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Something you may not know about chocolate

Authored by Cherie Wong


A brief discussion about the ingredients, the polymorph structures of chocolate and the appearance of fat bloom on the surface of the chocolate. How much do you know?


The evolution of chocolate

The earliest known cocoa plantations date back to 600 AD by Maya Indians and the Aztecs. The cocoa was recognized as the “food of the god” and the seeds were used as currency and produced a drink called ‘chocolatl’ (Mayan word ‘xocolatl’), which means ‘bitter water’. The cocoa beans were brought back to Europe in the 16th century. Sugar was added to overcome the astringent flavours, but cocoa beans were still served between the ruling classes. It wasn’t until the 17th century when the first chocolate drinking houses were established in London, but it remained a drink for the wealthy. At the end of the 19th century, chocolate was made as a plain block in Switzerland in the form of milk chocolate and it became available to a wider population. Following technological advancement, chocolate has developed into a variety of forms and has since remained very popular around the world. 1,2


Chocolate Ingredients


The key component in chocolate is cocoa butter, which originates from cocoa. Cocoa beans are harvested within cocoa pods (Figure 1). Fermentation is a critical step in the  production of quality chocolate. It kills beans, prevents germination and produces certain chemicals like theobromine and anandamide. When the chemicals are combined, they give the taste of cocoa. Fermentation is not an easy step; temperature, acidity and other factors like the involvement of yeasts have to be carefully monitored. Different ways of fermentation will give rise to different flavours. Drying is another important step (usually sun drying) for the production of chocolate as it prevents the beans from picking up water. Otherwise, it will become mouldy and destroy the delicate flavour of the bean.

Figure 2. The structure of sucrose (left) and lactose (right).

Most of the chocolate contains about 50% sugar, mostly in the form of sucrose(Figure 2, left). Lactose (Figure 2, right) as a part of cow’s milk is also used in milk chocolate. It sometimes replaces some sucrose content as it is less sweet than sucrose, leading to an overall decrease in the sweetness. Sugar alcohols (polyols) and polydextrose are used in many low-calorie or sugar-free products. These products as sugar substitutions have been developed for people who are unable to consume a lot of sugar, such as those with diabetes’


Milk and many dairy components are added to the chocolate as many people prefer a more creamy taste and texture. Milk fat and milk protein are vital in milk chocolate as it gives rise to its distinctive texture, flavour release and liquid flow properties. The creaminess of milk chocolate is due to the balance of milk proteins and the more acidic component from the beans. The amount of protein is proportional to the creaminess of the chocolate. It also adds the nutrient content of chocolate as the calcium in the milk is beneficial to health.


The ingredients and the manufacturing process are varied according to different companies: flavouring, food colouring and the main ingredients are different to create a specific ‘house flavour’.4


Polymorphs of chocolate

Some may recognize that the properties of chocolate change over time or depending on preservation conditions. For example, the chocolate bar is a crisp bar at room temperature, however, after putting in a fridge, it melts at room temperature. This interesting property is due polymorphism of cocoa butter. Polymorphism is the ability of a substance to crystallize in several different forms with their characteristic properties, with different melting points. Similarly, diamond and graphite are two different forms of polymorphs as they are both composed of carbon but have a different crystal structure.


Most of the fat inside chocolate is from the cocoa butter. That fat is made up of several triglycerides (Figure 3), each of which solidify at different temperatures and rates. There are 6 different ways the individual crystals can be packed: form I to VI (Table 1).


Table 1. The conditions of forming 6 different polymorphs of cocoa butter.5



Figure 3.The structure of triglyceride, palmitic acid, stearic acid and oleic acid.6

All fats are mixtures of triglycerides – they have three fatty acids attached to a glycerol backbone. In cocoa butter, oleic acid, stearic acid and palmitic acid are the three major acids which account for over 95% of fatty acid content (Figure 3). The chain of the fat in cocoa butter is relatively short and simple (with only alkane and alkene group), so they melt rapidly over a low range of temperature (room or mouth temperature). Stearic and palmitic acid are saturated acids (denoted as S) (hydrocarbon chains that do not contain C=C double bond) while oleic acid (denoted as O) is an unsaturated acid. The structure and composition of triglycerides affect the properties of the cocoa butter and thus the chocolate. For example, all saturated cocoa butter (with 3 saturated acids denoted as SSS) has a higher melting point than more common SOS (2 saturated acids and 1 oleic acid) due to a lower degree of unsaturation. The presence of C-C double bonds in triglycerides distorts the long fatty acid chains the molecular shape. Thus, the molecules are packed with less efficiency such that less energy (lower temperature) is needed to melt the crystal.6,7

