Curious Cook: The Maillard reaction

Want to know why that glossy-skinned, brown-roasted duck tastes so darn good?  Brace yourself for a chemistry lesson then. 

TO understand how a lot of good food is produced, one can start by understanding something called the Maillard reaction.

You may have heard of the Maillard (pronounced my-YAR) reaction in association with the browning of food during cooking. It also happens to be the most common group of chemical reactions in the world.

Maillard reactions are happening in our own bodies all the time – so it’s a bit odd that many people, cooks or otherwise, do not seem to know much about the reactions.

One of the main Maillard reactions occurs when food is dry-cooked – that is, fried, grilled or baked – in temperatures around 140°C or higher. So Maillard reaction is at work when meats and other ingredients turn brown during cooking. This happens because amino acids are reacting with a reducing sugar under the heat.

Meat as you know, is made up of strings of proteins, and proteins are made up of amino acids – rather long strands of amino acids, in fact.

Even though there are only 20 primary kinds of amino acids, the amino acid strands can combine in many different and complex ways – which is why different meats don’t taste the same. The proteins are different because the mix of amino acids are distinct for each animal.

You might like to think of amino acids as the alphabet and proteins are words and sentences; you can imagine the number of words and sentences that can be constructed by just combining the 26 letters of the alphabet.

A reducing sugar is just a simple sugar compound which can “reduce” other compounds by reacting with them and it does this by donating electrons to another chemical compound capable of accepting them, like amino acids under the right conditions.

Sugars are present in some degree or other in practically all food – but this is not necessarily the same sugar that you spoon into a cup of tea.

In meats and vegetables, the sugars present in the cells tend to be glucose, fructose and ribose. All three are reducing sugars (or monosaccharides), which explain why they work particularly well with the Maillard reaction.

The crystalline sugar for your tea is sucrose as it comes from sugar cane – and sucrose actually isn’t a reducing sugar. However, dry-cooking sucrose causes it to break down into the simpler sugars, fructose and glucose, which can contribute to the Maillard reaction.

Why is it always brown?

As mentioned, one of the most obvious manifestations of the Maillard reaction is when food turns brown in dry heat when cooked (optimally between 140°C to around 180°C) for selected periods of time.

Leaving food a little too long at these or higher temperatures produces another effect called caramelisation – and if food is cooked at even higher levels of heat, then the effect is called pyrolysis (or carbonisation or more accurately, charring and burning).

Both caramelisation or pyrolysis are not Maillard reactions – caramelisation is simply sugars darkening in heat and pyrolysis is your food burning and that will happen with or without sugars or amino acids.

So if your fire alarm goes off in the kitchen, you’ve basically gone past the Maillard reaction stage and it’s time to use a fire extinguisher, turn the cooker off or evacuate the place.

Deep-fried chicken. – Filepic

So why does food turn brown during the Maillard reaction? That is a very good question and the answer is cleverer than you think.

When the amino acids in the proteins combine with the reducing sugars under heat, they create new and very complex molecules – as an example, scientists have identified some 1000 new compounds created by the Maillard reaction when cooking meats. These new molecules impart the exquisite flavours of fried, grilled or roasted meat which we enjoy.

The amino acids which have reacted with sugars tend to arrange themselves in rings which form into bundles of larger and larger rings – and the light from these bundles of newly created rings reflect brown light best. So that is why cooked food that has undergone the Maillard reaction appears brown.

Actually, the numbers of new molecules created are not that many relative to the size of the cooked ingredients – the brown fried or grilled part of meat is usually much less than 1mm thick.

The Maillard reaction is a complicated jumble of chemical reactions producing hundreds of new molecules (or compounds), and it is pretty hard to know which compounds or combinations of compounds are crucial for the flavours that you enjoy. These new molecules are also detected and savoured by your olfactory senses (nose) and not just your tongue – remember that flavour comes from a combination of using both your nose and your tongue.

Using flavours created by the Maillard reaction

Obviously, cooks over the centuries have developed techniques to maximise and use the new flavours arising from the Maillard reaction, even without knowing the actual chemistry behind it. Pretty much all the great hawkers at your local food courts are like that, unless of course, you know one who has a degree in chemistry. Mainly, it started with trial and error and over time, the better techniques were adopted and the less useful efforts discarded – a little like culinary evolution.

The idea was to obtain the ideal Maillard-induced flavours desired during cooking – and of course, different cooks have different ideas about what tastes best.

Some techniques to control and enhance the flavours involve using different fats, using a combination of baking and searing, varying the cooking temperatures, covering the meats with marinades, using different spices – and even by adding certain alkaline chemicals.

The ripening of cheese in a cellar is a Maillard process.

It’s not all hot stuff

So far you are aware that the Maillard reaction occurs under high temperatures and only during dry cooking. Well, heat and time are definitely two of the factors that control the Maillard reaction – and there’s no doubt that heat does speed things up considerably, especially if you’re in a hurry to have a good meal.

However, the reaction also occurs at much lower temperatures – for example, the ripening of cheese in a cellar is a Maillard process and so is the curing of aged hams, and actually, also the process of rotting, though that is not such a nice thing.

Therefore, garden compost is also brown because of the Maillard process. Under temperatures below the boiling point of water (100°C), the Maillard reaction is much slower and enzymatic – it uses enzymes instead of heat to break down proteins and allow the resulting amino acids and reducing sugars to assimilate and react.

However, characteristically, the resulting compounds are still arranged in the particular series of rings which create the brown pigmentation.


As an illustration of a slow enzymatic Maillard process, I recall a story of two Malaysian women driving across the Australian outback, when one of them shouted suddenly, “Stop! Stop! I smell durian!” The other woman replied somewhat tersely, “Don’t be silly – where would one find durians here in the middle of nowhere?”

But the other lady insisted and due to their overpowering curiosity (and love of durians), they turned back – and found that they had just been passing a huge rubbish dump.

The Maillard reactions rotting the rubbish had come up with complex volatile molecules that closely resembled the smell of durians – and the ladies had driven through a little cloud of such air-borne compounds.The Maillard reaction is also the cause for the rich dark colour of soy sauce. It takes two to three months of gentle fermentation before the soy proteins are broken down by salt into amino acids which then interact with the sugars in the soy beans, producing melanoidin as one of the by-products.

This is what gives fresh soy sauce its distinctive red-brown colour. If left exposed to air, soy sauce will darken further but this is more likely to be due to oxidation than a further Maillard reaction.

Anyway, the enzymatic Maillard reaction is for this part more for information than for practical kitchen use as it is a rather slow chemical process.

And if most of this article has been gobbledygook to you, fret not; what you need to take away from this story is that browning is what makes food delicious and you should do it properly to maximise flavours in your fried or roasted foods. And if you want to impress your friends, remember that browning is the result of a chemical process known as the Maillard reaction.