Here’s the clever chemistry that can stop your food rotting

A hotel in Reykjavík has on display a McDonald’s burger and fries, seemingly undecomposed after 2,512 days – and counting. It was bought on October 30, 2009, the day that the last McDonald’s in Iceland closed. But you don’t have to go to Reykjavík to see it: it has its own webcam so you can watch it from your armchair.

What makes this meal so long-lived? Well, I haven’t examined this particular burger myself, but chemical reactions cause food to decay – and understanding them can help us to keep food better and for longer.

Let’s start with uncooked rice – in many peoples’ minds it’s a foodstuff that will keep for a long while. Experts reckon that polished white rice will keep for 30 years when properly sealed and stored in a cool, dry place. This means in an airtight container with oxygen absorbers that remove the gas that can oxidise molecules in the rice.

Hotter food goes off faster; as you may remember from school science lessons, chemical reactions are faster at high temperatures because hotter molecules have more energy and so are more likely to react when they collide. It’s one reason we have fridges. But there is a limit. Above a certain temperature (approximately 50-100°C), the enzymes in a bacterium get denatured – their “active site”, where its catalytic activity happens and it binds to molecules to carry out reactions on them, loses its shape and can no longer carry out reactions.

Back in the 19th century, Louis Pasteur invented the process that bears his name. Pasteurisation kills the bacteria that make food go off and today this is applied mainly to milk. Milk that has been pasteurised by heating to just over 70°C will keep for two to three weeks when refrigerated, while UHT milk, made by heating to 140°C, will keep in airtight, sterile containers for up to nine months. Raw milk left in the fridge would last only a few days.

Living off the land

The short life of food was the reason that medieval armies “lived off the land” by scavenging, but in 1809 a Frenchman named Nicholas Appert won a prize offered by his government for a process for preserving food. He showed that food sealed inside a container to exclude air and then cooked to a high enough temperature to kill microbes such as Clostridium botulinum kept for a long time.

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