Maillard Reaction, Strecker Degredation and Caramelisation

In the next of my apparent series of articles on all things coffee and chemistry I am going to tackle in a pleasant (I hope) depth these three types of reactions as they appear all the time in coffee articles. Obviously we are going to need to get into a bit of chemistry but hopefully it will remain understandable!

All of these are forms of non-enzymatic browning – opposed to reactions causing foods to go brown because of one of many enzyme actions, such as your apple turning brown when bitten into and left for a bit. This seems an obvious distinction, but I think it is important to include.

As usual – questions or mistakes in the comments!

The Maillard Reaction

Luis-Camille Maillard was the chemist after whom these particular reactions are named, as he was one of the first to investigate them in 1912. The term describes a very specific set a reactions – browning reactions occuring between amino acids and reducing sugars.
Amino acids are the building blocks of proteins, and just as the 26 letters of the alphabet can be used to write an infinite number of different texts, the 20 standard amino acids are put together to create a potentially infinite number of different proteins.
I won’t dwell on the chemistry but all amino acids have the same end to the chain:

The R symbolises the rest of the chain, and obviously this is the bit that is different from amino acid to amino acid.
So anything with protein in it is a candidate for the maillard reaction. All we need now is the presence of a reducing sugar.

The term reducing sugar implies something more interesting than what we find. Simply put a reducing sugar is any sugar with an aldehyde group. We often talk about aldehydes in coffee’s complex aroma chemistry (such as benzaldehyde – which has the characteristic smell of bitter almonds, and is used in just about any almond syrup), but to be an aldehyde means you have to have this arm tacked onto the molecule somewhere:

(again R is the the Rest of the molecule)
A list of reducing sugars is full of familiar names: fructose, glucose, lactose and maltose to name a few.
However, and this is important, sucrose – table sugar – is not a reducing sugar and takes no part in the Maillard Reactions (but does get involved in caramelisation).

These two react together and form new molecules with a brown colour – melanoidins (which you may recall being quite important to crema). Heat is not absolutely necessary, you could mix the two, put them in the fridge and they would turn brown – just very slowly! Heat helps speed the whole thing up. Equally they can happen with water present, but water slows it down. Without the sugar there can be no browning – this is why quakers stand out in roasted coffee, and are so pale.

Problems arise in the full description of reactions because different sugars and different amino acids produce different molecules, and then to complicate the picture these molecules begin to react and interact further. It is almost a cascading set of reactions. Therefore there are a great deal of different compounds created, both aromatic and non-volatile.

There is of course commonality between Maillard Reactions in different food products – coffee and chocolate share a great deal in terms of flavour as many of the same reactions have occured. Roast beef should also share some similar chemistry to roasted coffee, though whether there is enough commonality to make the combination palatable is debatable (though I have a collection of recipes for roasted and barbequed meat with a coffee crust!)
The Maillard reactions are the reason that milk left on a steam wand quickly turns brown.

These reactions are dependent on a few variables: the different amino acids available, the pH, the amount of water, any salts and the period of time held at a temperature.

The Strecker Degradation

This is one I thought I should include because I see it referenced quite a lot, in any text concerning roasting chemistry and is instrumental in the creation of the brown pigment as well as a myriad of volatile aromatics. It falls under the umbrella of, and requires compounds created by, the Maillard Reactions. Unfortunately this one isn’t so simple to explain.

It involves amino acids again, but instead of reacting with a sugar it is reacting with a molecule with two carbonyl groups, which look like this:

The compounds begin to trade parts of themselves, one of the carbonyl groups ending up on the amino acid (the acid having lost on carbon from its chain) creating an aldehyde as wll as an amino ketone. This is an intermediary stage and reactions continue to create many different volatiles.

I have refrained from making explicit descriptions of groups of aromatics. Ketones could be broadly described as having buttery, caramel flavours but key aromas in both raspberries and grapefruit are ketones.
Therefore I don’t think it is terribly helpful for me to claim that furans taste of this, and aldehydes taste of that….
Perhaps a description of each group of aromatics – esters, phenolics, terpenoids and thiols – would make an interesting article.


Much slower than Maillard reactions, and requiring much higher temperatures these reactions begin exclusively with sugars. They really begin up around 150C to 180C, with water being lost from the sugar molecule beginning the chain of events. In all cases the sugar is converted to a furfuryl. These are a type of furans that have a caramelly, slightly burnt and also slightly meaty notes. The same compound is produced via a different route in the Mailllard reactions. However it is with prolonged high temperature that many other types of aromas are generated.
Caramelisation is more predictable than Maillard reaction due to less variation in the starting compounds. Without the sulphur or nitrogen found in the amino acids caramelisation is unable to produce flavours as meaty as Maillard reactions

It is interesting to note how the sugar solutions taste changes in caramelisation. A sugar solution initially will be sweet with no aroma. Through caramelisation it becomes both sour and a little bitter, as a rich aroma develops. Generally the longer sugar is caramelised the less sweet it tastes.

