U5 - a + b Flashcards by Miranda Baillie

what is autoxidation

free radical reaction of unsaturated fatty acids with molecular oxygen

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true or false: autoxidation is spontaneous and self sustaining

true

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effects of autoxidation (4)

rancidity
loss of functionality
decrease in nutritive value
spoilage - possible toxic

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there are 11 factors affecting autoxidation

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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3 steps of autoxidation

initiation propagation termination

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what is initiation

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

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what 2 things must occur for autoxidation to happen

Hydrogen abstracted from fatty acid

2. molecular oxygen present

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how is hydrogen abstracted from the fatty acid chain

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

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equation for initiation

RH –> R. + H.

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how long does a free radical last

nanoseconds

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what happens if molecular oxygen isnt present

lipid free radicals recombine - no harm done

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what is propagation

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

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equations for propagation

R. + O2 –> ROO.

ROO. + RH –> ROOH + R.

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what is termination

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

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3 possible equations for termination

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

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

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

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primary products of autoxidation

hydroperoxides (#1) and peroxides

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true or false: hydroperoxides are the source of rancidity

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

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source of rancidity

low molecular weight aldehydes, alcohols, ketones, carboxylic acids

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what happens if propagation is more frequent than termination

accumulation of hydroperoxides

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are hydroperoxides stable

no - they degrade into low molecular weight rancidity molecules

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what 2 methods create rancidity molecules

monomolecular

2. bimolecular

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when does monomolecular dominate and what is its equation

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

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what is formed in monomolecular reaction

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

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how are aldehydes formed

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

how are carboxylic acids formed

oxidation of aldehyde

how are alcohols indirectly formed

reduction of aldedhyde

how are alcohols directly formed

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

how are ketones formed

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

when does bimolecular reaction dominate and what is its reaction

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

what do peroxy radicals prefer to do

abstract an H from a fatty acid

what do alkoxy radicals prefer to do

decompose into aldehydes, ketones, alcohols, carboxylic acids

what is the pace of autoxidation in relation to time>

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

3 methods to evaluate autoxidation

peroxide value TBA test AV test

definition of peroxide value

mEq of peroxide oxygen per 1000g fat

what does PV measure

only hydroperoxides

what is the primary autoxidation product

hydroperoxides

peroxide value method and equation

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

when does PV rise

when the bimolecualar reaction starts

when does PV fall

when the decomposition of peroxides outpaces their formation

when is the use of PV limited

when the fat is rancid

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

sensory data - taste panel

is there a rancid flavor associated with induction period

no - there is no accumulation of hydroperoxides

what is does the TBA test measure

a secondary breakdown product - malonaldehyde

what is malonaldehyde production directly proportional to

degree of autoxidation, which is correlated to flavor defects

what is malonaldeyde usually attached to

proteins since its highly reacive

equation for TBA

malonaldehyde + 2TBA --> protein + red complex

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

spectrophotometry - assess red color produced

what can malonaldehyde be directly quantified with?

HPLC

what is formation of malonaldehyde a function of

lipid makeup

what reacts in AV

p-anisidine reacts with aldehydes

what is measured/quantified in AV

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

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

alkoxy radical

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

peroxide value

The abstraction of a hydrogen converts a fatty acid into… a. Aldehyde b. Ketone c. Free radical d. alcohol

When a free radical reacts with molecular oxygen it will produce… a. Peroxy radical b. Hydroperoxide c. Alkoxy group

TBA test measures the concentration of…

malonaldehyde

True or false: the PV increases linearly with time

false

At low concentrations of hydroperoxieds, the dominant reaction is…

monomolecular

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

reduction

True or false: the formation of peroxides is infinite

false

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

Homolytic cleavage results in the formation of…

free radicals

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

homolytic cleavage

True or false: Autoxidation can be sustained in anaerobic conditions

false

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

true - oxygen is also replaced with nitrogen

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

rancidity

U5 - a + b Flashcards

(68 cards)