by Will Hartridge
Let me pose a question: what do a seared steak, roasted coffee beans, fried chips and pretzels have in common? On the surface, it appears that there is nothing to connect them together and this is just a randomly selected list of foods and drinks- after all, in what way can coffee be similar to steak? The connection lies in the fact that the same chemical process, the “Maillard reaction” to be specific, is responsible for the formation of the recognisable colours and appetising flavours in all of the items mentioned above. Furthermore, this is only a small subset of the countless examples where we have, knowingly or not, harnessed the Maillard reaction when cooking to improve the flavour of our food. I think that its true significance is demonstrated effectively by this quote from Jean-Marie Lehn; a chemistry Nobel Prize winner: “The Maillard [reaction] is, by far, the most widely practised chemical reaction in the world”. So what is this reaction and how does it give rise to such delicious foods?
It was initially stumbled upon accidentally by the eminent 20th-century French biochemist Louis-Camille Maillard: a researcher with an eye for perfection, excellence in writing and a diverse research spectrum. In 1912, whilst trying to determine the chemical mechanism for protein synthesis Maillard started to combine amino acids (the “building blocks” of proteins) with sugars as he believed they could be used as condensation agents, that is to say, aid the process of joining molecules together. Upon combining amino acids with hexose sugars (any sugar with the formula C6H12O6), he noticed an unfamiliar and unexpected reaction producing various compounds: some with brown colour and some that had savory aromas. He then went on to methodically characterise this reaction, hence why it was named after him. However, his research was not focused on the implications of this reaction in food, instead on the required reactants, conditions needed and products produced.
Because of the sheer complexity of the Maillard reaction and the large number of products that it forms, research stalled for many years and at that time it was not fully understood. During World War 2, the military initiated more research into the reaction as a more detailed understanding of its mechanism could be utilised in the production of food for soldiers that would not go off easily but was still pleasant enough to consume. This research was continued post-war and in 1953 John E. Hodge was the first to outline the chemical mechanism of the Maillard reaction in a paper titled: "Dehydrated Foods, Chemistry of Browning Reactions in Model Systems". The implications of this paper were, and still are substantial, to the extent that it is still amongst some of the most cited papers in the field of food science.
So now let us move on and discuss the mechanism by which it occurs. Initially, we know from the experiments Louis-Camille Maillard carried out that two components needed for the Maillard reaction to occur are amino acids and a sugar- but it has to be a specific type called a reducing sugar. What differentiates reducing sugars from non-reducing sugars is that they feature either aldehyde or ketone functional groups when converted to their open-chain forms, which are significant as the carbonyl group present in both plays a vital role in the Maillard reaction. These reactants explain why the Maillard reaction so often occurs in food- the two common components can easily be found in various foodstuffs since everything that was once living will contain protein and sugars can be naturally occurring in the food but are also often used in cooking.
Ketone and aldehyde functional groups
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| How the aldehyde group of glucose is exposed when in its linear form |
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A flow chart detailing the stages of the Maillard reaction |
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| The 3 initial stages of the reaction |
First, the reducing sugar reacts with an amino acid- the carbonyl group present on the open-chain form of the sugar reacts with the amine group on the amino acid resulting in the formation of a compound called an N-substituted glycosylamine and water
This glycosylamine is unstable therefore undergoes Amadori rearrangement: a reaction where glycosylamine is isomerized- meaning the atoms are rearranged but the chemical formula stays the same, to form a ketosamine (ketose- meaning a sugar with a ketone group and -amine meaning an amine group is present)
This ketosamine can then undergo transformation by different pathways depending on the conditions (acidic or alkaline) to form a few possible products
After these initial stages, the products formed further react and break down to form hundreds of different possible compounds, some of which are responsible for the flavours and aromas of the food. These groups of mostly heterocyclic compounds are shown in the diagram alongside what flavours they contribute to. Furthermore, brown coloured polymers can be formed, called melanoidins, which give some foods that undergo the Maillard reaction their distinct brown colour.
Some of the flavour
compounds produced and what they are responsible for
However, the reaction should not be confused with caramalization (another form of non-enzymatic browning that results in a brown colour) as it does not involve amino acids. There are also some notable downsides and potential dangers to the Maillard reaction, for example it is possible that a carcinogenic compound called acrylamide can be formed which has prompted much discussion and research in ways to remove it from food.
So now we know the mechanism by which the reaction occurs, is there any way to speed it up and produce more of those delicious compounds? Actually, there are many common cooking processes that we utilise, even if it is unknowingly, to harness the power of this reaction. Firstly, the most obvious thing to do is add more of the initial reactants: amino acids or reducing sugars. This is often achieved by glazing, for example with egg or milk glazes that contain proteins so allow for the Maillard reaction to form a glossy, brown surface on baked foods like bread. Milk also provides more reducing sugar (in the form of lactose) to further enhance the reaction and in other cases, like with meats, marinades can provide the sugar needed. Another variable is temperature: the reaction can occur at low temperatures however it is far too slow to have a meaningful effect, that is why we so often see its products after cooking food at high temperatures- temperatures in the range of 140-165°C will rapidly increase its rate as they result in the optimum (low but not too low) level of water for the reaction to take place. A final way to speed up the Maillard reaction is to change the pH to be more basic (higher)- this can be brought about by adding a common cooking ingredient: baking soda (sodium bicarbonate). An example use case of this is to make nicer sautéd onions: adding a pinch of baking soda results in more browned onions with a boosted savory taste. Pretzels are also another food that utilise a basic pH to enhance the Maillard reaction- they are sprayed with a dilute sodium hydroxide solution before baking resulting in their characteristic and recognisable brown shine. A higher pH alters the first stage of the reaction as it causes the amino group on the amino acid to become more nucleophilic and in some cases deprotonates it (removal of a proton- H+) thus increasing its reactivity.
So next time you tuck into any of the food items mentioned in this article, you have the Maillard reaction to thank for its delicious taste.
Sources:
Billaud, C. and Adrian, J., 2003. Louis‐Camille Maillard, 1878–1936. Food Reviews International, 19(4), pp.345-374.
https://cen.acs.org/articles/90/i40/Maillard-Reaction-Turns-100.html
https://www.masterorganicchemistry.com/2017/09/12/reducing-sugars/
https://www.futurelearn.com/info/courses/everyday-chemistry/0/steps/22336
https://www.chemistryworld.com/features/the-marvellous-maillard-reaction/3009723.article
https://khymos.org/2012/06/04/maximizing-food-flavor-by-speeding-up-the-maillard-reaction/
https://khymos.org/2008/09/26/speeding-up-the-maillard-reaction/
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