The Lab

Post-Harvest Coffee Processing: Anaerobic Fermentation and Carbonic Maceration

These processing styles are still new to the coffee industry. There is the greatest degree of experimentation in Costa Rica, but anaerobic fermentation is becoming increasingly popular around the world with varying degrees of success.

Anaerobic fermentation in coffee may refer to coffee that is depulped and only the mucilage-covered seeds are added to a sealed tank. Sometimes the pulp may be added back into the tank and mixed with the depulped seeds. Some farms mix the pulp of different varieties of coffee with the depulped seeds of another, attempting to increase the complexity of the coffee. Different spices or fruit are sometimes added as well. Cinnamon or Bananas might be mixed into the tank to impart their flavor onto the coffee.

The primary difference between carbonic maceration and anaerobic fermentation is how the coffee is added to a sealed fermentation tank. Carbonic Maceration is a technique adapted from wine-making in which whole grapes are fermented instead of being crushed. The same is true for coffee. Instead of being depulped, the whole cherry is added to the fermentation tank and sealed for the carbonic maceration process.

Both of these processing styles occur in a sealed environment like a metal drum or plastic container. The coffee will ferment in the drum for as long as three months. As the coffee ferments, the sugars are broken down by bacteria into carbon dioxide and alcohol. As more carbon dioxide is produced, pressure within the system increases, and a valve must be installed to allow for the excess pressure to be released, without the addition of oxygen into the chamber. The valve is most simply a hose submerged in a bottle of water that allows for carbon dioxide to bubble out, while fresh air cannot enter. More advanced processors may use a regulator to hold the pressure at a constant point before a valve allows the gas to escape. As fermentation progresses, the drum becomes increasingly carbon rich with minimal oxygen.

The rate of the fermentation is primarily driven by temperature. The tanks are best kept in a cool area since the breakdown of the mucilage generates heat that would accelerate the fermentation in a positive feedback loop. The hypoxic environment favors different communities of microorganisms like lactic-acid bacteria compared to traditional open air fermentation which favors yeast. These microorganisms will produce different aromatic compounds while breaking down the mucilage or cherry and those compounds are largely responsible for the final flavor. The increased pressure will also affect the migration of compounds into the seed, yielding more intense flavor.

Anaerobic Fermentation and Carbonic Maceration are still very new techniques for processing coffee, but some of the results are promising. Royal NY sources a limited amount of these processing styles. Contact a trader about availability.