Ceviche – Science of cooking fish with acids

Thought ‘cooking fish’ always involves some sort of heat? You might just be mistaken! You can also ‘cook’ fish using only acids, no gas stove required. It’s what makes so-called ‘ceviche’ and gives eating fish a whole new dimension. Despite the lack of heat, the acids can transform raw fish in a delicate ‘cooked’ fish. There’s quite a bit of science involved here, so let’s dive into why ceviche works, what ceviche is and end with a simple quick recipe. Ready for some ceviche science?

What happens when cooking fish

Before we can dive into ceviche science we have to start at the basics. What happens when cooking fish? We’ll start with the more common method of cooking fish: simply using heat.

When looking into chemical and physical processes in food we first have to know what we have to work with. In other words, what is a fish made? There are a lot of different types of fish with all a varying composition. When just looking at the fillets (thus not the bones), the by far most prevalent ingredient (just like in human beings) is water (often between 70-90%). In lean fish (those with only little fat) the next most prevalent ingredient are proteins. In fatty fish there is also quite some fat. Besides these two or three main ingredients there are still some minerals and vitamins in fish as well as a few carbohydrates.

When looking at what happens during cooking we mostly have to look at the proteins and water and in some cases the fat. During cooking the fish is heated, heating a fish will initiate several processes:

  1. Evaporation of moisture
  2. Denaturation of proteins; this is what actually ‘cooks’ the fish, it will turn white instead of translucent (similar to what happens when frying an egg)
  3. Melting of solid fats (if those are present, however, a lot of fats in fish can be liquid)
  4. Kill off any micro organisms present (once the heat is high enough)
  5. Change of colour (if applicable, shrimps are a good example)

It is mostly the denaturation of proteins that will make a fish look cooked. When proteins denature, their complex 3D structures unfold and get changed permanently. This different orientation drastically changes their appearance and ways of working.

What is ceviche?

Ceviche is fish that has not been heated, instead, it is cooked by immersing it in an acid. As we discussed above, one of the most important steps of cooking fish is the denaturation of proteins. This can be done by heat, but acids can do this as well! Acids will interact with the side chains of the proteins causing similar changes. Commonly used acids are lime & lemon juice.

Since the acid will not cause the evaporation of any moisture, the fish won’t be as prone to drying out. That said, it is possible to ‘overcook’ ceviche! If the proteins remain in the acid for too long, or if the acid is very acidic, the proteins will curdle and unfold too much. This causes the proteins to push out moisture, causing the ceviche to overcook.

Ceviche can be made with a lot of different types of fish. You can find heaps of recipes for salmon, shrimp and white fish (e.g. cod or tilapia) ceviche. All will work in similar way but tend to use different combinations of flavour.

Safety of ceviche

Since fish isn’t heated during the acid treatment, micro organisms aren’t killed by heat as usually is the case. The acidic environment might slow down the growth of some micro organisms, however, it does not necessarily kill the micro organisms (most will simply stop growing or slow down, but won’t necessarily be killed in this short time span). When making ceviche it is therefore very important to use high quality fish.

Using a freshly caught fish can be one example, however, in a lot of cases frozen is better than fresh for two reasons. One, fishermen tend to be at sea for several days in a row. They will freeze freshly caught fish on the boats immediately. Preserving the fish well. If this isn’t done the fish might actually already be a few days old when it comes off the boat ‘fresh’. The second important reason is that parasites in fish can be killed by freezing the fish. The low temperatures will kill of these parasites. So when using a fish that was frozen immediately, and thawed safely (don’t leave it on the countertop for 24 hours) you’ve got a good shot of having a fish suitable for ceviche.

ceviche with tilapia and tomatoes

Ceviche recipe

Below is a suggestion for a ceviche recipe, recipe can be varied a lot and the exact cooking time will depend on the limes and lemons you’ve used and their acidity. The more you dilute the lime & lemon juice, the less sour it becomes and the longer the cooking process will take. A few last ceviche science guidelines:

Ceviche science guidelines

  • The more acidic the ‘marinade’ (in other words, a lower pH value), the faster cooking will go
  • The smaller the fish pieces, the faster ‘cooking’ will proceed
  • Not all limes and lemons are equal, it is always best to look at the fish and see whether it’s done.
  • Ceviche can be overcooked, leaving the fish in the marinade for longer than requird doesn’t tend to improve the quality.

Now on to the recipe!

Ceviche - Science of cooking fish with acids
 
Prep time
Cook time
Total time
 
Author:
Recipe type: Fish
Serves: 2 large portions
Ingredients
  • 250g tilapia fillets (thawed)
  • 1 sjallot
  • 5 cherry tomatoes
  • Juice of a lime
  • Juice of two lemons
  • Juice of an orange
  • Salt and pepper (to taste)
Instructions
  1. Cut the fish in evenly sized pieces (if the size difference is very large some will cook a lot faster than others).
  2. Cut the sjallot and tomatoes in small pieces and mix with the tilapia.
  3. Pour in the juices and season with salt and pepper.
  4. Leave until the fish is fully cooked (some prefer their fish slightly raw in the center, take the fish out of the marinade sooner in that case), it took approx 30 minutes in our case.

Sources

FAO webpage on fish

Serious eats seems to have tested just about everything in food and cooking, including this ceviche.

Eating raw or undercooked fish increases the risk for foodborn disease and is thus not adviced for those with a weaker immune system. If in doubt, refrain from eating ceviche or search for expert advice.

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