B.B.Q. Salmonella

With summer comes the barbeque. And with the barbeque, all too often, comes food poisoning. As far as I can see there are two reasons for this:

1. Barbeques are almost always cooked by men.  This is a phenomenon that surely needs study by sociologists. At the first glimpse of summer, the man, who hasn’t set foot in the kitchen for six months, suddenly acquires a Michelin star and his own TV show, and becomes the undisputed king of cuisine. Women are banished to the outer reaches of the solar system, and the area around the barbeque becomes the domain of man.

2. (and here is where the physics lies) Barbeques, especially the traditional charcoal burning ones, are not particularly temperature controllable. This means it is easy to incinerate the outside of your chook-leg, but leave the inside almost raw.

To cook something correctly, you need the right temperature, and the right cooking time. Heat travels from the outside to the inside through the process of conduction, and just how quickly that happens depends on what material the item is made of. Physicists talk of a parameter called ‘thermal conductivity’ to describe this process – the more thermally conductive, the quicker the heat gets to the centre.

Unfortunately for cookery, the thermal conductivity of most meat is pretty low, meaning you need to allow time for the centre to get hot. In other words, putting all practicality aside,  sausages would make reasonable wall insulation for your house. (Think about that fatty layer of blubber on a seal – its role is to stop the seal losing heat to its icy environment – the same process of cookery but in reverse). This means that you can’t just whack them on high heat for a few seconds – all that will happen is that you’ll nicely cook the outer couple of millimetres but leave the rest untouched.

In metals, thermal conductivity is related to electrical conductivity, so it is no coincidence that, before its toxic properties were well understood,  saucepans and the like used to be made of copper – it nicely conducted the heat from the fire outside through to the water inside.

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