Summary: Flavour Generation
Lipid Oxidation
Fatty acids
• FAs can be saturated (no double bonds), monounsaturated (one double bond) or polyunsaturated
(two or more double bonds)
• More than one double bond → double bonds are methylene interrupted (in nature)
o Double bond - single bond - single bond – double bond
• During food processing (e.g. hydrogenation), the double
bond configuration may change from cis to a trans
• Nomenclature → 1/ the systemic name, 2/ the common
name & 3/ the short notation
• FAs are important building blocks of triglycerides
• Every food source has a unique FA profile
Lipid degradation reactions
• Lipids can be degraded by hydrolysis & oxidation
• Hydrolysis → triglyceride is broken down into three free fatty acids & a glycerol
o Hydrolysis can stimulate oxidation as free FAs are more sensitive to oxidation
• Lipid oxidation → reaction between an unsaturated fatty acid & oxygen
Lipid autooxidation
• Initiation → light, metals or ROS catalyse the
formation of an alkyl radical (L•) by hydrogen
abstraction from an unsaturated fatty acid (LH)
o Hydrogen is abstracted mainly from the
carbon in between two double bonds (lowest
dissociation energy)
o The more unsaturated → the more sensitive
to oxidation
o After a rearrangement of the double bonds
into a conjugated diene system (two
double bonds separated by one single bond)
→ two different alkyl radicals can be formed
• Propagation → alkyl radical reacts very quickly with
oxygen to form a peroxyl radical (LOO•); this LOO•
will react further with another unsaturated fatty acid
(LH) to form an hydroperoxide (primary lipid
oxidation production), along with a new alkyl radical
(L•), generating a propagation circle of reactions
• Termination → two radicals react together to form a
non-radical species
o Formation of non-radical secondary
oxidation products → dimers & polymers
,Decomposition of hydroperoxides
• Secondary oxidation → all the
reactions starting from the
hydroperoxides
• Decomposition of hydroperoxides is a
two-step reaction:
o Formation of an alkoxyl
radical
o Beta-scission of the alkoxyl
radical, which can occur on
both sides from the alkoxyl
group (position B is favoured)
Kinetic aspects
• Antioxidants → decrease as they are
consumed to prevent lipid oxidation
• PUFAs → decrease as they are oxidised
• Oxygen consumption → increase from
propagation phase
• Hydroperoxides → increase during
propagation and decrease during
termination as decomposition rate
becomes higher than the synthesis rate
• Non-volatile and volatile end products
→ increase during the termination
phase
Factors promoting lipid oxidation
• Oxygen
o Reaction of the FA alkyl radical with a triplet oxygen
o Singlet oxygen are highly reactive and can react directly on the double bonds of an unsaturated
fatty acid (photo oxidation type II)
• Light → Lipid oxidation catalysed by light is called photo-oxidation or light oxidation
o Type I → light activates a sensitizer type I that will then catalyse the hydrogen abstraction from
an unsaturated fatty acid leading to the synthesis of an alkyl radical
o Type II → the activated sensitizer (sensitizer type II) activates triplet oxygen into its excited
singlet oxygen form
• Metals (e.g. iron & copper)
o Electron transfer with oxygen or H2O2 generating radical species that can stimulate the initiation
phase of lipid oxidation
o Decomposition of hydroperoxides
• Temperature
o Higher temperature increases the rate of oxidation
o Accelerated shelf life → <40 °C (without affecting too much the pathways)
• Water activity
o Low aw (<0.2) → lipid oxidation is very fast
▪ Hydroperoxides are in closer contact and more available
▪ Metal ions are not isolated by a water jacket and can easily catalyse
▪ Oxygen migrates faster in fat than in water
o Middle aw (0.2-0.4) → decrease lipid oxidation
▪ Decreased mobility of hydroperoxides
▪ Isolation of hydroperoxides from pro-
oxidants (e.g. metals) via hydration
o High aw (>0.4) → increase lipid oxidation
▪ Higher mobility of molecules
o Very high aw (>0.8) → decrease lipid oxidation
▪ Dilution of pro-oxidant molecules in water
, Lipid oxidation in a multiphase food system
• The larger surface area in emulsions and the presence of pro-oxidants leads to a higher lipid oxidation
