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 twentieth century led to the questioning of the use and contamination of so much water – often 10 m³ per ton of green coffee – in the wet processing of coffee. The technological reactions to environmental concerns came in different ways.
Water Recycling – The first and more obvious reaction was to recycle waste water after some sort of “treatment”, primarily settling and/or filtering to remove solid matter that could otherwise clog the system.
Dry Transport – Another obvious reaction was to move away from transporting coffee with the help of water. The solution was to use gravity wherever possible (e.g.: dry reception hoppers) and, most importantly, to avoid the transport of coffee with water in channels and pipes using instead mechanized options, like conveyors and elevators.
Semi-dry and Dry Pulping – Next came the drastic reduction and even the elimination of water in pulping; since the pulp is rich in water, it was found that the friction or tearing process that removes the pulp from around the coffee bean could take place with the addition of little or no water. The low-water-consumption and dry pulpers used today are not very different from the conventional “wet” ones. The technical changes, started in the late 1980s, are mostly related to facilitating the flow of coffee and pulp in the absence of “lubricating” water. Several technologies and types of such new pulpers are now available in the market.
Mechanical Removal of Mucilage – The last frontier of water reduction and/or elimination remains in the removal of the mucilage that sticks to the parchment skin that involves the coffee bean. The original technique of breaking down the mucilage layer by means of natural fermentation in tanks with or without water followed by washing was progressively combined with and even replaced by the removal of mucilage by friction in several types of machines that were also water intensive in the past but are quite water efficient now. Although mucilage removers have been around for over a century, it was only in the last 2 or 3 decades that low water consumption models became available, with emphasis not only on water savings but also on full removal of mucilage and minimum damage to coffee.
Although there is little debate today about the impacts on coffee quality of water recycling and the reduction of its use in transport, floating and pulping, even though bean damage may increase with dry pulping, the debate is still far from settled in what regards water conservation in mucilage removal. This is the step that by far still consumes and contaminates the most water.
There are expert cuppers who insist that fermentation is the only way to enhance specific cup features, e. g.: acidity, especially in high-grown coffee. They argue that mechanical mucilage removal is yet to produce the same results in these cases although the cup differences may subsidize and even disappear as altitude drops. However, on the other side of the argument, there are studies that show that given the difficulties to control the natural process of fermentation, mechanical removal may, on the average, produce better quality coffee.
Yet other studies show that natural fermentation for a few hours followed by mechanical removal, which causes water consumption to fall substantially, may yield the same cup quality in most altitudes. It may become unavoidable that mainstream, high quality and even specialty washed coffee processing will progressively resort on full or partial mechanical removal of mucilage. Natural fermentation is likely to remain the option for less and less coffee whose price will have to eventually reflect the cost of treating the waste water generated by its process.