This is an essay I wrote as part of my coursework for Academic Writing. It is pure self-indulgence I'm afraid.
Crema is a creamy rich tan-coloured foam that sits atop an espresso coffee beverage. "A foam is a coarse dispersion of gas bubbles in a liquid continuous phase. In the case of espresso coffee, the gas phase is mainly the carbon dioxide generated during coffee roasting and entrapped within the cell structure." Crema is the terminus of a journey made by molecular carbon dioxide from the coffee bean.
The coffee bean is a seed of the coffee cherry (Coffea arabica or C. canephora v.robusta). After a cleaning, fermenting and drying process, the still whole green bean is in a stable storable condition with moisture content of 11%. The process of transforming the bean to espresso and crema first requires it to be roasted.
After roasting to a chosen temperature and time, coffee beans lose their moisture, undergo various stages of the Maillard reaction (a chemical reaction between sugars and amino acids resulting in a change of colour and flavour) and acquire a volume of carbon dioxide. They are darker in colour, lighter in weight and their chemical composition has changed via pyrolysis. The beans are then cooled quickly and packed. They evolve carbon dioxide. (Secure packaging commonly has valves to allow this release.)
Although there are various ways to create espresso coffee, usually the beans are milled (ground) to a fine powder. Hot water at a temperature between 92-94°C  is allowed to percolate under pressure through compressed grounds. If the resulting espresso has been made correctly it will have a surface of crema. "Grinding destroys the roasted coffee cell structure and this results in a remarkable release of carbon dioxide, with obvious consequences on crema" . Percolation needs to take place soon after grinding to avoid the escape of carbon dioxide that would otherwise aid the formation of crema.
Variations in the blend of coffee, the temperature and length of roast, fineness of grind, compression of grounds, temperature and duration of percolation, along with the distance of pour and type of receptacle can alter the colour, structure and longevity of the resulting crema. "The texture of the crema is a mark of quality, it must be stable, medium brown, and pervaded by wispy, lighter stripes ("tiger-skin effect"). Even after vigorous stirring the crema must again consolidate itself.".
The existence of a crema seems to be important organoleptically: "We observed a three-step ranking in intensity of the sensory attributes in the sensory evaluation: (i) high scores for the espressi (with crema), (ii) middle-range scores for the lungi (a lungo is an espresso with hot water added) with crema and (iii) lower scores for the lungi without crema.". The crema adds complexity to the mouthfeel as well as various volatile compounds to organoleptic analysis. The mouth area is coated with crema from the first sip. Aromatic and flavour components dissolved in crema are only gradually released. Receptors for bitterness are less sensitive. Chemviews suggests “After enjoying a cup of espresso with a good crema, one can actually still smell and taste it 20-30 minutes later.” Crema acts as a cap on volatile compounds retaining them in the assembly for longer. It also preserves heat, allowing the beverage to maintain its temperature for longer. According to Petracco, persistence is an important characteristic of crema and its disappearance is due to the limited elasticity of liquid films (lamellae) that entrap the gas. And so the journey ends.
The journey is summarised by Illy and Navarini who suggest that espresso brewing can be described as “a quick way to transfer carbon dioxide from roasted and ground coffee to a small cup by means of hot water under pressure”. This then leads to the facts that for crema in espresso coffee, carbon dioxide has to be:
– generated by roasting
– maintained in the bean by proper packaging
– maintained in the ground coffee
– solubilised in water
– released into the beverage.
 Illy E. and Navarini L. (2011) Neglected Food Bubbles: The Espresso Coffee Foam. Food Biophysics, 6: 336
 National Coffee Association USA (NCA). Available at: link
[Accessed 31 January 2016]
 Gloess A., Schönbächler B., Klopprogge et al. (2013) Comparison of nine common coffee extraction methods: instrumental and sensory analysis. European Food and Technology, 236: 609
 Chemviews Magazine Espresso Crema. 2013. Available at: link
[Accessed 31 January 2016]
 Illy E. and Navarini L. (2011) Neglected Food Bubbles: The Espresso Coffee Foam. Food Biophysics, 6: 346
 Roth K. Espresso – A Feast for the Senses. Chemie in unserer Zeit Wiley-VCH, 2010 Available at: link [Accessed 31 January 2016]
 Gloess A., Schönbächler B., Klopprogge et al. (2013) Comparison of nine common coffee extraction methods: instrumental and sensory analysis. European Food and Technology, 236: 624
 Petracco M. (2013). Foam. Trieste Coffee Cluster. Available at: link [Accessed 31 January 2016]