Appliance Science: The well-done physics and chemistry of the toaster

The humble toaster is a combination of physics and chemistry that produces a tasty treat. Find out all about the science of toasters and toast in appliance science.

The humble toaster is the unsung hero of the kitchen. You throw bread in there, and it comes out as tasty, tasty toast, ready for slathering with more tasty stuff to provide a healthy, nutritious breakfast. It sounds like a simple process, but there is a lot more going on in there than you realize. The humble toaster is both a great example of modern engineering and a fascinating piece of biochemistry.

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The heart of the toaster is one or more loops of nichrome wire. This nickel-and-chromium alloy has an unusual combination of properties: it is resistant to oxidation, has high electrical resistance and has a very high melting point. First patented in 1906, nichrome doesn't tarnish or rust, doesn't like to let electricity through and will stay solid until it is very, very hot (about 1,400 Celsius).

This combination means that when you run the right amount of electricity through it, the wire gets hot (but not hot enough to melt) and won't rust, break or get tarnished easily. This material is used in most electrical heating systems, including water heaters. For a toaster, you wrap this wire around a heat-resistant material like mica, and you have a heating element that heats up quickly when you flick the switch, giving off lots of nice infra-red heat energy and some visible light (the red light you see inside the toaster). When you turn the power off, it quickly cools.

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Combined with this is the pop-up tray, which is spring-loaded so that it wants to be at the top of the toaster. When you push the lever down, this pulls the springs and pushes the tray down, until it reaches a latch that holds it into place. Metal grills hold the bread vertically, so it doesn't fall over and touch the nichrome wires. The latch that holds the tray down is linked to a timer, which is controlled by a bi-metallic switch. In this, two metals are put next to each other that expand at different speeds. When you start the toaster, electricity runs through this switch and the metals heat up, expanding and eventually pushing apart, breaking the circuit. This releases the spring loaded tray latch, and your toast pops up.

The dial on your toaster controls how much electricity goes through this switch: a lower current means the switch takes longer to activate, so the more toasted your bread becomes. A higher current means more heating, so the switch is triggered quicker. This also explains why your toast comes out more well done on very cold mornings: if the bi-metallic switch is cold, it will take longer to heat up and break the circuit.

All of the complexity above is designed to do one thing: heat the bread with infra-red radiation to make toast. You might assume that this is a simple process, that the toaster is just burning the bread, carbonizing in a slower process than just setting light to it. But it's not that simple: toast tastes so good because it contains a complex mix of chemicals that are different to bread.

Rather than simply burning, the chemicals interact in unusual ways. In toast, the amino acids and sugars react, forming a range of complex chemicals such as acetyl tetrahydropyridine that aren't normally present in uncooked food. Humans can detect these chemicals by smell and taste in incredibly small amounts, and these chemicals give toast its distinctive smell and taste. While you may not know acetyl tetrahydropyridine (or, more precisely, 6-acetyl-1,2,3,4-tetrahydropyridine, to give it its full chemical name) by name, you know the smell: this is the unique (and wonderful) smell of toast and biscuits cooking.

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Appliance Science: The well-done physics and chemistry of the toaster