Flavour Generation - Maillard Reaction - flavour generation during thermal processing
67 important questions on Flavour Generation - Maillard Reaction - flavour generation during thermal processing
What is the main goal of flavour chemistry?
To understand the chemical reactions and interaction of flavour precursors and the generated flavour compounds.
What are function of cooking (heating)?
- Microbial safety: the microorganisms are destroyed, giving the food product a longer shelf life and rendering it safe for consumption
- Increase its digestibility
- Flavour and colour compounds are formed: giving food products a unique sense of smell and clear visual cues
What are the conditions Maillard reaction (can) take place according to the European legislation?
The heat treatment has to be lower than 180 C for a maximum of 15 min and not exceed pH 8 (safety reasons).
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Describe the flavour pyramid
- Top: top notes (mixtures of individual aroma compounds, small amounts will have an impact, first to be perceived when opening a package)
- Higher: Maillard reaction flavours (real direction to the flavour)
- Middle: complex tastes (amino acid mixtures)
- Bottom: basic tastes (salt, bitter, sour, sweet, umami)
Which gas is formed in meat flavour generation?
H2S, which immediately reacts with pentoses, resulting in the formation of the typical meat flavour.
In fresh meat, the residual enzyme activity is part of the ageing/maturation of meat, what happens to:
- proteins
- RNA
- glycogen
- Membranes/fats
- proteins --> peptides --> amino acids
- RNA --> nucleotides --> ribose & phosphate
- glycogen --> sugars --> acids
- Membranes/fats --> (phospho)lipids --> fatty acids
During cooking of meat, 3 important molecules are formed
- NH3 and H2S (furanones, pyranones, pyrites, thiophenes)
- Furfural (pentoses) and Hydroxymethylfurfural (hexoses)
- Melanoidins/browns
Is sucrose reactive in the Maillard reaction? Can it become reactive?
No, due to the lack of a reducing end and its equilibrium with the open form.
It can become reactive in the Maillard reaction after cleavage into glucose and fructose.
It can become reactive in the Maillard reaction after cleavage into glucose and fructose.
What happens when a reducing sugar reacts with an amino acid?
The reaction product will undergo
- Amadori rearrangement (for aldoses like ribose and glucose)
- Heyns rearrangement (for ketoses like fructose)
What product is formed after the Amadori/Heyns rearrangement?
The amino group of the sugar is cleaved off, forming a diketone (reduction) with two carbonyl groups.
What is the difference between a reducing and a non-reducing sugar?
Reducing sugar has a R-O-C-OH group that can form a carbonyl (C=O) group, a non-reducing sugar doesn't have this, but has a R-O-C-OR'
Is maltose a reducing sugar?
Yes
Is sucrose a reducing sugar?
No
Can a non-reducing sugar form imines?
No
What happens during the Amadori rearrangement with the imine?
It will be re-arranged to an amino ketone --> cleavage of amino acids --> (reactive) diketones formed, depending on pH which ones
How does pH influence the formation of diketones in the Amadori rearrangement of imines?
- Low pH --> first intermediate structure of the Amadori rearrangement can be attacked by a proton --> formation of iminium (1,2-enolization) --> can lose the amino acid by cleavage of the double bond --> 3 deoxy-osones (no oxygen on the third carbon atom)
- High pH --> no proton attack --> second intermediate of Amadori (2,3-enolization) --> third relocation of the double bond --> amino acid is cleaved off --> double bond undergoes final relocation --> 1-deoxy-osones
Both are formed at low and high pH, but more of the one mentioned at that pH.
What are 3-deoxy-osones and 1-deoxy-osones?
Both alpha-diketones, which are very reactive for follow up reactions
What are follow up reactions of the Amadori rearrangement?
Furanoid formation and fragmentation
What happens during furanoid formation at certain pH?
Compounds are formed that have a furan ring structure
- Low pH --> 3-deoxy-osones --> furfural with an aldehyde group connected to the furan ring
- High pH --> 1-deoxy-osones --> maltol or norfuraneol, both with a methyl group connected to the furan ring
Besides furanoid formation, deoxy-sones can also ...
- Undergo fission into smaller molecules.
- Be called reductones due to their reducing power
- be oxidised into a trike tone, which is even more reactive than the deoxy-osones (diketones) --> can degrade into smaller fragments
What is the effect of depolymerisation on reducing ends?
Depolymerization of the polymers can generate smaller molecules with more reducing ends, increasing the reducing power.
What is the flavour compound formed of the amino acid glycine? And what flavour does it have?
Formaldehyde --> fusal, chemical (not clear)
What is the flavour compound formed of the amino acid alanine? And what flavour does it have?
Acetaldehyde --> pungent, fresh, aldehydic, refreshing and green
What is the flavour compound formed of the amino acid valine? And what flavour does it have?
Isobutyraldehyde --> fresh aldehydic herbal green malty
What is the flavour compound formed of the amino acid leucine? And what flavour does it have?
Isovaleraldehyde --> fruity dry green, chocolate, nutty, leafy, cocoa
What is the flavour compound formed of the amino acid phenylalanine? And what flavour does it have?
