U5 - a + b
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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.)