The Chemistry of Chocolate

The Princeton vase (photo by Bruce M. White)

At the recent meeting of the Experimental Cuisine Collective, Stefan Bernhard, assistant professor of chemistry at Princeton University, discussed a topic appreciated by foodie and non-foodie alike: the chemistry of chocolate. Prof. Bernhard was introduced by Kent Kirschenbaum, associate professor of chemistry at NYU and cofounder of the Collective. Bernhard, a specialist in luminescent materials, uses the chemistry of food to get his students interested in chemistry. Using a presentation from his freshman seminar class, Bernhard began by talking about the origins of chocolate depicted in the Princeton vase (ca. 750 A.D.), which shows Mesoamericans using chocolate as a drink, which was the first use for cacao. Only in the very near past has chocolate been made into bars as we know it today.

Growing Region: Origin, Fermentation, and Drying

Bernhard continued his presentation with scientific background on chocolate, known by its scientific name as theobroma cacao, a fruit, which comes in three varieties: criollo, which is very rare; forastero, a hardier type; and trinitario, which is a hybrid of the previous two, commonly grown in the Caribbean. Cacao trees, which grow in the equatorial region, are very vulnerable and require a second tier of forest growth to shade them. This is why cacao trees are common in the tropical regions and rain forests, because they need the climate and protection that only a rain forest can provide. Still they are very susceptible to disease, but the cacao trees are crossbred to improve hardiness.

Cocoa beans in a cacao pod (photo by Keith Weller)

The cacao pods are harvested, split open, and the pulp and seeds (beans) removed. The pulp and beans are allowed to ferment for many days. After fermentation the beans are laid out in the sun and dried. Unwanted acids are removed in the drying process while unique flavor components are created. Both the processes of fermenting and drying are crucial in the making of good quality beans. The drying process can be difficult because of the moist climate of the rain forest. At all costs the onset of mildew must be avoided. After drying the beans are ready to be shipped north to companies that will manufacture chocolate. It is a misnomer to think that chocolate is produced in the growing regions.

Chocolate Production: Roasting, Grinding, and Refining

At the factories, the beans are cleaned and made ready for roasting. Roasting changes the flavor. Many times the same machines are used to roast the cacao beans as are used to roast coffee beans. Cacao beans are not uniform in size, so unlike coffee, where there are a number of different roasts, from light to dark, cacao beans are roasted for an indeterminate amount of time. The chocolate beans can be roasted whole in their shells or cracked into nibs. The beans are primarily made up of fat (55%), protein (12%), fiber (11%), and starch (6%). The roasting process removes unwanted fatty acids while also releasing unique flavor components.The roasted beans are ground and refined to become cocoa liquor, then pressed to separate the solids from the liquor. These solids are the cocoa butter, which at the molecular level resembles lard, and cocoa powder. To make chocolate, the cocoa liquor and cocoa butter are combined with sugar and conched to make dark chocolate. Milk powder is added to make milk chocolate. White chocolate is made without the cocoa liquor but vanilla or artificial vanillin may be added for flavoring. A melangeur, like a conch, can also be used to refine the chocolate; this device is predominantly used in Europe.

Taste and Texture: Snap, Melt, and Flow

Flavor depends on origin, drying, and roasting. There are many opinions regarding the best tasting chocolate. Americans tend to like Hershey’s over any other type, whereas Europeans prefer European chocolate from Switzerland or Belgium. There are more than fifty chocolate flavor molecules. These flavor molecules all depend on the circumstances mentioned above. Chocolate that is not well tempered is hazy in color, chalky in texture, and does not melt in your mouth. Good chocolate shows the properties of snap, melt, and flow. Snap and melt are both a result of fat crystallization, and flow depends on particle size and agglomeration properties.The fats and oils contained in chocolate are a mixture of four triacylglycerols (or fatty acids), originating in the cocoa butter; they are palmitic acid (25%), stearic acid (37%), oleic acid (34%), and linoleic acid (3%). At the molecular level, a chocolate bar is the result of organized crystallization of these acids. When chocolate is melted (37 °C), these acids become disorganized. Therefore it is very important to control the melting point of chocolate. The preferred form for melting chocolate is form V (ß2) (polymorph) (34–35 °C). The result is the formation of heat-stable fat crystals with good melt properties (mouth feel) and gloss. Artisanal chocolatiers typically melt chocolate at 50 °C, move it around on a marble to cool it to 27 °C (form IV), and then reheat the chocolate to 30–32 °C. The result is a stronger form of chocolate.

Chocolate is a distribution of particles, where a good distribution is critical. A particle size larger than 30 µm has a gritty mouth feel. Particles include sugar crystallite, cocoa particles, and cocoa butter. For good flow properties, particle attraction must be held under control. Cocoa butter is hydrophobic; sugar (sucrose) is hydrophilic. Viscosity in chocolate is important, that is why lecithin is added to control viscosity. Lecithin has an end that is hydrophobic (fat soluble) and another end that is hydrophilic (water soluble). Lecithin can hold the particles together properly.

Active Ingredients: Theobromine, Caffeine, Antioxidants

Chocolate contains theobromine and small traces of caffeine. Both caffeine and theobromine act as central nervous system and cardiac muscle stimulants and as diuretics, but the effects of theobromine are very mild. Chocolate also contains antioxidant compounds such as polyphenols and flavonoids, and anandamide, a blissful cannabinoid-like chemical. The higher the cocoa content in chocolate the more antioxidants. Therefore white chocolate has no antioxidants and milk chocolate dilutes the power of the antioxidants. The conclusion of Prof. Bernhard’s lecture: “Eat more chocolate, especially dark chocolate.”