Coffee

Coffee is the second most popular drink (after water) and the second most traded commodity (after oil). There are about 70 species of plant in the genus Coffea but 2/3 of commercially produced coffee comes from the arabica plant because of the superior taste and quality. The remaining 1/3 is mostly from the robusta plant which produces a more harsh flavour with a higher caffeine content. The arabica bush is about 1.5-2 metres tall and it grows in temperate climates. It is very sensitive to its environment and grows best at altitude in soil with low acidity. The plant is native to the highlands of Ethiopia, grows for about 50-100 years and starts to produce fruit after 4 years. The fruit resembles red berries and inside there is a small seed or bean.


The chemicals that give coffee flavour are all introduced during the growth process and nothing can be done to the beans once they have been grown. Coffee cultivation is difficult and involves many variables such as plant genes, soil, rainfall, sunlight and temperature ranges. Coffee is grown in over 70 countries principally Brazil, Colombia, Vietnam, Indonesia, Costa Rica, Mexico, India, and Puerto Rico. Human taste and smell are so sensitive (they evolved to protect us from bad and rotten food) that a single bad coffee bean in a batch of 50 ground to make a cup of espresso will taint the flavour. For this reason the best coffee producers employ workers to hand pick healthy ripe fruits. Coffee pickers can each pick 50-100kg of coffee fruits per day. The coffee cherries are dried and a machine is used to extract the raw green coffee beans. Any beans that are mouldy or defective are removed.

The next step is roasting in which the beans are heated to about 200 degrees C for between two and forty minutes. This drives a series of complex chemical reactions inside the bean that enrich the flavour. Green beans are mostly made of sugars with acids, fats, minerals, caffeine, proteins and amino acids making up the remaining 1/3. The cell wall of coffee beans is very thick and remains intact during roasting. Green beans have a 10% water content which is reduced to 5% during roasting. This water turns to steam and the inside of a bean is like a pressure cooker in which the sugars react with fats, amino acids and proteins to make melanoidins and glycosylamine which are brownish, bittersweet chemicals that give roast coffee its dominant taste. So the roasting process is essentially a caramelisation process. Alongside this a series of volatile, low mass aromatic compunds emerge that give coffee its familiar smell. If the beans are not roasted for long enough the aromas do not emerge and the coffee tastes acidic. Poor quality beans are roasted for longer periods to drive out the poor smells and tastes. This leaves a tasteless, bitter bean. The accepted roasting time for good quality beans is 12 minutes. Instant coffee is made using the leftover, crap beans by dry roasting them. The taste is bitter and there are often all sorts of unwanted products from the harvest thrown in to bulk it up.

Once the coffee is ground it is ready to be brewed. There are many techniques. The coffee can be placed in a filter and have hot water percolated through it over about 5 minutes. It can be placed in a container with the coffee for 5 minutes and then a plunger can be used to strain out the grounds as the liquid is poured out. These two methods allow more caffeine to be extracted but they also result in the dissolution of more of the soluble acids. The espresso techinque is favoured by connoisseurs. Here the grounds are compacted to a dense, fine powder. Hot compressed water is squirted through for about 30 seconds. A thin layer of dense, foamy liquor or crema forms on the surface of the coffee in the cup. If this is too light in colour then the extraction time was probably too short, the grounds too coarse or the water was not hot enough. A dark crema with a hole in the middle signifies over fine grounds in too great a quantity. Overextracted espresso gives a white spot in the center of the cup if the brewing time was too long or a white froth with large bubbles if the water was too hot.

The final result is a colloid of gas bubbles, oil droplets and solid fragments less than a few microns in size with water molecules bound to them. The coffee fluid has lots of body and it is very viscous. It coats the tongue with emulsified oils that sit there for about 20 minutes and continue to release flavours and aromas. A good cup of coffee should stay with you for about a quarter of an hour after the cup is empty.