What is lactic fermentation in coffee?

Lactic fermentation in coffee is a post-harvest process in which lactic acid bacteria (Lactobacillus spp., Leuconostoc spp.) break down sugars in the coffee mucilage into lactic acid, under anaerobic or semi-anaerobic conditions with little or no oxygen. This process produces characteristic cup profiles — creamy acidity, yoghurt or kefir notes, lingering sweetness — increasingly sought by micro-roasters and competition baristas.

Coffee fermentation is a complex microbiological process that occurs naturally on cherries or the mucilage (the sweet flesh surrounding the seed) after picking. Lactic fermentation is one of the fermentation pathways that producers can favour or control, as opposed to alcoholic fermentation (yeasts) or acetic fermentation (acetic bacteria).

Lactic Acid Bacteria (LAB) are ubiquitous in coffee post-harvest environments — on cherries, in tank water, on equipment. They thrive under anaerobic conditions and at temperatures between 18 and 35 °C. Without oxygen, they metabolise glucose and fructose from the mucilage to produce primarily lactic acid (homofermentative fermentation) or a mix of lactic acid, acetic acid and CO₂ (heterofermentative fermentation).

To promote lactic fermentation, producers use several techniques: fermentation in sealed tanks with low oxygen content (anaerobic), submersion of cherries under water with airtight lids, or temperature control to inhibit yeasts (which prefer 28–35 °C) and favour LAB (which tolerate lower temperatures better). Some producers — such as La Palma y El Tucán — directly inoculate selected strains of Lactobacillus plantarum or helveticus to steer the aromatic profile.

In the cup, a well-executed lactic fermentation produces soft, creamy acidity (unlike the bright, fruity acidity of yeast fermentations), with notes reminiscent of plain yoghurt, kefir, butter, or sometimes fresh mild cheese. This acidity is perceived as 'rounder' and less 'sharp' than standard fermentation. Lactic fermentation lots have achieved high competitive scores at barista championships, notably the World Brewers Cup.

Lactic vs alcoholic fermentation in coffee