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Why is coffee so important to so many people around the world, who use it to start the day well, and later to maintain alertness at work? The volume of coffee consumed in the USA alone each year is 66 billion cups. New Zealand ranks 47th among coffee drinking nations
Health factors relating to coffee consumption are relatively positive. Surveys from medical or scientific sources indicate benefits in reduced risk of stroke, Type 2 diabetes, heart failure and some forms of dementia. Some recent studies indicate those who drink coffee live longer.
Caffeine is only one of almost 1000 different organic molecules (components) detected by chemists in coffee, so it is one of the more complex beverages in common use. The sweetness of coffee comes from bean sugars, which have been caramelised during roasting. The bitterness of the caffeine helps balance the acidity of coffee. This balance of sweetness and bitterness is the key to the attractive character of coffee.
Coffee extraction is the process of dissolving flavours from ground coffee beans in water. Mainly sweet components are removed from the beans in the early stages of extraction, whereas mainly bitter components are extracted later. The precise timing of the extraction is critical. Fruit acids (especially citric acid but also malic and tartaric acids) produce the subtle acidity profile of the coffee. Chlorogenic acids (CGAs) degrade with roasting into sour organic acids (caffeic and quinic acids). Inorganic acids such as phosphoric acid react with these organic acids to give a pleasant tone.
Aromatics create the aroma and they form because of the reaction of sugars with amino acids. Sulphur containing molecules, including mercaptans and thiophene, contribute to a meaty aroma. Furans are a dominant factor among the aromatics and contribute a caramel aroma. Pyrazines and pyrroles contribute walnut and caramel aroma.
It is not surprising then, with so many variables involved, that preparing the best cup of coffee by a barista is a complex and subjective skill. Under-extraction and over-extraction are both considered a weakness. The aim is to produce the most appealing balance of sweetness and bitterness. There are several other sensitive taste factors, particularly the hardness or softness and pH (acidity/alkalinity) of the water used for extraction.
It is the caffeine in coffee and tea that delivers the distinctive bitter taste and stimulant effect. The Mayo Clinic reports the following caffeine content for various typical sized beverages (excluding decaf): espresso, 47-64mg; instant coffee, 63mg; latte, 63-126mg; black tea, 25-48mg; green tea, 25-29mg; cola soda, 24-46mg; energy drink, 27-164mg.
The coffea arabica plant, which produces the more popular and expensive form of coffee, has approximately half the caffeine by weight found in coffea robusta. Interestingly, humans are the only animal species attracted by caffeine, which is toxic to most other animals.
Professor Ralph Cooney
r.cooney@auckland.ac.nz
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