The Science of Mirepoix


Mirepoix, the mix of fried diced vegetables used as a base for cooking, is a matter of chemistry. Don't miss these tips to adjust its flavour intensity.
The Science of Mirepoix

Modern cuisine tends to discard all those pronounced and added flavours likely to cover the original taste of the main ingredient. Mirepoix is frequently accused of being a prime offender.

Mirepoix is a mix of gently fried diced vegetables used as a flavour base for cooking. A well-balanced mirepoix calls for one carrot, one celery stalk, one small white onion and a clove of garlic, as well as some excellent extra virgin olive oil. Apart from the garlic, the vegetables must be very finely chopped.

Whether it consists of onion and oil, garlic and oil or a mix of these ingredients enriched with vegetables and spices, there is always a fear that they will “dominate” the final flavour of the dish. While this is partly true, it must also be said that the taste and aroma of mirepoix is an integral part of traditional cuisine and therefore deserves due respect.

It is all a question of learning how to adjust the flavour intensity of these ingredients. Once again, science can come to our aid. 


Let's start with the onion based mirepoix. The chemical composition of the onion is mostly made up of water (92%). However, this is followed by 5.7% of sugar and 1% of proteins. You should know by now that sugar and protein are the two components necessary to provoke the Maillard reaction (the chemical reaction that gives browned food its distinctive flavour). We are used to seeing it take place with meat but, in actual fact, onion is one of the few vegetables which lend themselves to triggering it without the addition of anything else.

The Maillard reaction is not only affected by the temperature, which must be high, but also by the pH in the environment. The higher it is, the more intense the reaction will be. This results in our onion becoming browner in colour and sweeter in taste. For our purposes, we just need to add a pinch of bicarbonate of soda to the sliced or chopped onion. The more bicarbonate we add, the faster the reaction will be, resulting in a less “pronounced” taste. Take care not to exaggerate though: too much bicarbonate would make our mirepoix unpleasant to eat!


The same principle also applies to garlic, as its composition reaches values of 8% of sugar and 1% of protein, with an 80% of water. It is not surprising therefore that it is generally easier to burn garlic than onion: you just have to forget it for a second and the Maillard reaction goes too far!

So to get the most out of your garlic, it is advisable to take a few precautions, also because it is much more aromatic than onion. First of all, do not chop it but use it whole or, at most, cut in half. Many cooks use it with the skin on but in this case, the aroma is almost imperceptible. It is a much better idea to cut it in half and remove the green inner shoot, which contains the greatest concentration of those substances giving garlic its characteristic taste.


Having learned these few simple rules, it is now time to find out how to make an excellent mirepoix.

Remember that a mirepoix must not be fried but gently stewed. This is why the vegetables must go into the pan together with the oil (and bicarbonate of soda). The heat must be low and the ingredients should “sweat” slowly.

Do not be tempted to add more oil: all you need is enough to stop the vegetables sticking to the pan and the right amount of this quantity of vegetables is about one spoonful. This will also help you to mitigate the assertive flavours of the mirepoix.

Anything else? Use a wooden spoon to mix the ingredients for at least fifteen minutes. If the vegetables start to sizzle excessively, lower the heat or add a drop of water (no wine, since this would add aromatic notes which would cover the taste of the vegetables). How can we use this delicacy? For serving on some excellent toasted bread, even cold, with a pinch of salt and pepper. Simplicity first and foremost.