While I’ve discussed the Chemistry of coffee before and whether it was dangerous or delicious (or both!), that post didn’t discuss how coffee is made, or how different types and varieties are created.
From tree to bean
Coffee itself is made from coffee beans that grow inside red berry-like fruits called coffee cherries that grow on coffee trees. The trees are grown from green coffee beans that are not processed and are generally raised initially in a nursery before being planted outside and taking up to 3-5 years to reach maturity. The trees are evergreen and produce little white flowers, not dissimilar to those on citrus trees and would naturally grow 6-10 metres tall; however, for the purposes of harvesting, they are normally pruned and trimmed to a height of 2-3m.
The coffee berries themselves will take up to 11 months to mature and will ripen to a deep and vibrant red at all different times. This creates an issue in harvesting the berries as mechanical harvesters will not distinguish between ripe and unripe berries, while hand-picking the berries is extremely labour-intensive, but will result in better coffee. If the berries are machine-harvested, they must then be sorted to remove bad berries and debris, most commonly by water immersion where unripe berries float to the surface.
After harvesting, the berries must be processed, by either of two methods:
- Dry method: This is the older and traditional method of processing coffee that is used today when water resources are limited. It simply involves spreading the coffee cherries out in the sun and preventing them from getting wet at all until the cherry moisture content is under 12.5%, which may take up to several weeks.
- Wet method: This is a more modern method and has several seperate steps involved. Firstly, the cherries are pulped so that the flesh is removed and the beans are left with just a mucilage outer coating. The beans are put into large water tanks for 1-2 days where this mucilage skin breaks down before being washed off completely. At this point the beans have approximately a 57% moisture content and is dried, either by the sun or in a dryer until moisture reaches the ideal 12.5%.
Roasting and Grinding
At this point, the coffee is called green coffee and doesn’t taste anything like the finished product. In order to gain the rich tastes, flavours and aromas (from over 1000 different compounds), the coffee must be roasted. This is a fairly short process, generally involving the beans being roasted for 10-20 minutes or so at between 180ºC and 240ºC. A higher temperature or longer time will yield a darker roast coffee.
The coffee beans are then ground so that they can release their flavour into water and produce coffee. The fineness of the grind depends on its purpose as espresso coffee is a far finer grain than drip-brewed coffee. However, ground coffee will rapidly lose its delicious flavours and aromas. If the coffee is being packaged, it must occur quickly and be sealed in protective, UV resistant vacuum packing to avoid losing these aromas. Ideally though, the coffee could be ground just before brewing to truly exploit the flavours and make the whole place smell wonderful!
Instant coffee is made when the coffee is brewed to release a coffee extract (really just super-strong coffee). This extract is then either spray-dried by spraying the extract into hot air so it dries and falls down as a fine powder. The other method is freeze-drying where the extract is frozen and cut into granules. Whichever method is used, the instant coffee is then packaged, hopefully having preserved the flavours and aromas of the original coffee.
If the coffee is decaffeinated, it undergoes an extra step before roasting. The green beans are treated with a solvent, normally methylene chloride or ethyl acetate. They are normally rinsed with either solvent for about 10 hours. The caffeine bonds to these compounds and is removed along with them. There are also two chemical-free processes: the Swiss water method (involving multiple steps and is difficult) and CO2 method which is again difficult and not cost-effective.
Whichever process is used, the remaining beans are generally about 97% caffeine free and go onto to the rest of the roasting and grinding process.
And now, please go enjoy a lovely cup to celebrate International Coffee Day!