The majority of the world’s coffee — and practically all of the world’s robusta — is dry processed.
Brazil, the world’s biggest coffee producer, was responsible for 33% of the world’s arabica coffee production as of September 2018 (International Coffee Organization).
Strip picking leads to a big variation in ripeness between cherries. Winnowing and flotation can improve the efficiency of dry processing by allowing coffees of similar moisture content to be dried together.
Up-to-date statistics for exports by process are unavailable, but according to J. C. Vincent (1987), Brazil uses the natural process for 90% of their arabica coffee output.
High-scoring naturals are beginning to sell for higher premiums than washed coffees. The auction results of the winning lots from the Best of Panama competition from 2019 show that far higher prices were paid for naturals than for washed-processed coffees.
The main form of sugar in coffee beans is sucrose, making up anywhere from 5–9% of the seed and more than 90% of the total sugar content.
Washed processing triggers germination of the coffee seed. This process consumes the sugars stored in the seed, and it uses up the simplest sugars (glucose and fructose) first. As well as affecting the sucrose levels, germination in washed coffees increases the concentration of certain amino acids in the seed.
Naturals don’t germinate during processing; the pulp inhibits germination until the cherry is dried, at which point the seed’s metabolism shuts down entirely.
Little, if any, sugar passes into the seed during dry processing, but some compounds do seem to be absorbed into the seed. These are not the aromas from the fruit itself but the volatile compounds, particularly esters, that are created by microbes as the fruit begins to ferment.
With dry processing, the flora are far more actively involved in mucilage removal.
At the start of the natural process, the Gram-positive and Gram-negative bacteria are dominant.
Later in the process, after the cherries have begun to dry and the water activity (Aw) has dropped to below 0.8 Aw,