Mexican onion pickle science

This month is fruit & vegetable science month on my blog! All posts are somehow related to fruits and vegetables. We’re discovering the amazing science involved when cooking or baking with them! This post is all onions, more specifically: onion pickle science.

Tortillas are a regular favorite for dinner, they’re easy to prep (just heat in the microwave) and you can make tons of different fillings. Though, I wasn’t that good in creating a real Mexican style flavour (or what I would call Mexican, I’ve never been to Mexico). Until recently, when I discovered two recipes that so much improved the flavour of my tortillas! One of them is an onion pickle, easy and very very effective, inspired by Allerhande.

Mexican onion pickle science
  • 3 large red onions
  • 1 small red pepper (the final result won't be spicy though)
  • 100ml vinegar
  • 50g sugar
  • 1 tsp juniper berries
  • 1 tsp coriander seeds
  • 1 bay leaf
  1. Cut up the onions in half rings.
  2. Heat up a frying pan (for me a small frying pan worked best, but a small regular pot should also work) and add the onions, vinegar and sugar. Crush the juniper berries and coriander seeds (don't grind them, just break them) and add to the mixture together with the bay leaf.
  3. Add some water (approx. 100 ml) until the onions are well covered.
  4. Bring the liquid to the boil and boil the onions, covered with a lid, on a low heat for about 20-30 minutes, until they have bcome soft and translucent.
  5. You can eat it straight away, or store it in the refrigerator for a few hours later.

Of course, this wouldn’t be a Food Crumbles post if I would discuss ‘onion pickle science’!

The origin of pickles

Pickling is a technique originally used to preserve foods. By immersing vegetables in a very acid environment the growth of bateria, moulds and yeasts is inhibited. Most of these micro-organisms don’t grow in a low pH (a measure for the acidity, the lower the pH, the more acid it is) environment. So by immersing vegetables in an acid environment they could be kept for long periods of time during the year.

Mexican pickled onion

Onion pickle texture

The pickle I’ve made involves boiling the onions for some time. This makes the onion turn very soft and translucent and changes flavour and colour. Onions are made up of layers of tissue as you might have seen when cutting an onion. Each layer is pretty thin, which makes it very easy to heat the entire piece.

As you might have read a couple of days before, plants consist of plant cells which contain a pocket of water within a sturdy cell wall. Plants stay firm by keeping the water pockets full (think of a balloon for making papier-mâché). When heating the onion, the cell walls are broken down and the water is released, making the onion soft.

Red Onion Pickle - just before the cooking starts

Cooked onion flavour

Cooking onions also greatly changes the flavour of them. A raw onion has a pungent flavour which is caused by sulfur components, molecules that contain sulfur atoms. The onion uses these components to protect itself from animals and microbes. Luckily for us, this defensive system sets in quick and once its job is done, the pungency is gone. By heating the onion this will go even faster. The molecules will react and transform from being pungent into milder components.

Onions also tend to get sweeted when cooked. That is because onions store energy in long chains of fructose molecules, unlike a potato which uses starch for energy storage. By cooking the onion for a longer period of time these chains are broken down, making the onion sweeter! This evens out the acidity from the pickly nicely.

Mexican pickled onion 2

Red onion pickle colour

Last but not least, the colour of my red onion pickle has turned a beautiful even purple. A raw red onion however is white with a red skin. By breaking down the cell walls during cooking, the red colour of the onions has bled out of the broken cells and spreaded out evenly.

The molecule making red onions red, is a group called anthocyanins. They are pretty stable for temperature (that’s why they didn’t turn pink for instance). However, they are very sensitive to acidity. In an acidic environment (like this pickle for instance), they will be a dark pink/purple. But in a less acidic (thus more alkaline) food, they can turn blue, green, yellow or even colourless at a very high pH!

Uncooked onion pickle

You can also make an onion pickle by leaving out the cooking step and simply ‘marinating’ the onion in the vinegar mixture. This will give a more crunchy onion (the cell walls haven’t been broken down that much), but it will also be more pungent since the heat hasn’t been allowed to break down the sugars chains or the pungent sulfur molecules. Whatever you prefer!

Want to learn more on onions?

Here are some interesting sources I used that can give you more in depth details: NY Times, Making long term storage pickles, Cold pickled onions, On Food and Cooking from Harold McGee (when do I not use this book?), Scientific Literature on Onions!


Note, this recipe has been developed to use within a short time span. I haven’t tested this pickle for prolonged storage out of the fridge. If you want to make a pickle for long term storage make sure you use a recipe with enough vinegar (acidity) and work very clean to prevent contamination.

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