Phenylacetaldehyde --> green sweet floral hyacinth clover honey cocoa
What is the flavour compound formed of the amino acid isoleucine? And what flavour does it have?
2-Methylbutanal --> musty cocoa coffee nutty
What is the flavour compound formed of the amino acid methionine? And what flavour does it have?
Methional --> musty tomato, potato, vegetative, mold, ripened cheeses, onion, beefy broth and egg and seafood nuances
Sulphur containing amino acids (like cysteine) can follow different routes in the Strecker degradation. After the loss of carbon dioxide and subsequent rearrangements, the formed imine can follow 3 different routes
- The imine can cyclise again to form acetylthiazole precursors, which subsequently can be oxidized
- The imine can split into mercapto-acetaldehyde, which is similar to the other Strecker aldehydes, and an amino sugar
- The imine can undergo full cleavage into acetaldehyde, H2S, ammonia and a diktone. The diketone can again react further.
What kind of effect do cysteine and glutamic acid have?
Competitive effect, less formation of 2-methyl-3-furanthiol.
What kind of effect do cysteine and glycine have?
Synergistic effect, increased formation of 2-methyl-3-furanthiol.
Why could ammonia be added in the making of caramel?
Ammonia contains an amino group which could catalyze/factilitate the rearrangement into the diketones. These diketones are more reactive than the sugars in the caramel mixture. During the making of caramel, the water is evaporated. The molecules in the caramel mixture will also lose their retained water, resulting in the formation of furanoids, such as maltol. These furanoid compounds are responsible for the sweet aroma.
What is the relation between flavour formation and temperature?
Exponential
What is the relation between flavour formation and time?
Linear
What kinetics is expected for Maillard reaction?
Zero-order kinetics
Is the flavour formation in the Maillard reaction dependent on the precursor formation?
No
How much faster is the reaction rate when the temperature is increased with 10 C around 100 C?
2x faster
How does pH influence the formation of colour compounds in the Maillard reaction?
It gives an increased formation of colour compounds
A pH drop of the reaction mixture can be observed during the Maillard reaction. This pH drop is larger when the initial pH of the reaction mixture is higher. What causes this drop?
This drop is caused by the formation of organic acids (e.g. Acetic acid) during the Maillard reaction (e.g. The fragmentation of sugars i.e. Via retro-aldol formation). These organic acids are weak acids with a pKa ~ 4.8. At their pKa value, these organic acids behave as a buffer. Due to the buffering function, the pH of the reaction mixture will not drop below a pH ~ 4.8. The formation of these organic acids during the Maillard reaction is higher at higher pH.
Why does the Maillard reaction proceed faster at a higher pH?
During the Amadori reaction, a nitrogen of the amino acid should react with the carbon of the keto/adelhyde group of the sugar.
Due to the equilibrium, at low pH a small amount of the nitrogen will be deprotonated and able to react with the sugar, however to a low extent
- At a higher pH, the nitrogen of the amino acid is more deprotonated
- At a lower pH, the nitrogen is more protonated, which makes it difficult for the reaction to take place
Due to the equilibrium, at low pH a small amount of the nitrogen will be deprotonated and able to react with the sugar, however to a low extent
Why is the Maillard reaction not performed at pH levels below 4?
At low pH, the amino groups are protonated and cannot react. It could be possible to perform the Maillard reaction below pH 4 by addition of hydrochloric acid (HCl). The pH of the reaction mixture including HCl, will rise during the Maillard reaction due to e.g. Decarboxylation.
At which pH can meat flavour be produced the best?
At lower pH (pH < 7), the 2-methyl-3-furanthiol is then stable (however, less is formed).
At higher pH, what is also formed next to 2,3-enlisation?
Melanoidins
What is a pyrazine?
A six-membered ring including 2 nitrogen atoms in the ring structure.
What is the aroma of pyrazines?
Roasty, burned, bitter
What is the structure of melanoidins?
Many different structures have been identified. One of the suggested structures for melanoidins contains chromophores. The chromophores can be linked to a peptide/protein backbone and also make crosslinks between backbones. The crosslinks give the chromophores a high molecular weight.
Do chromophore structures have colour?
Yes, due to the nitrogen atoms and conjugation in the ring. They have absorption at visible light, can turn yellow, brown or even black.
An increase in pH gives an increase in Maillard reaction, in which 6 ways?
- More neutral amino groups reacts faster (imine & Amadori)
- More 2,3 enol (1-deoxy-oson) than 1,2 enol (3-deoxyoson)
- More hydroxide --> more fragmentation (retro-aldol)
- More pyridines, shifting to other furanoids
- More melanoids --> browning intenser
- Faster degradation of sulfur-furanoids (meaty)
How can the autoxidation reaction be accelerated?
By the presence of ions(II).
With products are formed in autoxidation reaction?
Aldehydes
The reaction between aldehydes and ammonia or H2S can form pentylpyridines. What are pentylpyridines?
Pentylpyridines are not strong aromas, but as quite reactive. This reactivity gives the pentylpyridine the ability to react with and remove other aroma molecules. The removal of other aroma compounds shifts the balance of the aroma compounds and corresponding flavour.
