Materials Science is not only about the ingredients from which objects are made but is also about what we eat. As I am from Belgium, I would like to tell you about our number one product we proudly spread around the world which is CHOCOLATE. It is our national product and let me also mention that the largest chocolate factory in the world is located in my country in Wieze, a small village in East Flanders region in Belgium.
Barry Callebaut produces around 270,000 tonnes a year, from cocoa bean to chocolate, making it the largest chocolate supplier in the world. In Wieze you will also find the first -in operation since 1988- of the 17 Chocolate Academy centers that have since been distributed all over the world. A completely new building was opened in 2014, making the Chocolate Academy center in Wieze the largest such center in the world. The Belgian chocolate sector exports its quality chocolate to the entire world. Two-thirds of both the industrial chocolate and the end products are exported abroad. That’s because there is a Belgian chocolate code developed in 2007. The Belgian chocolate code must ensure that the term ‘Belgian chocolate’ is only used for chocolate that actually comes from Belgium.
I am not a chocolatier, but I am materials science engineer, and even if I cannot say I am a big fan of chocolate, sometimes I do enjoy eating it and because we have many sort of of chocolate in Belgium, during the years of course I was curious to learn more about what this material is. I did my research, I studied and observed the process and I wish to share with you my experience with chocolate below. Here is how Chocolate works:
Take a piece of dark chololate and pop it in your mouth. For a few moments you will feel its hard corners against your palate and tongue but taste little in the way of flavour. It is almost impossible to resist the urge to give it a good bite, but very hard not to, so that you can experience what happens next: the lump becoming suddenly limp as it absorbs the heat from your tongue. As it becomes liquid, you will notice your tongue feels cooler, and then a combination of sweet and bitter flavours flood your mouth. These are followed by fruity and nutty sensations, and finally an earthy muddy taste down the back of your throat. For one blissful moment you will be in thrall to the most deliciously engineered material on Earth.
CHOCOLATE is designed to transform into a liquid as soon as it hits your mouth. This trick is the cumulation of hundreds of years of culinary and engineering effort, aimed initially at creating a popular drink that could hold its own against tea and coffee. An effort that failed miserably until chololate manufacturers realized that making hot chocolate in the mouth instead of in a saucepan was much more delightful, much more modern and far more widely liked: in effect they created a solid drink. The chocolate industry has never looked back. What made this possible was their understading and control of crystals – specifically, cocoa butter crystals.
THE COCOA BUTTER
COCOA BUTTER is one of the finest fats in the vegetable kingdom, slugging it out with dairy butter and olive oil for pole position. In its pure form it looks like fine unsalted butter and is the basis not just of chocolate but of luxury face creams and lotions. Don’t let this put you off – fats have always provided humans with more than just food, in the form of candles, creams, oil lamps, polish and soap. But cocoa butter is a special fat for many reasons. For one, it melts at body temperature, meaning that it can be stored as a solid but becomes a liquid when it comes into contact with human body. This makes it ideal for lotions. Moreover it contains natural antioxidants which prevent rancidity, so it can be stored for years without going off (compare that to butter made from milk, which has a shelf life of only a few weeks). This is good news for face cream makers but also for chocolate manufacturers.
Cocoa fat has another trick up its sleeve: it forms crystals, and these are what give chocolate bars their mechanical strength. The major component of cocoa butter is a large molecule called a triglyceride, which forms crystals in many different ways, depending on how these triglycerides are stacked together. It’s a bit like packing the boot of a car: there are many ways to do it, but some take up more space than others. The more tightly packed the triglycerides, the more compact the crystals of cocoa fat. And the denser the cocoa fat, the higher its melting point and the more stable and stronger it is. These denser forms of cocoa are also the hardest to make.
THE 6 POLYMORPHS OF CHOCOLATE
There are 6 different kinds of crystals in chocolate, but not all of them look good and taste good. So how do you get the chocolate that you buy at the store? Through material science! Each of the 6 crystals in chocolate are made of the same chemicals, but they have a different shape/structure. We call crystals with the same composition but different structure polymorphs.
For example, graphite and diamond are polymorphs. This means that the graphite in your school pencil and the diamonds found in jewelry are made of the same atoms (carbon atoms), but the carbon atoms are arranged in different crystal structures in graphite versus diamond.
In the case of chocolate, each of the 6 crystal polymorphs has a different melting temperature, so by controlling the heating and cooling rates of molten chocolate, we can melt the crystals we don’t want to obtain the type of chocolate crystals we do want.
