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Chemistry

Chocolate (Theobroma cacao)

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Chocolate eggs with structures of theobromine in a mortar and pestle.

With Easter just behind us most people (if you’re like the vast majority) have consumed a significant amount of chocolate, in the form of Easter eggs or creatively designed chocolate animals, but where does chocolate come from, can it actually benefit your health (when consumed in moderation) and why does it taste so good? 

1. The image above features some chocolate eggs in a mortar and pestle. The mortar and pestle are frequently used in chemistry laboratories, to grind chemicals in order to increase their surface area, for use in a particular reaction. Some reactions can be carried out in the solid phase by simply grinding two chemicals together. The molecule shown in this picture is theobromine, this belongs to a class of compounds known as alkaloids. The biosynthesis and some of the health benefits associated with it are described in Section 3.

For thousands of years chocolate has been produced from the dried and fully fermented seed (cacao or cacao bean) of the Theobroma cacao tree, which are native to Central and South America. Theobroma trees also exist in the West Indies, Nigeria, Ghana, Ceylon and Java; cacao trees typically grow in tropical climates, which lie close to the equator. The trees can grow taller than 8 meters, and unlike many plants the cacao fruits (or pods) are found on the trunk and large branches. The fruits contain the cacao beans and a sticky pulp.

The earliest known use of the cacao bean is thought to have been between 1400 and 1100 BC. This was a discovery by archaeologists from the University of Pennsylvania, who extracted residue from pottery excavated in Puerto Escondido, Honduras. Their findings show that the initial popularity of the cacao fruit arose from the fermentation of the pulp to form an alcoholic beverage. (PNAS, 2007, 48, 18937-18940). Beverages made from cacao beans, chillis, herbs, honey and flowers were the drinks of Mayan and Aztec kings and were commonly served at important ceremonies.

Once the cacao pods are ripe, as determined by their colour, they are harvested from the trees by hand. To produce the cacao butter and solids the pods are opened, the rind is disposed of, and the pulp and beans removed and dried in the sun for several days. The pulp ferments and becomes liquid as it ‘sweats’, which also allows easy separation from the beans. The beans are then fermented and dried. The process can take between 4-14 days depending on the climate; with the beans being constantly mixed and spread over a large surface area, throughout this period. Prior to fermentation, the cacao seeds are very bitter in taste. The fermentation process aids development of the sweet chocolate taste that we know and love, and produces cacao solids and butter. The final stage in the process, before the beans are shipped to factories elsewhere, is for the beans to be trampled on by the feet of the employees. To improve the appearance and preserve the beans on their journey, they are sometimes washed with a red clay and water mixture.

Each cacao pod contains an average of 40 beans. Depending on the type of chocolate desired (dark, milk or white), between 300-600 beans are needed to produce 1 kg of chocolate. Typically the beans are then roasted and then opened to give the chocolate nibs. The nibs are ground producing pure chocolate (chocolate liquor or cacao paste). To produce your average chocolate bar, additional cacao butter and sugar are added to the liquor, before it is refined, conched (a mixing process used to evenly distribute the cacao butter within the chocolate) and tempered.

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And here is what happens when you drip hot chocolate into water… fantasy pondscum.

2. Most familiar family chocolate bars contain  milk, sugar, cacao mass, cacao butter, vegetable fat, emulsifiers and various flavourings. For consumers, both taste (determined by the recipe) and texture (determined by how it breaks up in the mouth) of any type of food is important.  Of the ingredients in an average bar of chocolate, cacao butter is the key ingredient.  The main constituent of cacao butter is a triglyceride derived from palmitic, stearic, and oleic acids.  The arrangement of these fat molecules determines the structure of chocolate.

The structure of the triglyceride present in chocolate, and the molecules which react together to make it.

The structure of the triglyceride present in chocolate, and the molecules which react together to make it.

Triglycerides are esters of fatty acids (carboxylic acids with long hydrocarbon chains of between 10 to 30 carbons) and a trifunctional alcohol called glycerol.  Since glycerol has three alcohol functional groups, three fatty acids must react to make three ester functional groups.  The three fatty acids may or may not be identical. In the case of the triglyceride in cacao butter, three different fatty acids react.

The non-polar hydrocarbon alkane chain is an important counter balance to the polar acid functional group.  Fatty acids can be saturated (no carbon-carbon double bonds) or unsaturated (presence of one or more carbon-carbon double bonds).  The melting points of such fatty acids increase as the molecular weight increases.

The fats in cacao butter have at least six different crystal forms, or polymorphs.  The different arrangements can lead to different properties in the chocolate such as melting point, how easily it snaps, strength, glossiness and texture.  It is therefore important that manufacturers deliver a product with the correct polymorph to the consumer.  The polymorphism of cacao butter is still an active area of research.

The polymorphs have been classified according to their melting point as shown in Table 1 below (J.  Am. Oil Chem. Soc., 1998, 75(4), 425–439).  The melting point is determined by the efficiency of the stacking amongst the fat molecules.  The more ordered the stacking, the greater the density and hence more stable the polymorph is.

