Flavour Generation - Lipid oxidation in a multiphase food system and strategies for prevention
66 important questions on Flavour Generation - Lipid oxidation in a multiphase food system and strategies for prevention
What are the results of lipid oxidation?
- Decrease in nutritional value
- Formation of volatile compounds, associated with unpleasant off-flavours, such as rancid smell
Which lipids are polar and which are non-polar?
Polar --> phospholipids
Non-polar --> triacylglycerols
Non-polar --> triacylglycerols
Are phospholipids polar or non-polar?
Polar
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Are triacylglycerols polar or non-polar?
Non-polar
What are the 3 main functions of lipids?
- Structural as cell membranes are constituted by a phospholipid bilayer in which sterols and glycolipids can be incorporated
- Functional as they are precursors of hormones, vitamins and molecules involved in the regulation of inflammation
- Energetic as triglycerides can be stored as an energy reserve (e.g. In the adipose tissue of animals).
In foods, lipids contribute to:
- Texture, mouthfeel and melting properties, e.g. Tempering chocolate aims at forming fat crystals that have a melting temperature similar to the body temperature leading to chocolate melting on your tongue
- Structure of foods, e.g. Phospholipids can be used as emulsifiers due to their surface-active properties
- Nutrition both as a source of energy and as a source of essential fatty acids and vitamins
- Food technology (e.g. Frying)
- Flavour, both in a positive (e.g. Specific flavour in fried foods) and negative way (e.g. production of off-flavours following oxidation)
What are 3 different ways to refer to a fatty acid?
- The systemic name determined by the chemical structure
- The common name, most commonly used
- The short notation
What is the difference between the w or n-nomenclature and the delta nomenclature?
- w or n-nomenclature --> the position of the first double bond is counted from the methyl terminal
- delta nomenclature --> the position of the first double bond is counted from the carboxy terminal
How can lipids be degraded?
- Hydrolysis
- Oxidation
By what is hydrolysis favoured?
Water, enzymes, alkali
How can hydrolysis stimulate oxidation?
Free fatty acids (FFA) are more sensitive to oxidation compared to esterified fatty acids.
Which molecules are needed for lipid oxidation?
Unsaturated fatty acid and oxygen
How can lipid oxidation be stimulated?
O2, light, catalysts (enzymes)
What are the 3 steps of lipid autooxidation?
- Initiation
- Propagation
- Termination
Lipid oxidation is divided into two main oxidation phases:
- Primary oxidation that leads to the formation of hydroperoxides
- Secondary oxidation that comprises all the reactions starting form the hydroperoxides
Which hydrogen is abstracted during initiation?
The hydrogen on the position that requires the least energy to abstract the hydrogen (the lowest bond dissociation energy). This is often the hydrogen on the carbon next to the double bond. Preferably the hydrogen on the carbon between 2 double bonds in a 1,4-pentadiene structure.
How is the saturation of FA related to the reactivity?
Monounsaturated FAs are usually less reactive than polyunsaturated FAs. FAs with a higher number of double bonds have more sites sensitive for hydrogen abstraction and therefore oxidise faster.
More unsaturated --> more sensitive to oxidation
More unsaturated --> more sensitive to oxidation
How is the position of a functional group counted?
From the carboxyl terminal.
What kind of molecules are formed in termination?
Dimers and polymers, which will increase the viscosity of the oil.
What are the 2 steps in the decomposition of hydroperoxides?
- Formation of an alkoxyl radical LO.
- Beta-scission of the alkoxyl radical
- Position A
- Position B --> between double bond and alkoxyl group is favoured
Which molecule is used as a marker of secondary lipid oxidation?
Hexanal (results from beta-scission). It is a very potent volatile compounds, measured by gas chromatography.
Which factors can influence lipid oxidation?
- Oxygen
- Light
- Metals
- Temperature
- Water activity
In which ways can oxygen promote lipid oxidation?
- Triplet oxygen (3O2): reaction with alkyl radical
- Singlet oxygen (1O2): excited state of oxygen --> can add directly on the double bonds of unsaturated FA
What are the differences between the 2 types of photo-oxidation?
- Photo oxidation type I: Light activate a sensitiser type I will then catalyse the hydrogen abstraction from an unsaturated fatty acid leading to a synthesis of an alkyl radical. The lipid oxidation pathway is then a radical chain reaction as described earlier that used light as a catalyser.
- Photo oxidation type II: Activated sensitiser (sensitiser type II) activates triplet oxygen into its excited singlet oxygen form. Singlet oxygen reacts directly on the double bond leading directly to the formation of a peroxyl radical.
What are the 2 ways metals can catalyse lipid oxidation?