Types I and II crystals, as they are called mechanically are soft and quite unstable. They will, if given any chance at all, transform into the denser Types III and IV. Nevertheless they are useful for making chocolate coatings on ice creams, because their low melting point of 17°C allows them to melt in the mouth even when cooled by the ice cream.
Types III and IV crystals are soft and crumbly and have no brittle ‘snap’ when broken. The mechanical property of the snap is important to chocolatiers because it adds surprise and drame to our experience of the chocolate. For example, it allows them to create hard outer shells with which to encase soft centers, providing a textual contrast. From a psychophysics perspective, meanwhile, brittleness and the sound associated with cracking open a chocolate are linked with freshness, which again adds to the enjoyment of eating chocolate with a ‘snap’. Anyone who has tucked into a bar of chocolate expecting it to be hard and brittle only to find it gooey and melted knows just how dissapointing losing the snap can be. (Although it is fair to say that gooey chocolate has its place as well…).
For all these reasons chocolate makers tend to want to avoid Types III and IV crystals, but unfortunatelly they are the easiest to create: if you melt some chocolate and then let it cool down, you will almost certainly form Types III and IV crystals – this chocolate feels soft to the touch, has a matt finish and melts easily in the hand. These crystals will transform into the more stable Type V over time, but on the way they will eject some sugar and fat, which will appear as white powder on the surface of the chocolate – called bloom.
Type V is an extremely dense fat crystal. It gives the chocolate hard, glossy surface with an almost mirror-like finish, and a pleasing ‘snap’ when broken. It has a higher melting point than the other crystal types, meting at 34°C, and so only melts in your mouth. Because of these attributes, the aim of most chocolatiers is to make Type V cocoa butter crystals. Yet, this is easier said than done. They have to be created through a process of heating chocolate to a desired temperature to grow only one crystal structure (phase V), process called tempering chocolate (Fig. 1)
Type V is the crystal phase that gives chocolate its glossy, shiny appearance, its satisfying “snap” when you break it, and its smooth texture.
The diverse polymorphs are formed under different crystallization conditions. The thermodynamically most stable form, Type VI, has a dull surface and soft texture; only Type V shows the hardness and glossy surface appreciated by the consumer. Gourmets only accept chocolate in its crystal Type V, as it is this form that has the noble surface sheen, crisp hardness and the pleasant melting sensation in the mouth.The chocolate producer must achieve the physicochemical trick of making the chocolate crystallize not in the thermodynamically most stable Type VI but in the somewhat more energy-rich Type V. If he does not succeed, the chocolate is practically unsaleable, for 3 reasons:
1.The surface looks dull and shows a pattern reminiscent of frostwork. This renders the chocolate visually unattractive
2. Compared to form V with a melting point of 33.8 °C, crystal form VI, due to its higher melting point at 36.2 °C, melts only very slowly on the tongue and produces a coarse and sandy sensation on the tongue.
3. Crystal form VI has a soft texture. Compared to form V, biting into a bar of chocolate of form V does not feel crisp, but rather reminds of candle wax.
CONVERSION = The reason for the lower stability of crystal form V lies in the relatively loose packing of the lipid molecules, leaving empty spaces. In the solid state, crystal form V also tends to convert into the more stable form VI. The addition of milk fat retards the conversion so that the V→VI transition is less often observed in milk chocolate. At room temperature, the conversion takes place only slowly; it nevertheless limits the shelf life of chocolate to several months. Therefore, chocolate should always be stored in a refrigerated environment (15–18 °C). At higher temperature, e.g., in the sun or in a warmed up car trunk, the undesirable phase transition V→VI happens quickly, even faster than during unintentional melting and subsequent cooling. If the phase transition happens, the producer’s effort will have been in vain: The chocolate is dull, soft and melts only slowly in the mouth.
What is tempering? = Melt chocolate to 50°C. Cool it to 22°C so that only Form V is allowed to condensate at lower temperature), notice that Forms III, IV and V have similar melting points. Therefore, for best results, heat again to 31°C, which is warm enough to melt forms I,II, III and IV, then re-cool to 22°C to allow them to re-crystallise as Form V. Repeat again if greater Form V purity is desired.
However, there is another way to temper chocolate through introducing little bits of already tempered chocolate. These little bits of tempered chocolate act as seed crystals and initiate preferential crystal growth of phase V chocolate crystals (Fig. 2). These give the slower-growing Type V crystals a head start over the faster-growing Type III and IV crystals, allowing the whole liquid mass to solidify into the denser form of crystal structure before the Type III and IV crystals have a chance to get going.