Table showing the Melting Points of the Various Polymorphs.

Table showing the Melting Points of the Various Polymorphs (from J. Am. Chem. Oil Chem. Soc., 1998, 75, 425-439).

Room temperature is, on average, about 20 oC. Polymorphs I – IV are not suitable for making chocolate because they are unstable at room temperature. Polymorph VI is the most stable, but has been found to taste bland and be too brittle. Polymorph V has been found to have a far superior taste, texture and appearance compared to the others.  This form has a shiny appearance, produces an audible snap when broken, melts in the mouth, and has a smooth texture, which ticks all the characteristics required by any chocolate connoisseur.

However, polymorph V is, unfortunately, not the most stable of the six.  If the fat molecules have enough vibrational energy, they can realign and transform the crystal structure.  Vibrational transitions in molecules typically require an amount of energy that corresponds to the infrared region of the spectrum. We experience vibrational transitions in molecules, for example when we feel hot, or when we watch nature programmes that use infrared cameras, to record animals at night. Form V can convert to form VI over the course of several months.  The chocolate becomes harder and melts too slowly in the mouth due to the higher melting point.  This is why storing chocolate in a cool, dark place (i.e. a fridge) is advisable.

Another good reason to keep chocolate in the fridge is that if chocolate partially melts, the fats within it start to rise to the surface.   The chocolate may begin to appear to look dusty.  This change in the crystal structure is called ‘fat bloom’ (Miller, Teresa. “Milkfat Fractions Help Beat Blooming Chocolate“, College of Agricultural Life and Sciences, University of Wisconsin-Madison,  April 2007 and also Pierre Lonchampt and Richard W. Hartel, Eur. J. Lipid Sci. Tech., 2004, 106, (4), 241-274). In section 1, we see what happens when molten chocolate is added to water. The fat molecules in the cacao butter are insoluble in water and therefore clump together forming the ‘pondscum’ droplets. This is the same effect as pouring oil into water.

choc1The process of ‘tempering’ can help to stabilise the main structure of manufactured chocolate.  Tempering involves cooling melted chocolate very slowly which has the effect of increasing the amount of polymorph V that is formed (National Confectioner’s Association, “The Sweet Truth about Cocoa Butter“).  If melted cacao butter was allowed to cool naturally, a mixture of polymorphs I-V would be obtained. This is not ideal as these polymorphs have less desirable properties, that ultimately have an impact on the quality of the chocolate.  Once cooled, the chocolate is heated again to just below the melting point of polymorph V – this melts forms I-IV. When the chocolate is cooled again, it solidifies according to the pattern of existing form V crystals resulting in chocolate, with very little of the unwanted forms of structure present.molten

Inevitably, enjoying chocolate to excess can have detrimental, and somewhat obvious, effects on one’s health. However, chocolate can actually benefit your health!

Chocolate contains several compounds, in addition to the fats found in the cacao butter, including alkaloids: theobromine, caffeine and trigonelline; antioxidants: procyanidins and flavanoids; epicatechin. Procyanidins and flavanoids are thought to impart anti aging properties (Journal of GHR, 2012, 1(9), 171–195.)

3. In order to investigate some health benefits of cacao and flavanols, researchers at Harvard studied Kuna people, who are indigenous to Panama and Colombia, and importantly drink large amounts of cacao . The health of the Kuna people living on the islands was compared to those who inhabited the mainland. The islanders showed markedly lower rates of heart disease and cancer. The health benefits associated with increased consumption of cacao are attributed to the improved blood flow, which is a result of flavanol. (Int. J. Med. Sci.,2007, 4(1), 53-58.) The improved blood flow has implications for further reaching health benefits too, for example in brain activity and blood pressure.

Theobromine is found in large quantities in cacao and has several implications for health. It acts as a stimulant on the nervous system, in a similar way to caffeine. The molecular structures of theobromine and caffeine are very similar, only differing by one methyl group.

The Molecular Structures of Theobromine and Caffiene

The Molecular Structures of Theobromine and Caffeine

The synthesis of theobromine can be achieved from xanthosine (Advances in Botanical Research, 2013, 68, Chapter 4, 111-138). Heartburn often associated with chocolate consumption is caused by the presence of theobromine, which allows stomach acid to enter the oesophagus because it causes relaxation of the oesophageal sphincter. (Neth. J. Med., 2013 71(2), 63). However, due to theobromines ability to act as a vasodilator and a diuretic, the compound has been used as a drug to treat high blood pressure. Several other uses of theobromine exist, in fact its use as an anti-cancer agent has been patented (“Theobromine with an anti-carcinogenic activity“, US 6693104, 2004).

Contributors: Laura Rush (research, ideas, words, pictures), Julia Lister (research, ideas, words), Natalie Fey (images), Jenny Slaughter (pictures and editing).

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One thought on “Chocolate (Theobroma cacao)

  1. Several people have asked me recently about chocolate being poisonous for dogs – this might help: http://www.wired.com/2013/02/the-poisonous-nature-of-chocolate/. And if you want details: http://ngm.nationalgeographic.com/2007/10/pets/chocolate-chart-interactive

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