Expert chocolate-artist and chef, Nina Notman has been writing about the chemistry of cooking and tempering chocolate. Here’s her expert take on chocolate manipulation.

Armed with a palette knife, thermometer and marble worktop, I discovered for myself how hard an ingredient chocolate is to tame. Under the watchful eye of Bristol chocolatier Zara Narracott, I poured melted chocolate on the marble, pushed it back and forth with the flat blade of the knife while it rapidly cooled, before scraping it back into a bowl and stirring vigorously. It was all in vain; I succeeding only in producing something streaky and soft. It was not the uniform glossy chocolate we had been prepped on how to make at the start of the chocolate tempering workshop less than 30 minutes earlier.
Tempering in metallurgy refers to heating and cooling a metal, normally steel, to improve properties such as consistency, durability or hardness. The same is true for chocolate. Narracott passes me an untempered piece for tasting: it has a white coating and looks dry. Once in the mouth, it instantly crumbles rather than melts, but still tastes good. Next up is an extremely smooth and glossy-looking dark chocolate with a caramel centre. Biting into it gives a very satisfying crunch allowing the caramel to ooze out. Delicious.

Cocoa butter – the fat obtained from cocoa beans and mainly consisting of oleic, palmitic and stearic fatty acids – gives chocolate its physical structure. When tempering chocolate, it is the crystal structure of the cocoa butter that chocolatiers are manipulating. ‘Cocoa butter is a six-phase polymorphic crystal,’ explains chocolatier Richard Tango-Lowy, a physics graduate who now runs the Dancing Lion Chocolate shop in Manchester, US. The desirable crystal structure for chocolate is form V.

A taste of tempering
Tempering is required when making chocolate from its core ingredients: cocoa butter, cocoa solids, sugar, an emulsifier and flavouring. And, if you wish to retain the glossy look, it is also required every time chocolate is heated; for example, for moulding or adding different flavours. Artisan chocolatiers, including Narracott and Tango-Lowy, rarely make their own chocolate; instead, they buy in sacks of chips of high quality couverture chocolate, containing a higher percentage of cocoa butter than standard chocolate.

Prior to the workshop, Matt Hartings – who teaches a chemistry of cooking class at American University, Washington DC, US – had talked me through the basics of the tempering process. The first step is to melt the chocolate to destroy all the cocoa butter crystals that are present. ‘The chocolate is then cooled down very precisely, holding the temperature just below where the good cocoa butter crystal form is made,’ he explains. It varies from chocolate to chocolate, but generally form V crystals start to form just below 35°C, whereas undesirable form IV crystals crystallise out at around 27°C. Form VI crystals take too long to form and therefore aren’t created during the rapid tempering process, even though their higher melting point suggests they should be.

Theoretically, tempering should give you only form V crystals, but in practice some form IV seed crystals form too. ‘You then warm it up again a little, so that any bad [form IV] crystals that have formed re-melt and you’re left with just good crystals. Once that happens, then you can cool it down and pour it into moulds,’ Hartings explains. As the chocolate cools, the seed crystals present from the first step speed up the kinetics of form V crystal formation. While form V will be the dominant crystal structure for the cocoa butter triglycerides in the chocolate, a few of the lower melting point IV crystals or other crystal forms will form again as it cools.

Tempered chocolate takes several weeks to fully crystallise. ‘We typically mould the bars, allow them to crystallise in a cool environment overnight, then package them and store them in the same controlled environment for a week or two before putting them out for sale,’ explains Tango-Lowy. Filled chocolates are trickier due to the relatively short shelf life of their soft centres. ‘We typically put bonbons out for sale 3–4 days after production.’

There are many different ways to carry out the tempering process, but the one favoured by many artisan chocolatiers including Narracott and Tango-Lowy is the tabling method. The ‘table’ in this context is a cool surface such as a marble worktop or slab. Around three quarters of the melted chocolate from a bowl is poured onto the table and ‘worked’ while it cools using a palette knife or similar instrument. The cooled chocolate is then warmed a little by returning it to the bowl and stirring well with the remaining uncooled chocolate. ‘It sounds really simple and as it turns out it’s really hard to do,’ says Tango-Lowy. Hartings had also warned me before the workshop: ‘Chocolate is one of the more demanding things chemically to work with.’

At home, the easiest way to temper chocolate is to slowly add pieces of chocolate that have already been tempered to a bowl of melted chocolate and stir well until they have just melted. The tempered chocolate acts as seed crystals, feeding the formation of form V cocoa butter crystals as the chocolate cools. The proportions required are normally around a quarter solid chocolate to three quarters melted chocolate.

On an industrial scale, tempering chocolate involves extremely high-tech and expensive machinery. The Cadbury chocolate factory in Birmingham, UK, is one of the largest chocolate factories in the world. ‘In the factory we use tempering machines,’ explains Hayleigh Perks, a chocolate scientist at Cadbury. ‘The tempering machine takes the [already made] chocolate through the temperature [change] process. The chocolate melts, is cooled down and then heated back up to the desired temperature.’ The molten chocolate is poured into moulds – for Easter eggs or bars – straight from the sealed tempering machine and then immediately wrapped. The form V crystals formed in the machine then seed throughout the bar. ‘It takes three to four weeks for a chocolate bar from production to be fully crystallised,’ explains Perks.