(Dis)colouration of foods is a well beloved topic of food chemists. Colour pigments are highly complex, as are the reactions to form them. Browning especially, is a favorite. It’s interesting because there are various chemically completely different ways to turn something brown, with the same end-result: they’re brown (think old bananas, brown bread, fried steak, sugar caramel).
Today we’ll focus on one route to brown food: the Maillard reaction. It’s the one causing the brown bread and steak. Since the fundamental basics have been discussed before (when making gumbo), this post will do a deep dive into the hardcore chemistry, discussing the Maillard reaction mechanism.
Browning of food
Let’s get back to these different ways a food can turn brown. There are two main types of browning of foods: enzymatic & non-enzymatic browning. We covered enzymatic browning when talking about brown bananas. This type of browning is stopped by a high heat, so doesn’t apply to our steak or bread.
Non-enzymatic browning can again be split in two main categories: the Maillard reaction and caramellization. Caramellization requires high temperatures (above roughly 150°C) and only requires the presence of enough sugar. The Maillard reaction on the other hand can take place at room temperature (but speeds up quickly at higher temperatures, so don’t expect your bread to turn brown at room temperature) and requires two types of molecules to occur: both protein and sugars.
The Maillard reaction mechanism is great from a chemistry point of view, so we’ll do a real deep dive of this reaction’s chemistry!
If you’d like to learn more on the Maillard reaction, but at a more basic level, read my post on making ‘dark roux’, which includes an introduction to the reaction.
Maillard reaction chemistry – an overview
For the first description of the Maillard reaction mechanisms we have to go back to the beginning of the 20th century. The Frenchman, LC Maillard, hence the name, seemed to have brought together knowledge on several types of chemical reactions, that together are now referred to as Maillard reaction. Because, yes, the Maillard reaction is not just one reaction. Instead, it’s a series of consecutive reactions that can happen in all sorts of ways.
In short, the following steps take place during the Maillard reaction (below we will zoom in on the three steps in more details):
- It all starts with a sugar and a protein/amino acid. These react and form a compound.
- In more chemical terms: a reducing sugar (either an aldose or a ketose) reacts with an amine-group (an NH2-group, from the protein) to form a so-called Heyns or Amadori compound
- These molecules react further to form aromatic compounds.
- From a chemical structure perspective: most of these molecules contain a ring in their structure which is formed in this stage.
- In the final step large complex molecules, melanoidins are formed. These will eventually give the product a brown colour.
Now let us zoom in on these three steps of the Maillard reaction mechanism one by one; starting with the molecules that have to be present for the reaction to occur.
Maillard ingredient no.1: Amino-group
For the Maillard reaction to occur a so-called amino-group has to be present. An amino-group is made of one nitrogen atom and two hydrogen atoms: NH2. This group is attached to another part of a larger molecule (that is the amino acids for instance). In chemistry this is represented by the letter R, telling that there can still be various other atoms in that molecule. So this amino-group will be represented as follows: R-NH2.
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This type of group can be commonly found in proteins, peptides and amino acids. Amino acids are the building blocks of peptides and proteins (read more here). In the Maillard reaction the amino-group often comes from proteins, for instance milk proteins in butter.
Maillard ingredient no.2: Reducing sugar
Besides the amino-group we also need a so-called reducing sugar. This is a special type of sugar with a specific reactive group which also it to react in the Maillard reaction.
First the sugar, a sugar is a carbohydrate. Commonly mono- and disaccharides participate in the Maillard reaction. These are the smallest versions of carbohydrates.
But, when is such a sugar a reducing sugar? A reducing sugar is one that can give away electrons. In order for a molecule to give away electrons an atom has to have enough free floating electrons. As a matter of fact an oxygen atom which is bonded to a carbon atom with two bonds is such an atom. So sugars with such a group are reducing sugars (there’s a whole lot more chemistry here, but we’ll skip that).
