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.
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.
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.
This process produces hundreds of different chemicals which give foods their flavor. That's why some chefs refer to this as the flavor reaction, but chemists call this a Maillard reaction, after the French chemist who first identified the details of the process in 1912.
However, beware: the Maillard reaction only happens within a narrow temperature range, between about 285°F/ 140°C and 320°F/ 160°C. If the temperature is lower than that, the Maillard reaction doesn't start, and you get warm bread. Higher than that, and the sugars start to caramelize and pyrolize, where they break down into very small carbon-based chunks that have more to do with charcoal than toast. This is why toast burns if you do it for too long: the temperature of the bread gets too high and the tasty chemicals break down.
The Maillard reaction may also produce chemicals that are less appealing, such as the cancer-causing compound acrylamide. Although the way this is produced isn't yet quite understood, it does seem that the hotter the food is cooked, the more is produced. There is little cause for concern, though: in a 2002 report, the World Health Organization estimated that you would need to consume about 0.5 milligrams per kilogram of body weight to reach a level that might begin to cause problems, and that the average person eats less than 0.001 mg per kg of weight per day. So, unless you eat nothing but well-done toast (and an awful lot of it), I wouldn't worry about it.
The future of the toaster
Nichrome wire toasters have been around for over 100 years, but it seems that they aren't going away, as they remain the cheapest way to toast your bread. There have been experiments with laser toasters and hot-air gun toasters that can print images on your bread, but these remain the realm of the mad scientist and those with access to several-thousand-dollar laser cutting devices.
So, the next time you are waiting for your morning toast to pop up, consider how much is really going on inside the humble toaster. From the nichrome wire that can be heated up hundreds of times without breaking, to the complex chemical reactions that give your toast its taste, the humble toaster is a great piece of appliance science.