What is anaerobic fermentation in coffee processing?

Anaerobic fermentation means the coffee ferments in a sealed, oxygen-free environment - typically stainless steel tanks, food-grade barrels, or vacuum-sealed bags. Removing oxygen changes the microbial population: lactic acid bacteria and anaerobic organisms dominate over the mixed communities of open-tank fermentation.

The by-products are different too. In the right conditions, anaerobic fermentation produces lactic acid, ethanol, and various esters that interact with the bean and create flavour profiles impossible through conventional processing - tropical fruit, fermented berry, sometimes boozy or candy-like notes that can be striking or overwhelming depending on execution.

It requires careful management: temperature, CO₂ buildup, and timing all need close monitoring. Poor execution - too long, too warm, poorly sealed - produces vinegary or unstable results. When it works, it produces some of the most distinctive cups in specialty coffee.