Chocolate: Beneath the Wrapper

by Rachel Mowll, PhD student. Originally published in issue seven of Resonance.

ChocolateAt some point during our time studying science, most of us will learn a little about the chemistry of chocolate. As a topic it clearly has a wide appeal and is a good example of the applications of chemistry in everyday life. However one of the aspects of the chemistry of chocolate that I was not so familiar with from my studies is what interesting compounds it contains and what properties and effects they have. Is there a reason why we love it so much?

Indeed the chemistry of chocolate is surprisingly complex and teaching is often focussed on the manufacturing process, including the all-important process of tempering. Anyone who has watched the The Great British Bake Off will be familiar with this technique on a small scale, and as chemists we know that it is also performed on an industrial scale in the manufacture of chocolate. Its purpose is to ensure that the fat molecules from the cocoa butter solidify in the correct polymorph to give chocolate with the correct melting temperature.

Chocolate has long been known to make us feel good and is even considered by some to be an aphrodisiac. There are a few compounds present in chocolate which have been linked to this feel-good effect. However, is there any evidence for it or is it just wild speculation? Most of these claims are centred on molecules involved in the production and/or regulation of the neurotransmitters serotonin and dopamine in the brain, both of which are linked to feelings of happiness.

PhenylethylaminePhenylethylamine is present in chocolate at the relatively high level of 0.4 - 6.6 μg/g. When it occurs naturally in the brain it produces positive feelings by releasing dopamine and serotonin. It is even classed as a hallucinogen and is said to produce a high similar to ecstasy. However, when ingested in chocolate it is likely to be broken down before it can pass into the central nervous system, making it unlikely to be responsible for chocolate’s feel-good properties.

TryptophanTryptophan is an essential amino acid, meaning that humans are unable to synthesise it ourselves and must obtain it from food sources. It is a precursor in the synthesis of serotonin, so could clearly be linked to feelings of happiness. However, while chocolate can contain around 1300 μg/g, many other foods (often those containing high levels of protein in general) contain much higher levels, and indeed white flour contains the same quantity. It’s hard to imagine anyone snacking on a bag of flour to cheer themselves up. Like phenylethylamine it is also unlikely to enter the central nervous system unmetabolised after being ingested in chocolate and therefore probably isn’t the source of feel-good properties.

Chocolate also contains some of the diverse range of chemical structures known as Cannabinoids, the compounds responsible for the high produced by cannabis. Again, these compounds are also found naturally in the brain and are present in chocolate in such tiny amounts (0.05 μg/g) that they are unlikely to have any effect.

One psychological effect chocolate is known to have is as a stimulant. It is well known that it contains caffeine, with 100 g of chocolate containing an amount comparable to a cup of tea. And while caffeine may not exactly make you happy it may provide a boost to those in need of energy.

Chocolate also contains a related but less well-known stimulant, theobromine. As a chemist it is easy to assume that this compound contains bromine, but the name actually comes from the name of the Cacao Tree from which cacao beans are harvested, Theobroma Cacao. Incidentally, Theobroma literally means ‘food of the gods’. Theobromine is structurally similar to caffeine, with just one methyl group removed.

Theobromine is also responsible for another property of chocolate, its toxicity in dogs. While it is also toxic to humans in high enough doses, it would be near impossible for even a real chocoholic to eat enough chocolate for this to be dangerous. However this is not the case for dogs with the theobromine in chocolate causing nasty symptoms such as nausea and vomiting, diarrhoea, seizures, heart attacks and even death.

As should be becoming obvious, it’s unlikely that there’s one psychoactive compound in chocolate that makes it so popular. It is probably more to do with the high sugar and fat content, as well as the distinctive flavours and smooth, creamy textures.

Chocolate facts

66% of chocolate is eaten between meals with 22% eaten between 8pm and midnight.

1842, the year Cadbury created the first chocolate bar.

1875, the year Daniel Peter from Switzerland created milk chocolate.

White chocolate is not technically chocolate due to the absence of cocoa solids and chocolate liquor.

The Ivory Coast is the largest single producer of the world’s cocoa.

Nearly 70% of the world’s cocoa supply comes from Africa.

The flavour of chocolate comes from the cacao bean. Returning to the manufacturing process, cacao beans are fermented and proteins in the beans are broken down to amino acids. Then the beans are roasted, during which time some unpleasantly flavoured volatile compounds evaporate and a cascade of reactions occurs between the amino acids and sugars. These reactions produce a range of molecules including aldehydes, esters, ketones and furans which give flavour and colour.

As for the fats, the fatty acid molecules found in chocolate come from cocoa butter and are palmitic acid, stearic acid and oleic acid. The first two are saturated fatty acids while oleic acid is unsaturated, giving it a kinked structure which affects the packing of molecules and therefore the melting point of the chocolate. For this reason the proportions of the fatty acids are varied by manufacturers to give an optimum melting temperature. The previously mentioned six different polymorphs of cocoa butter have different melting temperatures, with form V being preferable with a melting point of 33.8 o C. This means it will melt in the mouth but not when stored at room temperature.

Of course, commercial chocolate does contain additives. For example, vanillin, the compound responsible for giving vanilla its flavour, is commonly used. It is anecdotally reported that American chocolate tastes sour, or even ‘like sick’! Interestingly, American brands often use butyric acid as an additive to give chocolate a sour note. So that mystery may be solved.

There are over 400 compounds in chocolate that have been identified and only a small number have been covered here. Upon googling ‘chemistry of chocolate’ to research this article I discovered a number of suggested google searches along the lines of ‘is chocolate a mixture or a compound’, or ‘is chocolate a pure substance’. I’ll admit that the thought of chocolate being a single compound (and the thought that some people would ask that question) did amuse me slightly, however I firmly believe that the reality is far more interesting.

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