In our Raw Materials series, Merit's Director of Green Coffee Jamie Isetts writes on green coffee, origin, and the finer details that make our coffees and partners so unique.
Conventional wisdom for coffee processing is that cherries are processed the same day that they are picked off the tree. This rule of thumb is so rigid that mills often run late into the night during the harvest season.
However, I’ve noticed an interesting trend at farms around the world in the last few seasons. More and more farmers are adding a “resting” period, where whole cherries sit undisturbed without drying for up to three days. They can then be processed as a washed or honey coffee or taken to sunny drying beds to become naturals. Once perfected, this technique requires little investment in infrastructure and no advanced technology.
So, what’s going on with this method?
Cherries rest before processing at Guatemala's Finca La Senda.
As far as I know, there’s no research-based consensus on what exactly is happening during this “cherry resting” period. Going forward, the generalizations I make are based on my own anecdotal experience working with specific farms and coffees. Another key resource was Schwan and Fleet’s excellent book, Cocoa and Coffee Fermentations. If you would like to debate something I mention here or just discuss more about this topic, please reach out at email@example.com.
I first came across “resting” while visiting Enrique López of Mexico’s Finca Chelin. Enrique is a die-hard processing nerd who, oddly, has a professed love for grape soda. He loved the fruity flavors so much that he began to look for ways to amplify them in his coffee. The answer? Resting or “reposando” the cherries for one or two nights before processing.
The drying patio at Enrique López's Finca Chelín brims with a variety of processing experiments, which dry to different colors.
Coffees that were rested before processing tend to have more grape, peach, and green apple flavors, along with tartaric and malic acidity. These acidities pair with the more standard citrus acidity to give coffees dimension and strong fruit character. I’ve also noticed that these coffees have a more syrupy body, decreased astringency, and greater sweetness. The overall effect is a rounder, juicier coffee.
These flavors mostly come from the fermentation that happens as these whole cherries are sitting still, waiting to be processed. Microorganisms in the environment consume the sugars present in the fruit and release flavor compounds like esters that are small enough to penetrate the cell membranes of the seed (green bean) inside.
Of course, fermentation occurs in all “normal”, unrested coffees, too. However, the step of removing part or all of the fruit through depulping takes away some of the material available for microbe fuel. On top of that, the coffee seed begins to germinate when stripped of its skin. At this stage, the seed itself is also consuming sugars that could be otherwise be used by microorganisms. Having a rest period before depulping offers more material for fermentation.
And what about naturals? This same effect is happening, but the environment is a little different. Many producers of fine naturals use a thin layer of cherries dried on a raised bed by sun or air currents. Fermentation can occur until the moisture needed for microbes to survive has evaporated. The faster and more evenly that a natural lot is dried, the less of these berry/boozy flavors we’ll taste. In the case of “resting”, the cherries are kept closer together in a fermentation tank, a bag, or even under a sheet of plastic.
There’s a reason this practice isn’t used by everyone. Resting requires pretty tight control over a farmer’s inputs, and lots of trial and error.
As with naturals and honeys, consistency is an issue. The amount of available sugar, exposure to microorganisms, and access to oxygen will vary between each cherry in the tank. Immaculate cherry selection is critical to avoid off flavors and produce a consistent final lot.
Temperature is an important factor in determining the rate of fermentation. Since nearly all coffee processing takes place outside, the weather will dictate the temperature and humidity on a given day. Overfermentation can happen very quickly where the climate is naturally hot. Resting tends to work best at high altitudes where the temperatures are cooler, allowing for easier control of the fermentation process. However, it is possible to be too cold. Ethiopia’s Abana Estate is located at roughly 2000m above sea level in western Ethiopia. Owner Michael Omran sped up his cherries’ reactions by covering them with tarp to create a hot-house effect. The ideal conditions and length of resting time vary greatly from farm to farm.
Michael Omran, owner of Abana Estate in Ethiopia. Omran employs the resting, or cherry fermentation, technique at his high altitude washing station. Unfortunately we don't have a good picture of his plastic-covering method!
Other Rested Coffees to Try
In addition to both coffees in Abana’s Anderacha series, we’ll offer a few other coffees this year that have been “rested” before processing. Look out for our range of coffees from Guatemala’s Finca La Senda. Owner Maria Eugenia Pérez worked with consultant Thomas Pingen to design a variety of unique processes that include an open-air resting period before being depulped or dried.
Maria Eugenia Pérez checks some drying beds at Finca La Senda. She oversees the processing at her farm, which employs the resting technique in many of its coffees.