Chemistry World Blog

Over at CENtral Science, they’re having a food chemistry blog carnival in the run up to Thanksgiving. As my contribution, I thought I’d share a recent food chemistry encounter with you all…

Whether you prefer butterscotch, toffee, honeycomb hokey pokey, spun sugar, nut brittle or the unctuous dulce di leche, caramelised sugar is a sticky treat that goes straight to the heart of most people’s idea of pleasure on a plate (or in a bowl of icecream…)

The chemistry of caramelisation is fascinating, but the other day, while I was settled in front of the TV to catch up on the Great British bake off masterclass on crème caramel, the description of the process had me shouting indignantly at the screen.

Now, no-one could describe the bake off as a science show. But that doesn’t excuse this ‘explanation’ (skip to 39:18):

‘When sugar is heated, each crystal is broken down into carbon, hydrogen and oxygen. The hydrogen and oxygen recombine to make water, which evaporates, leaving the carbon, which becomes caramel.’

If this was truly the case, no one would ever eat caramel. I don’t know many people who like the taste of pure carbon, and charcoal powder is not going to be as aesthetically pleasing on a plate as a rich, nutty-smelling, golden brown sugary sauce.

It’s a shame, because some of the other advice in the programme is soundly based in science – queen of home-bakers Mary Berry advises us not to be tempted to put a spoon into the boiling mixture, ‘because it will immediately cloud over and begin to crystallise’. Fair enough – stirring will doubtless introduce nucleation sites, and lead to a grainy, disappointing caramel. Lots of top tips for handling the caramel as well, as it is a tricky process to get right, and a nightmare to clean up if you let it cool in the pan.

However, I also feel the need to question the advice to avoid using a non-stick pan because the mixture will crystallise. From my own chemical instinct and a quick survey of the web, I can see no reason why this should be the case. Surely the non-stick coatings of modern pans will be at least as smooth as stainless steel (as long as they’ve been properly looked after). I can understand the argument that one shouldn’t heat older (or cheaper) teflon-coated pans over the kind of heat that making caramel requires, but modern non-stick coatings like hard-anodised aluminium shouldn’t have any problems at all.

So what is actually going on when you make caramel?

First of all, the sugar is simply dissolving in the water (at least for a wet caramel like this one). As Mary demonstrates, stirring at this point is fine – in fact it will aid dissolution, as any good chemist knows. This process is also accelerated by acid, which is why some caramel recipes include a squeeze of lemon juice or some citric acid.

As the water evaporates, the boiling point of the solution is raised, and the sucrose undergoes hydrolysis. While this could be described as a kind of decomposition, it certainly doesn’t give carbon, hydrogen and oxygen. It simply separates the sucrose dimer into its glucose and fructose monomers. Again, acid will help here.

After that, things get a little more complicated. As the mixture gets hotter, different reactions become favourable. The sugars begin to break down further into a whole range of smaller, more volatile compounds. These give caramel its spectacular colour and aroma.

But caramel is not just a brown and aromatic version of sugar. It is thick and sticky. This consistency comes from the sugar molecules joining back together and dehydrating (so yes, at some point the process does produce more water, which in turn evaporates, but it’s not through direct combination of hydrogen and oxygen…). This makes oligomers, called caramelan, caramelin and caramelen, which assemble into particles and networks and give the caramel its texture.

For anyone who wants to delve a little deeper into the chemistry, I can recommend Matthew Hartings’ write-up at his Sciencegeist blog.  It comes complete with a recipe for caramel sauce, which you can pore over to get something to pour over a festive treat this holiday season…

Phillip Broadwith