In short, an anaerobic coffee is one that has undergone some period of time fermenting in an environment depleted of oxygen. The amount of time spent without oxygen, type of fermentation tank, and even additives like fruit or yeast will vary from farm to farm, even lot to lot, all in ways that can impact taste.
As the fermentation starts to occur, carbon dioxide is produced, creating a completely high pressure, anaerobic environment. These factors affect the coffee flavour and taste profile in two important ways.
- An anaerobic environment favours a very different set of fermenting bacteria and yeast, leading to a dominant lacto-fermentation.
- The pressure also forces coffee juices into the seed / bean itself, adding more fermentable sugars to continue the process.
Referring only to the fermentation environment, an anaerobic coffee could undergo either a washed, natural, or honeyed process.
The Volcán Azul WAE Geisha has been naturally processed following the fermentation period of seven days. Once fermentation has been complete, the cherries are spread out onto a drying patio and slowly dried over the course of 2 weeks.
In the case of the Shantawene Gatta this coffee has been processed via the 'Honey Process' where the coffee cherries are first pulped mechanically, removing most of the fruit. This parchment coffee - with it's gel-like, sticky mucilage - is then packed tightly into small fermentation tanks. Once completed, the coffee is then dried with the mucilage still attached, as with a honey processed coffee. All of this adds layers of complexity to the final cup.
See the video below from Cafe Exports for an excellent overview of anaerobic fermentation.