1.1 Lipid Oxidation Flashcards
What’s the difference between a triacylglycerol and a phospholipid?
T = glycerol backbone + 3 esterified FA
P = glycerol backbone on which 2 FAs and 1 polar phosphogroup are esterified
P are also amphiphilic molecules
What’s the natural configuration of double bonds?
cis
become trans when hydrogenated
What’s the motif that polyunsaturated FA follow?
1,4-pentadiene motif
How can FA composition be measured?
first extract the lipids and then do GC
What’s the FA composition of milk? Vegetable oils? Fish oil?
Milk: short chain FA
Vegetable oil: C18 FA
Fish: EPA (C20:5 met3) and DHA (C22:6 met3)
How can lipids be degraded?
hydrolysis
oxidation
How does hydrolysis work?
From the triglyceride, 3 FA + glycerol are released
- occurs with water
- can be catalysed by alkali, acids or enzymes (lipases)
What’s the main reaction in lipid oxidation?
reaction between unsaturated FA and oxygen
What’s the main issue in food systems?
Autoxidation
What are the main steps in lipid oxidation? Shortly describe what happens in each step
INITIATION:
- formation of an alkyl radical (L.) by hydrogen abstraction form uns FA
PROPAGATION:
- alkyl radical reacts further with another unsaturated FA to form hydroperoxides
- this generates other alkyl radicals
TERMINATION:
- combination of 2 radicals to form non-radical secondary oxidation product
What catalyzes the initation reaction?
light, metal, ROS
Where does the H abstraction occur?
C atom next to a double bond
for polyuns FA: abstraction occurs form carbon in between 2 double bonds
Which conditions are needed for termination to occur?
- low oxygen pressure
- very high oxidation and accumulation of peroxy radicals
What is a peroxyl radical? (LOO.)
formed when alkyl radical reacting very fast with oxygen
What are the 2 main steps in the decomposition of hydroperoxides?
- formation of alkoxyl radical LO.
- beta scission of the alkoxyl radical
Where does beta scission occur?
position between the alkoxyl group and the double bond is favored
What are the products of beta scission?
aldehyde (hexanal) + radical
What are the factors promoting lipid oxidation?
oxygen, light, metals, temperature, water activity
What do triplet oxygen and singlet oxygen react with?
triplet oxygen: alkyl radical
singlet oxygen: double bond of unsaturated FA
In which type of photo-oxidation/light oxidation is singlet oxygen involved?
type II
What’s the difference between photo oxidation type I and type II?
TYPE 1:
- Sen* catalyses H abstraction from uns FA leading to synthesis of alkyl radical
TYPE 2:
- Sen* reacts with triplet oxygen causing the formation of singlet oxygen
- this singlet oxygen then reacts with double bond from uns FA to form peroxyl radicals
What are the main products of photo oxidation I and II?
I: alkyl radicals
II: peroxyl radicals
What are the 2 ways metals catalyse photo oxidation?
- autoxidation of reduced metals: electron transfer (with oxygen or H2O2) generates radical species that stimulate initiation
- decomposition of hydroperoxides (into alkoxyl/peroxyl) that can take part in propagation phase
What temperatures promote lipid oxidation?
high T
