The other day we made an onion tart that was so incredibly sweet, it was hard to believe the topping was made of just about nothing but onions. It tasted nothing like raw pungent onion and definitely reminded me that onions are so multi-functional.
Onions can be sweet, tangy, sour, soft or crispy. Onions have so many different faces, and all of them show up in different situations. Nevertheless, all originate from the same onion and the same molecules to start with. It’s a great example of food chemistry at play!
It starts with a raw onion
Onions are quite a unique vegetable. They grow underground and are built up of a lot of different layers. Since onions are bulbs, they are meant to store plenty of energy for the plant to grow from, which is stored in the form of sugars. Onions are closely related to garlic, which are also bulbs, and all of these belong to the allium family.
Onion chemistry – Thiosulfinates
The chemistry of onions (and allims in general) is mostly determined by molecules that contain a sulphur (S) atom in them, sulphuric compounds. The most important group of those for the flavour and odour of onions are the thiosulfinates. It’s these components that cause you to tear up when cutting an onion, but also give it its main flavour and odour.
What is interesting though is that the raw onion doesn’t even contains these molecules yet. Instead, they are only formed once an onion is damaged (e.g. sliced)! The slicing action breaks up cell structures, releasing all the different molecules inside. As a result, specific enzymes (allinases) get into contact with molecules in the onion that are precursors to those thiosulfinates. The enzymes catalyze a reaction which results in the formation of thiosulfinates. Within 30s after cooking all these precursors have reacted into the pungent thiosulfinates!
It’s these thiosulfinates that can cause you to start crying while slicing an onion. One of the more potent molecules is propanethial-S-oxide. It also contributes to the characteristic onion flavour.
Cutting an onion is the first measure of control for thiosulfinates. Since the odour and flavour components are only formed once the flesh is cut into pieces, cutting an onion more finely will release more flavour. The opposite is also true, if you boil an onion, you will break down the enzyme before it even has a chance to catalyze the chemical reactions. As a result, this onion will lack a lot of the onion flavour.
Once the onion has been cut, you can control thiosulfinates by subjecting the onion to different heat treatments. A freshly cut onion, that isn’t heated at all will be pungent and strong, it might be desirable for a guacamole. Heating and cooking the onions will initiate a whole range of chemical reactions. These reactions are different when they occur at room temperature, versus the temperature of boiling water. Also, they take a lot longer than that initial set of reactions that occurred after slicing. As a result, some longer cooking times are necessary to break down the pungent smells into the milder, more savoury notes.
The importance of soil for flavour
Sulphur is an essential atom for all those flavour and odour molecules. But onions can only get a hold of sulphur through the soil. As a result, soils with more sulphur (up to a certain limit of course) will make onions with stronger flavours and odours than those grown on sulphur poor land. Of course, it also depends on the variety you’re growing, some onions simply take up less sulphur than others.
Another important way to develop and control flavorus in onions is to caramelize them. When sauteing onions on a moderate heat they will turn a light brown. This is caused by the Maillard reaction, a reaction between sugars and proteins.
Creating sweet onions
So why where these onions on this onion pie so beautifully sweet? What had happened? Well, the onions had simmered in some butter with a little salt and sugar for over an hour. As a result, they had softened and broken down considerably. The sugars had come free and all those sulphuric components had continue to react and mellow down. None of the pungent flavour was still left!
Compound Chemistry, The chemistry of an onion, 2014, link
Cook’s Illustrated, Cook’s Science, Chapter 29 Onions, 2016
Cooking science guy, The science of onion flavour, 2012, link
Noureddine Benkeblia, Virginia Lanzotti, Allium Thiosulfinates: Chemistry, Biological Properties and their Potential Utilization in Food Preservation, Global science books, 2009, link