U5 - a + b Flashcards
what is autoxidation
free radical reaction of unsaturated fatty acids with molecular oxygen
true or false: autoxidation is spontaneous and self sustaining
true
effects of autoxidation (4)
- rancidity
- loss of functionality
- decrease in nutritive value
- spoilage - possible toxic
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
3 steps of autoxidation
initiation propagation termination
what is initiation
homolytic cleavage of a covalent bond, forming 2 free radicals (the electrons are split evenly)
what 2 things must occur for autoxidation to happen
- Hydrogen abstracted from fatty acid
2. molecular oxygen present
how is hydrogen abstracted from the fatty acid chain
light/ionizing radiation heat metallic cations enzymes reactive singlet oxygen combination
equation for initiation
RH –> R. + H.
how long does a free radical last
nanoseconds
what happens if molecular oxygen isnt present
lipid free radicals recombine - no harm done
what is propagation
reaction of the fatty acid free radical with molecular oxygen, formation of peroxy radical. self-propagation occurs
equations for propagation
R. + O2 –> ROO.
ROO. + RH –> ROOH + R.
what is termination
when 2 free radicals react an unreactive species is formed and the free radical chain stops
3 possible equations for termination
ROO. + H. –> ROOH
ROO. + ROO. –> ROOR + O2
ROO. + ROOR
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
primary products of autoxidation
hydroperoxides (#1) and peroxides
true or false: hydroperoxides are the source of rancidity
false - they can degrade into what actually forms rancid odors and flavors
source of rancidity
low molecular weight aldehydes, alcohols, ketones, carboxylic acids
what happens if propagation is more frequent than termination
accumulation of hydroperoxides
are hydroperoxides stable
no - they degrade into low molecular weight rancidity molecules
what 2 methods create rancidity molecules
- monomolecular
2. bimolecular
when does monomolecular dominate and what is its equation
early in autoxidation. ROOH –> RO. + .OH
what is formed in monomolecular reaction
alkoxy radical (RO.) and hydroxy radical (OH.)
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
UV
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
c
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
c
When a free radical reacts with molecular oxygen it will produce…
a. Peroxy radical
b. Hydroperoxide
c. Alkoxy group
a
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
b
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