Figure 4. (a) Double and triple length packing configuration. (b) Crystal packing of triglycerides for, and polymorphs.8

Apart from the different types of triglycerides, a single type of oil molecule can be packed in different ways to make various polymorphs. The triglycerides can fit into each other much like stacking chairs (Figure 4), and these small stacks have to fit together with other stacks. The angle at which they fit together determines their stability. Polymorph  only forms at low temperature and can transform into other forms easily; like chairs stacked straight up easily fall over. Cocoa butter can form 6 different stacking patterns with distinct melting point differences (Table 1).


Among the six polymorphs, only one of these will give the suitable properties that are useful for chocolate. For bar chocolate, it is usually in Form V due to its glossy appearance and hardness with a good snap. Although Form VI is the most stable type, it usually transforms from solid to solid state but not from the liquid transformation in the manufacturing process. In contrast, Form I is very unstable and has a melting point at 17 °C and is therefore only suitable for ice-cream coatings, and will rapidly transform to Form II and Form III and so on.4,6,7

Figure 5. The fat bloom of chocolate.9

Sometimes, a white powdery surface can be seen on chocolate after weeks of preservation. It is called “fat bloom” which is made up of large fat crystals, not mould (Figure 5). There are a few ways for the chocolate fat bloom to form. But the most common one is that when a more stable polymorph is formed, the chocolate becomes denser and contracts. The energy given out by stabilization can push some liquid cocoa butter out of the chocolate surface, which crystallizes as white powdery crystals on the surface. Chocolate makers take precautions to reduce fat blooming. They can insert another hard layer to prevent the liquid cocoa butter from reaching the surface or add an anti-bloom fat to slow down the transformation from Form V to Form VI.6,10


“Chocolate substitution”

In the First World War, companies like Rowntree used vegetable fats in their chocolate as they were unable to import cocoa butter. Later, research has shown that some vegetable fat consists of the same triglycerides as cocoa butter, leading to the patent of Unilever in 1956 of producing a fat from some vegetable sources that are nearly identical to cocoa butter. Nowadays, there are products made from cocoa butter equivalents (CBEs); some countries can even allow all the cocoa butter to be replaced by other fats, which are known as cocoa butter replacers (CBRs). They all have the chocolate flavour, but actually, are not made from cocoa.[iv]


Conclusion

This article has introduced the brief history and the ingredients of chocolate, as well as different forms of chocolate known as polymorphs. However, there are a lot more theories behind the science of chocolate including the manufacturing process and the flow properties of liquid chocolate. I highly recommend all the chocolate lovers to read more about these topics. The next time you bite a piece of chocolate, you will understand what you are actually eating and the science behind the confection.


References:

[1] Cadbury Dairy Milk - Discovering Chocolate, https://www.cadbury.com.au/about-chocolate/discovering-chocolate.aspx, (accessed 23 Mar 2019).

[2] Godiva - The History of Chocolate: The Mayans and Aztecs, https://www.godivachocolates.co.uk/the-history-of-chocolate-mayans-aztecs.html, (accessed 23 Mar 2019).

[3] Swift2go, https://medium.com/swift2go/how-to-create-your-own-cocoapod-simple-easy-steps-b510314d80d, (accessed 23 Mar 2019).

[4] S T Beckett, Science of Chocolate, Royal Society of Chemistry, Cambridge, 2000.

[5] The importance of structure: chocolate, http://www.rsc.org/Education/Teachers/Resources/Inspirational/resources/4.1.4.pdf, (accessed 23 Mar 2019).

[6] Chocolate (Theobroma cacao), https://chempics.files.wordpress.com/2014/05/fattyacidstructure.jpg, (accessed 01 Apr 2019).

[7] A. Ali, et al., Food Chemistry, 2001, 72, 491-497.

[8] Nissim Garti, Neil R. Widlak, Cocoa Butter and Related Compounds, Academic Press and AOCS Press, 2012, 63-71.

[9] Inside Science, https://www.insidescience.org/news/white-film-chocolate-blame-fat, (accessed 23 Mar 2019).

[10] Serafim Bakalis, et al., Procedia Food Science, 2011, 1, 340-346.