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FDSC251 (Food chem 1) > Lipids lecture 7 + 8 > Flashcards

Lipids lecture 7 + 8 Flashcards

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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”.
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2
Q

what is randicity?

A

undesirable off-flavors and odors

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3
Q

what does extensive autoxidation lead to?

A

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

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4
Q

overall result of autoxidation?

A

spoilage of edible fats and oils - usually become inedible.

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5
Q

3 steps of aotuoxidation rxn?

A

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

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6
Q

2 actions in atuoxidation?

A

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

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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

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8
Q

describe homolytic cleave of a covalent bond to produce free radicals

A

RH –> R. + H.

where R is rest of the FA

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9
Q

how long do free radicals survive for?

A

10^-9 to 10^-12

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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

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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

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12
Q

what is formed in a propagating rxn?

A

peroxy radical

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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

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14
Q

describe termination step

A

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

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15
Q

describe accumulation of hydroperoxides

A

when propagation reaction is more frequent than the termination reaction

(many peroxy radicals being foromed = accumulation)

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16
Q

what are primary oxidation products?

A

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

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17
Q

hydroperoxides vs peroxides?

A

hydroperoxides (R-O-O-H)

peroxides (R-O-O-R).

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18
Q

flavor/odor of hydroperoxides?

A

not source of flavors/odors, but are their precursors

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19
Q

what are compounds responsible for off flavors?

A

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

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20
Q

stability of hydroperoxides?

A

not chemically stable

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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.

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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
Q

describe alcohol generation

A

The alkoxy radical steals a hydrogen from R3H to form the alcohol = free radical is formed to carry on the rxn

26
Q

describe formation of a ketone

A

H is abstracted from radical chain

rxn is a termination step

27
Q

describe bimolecular rxn

A

reaction takes place at higher hydroperoxide concentrations

forms RO. + ROO. + H2O (alkoxy and peroxy radicals)

28
Q

what do bimolecular vs monomolecular rxns form?

A

mono: alkoxy and hydroxy
bi: alkoxy and peroxy

29
Q

what do peroxy radicals prefer to abstract a H from?

A

FA

30
Q

what can decomposition of unstable alkoxy radicals lead to?

A

chain scission and the formation of aldehydes, ketones, alcohols, or carboxylic acids.

31
Q

describe breakdown of hydroperoxide?

A

first in monomolecular rxn (alkoxy radical) = decomposes to short chain deldehydes and ketones and more hydroperoxides

32
Q

what are propagators of the autoxidation rxn?

result of rate of rxn?

A

formation of peroxy

net result: rxn starts slow and increases exponentially

33
Q

what is PV?

A

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
Q

what are hydroperoxides considered?

A

primary oxidation products

35
Q

what do hydroperoxides release in acidic environments?

how is amount of I2 released measured?

A

releases I2 from KI

I2 is measured by titration with standardized sodium thiosulfate.

36
Q

what is the induction period in accumulation of hydroperoxides?

A

monomolecular rxn

slow initial accumulation of ROOH

37
Q

what rxn is at the rapid rise in the PV?

A

bimolecular rxn

38
Q

what causes decrease in PV after propagation step?

A

more decomp of hydroperoxides than formation

39
Q

what does metamorphosis of aldehydes and ketones cause?

A

flavor and odors = causes lipids to be organoleptically unacceptable

40
Q

limits of the PV test?

A

rancid and fats that have just began the reaction will both have low PV

41
Q

is there off flavor in the induction period?

A

no b/c ROOH are mostly being formed, not broken down (off flavor occurs during breakdown)

42
Q

what is the TBA test?

A

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
Q

reactants and products of the TBA test?

A

malonaldehyde reacts with 2 molecules of TBA (thiobarbituric acid).

produces a red complex (can be tested spectrophotometrically)

44
Q

result of high reactivity of malonaldehyde?

A

it is often attached to proteins and needs to be released with acid

45
Q

what can malonaldehyde also be quantified with?

A

HPLC

46
Q

malonaldehyde formation is a function of….

A

the lipid makeup

47
Q

what are other secondary oxidation product tests besides TBA?

A

AV (anisidine value): reaction of p-anisidine with aldehydes, THEIR reaction adduct measured in the UV

carbonyl number

48
Q

define rate of autoxidation

A

function of the degree of unsaturation of the fatty acids in the lipid system

49
Q

factors that initiate oxidation of FA

A

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
Q

where are H abstracted from?

A

alpha position, next to DB b/c more labile (constantly changing)

51
Q

what may occur during formation and decomp of peroxy radicals?

A

cis/trans isomers

52
Q

natural form of linolenic?

A

unconjugated

readily converted into mixed bonding system

53
Q

how many methylene carbons in linoleic? linolenic? arachidonic?

A

linoleic: 1
linolenic: 2
arachidonic: 3

FDSC251 (Food chem 1) (8 decks)

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