Caramelisation is a term that is really bandied about within the cooking world. Caramelised apples, onions, carrots, sugar, vinegar – we have all heard it, but when it comes to understanding it where exactly are we? I have put together this little primer on caramelisation which should be a helpful introduction for some and perhaps a refresher for others.
Caramelisation belongs to a suite of processes known as “browning reactions” that are best described as, well, reactions that turn foods brown. Sometimes science is simple, isn’t it? While the colour may be the same in all cases, the path that leads there is quite different. You have already heard me mention another browning event, the Maillard reaction, which I told you was a reaction of proteins with reducing sugars. The Maillard reaction is important in foods that contain good levels of protein, such as meats. Caramelisation sets itself apart in that proteins are not involved at all, only sugars. The most common sugar you will encounter in the kitchen will be sucrose, though you may have come across other ones as well. Fructose, glucose, lactose, and galactose are all sugars that are encountered in some form or another in cooking, and they will all caramelise under the right conditions.
What are the right conditions? Heat, and lots of it. Take the case of granulated sugar, sucrose. As it is heated it passes from a white powder into a brown melt that rapidly turns black. At around 160ºC, below its melting point, the sugar is already breaking down into smaller molecules that are themselves combining and recombining to form more and more elaborate substances. Hundreds of different chemicals are formed that lend the colour and flavours we think of when we hear the word “caramel”. The buttery scent comes from a small molecule commonly named diacetyl (less commonly called 2,3-butanedione; a real mouthful). The nutty odour is a property of the molecule called furan. The odour of very ripe fruit comes from ethyl acetate. These are only three of a huge number of compounds formed within seconds at high temperatures in a pan or oven. The important thing to remember is that temperatures above the boiling point of water are required, which is why you need a fairly dry environment or at the minimum a very concentrated mixture of sugar in water. The high concentration of sugar increases the boiling point of the mix so that temperatures above 100ºC occur. This is the case when you make fudge. The caramelisation is happening at the very bottom of the pot where the temperatures are highest, which is why you will often see dark reddish brown bubbling to the surface as you stir your candy.
Caramelisation is also important during the roasting of green coffee beans. It may surprise you to know that raw coffee beans are naturally green. The brown colour is produced mainly through caramelisation and the Maillard reaction, and the deliciously enhanced flavour of roasted coffee is due to the multiplicity of aromatic molecules created. The famous “second crack” of coffee roasting can be attributed to the rapid expansion of the bean under the force of carbon dioxide pressure. This gas is a happy consequence of the decomposition of sugars in the coffee and serves to provide an oxygen-free environment for the precious and fragile flavours that would otherwise be gradually destroyed if left open to the air. Perhaps now is the time to remind you to always grind just before making your cup of coffee so that you get the freshest and fullest flavour possible.
Although it is often impossible to definitely ascribe a toasty flavour or tawny colour to either the Maillard reaction or caramelisation, in practice high protein foods such as meats and lower temperatures will produce more Maillard products while high sugar foods such as candy and some fruits and vegetables and higher temperatures will produce more caramelization products. In the end, though, only your palate can decide which one is more important!