Seed crystals are often used in ceramic and metal processing to initiate a desired crystal phase to grow. Material scientists call seed crystals nucleation sites. Nucleation sites are areas in a material that make it easy for crystals to grow. Using nucleation sites to cause preferential crystal growth is a very important process in material science as it allows scientists and engineers to optimize material properties.
When you put pure dark chocolate into your mouth and sense it start to liquefy, what you are feeling are the Type V cocoa butter crystals that are holding the chocolate together starting to wobble. if they have been cared for properly they will have spent their entire life at temperature below 18°C. Now, in your mouth, they experience higher temperatures for the first time. This is the moment they have been created for. It is their and last performance. As they warm up and reach the threshold of 34°C they start to melt.
This change from solid to liquid – a so-called ‘ transformation of state’ – requires energy to break the atomic bonds that are holding the molecules of a crystal together, thus freeing them to move around as a liquid. So as the chocolate reaches its melting point, it takes this extra energy that it needs from your body. The chocolate gets this energy in the form of latent heat, as it is called, from your tongue. You perceive it as a pleasant cooling effect, similar almost to sucking a mint. It’s the same cooling effect that is produced when you sweat, but rather that a solid becoming a liquid, instead a liquid (your sweat) changes state into a gas, absorbing the latent heat required to do so from your skin. Plants use the same process to cool themselves down.
In the case of cocoa crystals, the coolness of the chocolate melting is accompanied by the sudden production of a warm thick liquid in the mouth, and it is this wild combination of impressions that is responsible for the unique feel of chocolate in the mouth – it is the beginning of the hot chocolate experience.
What happens next is that the ingredients of the chocolate once bound together by the rigid cocoa butter matrix, are now free to flow to your taste buds. The grains of the cocoa nut, which were once encapsulated in the solid cocoa butter, are now released. Dark chocolate usually contains 50% cocoa fat and 20% cocoa nut powder (referred to as “70% cocoa solids’ on the packaging). Almost all the rest is sugar. 30% sugar is a lot. It’s the equivalent o putting a spoonful of sugar in your mouth. Nevertheless dark chocolate isn’t overly sweet; sometimes it’s not sweet at all. This is because at the same time that the sugars are released by the melting cocoa butter, so are chemicals known as alkaloids and phenolics from the cocoa powder. These are molecules such as caffeine and theobromine, which are extremely bitter and astringent. They activate the bitter and sour taste receptors and complement the sweetness of the sugar. Balancing these basic tasted to give the chocolate a rounded flavour enhancer, as well as adding another dimension to modern chocolates, has in turn led to chocolate being used as an ingredient in svoury dishes: it is the basis of the Mexican dish pollo con mole, which is chicken cooked in dark chocolate.
Although we may not always think of chocolate as a common scientific material, tempering chocolate is actually quite similar to synthesizing and processing metals and ceramics. Tempering chocolate is a great way to learn about the important role crystal structure plays in material properties. Even the desired properties of chocolate such as its smoothness and shininess are obtained through controlling the crystal structure of chocolate. By performing this activity, we can all learn to be material scientists in our kitchens!
However; cooked chocolate tastes different from eating chocolate for another reason, though. Although basic taste is general on the tongue by the taste buds, which distinguish between bitter, sweet, salty, sour and umami (meaty or savoury), most flavour is experienced through smell. It is the smell of chocolate from within your own mouth that is responsible for its complex taste. When you cook chocolate, many of its flavour molecules evaporate or are destroyed by the cooking. This is a problem not just for hot chocolate but also for coffee and tea. It is why you need to drink those drinks within minutes of being brewed, otherwise the flavour disappears into the air. It is also why you lose much of your sense of taste when you have a cold – because the smell receptors in the nose are covered by mucus. The genius of creating hot chocolate in the mouth is that the cocoa butter encapsulates the flavour molecules until the moment that you eat it, and only then does it release its cocktail of more than 600 exotic molecules into your mouth and up your nose.
Some of the first flavours that you detect up your nose are fruity ones belonging to the “ester” family of molecules. These molecules are responsible for the ripe smell of beer, wine and more obviously, fruits. But these esters are not present in the raw cocoa bean. I know this becasue I have eaten a raw cocoa bean and it tastes horrible: it is fibrous, woody, bitter and bland; there is no fruitiness, no hint of a chocolate taste, and certainly no reason to taste one again. It takes quite a bit of engineering to turn these rather exotic-looking but dull-tasting beans into chocolate. So much so, in fact, that it gets you wondering how it was ever invented at all.