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Food Chemistry - FDSC251 > U5 - a + b > Flashcards

U5 - a + b Flashcards

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1
Q

what is autoxidation

A

free radical reaction of unsaturated fatty acids with molecular oxygen

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2
Q

true or false: autoxidation is spontaneous and self sustaining

A

true

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3
Q

effects of autoxidation (4)

A
  1. rancidity
  2. loss of functionality
  3. decrease in nutritive value
  4. spoilage - possible toxic
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4
Q

there are 11 factors affecting autoxidation

A

energy (light and heat), catalysts, double bonds, enzymes, chemical oxidants, oxygen content and types of oxygen, natural antioxidants, phospholipids, fatty acids, mono and di glycerides, polymers

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5
Q

3 steps of autoxidation

A

initiation propagation termination

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6
Q

what is initiation

A

homolytic cleavage of a covalent bond, forming 2 free radicals (the electrons are split evenly)

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7
Q

what 2 things must occur for autoxidation to happen

A
  1. Hydrogen abstracted from fatty acid

2. molecular oxygen present

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8
Q

how is hydrogen abstracted from the fatty acid chain

A 
light/ionizing radiation
heat
metallic cations
enzymes
reactive singlet oxygen 
combination
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9
Q

equation for initiation

A

RH –> R. + H.

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10
Q

how long does a free radical last

A

nanoseconds

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11
Q

what happens if molecular oxygen isnt present

A

lipid free radicals recombine - no harm done

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12
Q

what is propagation

A

reaction of the fatty acid free radical with molecular oxygen, formation of peroxy radical. self-propagation occurs

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13
Q

equations for propagation

A

R. + O2 –> ROO.

ROO. + RH –> ROOH + R.

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14
Q

what is termination

A

when 2 free radicals react an unreactive species is formed and the free radical chain stops

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15
Q

3 possible equations for termination

A

ROO. + H. –> ROOH
ROO. + ROO. –> ROOR + O2
ROO. + ROOR

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16
Q

which of the 3 termination reactions is most probable and what does this lead to

A

hydrogen abstraction (ROO. + H. –> ROOH) leading to an accumulation of hydroperoxides

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17
Q

primary products of autoxidation

A

hydroperoxides (#1) and peroxides

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18
Q

true or false: hydroperoxides are the source of rancidity

A

false - they can degrade into what actually forms rancid odors and flavors

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19
Q

source of rancidity

A

low molecular weight aldehydes, alcohols, ketones, carboxylic acids

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20
Q

what happens if propagation is more frequent than termination

A

accumulation of hydroperoxides

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21
Q

are hydroperoxides stable

A

no - they degrade into low molecular weight rancidity molecules

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22
Q

what 2 methods create rancidity molecules

A
  1. monomolecular

2. bimolecular

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23
Q

when does monomolecular dominate and what is its equation

A

early in autoxidation. ROOH –> RO. + .OH

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24
Q

what is formed in monomolecular reaction

A

alkoxy radical (RO.) and hydroxy radical (OH.)

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25
Q

how are aldehydes formed

A

chain scission
alkoxy radical steals an electron from the R2to form a double. forms an aldehyde and releases a free radical - propagation

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26
Q

how are carboxylic acids formed

A

oxidation of aldehyde

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27
Q

how are alcohols indirectly formed

A

reduction of aldedhyde

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28
Q

how are alcohols directly formed

A

alkoxy radial steals a hydrogen from R3H to form an alcohol and release a free radical - propagation

29
Q

how are ketones formed

A

hydrogen is abstracted from the radical chain. this is a termination step reaction - free radicals combine; none form