What are products in the Maillard reaction?
Fragments retro-Aldol (& retro Strecker) to C2, C3, C4, pieces, NH3 & H2S to melanoidins, pyridines, furans
What are the conditions of the Maillard reaction?
- Temperature 10 C higher, 2x faster
- pH higher, stronger Maillard reaction
What is a reducing sugar? (more than one answer may be correct)
- It is an electron donor chemical compound.
- It is an electron acceptor chemical compound.
- It is a compound that gains electrons and is reduced in a chemical reaction.
- It is an electron donor chemical compound.
- It is an electron acceptor chemical compound.
- It is a compound that gains electrons and is reduced in a chemical reaction.
Which statements on the effect of pH on Amadori degradation are true? (more than one answer may be correct)
- At pH 7 the Amadori degradation follows mainly 1,2-enolisation with the formation of furfural or hydroxymethylfurfural.
- At pH higher than 7 the Amadori degradation follows mainly 1,2-enolisation with the formation of furfural or hydroxymethylfurfural.
- At pH 7 the Amadori degradation follows mainly 2,3 enolisation with the formation of compounds such as 4-hydroxy-5-methyl-2,3-dihydrofuran-3-one, acetol, pyruvaldehyde and diacetyl.
- At pH 7 the Amadori degradation follows mainly 1,2-enolisation with the formation of furfural or hydroxymethylfurfural.
- At pH higher than 7 the Amadori degradation follows mainly 1,2-enolisation with the formation of furfural or hydroxymethylfurfural.
- At pH 7 the Amadori degradation follows mainly 2,3 enolisation with the formation of compounds such as 4-hydroxy-5-methyl-2,3-dihydrofuran-3-one, acetol, pyruvaldehyde and diacetyl.
Which statement on amino acids are true? (more than one answer may be correct)
- Amino acids contain a central carbon (C) atom to which both an amino and a carboxyl group are attached.
- A free amino group of an amino acid can condense with a reducing sugar.
- Only the a-amino groups of terminal amino acids can condensate with a reducing sugar.
- Amino acids contain a central carbon (C) atom to which both an amino and a carboxyl group are attached.
- A free amino group of an amino acid can condense with a reducing sugar.
- Only the a-amino groups of terminal amino acids can condensate with a reducing sugar.
Which statement on Strecker degradation is true?
- Dicarbonyl compounds can react with free amino group of amino acids forming of aldehydes and a-aminoketones.
- Dicarbonyl compounds can react with free carboxyl group of amino acids forming of aldehydes and a-aminoketone.
- Dicarbonyl compounds can react with free amino group of amino acids forming of aldehydes and a-aminoketones.
- Dicarbonyl compounds can react with free carboxyl group of amino acids forming of aldehydes and a-aminoketone.
Which of those phenomenon were observed by Martins et al. (2000) (Reaction Routes) by heating a glucose/glycine model system at pH 7? (more answers can be correct)
- Partial isomerization of glucose via 1,2-enolization.
- Partial degradation of sugars into organic acids.
- Formation of Amadori products.
- The complete reduction of glycine concentration, i.e. concentration below detection limit, caused by condensation with Amadori products
- The interaction of breakdown products with amino groups into advanced Maillard products known as melanoidines.
- No formation of hydroxymethylfurfural.
- Partial isomerization of glucose via 1,2-enolization.
- Partial degradation of sugars into organic acids.
- Formation of Amadori products.
- The complete reduction of glycine concentration, i.e. concentration below detection limit, caused by condensation with Amadori products
- The interaction of breakdown products with amino groups into advanced Maillard products known as melanoidines.
- No formation of hydroxymethylfurfural.
Heating of a protein with starch will give a Maillard reaction. T/F
False, only free (non-bound) amino acids and non bound sugars are reactive. Not when in polymer. If there would have been enzymes, it would have been correct.
Increasing the pH increases Maillard reaction.
True. Higher pH, more -OH, so more attack on carbonyls, also amino group for imine formation more neutral.
All amino groups give Amadori type products. T/F
Ture. Amino group does not need to be amino acid, also ammoniak will react.
Fragmentation of sugars (high pH) is an equilibrium reaction. T/F
False. The small fragments formed are very reactive and form again other molecules like pyridines, melanoid. Amadori was already not equilibrium anymore, this fermentation is even after that.
Fragmentation of sugars (high pH) is an equilibrium reaction. T/F
False. The small fragments formed are very reactive and form again other molecules like pyridines, melanoid. Amadori was already not equilibrium anymore, this fermentation is even after that.
Higher pH best for meat flavour formation. T/F
False. Higher pH gives a faster reaction, however also more efficient degradation, especially some sulphur compounds (MFT).
Zero-order kinetics means that precursor becomes zero in concentration. T/F
False. Zero means zero in change of concentration (not significant).
Exponential order kinetics describe atomic bomb explosions. T/F
True. Reaction starts with neutron, but with each reaction produces 2 neutron: 1, 2, 4, 8 etc.
Lipoxidation, (catalysed by iron) ions best in water.
True. Water is the best inert solvent for radical reactions as OH-bond very strong (homolysis in 2 radicals -H and -OH).
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