The Science of Slicing Garlic and Onions

The Science of Slicing Garlic and Onions


Garlic and onions are the essential ingredients of many dishes. Some chefs prefer to do without them, knowing that they have a very strong flavour and tend to mask all the other ingredients. But this is as always a matter of method and quantity: garlic and onions, in fact, are a concentrate of substances which if correctly used can positively enhance the flavour of the other ingredients.

Let us start from the basics: we often speak of garlic and onion together because they are considered to be similar. And that is indeed the case: both in fact belong to the amaryllidaceae family. Their chemical and culinary characteristics are therefore very similar. Each of them, technically, do not have aroma; that is to say, they do not contain odorous substances.


The inside of each garlic clove is called the "germ" which is also the youngest part of the bulb and therefore rich in the substances that give this vegetable its typical flavor. In fact, the outer part of the garlic is actually the oldest, which, with the passage of time, loses its aromatic intensity. Instead, the germ, which is "the last born", tends to be much more aromatic. If the odor and the taste of garlic are too strong, removing germs makes the flavour less intense and more delicate.

I imagine that this news will astonish you, but the odour of garlic in fact depends on a reaction caused by slicing. When we slice garlic an aminoacid is released which is rich in sulphur and immediately comes into contact with an enzyme called alliinase. So the two create allicin: this is the substance that gives the garlic the odour and taste for which it is used.

From this we derive an important lesson: the more we cut off a slice, the more we encourage the reaction between aminoacid and enzyme, increasing the production of odours and flavours. Not only that: the slicing ‘only’ allows the reaction to happen, but the reaction then continues with the passage of time. Therefore, to sum up, the more we cut a slice and the more we expect to use the sliced garlic, the stronger the odour and taste will be.

Therefore if we are to put a mere trace of garlic in our dish, it is better to cut one slice in half and use it straight away. If however we want as much intensity of flavour as is possible and imaginable, we cut the slice into tiny pieces and wait for about 10 minutes before using the chopped ingredients. But we should bear in mind that once the garlic starts to cook, the allicin will change into a series of compounds with a more delicate aroma.

You will not be surprised to learn that much the same description applies to the onion. In this case too we are faced with a vegetable without aromas, since these form only after the onion is sliced. Obviously there is always an aminoacid on a sulphuric base, very similar to allicin, beneath the phenomenon. The aminoacid, after the onion is sliced, always comes into contact with the enzyme alliinase, forming the compounds that give the onion its characteristic aroma and taste. Furthermore, the enzyme LF-synthase forms propanethial S-oxide, the substance that causes tears.

It goes without saying that the same identical advice concerning garlic applies equally to the onion. With one more thing. Various studies have shown us in fact that cutting the onion lengthways, that is to say from the top to the bottom instead of across the middle, reduces the intensity of the aroma and also the quantity of tears. Not only that: for both garlic and onion, the use of a food processor tends to intensify the aromas.

To put into practice all that we have learnt, all that remains is to try an easy and delicious recipe. This is the fabulous garlic and onion soup. Slice two large onions and cut up about ten cloves of garlic into small pieces. Sauté the garlic and onions in plenty of oil for about 10 minutes, then add a glass of white wine, soften the mixture and add a litre of vegetable stock. Bring to the boil and simmer on a low flame for 40 minutes. Turn off the heat and leave the soup to cool for 15 minutes, then add 150 ml of fresh cream, stir it into the soup and serve it with slices of toast.

Obviously you now know how to increase or decrease the intensity of the taste of garlic and onion!