30
Q

when does bimolecular reaction dominate and what is its reaction

A

when ROOH accumulates later in autoxidation. 2ROOH –> RO. + ROO. + H2O

31
Q

what do peroxy radicals prefer to do

A

abstract an H from a fatty acid

32
Q

what do alkoxy radicals prefer to do

A

decompose into aldehydes, ketones, alcohols, carboxylic acids

33
Q

what is the pace of autoxidation in relation to time>

A

slow at first, then fast. monomolecular then bimolecular, so when both alkoxy and peroxy radicals are formed the reaction can propagate more

34
Q

3 methods to evaluate autoxidation

A

peroxide value
TBA test
AV test

35
Q

definition of peroxide value

A

mEq of peroxide oxygen per 1000g fat

36
Q

what does PV measure

A

only hydroperoxides

37
Q

what is the primary autoxidation product

A

hydroperoxides

38
Q

peroxide value method and equation

A

2KI + 2ROOH –> I2 + 2KOH + 2ROH, then measure amount of I2 released by titrating with sodium thiosulfate

39
Q

when does PV rise

A

when the bimolecualar reaction starts

40
Q

when does PV fall

A

when the decomposition of peroxides outpaces their formation

41
Q

when is the use of PV limited

A

when the fat is rancid

42
Q

what must PV be correlated to in order to provide indicator of accessability

A

sensory data - taste panel

43
Q

is there a rancid flavor associated with induction period

A

no - there is no accumulation of hydroperoxides

44
Q

what is does the TBA test measure

A

a secondary breakdown product - malonaldehyde

45
Q

what is malonaldehyde production directly proportional to

A

degree of autoxidation, which is correlated to flavor defects

46
Q

what is malonaldeyde usually attached to

A

proteins since its highly reacive

47
Q

equation for TBA

A

malonaldehyde + 2TBA –> protein + red complex

48
Q

what test is performed on the products of malonaldehyde-protein breakdown

A

spectrophotometry - assess red color produced

49
Q

what can malonaldehyde be directly quantified with?

A

HPLC

50
Q

what is formation of malonaldehyde a function of

A

lipid makeup

51
Q

what reacts in AV

A

p-anisidine reacts with aldehydes

52
Q

what is measured/quantified in AV

A

UV

53
Q

To initiate an autoxidation reaction, hydrogen has to be abstracted from a fatty acid from

a. Carboxylic and
b. Methyl end
c. Double bond within the FA

A

c

54
Q

An alkyl, carbon and hydrogen chain, group singular bonded to oxygen is…

A

alkoxy radical

55
Q

Milli-equivalents of peroxide oxygen per 1000g of fat is…

A

peroxide value

56
Q

The abstraction of a hydrogen converts a fatty acid into…

a. Aldehyde
b. Ketone
c. Free radical
d. alcohol

A

c

57
Q

When a free radical reacts with molecular oxygen it will produce…

a. Peroxy radical
b. Hydroperoxide
c. Alkoxy group

A

a

58
Q

TBA test measures the concentration of…

A

malonaldehyde

59
Q

True or false: the PV increases linearly with time

A

false

60
Q

At low concentrations of hydroperoxieds, the dominant reaction is…

A

monomolecular

61
Q

Aldehydes resulting from the decomposition of the alkoxy radical can produce alcohols by…

A

reduction

62
Q

True or false: the formation of peroxides is infinite

A

false

63
Q

The peroxy radical is formed as a result of …

a. Abstraction of hydrogen from the fatty acid
b. Reaction fatty acid radical with oxygen
c. Both
d. none

A

b

64
Q

Homolytic cleavage results in the formation of…

A

free radicals

65
Q

When the two electron in a cleaved covalent bond are divided equally between the products, the reaction is called…

A

homolytic cleavage

66
Q

True or false: Autoxidation can be sustained in anaerobic conditions

A

false

67
Q

True or false: oils are bottled in dark glass containers to protect them from autoxidation

A

true - oxygen is also replaced with nitrogen

68
Q

The undesirable off-flavors and odors associated with oxidized fats and oils is called…

A

rancidity

Food Chemistry - FDSC251 (10 decks)

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