Geisha Carbonic Maceration : Coffee

This video shows our farm’s first attempt at an anaerobic fermentation process known as carbonic maceration. This process, borrowed from the wine industry, involves rapidly replacing the air in the fermentation tank with pure carbon dioxide, and is a fairly new practice in the coffee industry. A close friend has used this process to consistently produce full-bodied, exotic (think papaya, ripe red fruits), 89-90 point coffees—scores usually reserved for very high altitude (1800MASL+) Geishas—even though his farm tops out at 1500MASL. We cupped six of his incredible Geisha Carbonic Maceration samples before our curiosity got the better of us and we procured a CO2 tank of our own.

The gradual replacement of oxygenated air with CO2 occurs in normal anaerobic fermentation as the yeasts release CO2 as a byproduct, which sits below the ambient air and either pushes it out a hose in the top of the tank or builds pressure until air begins leaking out in the case of sealed tanks. Carbonic maceration differs in that oxygen is never present during fermentation; this means the only bacteria fermenting the coffee are those that function without oxygen. By changing the bacterial makeup of the fermentation process in this way, we may be able to dramatically alter the cup profile and quality of our coffee, for better or for worse.

We first tapped a few of our normal airtight fermentation tanks to make an attachment for the hose. We filled one tank with Catuai cherries, which cup on average at an 88.5 when fermented normally, and one tank with the finest Geisha on the farm. This 1900m Geisha is extremely sweet, complex, and valuable; it rarely cups under a 90. Needless to say, I worry about butchering this coffee with an unfamiliar fermentation method, but with the help of our experienced friends I’m confident we’ll get the results we want.

We added a one-way valve to make sure the regular air could leave the tank once we started adding CO2, but we didn’t see the bubbler bubbling because our tanks leak under pressure to prevent their exploding. We tried Seran wrapping the leaky seal to direct the air through the valve, but this turned out to be more trouble than it was worth since it doesn’t really matter if the air escapes through the seal or the valve as long as it escapes somehow. After a few minutes running the CO2, we let the gasses settle and then came back to repeat the process.

These coffees will now sit in the sealed tanks for five days, exposed only to carbon dioxide. Then, we’ll send the cherries to dry for around twenty days while I take some of the juice back to my kitchen to pasteurize and taste test against the juice from other fermentation methods. Fingers crossed we end up with a mind-blowing coffee and a good gin drink to go with it.

The last clip is looking through our refractometer at the fresh juice of our low-altitude Catuai coffee, which read at 11% sugar concentration. It was interesting to compare sugar concentration readings from our different coffees; high-altitude Geishas were much more sugary at 16%. However, Brixier doesn’t always mean better. Costa Rican coffees that don’t score as well as Panama Geishas can have upwards of 20% sugar concentration. Brix is another example of one of the thousands of factors that benefit but don’t define the quality of a coffee.

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