Anaerobic fermentation is a processing method where the coffee is processed in a fully sealed and oxygen deprived fermentation tank.
After selective handpicking of fully ripe cherries, they are checked in water basins to sort out underripe floaters. Coffees enter the anaerobic process in depulped cherries (honey). The beans are placed in air-sealed barrels or stainless steel tanks that need to be in a cooler environment (wind or even fridges), as the fermentation process creates heat.
After around 18-24 hours, the anaerobic process has started causing a breakdown in the mucilage and a buildup of CO2 pressure in the tank. This pressure forces the flavours of the juicy mucilage into the coffee parchment. Pressure is released through one-way valves at the top of the barrel - or oftentimes released through pipes that surpass water basins. The anaerobic fermentation has many levels and can take up to 120 hours, sometimes even 240 hours.
The types of microbes able to survive and participate in fermentation is limited by the lack of oxygen. The result is a very expressive flavour profile that oftentimes has notes of cinnamon, bubble gum or poached pear.
Once carefully removed from the tank, the coffee is dried to ensuring a halting of the fermentation stage. This experimental process yields unexpected and complex flavours, while also giving the producer great control over the sugars, temperature, pressure, pH and length of the ferment.
The main difference between anaerobic fermentation and carbonic maceration is that in carbonic maceration the cherry is left in tact, and not pulped prior to entering a sealed tank. Carbonic maceration is a term used in the wine industry, where whole grapes are fermented without pressing the juice. This tends to produce fresh and fruity wines that may not age well but can be consumed quite soon after their production.
In carbonic coffee fermentation the skins of the cherries are left in tact and therefore fermentation takes longer, ie pushes flavour profiles even more than anaerobic styles. The varying levels of pressure in the tank create different available sugars and pectins for the microbes to macerate. Near the bottom of the tank, the coffees are gently pressed over time by gravity, whereas cherries near the top of the pile remain unpressed and fermentation will occur slowly and almost entirely inside the skin of the fruit.