Which lever works best in managing fermentation? Not time, not how tightly you seal, but temperature, I think. Temperature changes the speed of fermentation, and not only the speed: it changes the cast of microbes doing the work inside.
There is a controlled experiment that treated temperature as an independent variable, comparing three varieties at 15 °C, 30 °C and natural fermentation (measured 23 to 31 °C). The result was clear.
Fermentation time was shortest for natural fermentation and longest at 15 °C, by more than 24 hours. At 15 °C the pH levelled off at about 4.0 after 20 hours and stopped falling; at 30 °C and in natural fermentation it kept falling below 3.5. Sugar consumption was slower in the cold too, with glucose remaining high at the end (Peñuela-Martínez et al., Fermentation 2023).
The most interesting part is the cast of microbes. In the same study, mesophilic bacteria and lactic acid bacteria were most numerous at 30 °C and fewest at 15 °C, while yeasts were relatively more numerous at 15 °C. So temperature is a knob that speeds up fermentation and, at the same time, a switch that decides which microbes get the advantage.
In another study comparing anaerobic fermentation at different temperatures, Acetobacter, which makes acetic acid, dominated at high temperature (37 °C) and pushed the coffee toward vinegar, while at lower temperature the lactic acid bacterium Leuconostoc led a gentle fermentation (Lee et al., Foods 2023).
This matches what we feel in the field. A lot left on a hot day takes on a sharp acidity that hints at vinegar. A lot that proceeded slowly on a cool day, by the same method, has roundness. Temperature does not change the strength of the taste; it changes the kind.
Oxygen is worth thinking about together with temperature. In an experiment sealing a natural under CO2, sucrose consumption was held back, the sources of sweetness and the chlorogenic acids were preserved, and the sensory score was higher (Garzón-García et al., 2024). Low temperature and sealing do the same thing: they slow the consumption of sugar.
So in our anaerobic lots we are especially conscious of the cool side. Seal and let it get hot, and fermentation races ahead and tips toward vinegar. Sealing is not a safety device.
A farm in Laos has no water-jacketed bioreactor. So what we can do is limited: add cold water, set up in the shade, set up after evening when the temperature drops. Those three.
Modest, but the effect is not small. Just shifting the set-up time to the evening makes the start of fermentation gentler. Set up in the hottest hour of the afternoon and the early fall in pH is steep, and adjustment afterwards is hard.
One more: if the temperature rises during fermentation, add cold water. I want to confirm again and again that the purpose here is not to manipulate pH. pH falls by itself. What we lower is the temperature; what we gain is time for the aroma to develop.
If you do not measure temperature, it ends with "the taste was different this year". Same recipe, same time, and a 3 °C difference in air temperature makes a different fermentation. So the lot sheet records air temperature, liquid temperature and the time of set-up. With those three, you can reread the conditions of a good year and reproduce them.
The cool side is easier to handle. At 15 °C fermentation proceeded slowly and pH stopped falling at about 4.0, while at 30 °C it fell below 3.5. But cold more than doubles the fermentation time.
When hot, acetic-acid-producing bacteria dominate and the coffee tips toward vinegar. When cool, lactic acid bacteria lead, giving gentle, clean acidity.
On a small farm, realistic methods are adding cold water, setting up in the shade, and setting up in the cool of the evening. In every case the aim is not to manipulate pH but to slow the fermentation.
For the whole picture see The four stages of fermentation; for reading pH, Reading the fermentation pH curve; for anaerobic fermentation, What is anaerobic fermentation.
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