rate compared to bulk oil
• Oil phase
o More unsaturated oil → more sensitive to lipid oxidation
o Antioxidant compounds (e.g. tocopherols & carotenoids)
o Surface active compounds (e.g. FFAs) → migrate to
interface or form colloidal structures (entrap water)
o Increasing the oil fraction may reduce lipid oxidation
(lower level of aqueous pro-oxidants)
• Aqueous phase
o pH can have an effect on lipid oxidation because of:
▪ Charge of molecules with ionisable groups (proteins)
▪ Precipitation and solubility of molecules (metals)
▪ Activity of chain-breaking antioxidants
o Excess emulsifiers remaining in the aqueous phase reduce lipid oxidation
▪ Proteins containing phosphoseryl residues → act as metal chelators
▪ Proteins rich in cysteine (Cys), methionine (Met), Tyrosine (Tyr) or Tryptophane (Trp)
residues → act as radical scavengers
▪ Surfactants → form micelles in the aqueous phase that could trap other compounds
• Interface
o Effect of droplet size unclear
o Surface electrostatic charge (surfactants) → modulating
electrostatic attraction or repulsion with the interface
environment
o Thicker interface → reducing the interactions between
lipids and water-soluble pro-oxidants
Strategies to prevent lipid oxidation
• Packaging to protect against light and/or under vacuum or nitrogen to limit oxygen exposition
• Cold chain storage
• Antioxidants addition
o Preventive/secondary antioxidants act on pro-oxidants present in the environment
▪ Metal chelation, singlet oxygen quenching & oxygen scavengers and reducing agents
o Chain-breaking/primary antioxidants inhibit or delay lipid oxidation
by interacting directly with the lipid radicals
▪ A chain-breaking antioxidant should react fast, be food-grade &
be able to donate a hydrogen atom to a lipid radical
o Polar paradox → in bulk lipids, polar antioxidants (AOs) are more efficient than non-polar AOs,
while in oil-in-water emulsions, non-polar AOs are more efficient than polar Aos
o Cut off effect → highest antioxidant capacity can be found for amphiphilic molecules with an
intermediate chain length
o Antioxidant synergy → antioxidants can reinforce the effect of each other
Lipid Oxidation
Fatty acids
• FAs can be saturated (no double bonds), monounsaturated (one double bond) or polyunsaturated
(two or more double bonds)
• More than one double bond → double bonds are methylene interrupted (in nature)
o Double bond - single bond - single bond – double bond
• During food processing (e.g. hydrogenation), the double
bond configuration may change from cis to a trans
• Nomenclature → 1/ the systemic name, 2/ the common
name & 3/ the short notation
• FAs are important building blocks of triglycerides
• Every food source has a unique FA profile
Lipid degradation reactions
• Lipids can be degraded by hydrolysis & oxidation
• Hydrolysis → triglyceride is broken down into three free fatty acids & a glycerol
o Hydrolysis can stimulate oxidation as free FAs are more sensitive to oxidation
• Lipid oxidation → reaction between an unsaturated fatty acid & oxygen
Lipid autooxidation
• Initiation → light, metals or ROS catalyse the
formation of an alkyl radical (L•) by hydrogen
abstraction from an unsaturated fatty acid (LH)
o Hydrogen is abstracted mainly from the
carbon in between two double bonds (lowest
dissociation energy)
o The more unsaturated → the more sensitive
to oxidation
o After a rearrangement of the double bonds
into a conjugated diene system (two
double bonds separated by one single bond)
→ two different alkyl radicals can be formed
• Propagation → alkyl radical reacts very quickly with
oxygen to form a peroxyl radical (LOO•); this LOO•
will react further with another unsaturated fatty acid
(LH) to form an hydroperoxide (primary lipid
oxidation production), along with a new alkyl radical
(L•), generating a propagation circle of reactions
• Termination → two radicals react together to form a
non-radical species
o Formation of non-radical secondary
oxidation products → dimers & polymers
,Decomposition of hydroperoxides
• Secondary oxidation → all the
reactions starting from the
hydroperoxides
• Decomposition of hydroperoxides is a
two-step reaction:
o Formation of an alkoxyl
radical
o Beta-scission of the alkoxyl
radical, which can occur on
both sides from the alkoxyl
