Mexican pickled onion a quick and easy recipe.

What happens when pickling – A Mexican onion pickle

Pickling is one of those cooking techniques that can completely transform or upgrade your food without a lot of trouble. Simply put the vegetables, or whatever you’ve decided to pickle in an acidic brine for anywhere between 5 minutes and days to transform that vegetable into something very different.

Raw onions can be a bit harsh by themselves. Cooking them gently works to sweeten them, but pickling can also remove that harshness of a raw onion very well. It’s a great condiment when eating tortillas so about time to discuss the science of pickling onions.

What is pickling?

A pickled food, often a vegetable, has been submerged in an acid liquid for a certain amount of time. A liquid is acidic when the pH value is less than 7. There are roughly two main types of pickling:

  1. Submerge the vegetable completely in an acid (e.g. vinegar)
  2. Fermentation of the food, through micro organisms which produce acids (mostly lactic acid bacteria)

The first pickle tends to be quite short and easy. The main process that occurs is a change in flavour and a slight change in texture. The acid will sit in the food at the end of the pickle. This type of pickle can be as short as several minutes to several hours. Since this type of pickling will not prevent spoilage that well, it is often used in combination with heat to properly kill all the micro organisms.

Fermentation of foods is a more intense pickling method. Often fermentation starts by mixing the cut product with salt. It is then kept for a certain period of time, which will depend on the product type. During this time the product will ferment. Lactic acid bacteria will grow in the mixture. There bacteria produce lactic acid, which makes the food sour, but they can also produce vitamins and will greatly change the flavour of the food. An example of such a fermented food is sauerkraut, but also yogurt and certain cheeses.

In this post we’ll focus on the first method: the quick pickle through addition of an acid directly. This is generally a quick pickle and often cannot be kept as long as a fermented pickle. We did discuss both methods in more detail when discussing pickled cucumbers.

The origin of pickling

Pickling is a century old preservation technique. Without having any refrigeration it is hard to keep vegetables throughout the year. The vegetables will spoil because of break down by micro organisms. However, most micro organisms cannot grow in a very acidic environment. By immersing the vegetable in the acidic liquid, the brine, the growth of these micro organisms is prevented. Pickling was thus a way to keep the vegetables from spoiling and could at the same time even improve the texture and flavour of the foods.

A quick pickle

A fast pickle is generally made by cutting up a product in small slices before adding vinegar (an acid). To enrich the flavour sugar and spices can be added. This type of pickle will give a nice product, but it will generally not be shelf stable. Therefore these pickles tend to be heated for prolonged storage.

In the recipe discussed below the onion is both pickled and cooked simultaneously, resulting in a lot of great textural and flavour changes.

Red Onion Pickle - just before the cooking starts
Preparing the onion pickle, before the last bit of water is added.

Effect of cooking an onion pickle

It is possible to pickle onion without heating it. The structure of the onion won’t change a lot. It will still be crispy and crunchy, maybe even a bit more so because of the diffusion of water into the onion slices.

By cooking the pickle as well, not only does the flavour of the spices enter the onion more rapidly or does the onion get that sour note, it will also change in structure. Heating the onions makes them turn very soft and translucent. The heat breaks down the plant cells and the ‘glue’ that keeps these together. As a result water can escape from the cells. This makes a cooked vegetable softer due to the loss of turgor.

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 sweeter when cooked. That is because onions store energy in long chains of fructose molecules. By cooking the onion for a longer period of time these chains are broken down, releasing the very sweet fructose! This evens out the acidity from the pickle nicely.

Red onion pickle colour

When cooking vegetables the colours tend to change or diffuse as well. This can be seen very well in the case of red onions. A raw red onion has red and white layers. However, by breaking down the cell walls during cooking, the red colour will escape. As a result it will spread throughout the entire onion. The red and white lines are gone now, instead the whole onion has turned a dark purple.

Not only does the red colour move around, it also turns slightly more pink than red. It is a phenomenon very similar to what we saw for red cabbage. The molecule making red onions red, are anthocyanins. These colour molecules are stable at increased temperatures (which is why the onion won’t turn white). 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 or more greenish.

Mexican Onion Pickle Recipe

Is it a real Mexican pickle? The most Mexican element seems to be that spicy red pepper. That said though, it works great with Mexican food, tortillas for instance. The pickle works great if you use it directly after you’ve made it. However, if you’ve cleaned your jars properly and work clean it will keep in the fridge for a few weeks. Just be sure to add enough vinegar and to submerge all of the onion in the pickle.

Mexican pickled onion a quick and easy recipe.

Mexican onion pickle

Yield: 6 portions
Prep Time: 10 minutes
Cook Time: 30 minutes
Total Time: 40 minutes

A quick tasty onion pickle.


  • 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. Take a small pan and add the onions, vinegar and sugar.
  3. Crush the juniper berries and coriander seeds (don't grind them, just break them). Add, with the bay leaf and red pepper, to the onions.
  4. Add water (approx. 100 ml) until the onions are well covered.
  5. Bring the liquid to the boil and, on a low heat, simmer the onions, covered with a lid, until they're soft and translucent.
  6. The final result will be a soft onion with plenty flavour, great to top up some tortillas or wraps.

Prefer a non-cooked onion pickle? Have a look here. Two other great resources are an article by the NY Times and of course, On Food and Cooking from Harold McGee.

If you’d prefer to experiment somewhat more with fermentation pickling, check out this guide or this phickle website.

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