What you need
White chocolate
Dark chocolate
Thermometer
Hazelnuts
Coffee beans
Sea salt
Advertisement
IF THERE is someone you would like to impress on Valentine’s day, this swirly chocolate slab could be just the thing. Simply melting and mixing the chocolates won’t do, however. To make glossy, smooth chocolate with a pleasing snap, you need a little knowledge of its chemical structure.
The chocolate we know is a far cry from the astringent cocoa bean from which it originates. The cacao tree has been cultivated in Central America for more than 3000 years. Archaeologists have found chemical traces of cacao on pottery dating to 1600-1800 BC.
Cocoa beans grow in large pods packed with fibrous, sugary pulp. To make chocolate, the beans are first fermented in the pulp. Bitter phenolic compounds react and form less bitter complexes, while enzymes in the beans break down sugars and proteins into compounds that react more easily in the next step: roasting. Maillard reactions then produce the full, complex flavour of chocolate.
The roasted beans are ground, breaking the cells into particles suspended in fat, known as cocoa butter. The mixture can be turned into chocolate by adding ingredients such as sugar and milk solids. It is then mechanically worked in a process called conching, before being cooled into solid bars.
The texture of chocolate is determined by the crystal structure of the fat molecules. The glossy surface and pleasing snap of a good bar of chocolate arise when the fat molecules align in a dense network of compact crystals. If chocolate melts and resolidifies in an uncontrolled way, the molecules form loose, irregular crystals that result in soft, mottled, crumbly chocolate.
Tempering is a way of creating the right crystalline structure. You will need a thermometer because the key is to hold the temperature at 31 to 32°C, which is below the melting point of the right crystals and above the melting point of the wrong crystals.
Break or chop 300 grams of dark chocolate into small pieces and hold a few in reserve. Melt the rest in a bowl over a pan of hot but not boiling water. Once it is all melted, remove from the heat and stir in the reserved chocolate, which will seed the melted chocolate with the right kind of crystals. Keep stirring until the temperature falls to 31 to 32°C. Hold it at this temperature until you are ready to use it.
Do the same with 300 grams of white chocolate in a separate bowl, but keep it at 27 to 28°C.
Line a tray with baking paper. Pour in the dark chocolate, then pour the white over the top. Use a chopstick to make patterns and ensure there are no holes in the slab. Drop in some hazelnuts and coffee beans – or anything else you like – along with flakes of sea salt. Let it cool at room temperature to allow the crystalline structure to develop fully. Wrap up the whole slab as a gift, or break it into pieces.
For next week
Kombu (dried kelp)
Katsuobushi (tuna flakes)
Miso paste
Silken tofu
Spring onion
Next in the series
1 The scientific shortcuts to cooking delicious caramelised onions
2 How to make halloumi and ricotta cheese using ancient biotechnology
3 How to cook perfect chips: Learn the science of crispiness
4 Here’s how to make your own tofu for Chinese New Year
5 Use the science of curing to turn salmon into gravlax at home
6 Tempering chocolate
7 Umami and flavour
How to maximise that rich, savoury taste
8 Perfect pancakes
9 Kimchi and fermentation
10 Sourdough bread
All projects are posted at Email: cooking@newscientist.com