group (position B is favoured)
Kinetic aspects
• Antioxidants → decrease as they are
consumed to prevent lipid oxidation
• PUFAs → decrease as they are oxidised
• Oxygen consumption → increase from
propagation phase
• Hydroperoxides → increase during
propagation and decrease during
termination as decomposition rate
becomes higher than the synthesis rate
• Non-volatile and volatile end products
→ increase during the termination
phase
Factors promoting lipid oxidation
• Oxygen
o Reaction of the FA alkyl radical with a triplet oxygen
o Singlet oxygen are highly reactive and can react directly on the double bonds of an unsaturated
fatty acid (photo oxidation type II)
• Light → Lipid oxidation catalysed by light is called photo-oxidation or light oxidation
o Type I → light activates a sensitizer type I that will then catalyse the hydrogen abstraction from
an unsaturated fatty acid leading to the synthesis of an alkyl radical
o Type II → the activated sensitizer (sensitizer type II) activates triplet oxygen into its excited
singlet oxygen form
• Metals (e.g. iron & copper)
o Electron transfer with oxygen or H2O2 generating radical species that can stimulate the initiation
phase of lipid oxidation
o Decomposition of hydroperoxides
• Temperature
o Higher temperature increases the rate of oxidation
o Accelerated shelf life → <40 °C (without affecting too much the pathways)
• Water activity
o Low aw (<0.2) → lipid oxidation is very fast
▪ Hydroperoxides are in closer contact and more available
▪ Metal ions are not isolated by a water jacket and can easily catalyse
▪ Oxygen migrates faster in fat than in water
o Middle aw (0.2-0.4) → decrease lipid oxidation
▪ Decreased mobility of hydroperoxides
▪ Isolation of hydroperoxides from pro-
oxidants (e.g. metals) via hydration
o High aw (>0.4) → increase lipid oxidation
▪ Higher mobility of molecules
o Very high aw (>0.8) → decrease lipid oxidation
▪ Dilution of pro-oxidant molecules in water
, Lipid oxidation in a multiphase food system
• The larger surface area in emulsions and the presence of pro-oxidants leads to a higher lipid oxidation
rate compared to bulk oil
• Oil phase
o More unsaturated oil → more sensitive to lipid oxidation
o Antioxidant compounds (e.g. tocopherols & carotenoids)
o Surface active compounds (e.g. FFAs) → migrate to
interface or form colloidal structures (entrap water)
o Increasing the oil fraction may reduce lipid oxidation
(lower level of aqueous pro-oxidants)
• Aqueous phase
o pH can have an effect on lipid oxidation because of:
▪ Charge of molecules with ionisable groups (proteins)
▪ Precipitation and solubility of molecules (metals)
▪ Activity of chain-breaking antioxidants
o Excess emulsifiers remaining in the aqueous phase reduce lipid oxidation
▪ Proteins containing phosphoseryl residues → act as metal chelators
▪ Proteins rich in cysteine (Cys), methionine (Met), Tyrosine (Tyr) or Tryptophane (Trp)
residues → act as radical scavengers
▪ Surfactants → form micelles in the aqueous phase that could trap other compounds
• Interface
o Effect of droplet size unclear
o Surface electrostatic charge (surfactants) → modulating
electrostatic attraction or repulsion with the interface
environment
o Thicker interface → reducing the interactions between
lipids and water-soluble pro-oxidants
Strategies to prevent lipid oxidation
• Packaging to protect against light and/or under vacuum or nitrogen to limit oxygen exposition
• Cold chain storage
• Antioxidants addition
o Preventive/secondary antioxidants act on pro-oxidants present in the environment
▪ Metal chelation, singlet oxygen quenching & oxygen scavengers and reducing agents
o Chain-breaking/primary antioxidants inhibit or delay lipid oxidation
by interacting directly with the lipid radicals
▪ A chain-breaking antioxidant should react fast, be food-grade &
be able to donate a hydrogen atom to a lipid radical
o Polar paradox → in bulk lipids, polar antioxidants (AOs) are more efficient than non-polar AOs,
while in oil-in-water emulsions, non-polar AOs are more efficient than polar Aos
o Cut off effect → highest antioxidant capacity can be found for amphiphilic molecules with an
intermediate chain length
o Antioxidant synergy → antioxidants can reinforce the effect of each other
