Lipids lecture 7 + 8 Flashcards Preview

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1

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”.

2

what is randicity?

undesirable off-flavors and odors

3

what does extensive autoxidation lead to?

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

4

overall result of autoxidation?

spoilage of edible fats and oils - usually become inedible.

5

3 steps of aotuoxidation rxn?

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

6

2 actions in atuoxidation?

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

7

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

8

describe homolytic cleave of a covalent bond to produce free radicals

RH --> R. + H.

(where R is rest of the FA)

9

how long do free radicals survive for?

10^-9 to 10^-12

10

what happens to free radicals in absence of O2?

free radicals quickly recombine

H. + H. --> H-H (H2)

thus, minimal changes occur to FA

11

when does a propagating rxn start?

in presence of oxygen

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

12

what is formed in a propagating rxn?

peroxy radical

13

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

14

describe termination step

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

15

describe accumulation of hydroperoxides

when propagation reaction is more frequent than the termination reaction


(many peroxy radicals being foromed = accumulation)

16

what are primary oxidation products?

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

17

hydroperoxides vs peroxides?

hydroperoxides (R-O-O-H)

peroxides (R-O-O-R).

18

flavor/odor of hydroperoxides?

not source of flavors/odors, but are their precursors

19

what are compounds responsible for off flavors?

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

20

stability of hydroperoxides?

not chemically stable

21

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.

22

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)

23

decomposition reactions of the alkoxy radical?

1. aldehyde generation

2. alcohol generation

3. formation of ketone

24

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

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.