Lipids lecture 7 + 8 Flashcards

1
Q

what is autoxidation?

A
  • 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”.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

what is randicity?

A

undesirable off-flavors and odors

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what does extensive autoxidation lead to?

A

losses in functionality and a decrease in nutritional value - the oil may eventually become toxic

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

overall result of autoxidation?

A

spoilage of edible fats and oils - usually become inedible.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

3 steps of aotuoxidation rxn?

A

(1) Initiation
(2) Propagation
(3) Termination

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

2 actions in atuoxidation?

A

(1) A hydrogen must be abstracted from the fatty acid chain (2) Molecular oxygen must be present

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Hydrogen can be abstracted from the fatty acid chain by…

A

• 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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

describe homolytic cleave of a covalent bond to produce free radicals

A

RH –> R. + H.

where R is rest of the FA

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

how long do free radicals survive for?

A

10^-9 to 10^-12

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what happens to free radicals in absence of O2?

A

free radicals quickly recombine

H. + H. –> H-H (H2)

thus, minimal changes occur to FA

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

when does a propagating rxn start?

A

in presence of oxygen

O2 reacts w/ free acid free radical and forms a new PEROXY RADICAL

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

what is formed in a propagating rxn?

A

peroxy radical

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

what does the peroxy radical do? what is the result of this?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

describe termination step

A

when 2 free radicals react with each other = neutral/unreactive species is formed = free radical chain rxn terminates

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

describe accumulation of hydroperoxides

A

when propagation reaction is more frequent than the termination reaction

(many peroxy radicals being foromed = accumulation)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what are primary oxidation products?

A

hydroperoxides (R-O-O-H) and peroxides (R-O-O-R).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

hydroperoxides vs peroxides?

A

hydroperoxides (R-O-O-H)

peroxides (R-O-O-R).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

flavor/odor of hydroperoxides?

A

not source of flavors/odors, but are their precursors

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

what are compounds responsible for off flavors?

A

low-molecular- weight aldehydes, alcohols, ketones and short-chain fatty acids.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

stability of hydroperoxides?

A

not chemically stable

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

2 hydroperoxide breakdown mechanisms?

when does which one predominate?

A

(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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

describe the monomolecular rxn

A

ROOH –> RO. + OH.

formation of an ALKOXY radical and a HYDROXY radical (has more reactivity than peroxyradicals. has short life span)

23
Q

decomposition reactions of the alkoxy radical?

A
  1. aldehyde generation
  2. alcohol generation
  3. formation of ketone
24
Q

describe aldehyde generation

A

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

25
describe alcohol generation
The alkoxy radical steals a hydrogen from R3H to form the alcohol = free radical is formed to carry on the rxn
26
describe formation of a ketone
H is abstracted from radical chain rxn is a termination step
27
describe bimolecular rxn
reaction takes place at higher hydroperoxide concentrations forms RO. + ROO. + H2O (alkoxy and peroxy radicals)
28
what do bimolecular vs monomolecular rxns form?
mono: alkoxy and hydroxy bi: alkoxy and peroxy
29
what do peroxy radicals prefer to abstract a H from?
FA
30
what can decomposition of unstable alkoxy radicals lead to?
chain scission and the formation of aldehydes, ketones, alcohols, or carboxylic acids.
31
describe breakdown of hydroperoxide?
first in monomolecular rxn (alkoxy radical) = decomposes to short chain deldehydes and ketones and more hydroperoxides
32
what are propagators of the autoxidation rxn? result of rate of rxn?
formation of peroxy net result: rxn starts slow and increases exponentially
33
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
34
what are hydroperoxides considered?
primary oxidation products
35
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.
36
what is the induction period in accumulation of hydroperoxides?
monomolecular rxn slow initial accumulation of ROOH
37
what rxn is at the rapid rise in the PV?
bimolecular rxn
38
what causes decrease in PV after propagation step?
more decomp of hydroperoxides than formation
39
what does metamorphosis of aldehydes and ketones cause?
flavor and odors = causes lipids to be organoleptically unacceptable
40
limits of the PV test?
rancid and fats that have just began the reaction will both have low PV
41
is there off flavor in the induction period?
no b/c ROOH are mostly being formed, not broken down (off flavor occurs during breakdown)
42
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
43
reactants and products of the TBA test?
malonaldehyde reacts with 2 molecules of TBA (thiobarbituric acid). produces a red complex (can be tested spectrophotometrically)
44
result of high reactivity of malonaldehyde?
it is often attached to proteins and needs to be released with acid
45
what can malonaldehyde also be quantified with?
HPLC
46
malonaldehyde formation is a function of....
the lipid makeup
47
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
48
define rate of autoxidation
function of the degree of unsaturation of the fatty acids in the lipid system
49
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
50
where are H abstracted from?
alpha position, next to DB b/c more labile (constantly changing)
51
what may occur during formation and decomp of peroxy radicals?
cis/trans isomers
52
natural form of linolenic?
unconjugated readily converted into mixed bonding system
53
how many methylene carbons in linoleic? linolenic? arachidonic?
linoleic: 1 linolenic: 2 arachidonic: 3