Modeling the Fermentation
This page describes a set of models we built to turn the more than 10,000 measurements from around 50 different fermentations into something more useful: a compact mathematical fingerprint of how each fermentation unfolded.
The Basics
When the grape must reaches the fermentation tank, a two- to three-week scramble begins. Yeast converts sugar into alcohol; color and tannin migrate out of the skins and seeds into the juice; and the acid balance shifts. We follow all of this with a spectrometer and a handful of bench measurements, sampling each fermenting batch every day or two. The result is a thicket of numbers — for a single ferment, a dozen quantities each measured a dozen or more times.
The model's job is to replace that thicket, for each quantity, with one smooth curve and a few numbers that describe it: when color peaked, how fast the sugar fell, how much tannin we ended up with. Once every fermentation is summarized in the same way, we can compare vintages on an equal footing, flag a batch that is behaving oddly while we can still do something about it, and — eventually — predict where a ferment is heading from its first few days. We call these curves trajectories, and the collection of them a kinetic model — kinetic because it is about rates, about how fast things happen.
What We Measure
For each fermentation batch we track ten quantities. In plain terms:
Sugar (°Brix) — the food supply, falling as the yeast consume it.
Alcohol — rising as the sugar disappears.
Free, bound, and total anthocyanins — the red pigments.
Tannins and total iron-reactive phenolics — structure and mouthfeel.
Titratable acidity and pH — the acid balance.
Volatile acidity — the faint whiff of vinegar we hope stays low.
We also record the temperature at every sampling, which turns out to matter more than anything else (see below). All told, the years from the late-1990s to today add up to 49 fermentations and just under a thousand individual measurements consistently gathered.
The Right Clock: Temperature-Adjusted Time
Here is the single most important idea on this page. Chemistry runs faster at elevated temperatures. A day of cold soak at 50 dF barely moves the needle; a day of vigorous fermentation at an excessive 95 dF accomplishes a great deal. So calendar time — days since harvest — is the wrong yardstick for how far a ferment has progressed. Two batches that are three days in can be in entirely different places if one was kept cold and the other warm. That temperature governs extraction is well established in the winemaking literature [2, 3, 5].
Figure 1. (A) The chemistry runs faster when it is warm: each observed temperature becomes a speed multiplier. A cold-soak day at 50 °F counts for about a tenth of a reference day; a warm fermentation day at 95 °F counts for five. (B) Adding those multipliers up gives “thermal time” τ. For this 2023 Cabernet the cold soak barely advances the clock, then warm primary fermentation races ahead of the calendar.
We fix this by stretching and compressing the clock according to temperature. Every observed temperature is turned into a speed multiplier:
f(T) = Q10 ^ ( (T − 77 °F) / 18 )
Three numbers set the scale: a reference temperature (77 dF, where the multiplier is exactly one), the 18 dF that corresponds to a 10 °C step (the rule is conventionally stated per 10 °C), and Q10 — the factor by which the rate rises for each such step. Summing these multipliers over the fermentation gives thermal time, written τ (“tau”). A warm day contributes several thermal days; a cold-soak day a fraction of one.
How temperature-sensitive is the chemistry — what is Q10? The textbook default is 2, meaning the rate doubles every 10 °C. We did not want to assume it, so we let our own data choose: we refit every batch across a range of Q10 values and asked which produced the cleanest curves.