The Chemistry of Chocolate

In this section, we will discuss about the chemistry behind chocolate. In more detail, we have a discussion about what chocolate is from chemistry point of view and how it is manufactured, as the processing determines the quality of chocolate product. To begin our discussion, we will have a brief history about chocolate.

John Cadbury

In the earliest history of chocolate was a cold, bitter, frothy drink known as kakawa or cacahuatl used by the Mayans and Aztecs in 1500 BC - 1500 AD. The word of chocolate was come from words 'Choco' means foam and 'atl' means water, so 'chocolate is literally foamy water. In 1544, the first known shipment of cocoa beans to Europe, mainly to Seville, by the conquistador; later on the cocoa plant was named scientifically as Theobroma cacao by Carl Linnaues. After that, chocolate houses acquired a bawdy reputation as depicted by Hogarth's A Rake's Progress in 1735. In 1831, John Cadbury starts selling drinking chocolate and cocoa, and followed by first chocolate bar manufactured by Fry & Sons in 1847. In 1876, Nestle manufactured the first milk chocolate and in 1879 Lindt introduced new process called 'conching' to improve flavour.

In general, there are 4 main formulations of chocolate which are cocoa solids, cocoa butter, sugar, and milk solids. The composition of chocolate depends on type of chocolate bar for example if milk solids is removed it becomes dark chocolate and if the cocoa solids is removed it becomes white chocolate. Besides that, the composition of chocolate bar also depends on the brand as illustrated below.

As your information, cocoa solids and cocoa butter are the most expensive ingredients in chocolate bar.

Cocoa beans processing

The main ingredient of chocolate bar is obviously cocoa beans from the Theobroma cacao tree. The cacao tree produces cocoa pods as each cocoa pod can be 0.2 - 1.0 kg weight as the annual production of cocoa beans is 600 000 tonnes and each pod contains 30 - 40 cocoa beans. Then, cocoa beans are fermented for 5 - 7 days until the beans turn brown. This fermentation process produces around 30 new volatile chemicals which are mostly amino acids and alcohols, but also a lot of acetic acid.Then, the beans are dried for 1 - 2 weeks prior to shipment.

Map of cocoa beans producers

The cocoa beans are commonly grown across the tropical countries such as Ivory Coast, Indonesia, Ghana, and Nigeria as the top 4 cocoa beans producers (source: FAO).

Roasting process

The processing of cocoa beans into chocolate bar is such a long way. The first step is the dried, fermented beans are roasted at 110 - 140°C for 45 - 60 minutes to employ the Maillard reaction. This reaction gives the different taste and smell of roasted beans. Maillard reaction is a complex set of reactions that occur on heating carbohydrates (sugars) and proteins (amino acids). This reaction was recognised by Maillard around 1910 and known as non-enzymatic browning and the initial step of this reaction is shown below.

Maillard reaction

This reaction involves at least 7 further steps as it converts flavourless amino acids into aldehydes and eventually form brown nitrogen-based polymers called melanoidins. Furthermore, it can produce more than 1000 volatile flavour chemicals depending on precise temperature, pH, and time and also responsible for aromas such as malt, bread, coffee, caramel, roasted meats, etc. Therefore, roasting process produces many more complex chemicals that add flavour.

Conching process

After that, roasted beans (cocoa mass) are either pressed to make cocoa butter and cocoa powder, or ground to make cocoa solids. Then, cocoa solids is conched for up 4 days; the cocoa beans are ground up with sugar and heated gently while blasted with air in order to remove volatile compounds (e.g. acetice acid). Then, extra cocoa butter is added to coat cocoa solids and sugar so as to produce 'chocolate liquor'. Moreover, conching develops taste, dictates sensory properties.  

The conching process is the most important in chocolate processing. In the simplest way conching can be described as sugar crystals, cocoa solids, milk solids (depends on the type of chocolate is made), molten cocoa butter, and lecithin are mixed together and heated at 54°C for 72-96 hours (3-4 days) to produce stable emulsion of chocolate liquor. The final chocolate liquor is a suspension of 2 (or 3) types of finely divided solid particles in liquified cocoa butter.

Lecithin

As you notice, there is one of the ingredient of chocolate is lecithin, and lecithin is a natural zwitterionic surfactant which has a function to disperse sugar molecule (hydrophilic) into cocoa butter (hydrophobic). The sugar molecule is bound to polar head of lecithin as the hydrophobic tail binds into cocoa butter.

Besides that, the size of these solid particles affects the sensory feel of chocolate as if particles are too large (> 30 µm), chocolate tastes 'gritty'. In the UK, the preferred particle size is approximately 15 µm (Cadbury's Dairy milk is around 25 µm and Galaxy is around 17 µm). In continental Europe, preferred particle size is approximately 2 µm, so it is more expensive as longer conching times is needed. Hence, smaller particles make chocolate liquor more viscous; which means melts more slowly in mouth.

Chocolate packaging

The next step is the 'conched' chocolate liquor is tempered, then cooled and poured into moulds to make bars. The tempering process is also one of the importance of processing in chocolate manufactured. The background of this process is from X-ray diffraction studies confirm cocoa butter is polymorphic, with 6 crystalline phases as shown below.    

