What is the target pH during coffee fermentation?

In coffee fermentation, pH is the primary indicator of process progress and microbial health. A target pH of 3.8 to 4.5 is generally sought for washed fermentations: below 3.5, the risk of over-fermentation with vinegar-like notes is high; above 5, fermentation is too slow or stagnant. Monitoring pH allows timely intervention to halt the process and preserve the aromatic integrity of the lot.

pH measures the concentration of hydronium ions (H⁺) in a solution on a logarithmic scale from 0 to 14. During coffee fermentation, lactic and acetic bacteria convert the sugars in the pulp and mucilage into organic acids — primarily lactic acid and acetic acid — progressively lowering the pH of the fermentation water. This acidification process is beneficial within limits: it effectively breaks down the mucilage, encourages beneficial bacteria over pathogens, and contributes to the final acidity in the cup.

The starting pH of the fermentation water — before cherries are added — is generally between 6.5 and 7.5 (neutral to slightly alkaline). It falls progressively during fermentation. The speed of descent depends on several factors: temperature (at 25 °C, fermentation is roughly twice as fast as at 18 °C), the sugar content of the cherries (a function of ripeness), and the initial microbial load. In high-altitude zones (above 1,800 m), cool overnight temperatures naturally slow fermentation, allowing longer time windows and more complex profiles.

For natural (dry) fermentations, measuring pH is more challenging since fermentation occurs in a solid substrate. Some advanced producers embed probes within cherry layers to monitor internal acidity. For anaerobic and carbonic tank fermentations, the pH of the residual water or expelled juice is measured at regular intervals (every 12 to 24 hours). A little-known but important fact: the final fermentation pH does not directly translate to cup pH — roasting destroys a large portion of the organic acids formed and generates new ones through Maillard reactions, notably chlorogenic acid degraded to quinic acid. A coffee fermented to pH 3.8 will not necessarily produce a very acidic cup.

pH ranges during coffee fermentation