Naked Scientists - Kitchen Science - Why sausages split lengthways
With Derek Thorne and Hugh Hunt

4th February 2007 - Transcript of the live radio show

Although it's freezing in England right now, on the other side of the world the sun is shining. So in honour of the sun-drenched Antipodes, Derek goes for a BBQ at the house of our very own Ozzie Hugh Hunt.

To do this experiment, you will need:
Some sausages
Willpower (you can't eat the sausages before the experiment is over!!)

How to do the experiment:
1 - Light a BBQ - ask a sensible adult to make sure everything's safe
2 - Throw on a few sausages and let them cook
3 - Keep an eye out for sausages with split skins. In which direction do they split?

So what happens?

Sausage skins split lengthways down the sausage.


You can think of a sausage skin as being like a pressure vessel. It's actually holding in all the contents that are expanding with the heat. When the skin suddenly bursts, it's going to burst in the direction where the stress is highest. The stress is really just the amount of tension there is the skin. It turns out that for a pressure vessel, the stress going around circumferentially is twice as high as the stress going lengthwise. So that means that it tends to rip along the length because the circumferential stress is twice as high as the longitudinal stress. The same thing happens when you put a can of beer or a can of coke in the freezer: it will also burst along its length.

We can see this effect in other ways too. For example, you have gas pipe lines going thousands of miles across Alaska - a bit like a giant sausage. The one thing you have to be really careful about is that you don't get a burst pipe. If it starts to burst, the burst will go right along the pipe for hundreds of miles. The people in the know will say that the pipe is 'unzipping'. It's absolutely catastrophic because it releases gas over a large area.

But going back to our sausages - the best way to prevent them splitting is poking them, but that lets out all the delicious juices. So you really want to find a compromise, and this is to cook them at a nice gentle rate so they don't burst.