaldehyde reacts with 2 molecules of TBA (thiobarbituric acid).
produces a red complex (can be tested spectrophotometrically)
result of high reactivity of malonaldehyde?
it is often attached to proteins and needs to be released with acid
what can malonaldehyde also be quantified with?
HPLC
malonaldehyde formation is a function of….
the lipid makeup
what are other secondary oxidation product tests besides TBA?
AV (anisidine value): reaction of p-anisidine with aldehydes, THEIR reaction adduct measured in the UV
carbonyl number
define rate of autoxidation
function of the degree of unsaturation of the fatty acids in the lipid system
factors that initiate oxidation of FA
heat, radiant energy, enzymes divalent metal ions and singlet oxygen
molecules with DB = electrons from DB can resonate and stabilize free radical structure = more susceptible to abstraction of a hydrogen, because the LIFETIME of the free radical formed is extended
where are H abstracted from?
alpha position, next to DB b/c more labile (constantly changing)
what may occur during formation and decomp of peroxy radicals?
cis/trans isomers
natural form of linolenic?
unconjugated
readily converted into mixed bonding system
how many methylene carbons in linoleic? linolenic? arachidonic?
linoleic: 1
linolenic: 2
arachidonic: 3
