Bitter bugs

Warning: Eating undercooked seafood carries a risk of food poisoning. If you plan on eating this dish, use fresh, chilled, unfrozen prawns from a good fish shop. Eat it within 24 hours of preparation and keep it chilled until serving.

You will need

• Fresh, uncooked prawns
• Lemon juice
• Bowl
• Fridge
• (Optional – red onions, lime juice, tomatoes, chilli, salt, coriander)

What to do

Fully shell the prawns, wash them under running water and place them in a clean bowl. You can try just one prawn to try this experiment. (Helpful hint – store the shells in the freezer until garbage collection day to avoid stinking up your bins).
Completely cover the prawns in lemon juice so no bits of prawn are sticking out of the top. Stir the prawns to make sure they are completely coated in lemon juice.
Place the bowl in the fridge to marinate for at least twelve hours.
What colour changes do you see in the prawns?
Optional – If you decide to make this dish to eat, finely dice the onions, tomatoes and coriander and add them to the bowl before placing it in the fridge. Feel free to add chilli, salt and some lime juice for more flavour. If any sections of prawn are still blue-green after the twelve hours, stir the mixture again and return it to the fridge for another six hours.

What’s happening?

The dish you’ve just made is called ‘ceviche’ (se-bee-chay), and is a popular way to prepare fresh seafood in middle and South American countries. Rather than cooking it in the usual way, the process uses citric acid to soften the meat and add flavour.
There are many different coloured pigments giving crustaceans such as crabs, lobsters and shrimp their colour. Mature prawns usually have blue-green stripes that help them blend into their aquatic environment. This pigment, a type of protein called beta-crustacyanin, is wrapped up with a red coloured protein called astaxanthin.
Both heat and acid cause the strings of amino acid that make up proteins to unravel. The acid in the lemon juice changes the shape of the beta-crustacyanin, making it let go of the astaxanthin, turning the shell red.

Applications

Humans have been cooking food over a fire for a long time now – there is evidence that our ancestral relatives were doing it nearly two million years ago. Cooking with heat does two things; it changes the chemicals in the food, making them taste better, and it also breaks apart the proteins so they can be chewed up and digested with less effort.
Using acid, such as vinegar or lemon juice, as a marinade is another common method for tenderising meat. Yet while we might describe the sensation of a strong acid on the skin as ‘burning’, it does not burn in the same way as heat might.
Both acid burns and heat burns are forms of chemical reactions that will change the chemicals making up the cells of your skin. However, extreme heat generally causes the chemicals to change shape and react with oxygen in the air, forcing them fall apart. Acid, on the other hand, donates hydrogen ions or a ‘protons’ to the proteins, leading to a different kind of chemical reaction. The end result is often the same, however. Since you’re made up of chemicals such as proteins, fats and carbohydrates, extreme heat and acid can both damage the cells that form your body.