From table above, only phases V and VI give the 'snap' and 'mouth feel' needed for good-quality chocolate as those phases have melting point close to body temperature which makes chocolate melts in the mouth. Tempering process involves slowly heating chocolate up to 44°C, then slowly cooling to 31°C. This thermal cycle allows cocoa butter to crystallise into phases V and VI exclusively.

Another aspects that need to be controlled in chocolate manufacture is the viscosity. Viscosity can be defined as a measure of the resistance of a fluid to deform under shear stress; usually perceived as the resistance to flow. For example, golden syrup has a constant viscosity and tomato sauce has a 'Bingham yield stress'. Chocolate liquor is a complex fluid whose viscosity depends on the stirring rate; it is a 'shear-thinning' non-Newtonian fluid. Therefore, the harder chocolate is stirred, the easier stirring becomes. The viscosity of those samples are shown on the graph below.

Furthermore, chocolate liquor viscosity determines coating thickness, since viscosity is always changing, it can be difficult to control during manufacture. Hence, if viscosity is too low, the chocolate 'feet' can be formed.

Chocolate 'feet'

Until this far, we know the manufacturing of chocolate as the chemistry can be applied behind the process. Now, we will see the chocolate from the point of view of chemistry. Chocolates can be described as a complex mixture of organic compounds as it can contain around 400 chemicals with the description is shown below.

In this part, we will see some prominent chemicals inside chocolate such as theobromine, caffeine, phenylethylamine, serotonin, and polyphenol.

First component is theobromine which comprises 1.5-2.7% chocolate and this is the major component in chocolate. Theobromine is a type of methylxanthine which is a weak stimulant for the central nervous system. As you notice from the structure, there is no Br in theobromine, as its name means 'food of the gods'. Theobromine also a diuretic, a cardiac stimulant and causes dilation of arteries, but its metabolic half-life is only 6 - 10 hours, so its effect is rather short-lived. Furthermore, dogs cannot tolerate theobromine; just 50 g of chocolate can kill a small dog.

Theobromine (left) and caffeine (right)

Secondly is caffeine which is an alkaloid drug found at low levels (0.1%) in chocolate and you can see that it is very similar chemical structure to theobromine (methylated theobromine). Caffeine is known as the world's most widely used 'psycho-active' substance and it is a stimulant for the heart and central nervous system. Caffeine also promotes wakefullness and increases mental activity, but these effects are partially negated by heavy use. Besides that, it is also a diuretic, can cause headaches, and constriction of blood vessels. For your information, caffeine content of a chocolate bar is 3-5 times lower than that in coffee.

Phenylethylamine

The third prominent ingredient of chocolate is phenylethylamine which raises blood pressure and blood glucose levels and it makes us feel more alert and contented. Phenylethylamine is related to amphetamines; known as the love drug. Besides that, phenyethylamine is thought to cause release β-endorphin, an opioid peptide, and it is also claimed to be an aphrodisiac: mimics the brain chemistry of someone in love. However, phenylethylamine only found at relatively low levels in chocolate and its metabolic rate is simply too high for such 'brain chemistry' effects.

Serotonin (left) and prozac (right)

After that, there is chemical called serotonin which acts as a neurotransmitter (similar to prozac) and it has a calming and relaxing effect. Besides that, it causes neuron excitation in the central nervous system and if our serotonin levels are low, we crave sweet, starchy foods such as chocolate. Serotonin is believed to be important for the regulation of anger, aggression, mood, body temperature, sexuality, vomiting, sleep, and appetite.

Apigenin, one of a family of polyphenols 

The last chemical that is discussed in this part is polyphenols. Polyphenols are known as anti-oxidant as anti-oxidant reacts with free radicals to reduce oxidative stress in the body. Polyphenols also reduce heart disease by preventing oxidation of low-density lipoproteins, which otherwise cause clogged arteries. For your information, 1 square of milk chocolate has same amount of polyphenols as a glass of red wine and dark chocolate contains more polyphenols than milk chocolate. Polyphenols also act as a natural preservative to prevent chocolate turning rancid. However, polyphenols are complex mixtures that are notoriously difficult to study, so it is doubted that polyphenols equally effective as anti-oxidants.

One thing from chocolate product occasionally happens is chocolate blooming, so chocolate can develop a white powdery surface due to fat bloom or sugar bloom. This problem is due to incorrect storage of chocolate; either too warm or too cold. Firstly is fat bloom which occurs when cocoa butter either does not crystallise properly or undergoes a phase transition when stored in warm or humid conditions. Fat-bloomed chocolate feels 'oily' or 'greasy' due to surface layer of fat. Luckily, fat bloom can be reversed by warming or cooling in similar way of tempering process.

Chocolate blooming

In the other sides, sugar bloom occurs when sugar dissolves in surface moisture and recrystallises, so do not store unwrapped chocolate in the fridge. Sugar-bloomed chocolate feels 'grainy' due to sugar deposits on the surface. However, sugar bloom is irreversible, so sugar-bloomed chocolate cannot be fixed (either you can eat it or throw it away).

To summarise this section, the overview of chocolate manufacturing process is shown below.

The overview of chocolate manufacturing process