In chemistry we call these specific groups aldehyde (-COOH) and ketone (-CO-) groups. In practice, all monosaccharides (glucose and fructose for example) are reducing sugars, as is lactose (a disaccharide). Sucrose (regular granulated sugar) however is not.
Maillard reaction mechanism – step 1 – Amadori & Heyns
Now that we have a reducing sugar and an amino group the Maillard reaction can start. In the first step these two will react and form one molecule.
Over the course of a total of 4 reactions they will be rearranged into either an Amadori or a Heyns compound. Which one of the two is formed depends on the type of sugar that participates. If it’s a sugar with an aldehyde group (-COOH) it will be an Amadori. If it’s a sugar with a ketone group (-CO-) the Heyns compound will be formed.
The actual chemical reactions involve quite a lot of organic chemistry. This means a lot of structural drawings of molecules and arrows indicating the compounds that are formed, added and the way reactions take place. Below the formation of the Amadori compound is shown.
The first step of the Maillard reaction mechanism. In this step a sugar (in this case glucose) reacts with an amino group (which can be attached to a protein) to form an Amadori compound. It all starts with the sugar and amino group that form one molecule. This one molecule then splits of water before rearranging into the Amadori compound. The reactions with arrows in both sides indicate that the reaction is reversible.
Maillard reaction mechanism – step 2 – Rearrangements
So far, the choice of reactions was limited. Roughly two different reactions could take place, one forming the Amadori compound, the othera Heyns compound. Both occur in a very similar way so no need to zoom in on both. But from here it starts getting more messy. More than reaction can take place here, but for simplicity I will stick to one to show a mechanism.
The molecule on the far left is the Amadori compound from the previous reaction. If this molecule is in an acid environment (hence the H+), this reaction can take place. It will result in the formation of a molecule with two double bound oxygen molecules. It’ll depend on the conditions how many -OH groups are left over, this depends on the mechanism and conditions. More -OH groups might split of in the form of water.
Once these molecules with two double bonded oxygen atoms have been formed, they can participate in the Strecker reaction. During the Strecker reaction aromatic molecules are formed and the variation in reaction pathways also starts to increase drastically.
As you’ve seen with the previous reactions, quite from the start there are several ways for this reaction to proceed. This is the step where it starts getting very messy. The two products formed in this reaction can do all sorts of things. One option is that the aminoketon (upper product) can react with another aminoketon forming a ring, this results in the formation of pyrazines, which are very aromatic.
The last step of the Maillard reaction mechanism
A lot of different reactions take place now, with products from all different reactions reacting with one another, just with whichever one is close by and which reaction is encouraged by the current conditions. You’ve already read some of the possible routes for aroma formation. This is the point where the brown colour is formed as well. These molecules will react and form complex structures with a lot of rings in them.
An example of a coloured molecule that can be formed during the Maillard reaction.
So, let’s summarize that quickly. You’ve just seen that the maillard reaction is one big heap of chemical reactions. It’s not very controlled and many different molecules are formed and react again with others. It’s always been a real challenge to figure out what exactly happens. With these few chemical reactions you’ve gotten enough of a grasp of the reaction to improve your basic understanding.
Using this knowledge on the Maillard reaction mechanism you can now start reasoning as to how we can control this reaction in food.
A closing note on the name
There’s an ongoing debate as to whether the Maillard reaction should be called the Maillard reaction. We decided to stick with the name since it’s so commonly referred to. But, from a chemist’s perspective, there are more suitable names.
All reaction schemes have been made by myself, using the French summary on the Maillard reaction mechanism, as well as several food chemistry lecture slides I still had lying around.
If you’d like to learn more about the Maillard reaction, have a look at this interesting post from Khymos.
For this article I used the lc-maillard website. Unfortunately, since then this website, which contains the complete thesis on this reaction has gone away.
This, Herve, Don’t speak of Maillard reactions : they are glycation reactions, 2021, link