- Electron transfer with oxygen or H2O2, generating radical species that can stimulate the initiation phase of lipid oxidation
- Decomposition of hydroperoxides. In products containing iron, iron metals can catalyse, through their redox cycle, the decomposition of hydroperoxides into alkoxyl (LO.), but also peroxyl (LOO.) that can then take part in the propagation phase.
Why should the temperature not be too high when looking at the influence on the rate of lipid oxidation?
A much higher temperature (above 40 C), will influence the reaction pathways.
How can you measure the amount of unsaturated lipids?
With GC-FID (gas chromatography flame ionisation detector) you can measure the loss of unsaturated FA
How can you measure the amount of LOO. Formed?
Measure radical formation (ESR (electron spin resonance))
How can you measure the formation of non-radical compounds?
Measure dimers/polymers (e.g. HPSEC)
How can you measure secondary oxidation products?
Measure total aldehydes, volatile (GC), specific products (e.g. MDA). Sensory analysis
Is lipid oxidation higher or lower in an emulsion compared to bulk oil?
In an emulsion you have a larger surface area and the presence of pro-oxidants (e.g. Iron, high aw) leads to a higher lipid oxidation rate compared to bulk oil.
Which factors are related to the oil phase?
- Minor components in the oil
- Antioxidants
- Surface-active impurities (e.g. FFAs, PLs)
- Oil fraction
Which minor components are present in oil? And what is their effect?
- Antioxidants (e.g. Tocopherols, carotenoids) --> slow down or inhibit lipid oxidation
- Surface-active impurities (e.g. FFAs, phospholipids)
- migrate to interface and affect its properties (e.g. Charge)
- may form colloidal structures (e.g. Reverse micelles) which can entrap small volumes of water and water-soluble pro-oxidants (e.g. Metals)
Which minor components are present in oil? And what is their effect?
- Antioxidants (e.g. Tocopherols, carotenoids) --> slow down or inhibit lipid oxidation
- Surface-active impurities (e.g. FFAs, phospholipids)
- migrate to interface and affect its properties (e.g. Charge)
- may form colloidal structures (e.g. Reverse micelles) which can entrap small volumes of water and water-soluble pro-oxidants (e.g. Metals)
How does the oil fraction influence the lipid oxidation?
Increasing the oil fraction from 5 - 40% reduces lipid oxidation. Reducing the aqueous phase is associated
- with a decrease in level of aqueous pro-oxidants such as metals, leading to a decrease in the lipid oxidation process.
- with less physical destabilisation. Due to a higher viscosity (more oil), it is less mobile and more stable. HIPEs are even more stable (higher packing density).
To what is the effect of pH on the aqueous phase related to?
- Charge of molecules with ionisable groups: proteins as emulsifiers can change the charge at the interface and affect lipid oxidation
- Precipitation and solubility of molecules: low pH metals more soluble, more pro-oxidant action --> more lipid oxidation
- Activity of chain-breaking antioxidants: low pH, higher H-donating capacity, increases antioxidant activity --> less lipid oxidation
At what pH are metals more pro-oxidant?
At a low pH, because they are more soluble then.
At what pH is the activity of chain-breaking antioxidants higher?
At a low pH, they have a higher H-donating capacity, so this increases their antioxidant activity.
Which factors are related to the aqueous phase?
- pH
- Excess emulsifiers
How does the presence of excess emulsifier influence lipid oxidation?
Excess emulsifiers remaining in the aqueous phase significantly reduces lipid oxidation by:
- Chelate metal ions (protein)
- Scavenge free radicals, they can give away hydrogen (protein)
- Form colloidal structures in the aqueous phase (e.g. Micelles). They can solubilise certain components (fatty acids, antioxidants, hydroperoxides). (surfactant)
Which factors are related to the interface?
- Droplet size
- Surface electrostatic charge
- Emulsifier
- Thickness/density of the interface
How does the droplet size influence the lipid oxidation?
Lower droplet size --> more surface --> increases lipid oxidation. However, many contradicting literature.
How does the electrostatic charge of the interface influence lipid oxidation?
- An anionic surfactant (= negatively charged, e.g. SDS) tends to attract positive charged metal ion from aqueous phase, this will increase the interaction with polar hydroperoxides and stimulate lipid oxidation
- A positively charged surfactant would repeal the metal ions and therefore limit lipid oxidation
Are proteins or surfactants better emulsifiers to reduce lipid oxidation?
- General case, no control of excess emulsifier concentration --> proteins better
- Emulsifiers only at interface --> surfactants better
How can interfacial thickness be increased?
- Using bigger emulsifiers
- Deposing more layers around the droplet
How does a thicker interfacial layer help prevent lipid oxidation?
It reduces the interaction (by enlarging the distance) between lipids and pro-oxidants.
What are 3 strategies to prevent lipid oxidation and what are their drawbacks?