Extended Reading:
General Book: The Maillard Reaction
Website: Course Information and A nice Diagram
Coffee Specific: The Science of Quality

Hopefully this will give the reader an insight into the basics of the different types of chemistry going on when you apply heat to food and it turns brown. Because of the broad range of applications I haven’t focused this one completely on coffee, though no doubt that will change when I come to review it. Any mistakes or obvious omissions then please let me know!


  1. If the mailard reaction & caramalisation happens during roasting; during extraction, what reactions cause the shot to become more or less “syrupy”?

  2. That would be down to the quantity of solubles affecting the viscosity. I think.
    Quality/quantity of extraction basically.

  3. So there is no “caramalisation” process during extraction? I’m wondering if the sweetness being brought out of the coffee during extraction can be analogous to the caramalisation of an onion being cooked slowly? (not sure if this analogy is too weird to dignify a response). Obviously the heat and flow rate have an effect on the amount of sugars extracted from the bean – I’m wondering if the heat subjected to the coffee during extraction further contributes to the maillard reaction of caramalisation, or if there is another chemical reaction taking place…?

  4. There are no reactions taking place because the temperature is way too low – you need to be beyond 150C/300F before anything happens.

    When you are slow cooking water you are driving off as much water as possible (as this generally inhibits these reactions – same with the first stage of roasting where the beans dries before starting to colour) and then there are quite a lot of complex reactions going on within the onion, along with caramelisation/browning reactions.

    There are obviously lots of reactions taking place during extraction, but most of them are compounds going into solution or water binding to various compounds within the coffee.

    The reason we get the texture is simply that espresso is that there are a lot more solubles per ml than any other brew, giving it that syrupy rich texture.

  5. If the Maillard reaction, or carmelization were taking place in a product, would you expect it to happen uniformly throughout the product, i.e would the product turn brown throughout the product or is it possible ? We produce a baked product with a high sugar concentration that we occasionally encounter problems with brown specks forming in the product, primarliy at or near the surface. Someone has suggested the Maillard reaction as a possible explanation, but we are uncertain if this is really a possibility, given the peck formation.

  6. hello i would like to know the reactions including the figures of the chemical compounds formed which take place during the caramelisation of sugars

  7. oohh GOD thank finnaly i find what iam looking. thanks .. this article really help me doing my homework abt mailard reaction

  8. i need help.
    dear admin, i need the maillard reaction step by step with all of component that includes for this reaction.
    with my honour please reply to my email. thanks before

  9. Hi Syukur,

    I am afraid this article was just a general introduction to these reactions from a coffee perspective. For full reaction pathways I can’t really help you and you may need to find a text dedicated to the subject. The Royal Society of Chemistry probably have something.

  10. sir,
    can you please give me the answers of the following questions,with step by step chemical reactions with chemical structures.the questions are:
    1.what is maillard raection,strecker drgradation,caramelisation?give definitions and chemical reactions taking place.

  11. Hello. Ur article really helped me to understand the Maillard reaction. however, u said: “These reactions are dependent on a few variables: the different amino acids available, the pH, the amount of water, any salts and the period of time held at a temperature.

    I would like to know how exactly the type amino acids and the sugars present can affect the reaction.

  12. I’ll be completely honest and admit that I don’t know and I am not sure where to look for the massive variety of reaction pathways that would be possible. The RSC might have some good books on the subject?

  13. hellow every body working on caramelization
    can any one tel me about the catalyst used in the caramelization of apple juice

  14. I am taking a food science class. Is it true that heat is not required for Malliard Reaction? Why?

  15. yes, while storing foods if the conditions are favourable for the reaction i.e carbonyl compound and amine group contain lone paired electron the reaction can take place.

  16. This is a question regarding the improvement in flavors that occur when resting roasted coffee for a few days, I’ve seen many people describe this as simply staling and that some amount of staling is good up to a peak of flavors and then the beans deteriorate, but this doesn’t seem right. As I’ve recently been roasting almonds in my roaster as well, I’m noticing that the almonds also do not taste their best right after roast and require a day or two of rest before their full roasted flavors are at their peak. I haven’t been able to find any literature on this (though I did come across an interesting article on how to keep nuts fresh longer by roasting at a lower temperature or doing a two step roast which seems to slow down staling, which I now cannot find), I’m interested in any feedback on this.


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