Warm weather pairs perfectly with a cool meal, and we have just the recipe. Ceviche (suh-VEE-chay) is a popular dish from Latin America that relies on lime juice to “cook” seafood without any heat.

Just the idea for kids in the kitchen on or before Christmas and new year’s feasts.

The team at Double Helix kids’ science magazine has adapted a ceviche recipe to illustrate how even weak acids can change the chemical properties of meat proteins.

Try this activity with kids and watch as the fish changes colour before your eyes. You’ll end up with a refreshing appetiser that will satisfy both the curiosity and the palettes of scientists of all ages.

You will need

• 150 grams sashimi grade fish (can be kingfish, salmon, tuna, trevally, or any soft-textured fish)
• 2 limes
• ¼ red onion
• ½ jalapeño (optional)
• 1 garlic clove
• ¼ cup fresh coriander
• ½ cup cherry tomatoes
• 1 tablespoon extra-virgin olive oil
• ½ avocado
• Salt
• Pepper
• Corn chips
• Juicer
• Bowl
• Knife
• Cutting boards
• Refrigerator
• Camera (optional)

What to do: Make ceviche

First, make sure you can source some sashimi grade fish for food safety reasons. This may be available at the fish counter at your local supermarket or IGA. You can also buy sashimi from take-away sushi restaurants.

Wash your hands with soap and water and use clean cooking equipment, of course.

Juice your limes into a large bowl. They should make about ¼ cup juice.

Next, chop your vegetables: dice the red onion, dice the jalapeño, dice the garlic, chop the coriander leaves, and quarter your cherry tomatoes. Add them all to the bowl, with some salt and pepper to taste.

On a separate cutting board, chop up your fish into 1.5 cm cubes. Make note of the colour of the fish cubes. You might like to take a ‘before’ photo.

Add your fish and olive oil to the bowl. Give it a good stir!

Cover the bowl with a lid or cling wrap and then put it in the refrigerator for 30 minutes.

After 30 minutes, check on your ceviche – has the colour of the fish changed? Take another photo and compare it to the before photo. When you’re done, give the ceviche a quick stir and then leave it in the refrigerator for another 30 minutes.

When the timer goes off, chop your avocado into small chunks, on your vegetable chopping board.

Take your ceviche out of the refrigerator – what colour is the fish now? Take an ‘after’ shot.

Add the avocado and stir. Give it a taste and add more salt and pepper if you want.

Open the bag of corn chips and use them to scoop up the ceviche. Buen provecho!

(Be sure to eat your ceviche on the same day you make it. It doesn’t keep well as too much time in the lime juice makes the seafood go tough.)

What’s happening?

Meat is mostly made of proteins, and each protein is made of a long chain of building blocks called amino acids. The 3D shape of each protein is very important to how it works. But this 3D shape is held together by weak bonds between the amino acids. It’s a bit like origami – and scientists even call it “protein folding” when talking about how a protein gets its 3D shape.

Heat adds a lot of energy to food. So as you cook meat, that energy breaks the weak bonds between the amino acids. As a result, the protein unravels, and the 3D shape is lost. Imagine an amazing origami dragon being completely unfolded into a piece of creased paper. Scientists call this process “denaturing”.

But heat isn’t the only way to denature a protein. As you discovered with your ceviche, the acid in the lime juice can also destroy the weak bonds holding proteins together. And as the protein becomes misshapen, it changes colour and texture. What changes did you observe as your ceviche cooked? Did it remind you of seafood cooked with heat?

Buen provecho!

“Buen provecho” is Spanish for “enjoy your meal” and it’s a polite thing to say before tucking in, especially in Latin America. There are different versions of ceviche across Latin America, but the dish probably originated in Peru. This recipe is most like the avocado-filled ceviche of Mexico.

Citrus fruits are native to Asia and came to Latin America only recently, with the arrival of Europeans in the late 1400’s. But archaeologists have found evidence that suggests Peruvians were making ceviche with acidic chilli peppers and seaweed, possibly for thousands of years.

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