- Packaging
- to protect against light
- and/or under vacuum or nitrogen to limit the exposition to oxygen. Drawback: not efficient once the product is opened
- Cold chain storage (fridge, freezer). Drawback: not sustainable
- Antioxidants additions. All antioxidants present the characteristic to react rapidly in order to prevent lipid oxidation. In most cases, synthetic antioxidant such as EDTA that are shown to be very efficient are preferred.
How do preventive anti-oxidants act?
They act on pro-oxidants. They are also called secondary antioxidants, as they do not act directly on the lipid radical, but on pro-oxidants molecules present in the environment.
What are 3 types of preventive antioxidants and what are their modes of action?
- Metal chelation. They interact with metal ions in order to prevent their pro-oxidant effect. Common examples are citric acid, phosphoric acid, ethylenediamine tetra acetic acid (EDTA)
- Singlet oxygen quenching. They would prevent lipid oxidation type II in which singlet oxygen is involved. Carotenoids such as beta-carotene, lutein or lycopene are the main singlet oxygen quenchers. The principle of quenching is an energy transfer from the singlet oxygen to the carotenoids.
- Oxygen scavengers and reducing agents. E.g. Ascorbic acid and derivatives.
Which 2 types of antioxidants are there?
- Primary antioxidants: chain-breaking --> act on lipid radicals directly
- Secondary antioxidants: preventive antioxidants --> act on pro-oxidant factors
How do radical scavengers/chain-breaking antioxidants act?
They inhibit or delay lipid oxidation by interacting directly with the lipid radicals.
How do radical scavengers/chain-breaking antioxidants act?
They inhibit or delay lipid oxidation by interacting directly with the lipid radicals.
What should the properties of a chain-breaking antioxidant be?
- React fast
- Be food-grade (safe for consumption)
- Be able to donate a hydrogen atom to a lipid radical in order to stabilize it --> produce a stable product
- Be soluble and non-toxic
E.g. Compounds with a phenolic hydroxy group. Such compounds can give away the hydrogen from their hydroxy group to a lipid radical, converting themselves into a radical that is stabilised by resonance within the benzene ring.
What kind of antioxidant is alpha-tocopherol?
A natural chain breaking antioxidant
What kind of antioxidants are BHT or BHA?
Synthetic chain breaking antioxidants
What is the polar paradox?
- In bulk lipids --> polar antioxidants are more efficient than non-polar antioxidants
- In oil-in-water emulsions --> non-polar antioxidants are more efficient than polar antioxidants
What is the cut-off effect?
Related to the hydrophobicity of antioxidants in regards to their efficiency. Longer alkyl chain --> more hydrophobic
- Compounds with short alkyl chain (i.e. Low hydrophobicity) have limited antioxidant efficiency
- Increasing the alkyl chain length (i.e. Increasing hydrophobicity) increases the antioxidant efficiency until a certain point, after which the antioxidant efficiency decreases drastically
Explain how the hydrophobicity and the antioxidant efficiency are related?
- Low hydrophobicity --> stay in the water phase, too far from the lipids to be able to prevent efficiently their oxidation
- Intermediate hydrophobicity --> found at the interface where the antioxidant activity is optimal
- High hydrophobicity (long chain length) --> dissolve in the oil droplet, which decreases their antioxidant efficiency
How do alpha-tocopherol (vit. E) and ascorbic acid (vit. C) synergize?
Alpha-tocopherol is chain breaking, so it gives away hydrogen and becomes a radical. Then ascorbic acid gives back -H to tocopherol that can make LOOH of radical L.
What is accelerated shelf life testing?
Storage in elevated stress conditions that will fasten the lipid oxidation process.
Which conditions can be used to accelerate shelf life?
- High temperature
- High oxygen level (often combined to high temperature)
- Free radical source
- Use of catalysts such as metals
What is the mechanism of prevention of lipid oxidation by excess of emulsifiers?
- Non-absorbed proteins
- Chelate metal ions (phosphoseryl residues in caseins)
- Scavenge free radicals by donating a proton to lipid radicals
- Non-absorbed surfactants
- Colloidal structures formed by surfactants in the aqueous phase (e.g. Micelles) can solubilise certain components --> sequestration of certain molecular species (e.g. Fatty acids, antioxidants, hydoperoxides)
Which sample oxidises the most:
- Soybean oil used for frying
- Soybean oil used for salad dressing
- Soybean oil used for frying
- Soybean oil used for salad dressing
For which purpose are sunflower oil and flaxseed oil the best used?
- Sunflower oil: heat stable, suitable for various applications
- Flaxseed oil: high in PUFAs and low smoking point, best for salad dressing
How are the degree of saturation of a fat and the sensitivity to lipid oxidation related?
A higher degree of unsaturation (more double bonds), the more sensitive to lipid oxidation.
How can you measure non-volatile compounds?
Liquid chromatography (LC)
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