Water entered coffee processing when Arabicas began being planted in areas that were hotter and/or more humid than the highlands of Ethiopia. Recently harvested coffee cherries fermented rather quickly in these new areas, with adverse impacts on cup quality. The solution at the time was to remove the sugar-rich, fermentation-prone pulp and mucilage in what was to be called ‘wet milling,’ thus giving birth to washed coffee in opposition to the dry system that produced naturals at the birth place of Arabica coffee. It was also found that coffee processed by the two methods had cup features that were different, with body and sweetness stronger in natural coffees as opposed to the aroma and acidity in washed coffees.
The processing of natural Arabicas require little or no water at all, and where water is used – in Brazil, for example, for flotation – it can be recycled for several days and the degree of contamination is very low, if any.
For well over a century, the wet milling of coffee was a water-intensive process, as the name indicates, and there was little concern for water consumption and contamination. In the conventional technology then prevailing and still used in many areas today, water is used in flotation, to separate over-ripe and dry cherries from the ripe ones, in pulping, to remove the pulp, in mucilage removal, carried out by natural fermentation or friction in machines, and in the transport of coffee and by-products (e.g., pulp). The growing concern for the environment in the last quarter of the last century led to questioning the use and contamination of so much water – often 10,000 m³ per ton of green coffee – in the wet processing method. The pollution load in the wastewater from the wet milling of coffee can be 30 to 40 times greater than the one found in urban sewage!