Before dawn, while it is still dark, I look into the fermentation tank. I smell it. The acidity seems a little sharper than yesterday. But smell and instinct alone cannot settle when to stop. The same smell means different progress inside on a hot day and a cool one. So I put a pH meter into the tank. Fermentation shapes the taste of coffee enormously, and the awkward thing is that you cannot see what is happening inside. We read that invisible progress with a speedometer: pH.
Fermentation is the process in which microbes eat sugar and turn it into something else. Coffee fruit already carries a team of microbes: lactic acid bacteria, acetic acid bacteria, enterobacteria, yeasts and filamentous fungi. They break down sugar and make acids such as lactic and acetic acid, ethanol, and the esters that become fruity aromas. The more sugar turns into acid, the lower the pH of the liquid. So the number on the pH meter is a mirror of how far fermentation has gone. Guess by smell; check the answer with the number.
pH during fermentation is not decided by time alone. As a reference line, fermentation pH has been reported to fall to about 4.5 at 16 hours and about 4.0 at 36 hours (De Bruyn et al., 2017). But the same end pH arrives sooner at higher temperature. The optimum range is roughly 24 to 32 °C and 35 to 45 hours; within it, cupping scores (SCA, out of 100) of 81 to 85 have been obtained (Vaz et al., 2023). What is happening as it falls is known too. Within an appropriate fermentation time, sugar is almost used up and good acids such as lactic, acetic and malic acid are produced, while off-flavour acids such as butyric and propionic acid are not detected unless fermentation goes too far (Martins et al., 2019). So pH is a speedometer that speaks of quality and spoilage risk at the same time. Watch only the clock and on a hot day you overshoot.
What matters is how you read the meter. The steep fall eases, and the curve nears the bottom. The good acids are all in, one step before the bad ones appear. That is the target. On a hot day you reach the same pH sooner, so the stopping point differs between morning and noon. So it is not "stop after so many hours" but "stop when the pH nears the target". Time is a guide; pH is the goal. Keep this reading in the record for every lot and you can set the aim again at the same time next year.
"We do not cut fermentation by time. We record the hour we started, the air temperature and the way the pH falls, all together, and stop when the curve nears the bottom and flattens. On a hot day the same pH arrives sooner, so we look back at the record for the same time last year and set the target pH for that day's temperature."
"We do not stop fermentation by time. We stop just before the pH reaches the bottom."
For Typica Washed, for example, this season we are aiming at 36 hours around 25 °C, reaching pH 4.0.
The measured values of our lots are listed on the DATA BANK page. For the thinking behind keeping numbers, see What is the Data Bank; for the drying that follows fermentation, The science of drying.
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