What roasting does, and what it does not do

Münchow puts forward elsewhere a claim that runs against the idea that color would sum everything up: at equal final color, a fast roast will have swollen more than a slow one. He gives 50 percent expansion in volume for light roasts and up to a doubling for dark ones.

If the first claim is correct, two coffees of the same Agtron value would have neither the same density nor the same solubility, that is, neither the same ease in giving up their compounds to water. Color would then cease to be a sufficient summary of what will happen at extraction, and the 80/20 split would hold for the cup tasted under a fixed protocol, not for all preparations.

This claim is stated flatly, with neither measurement nor source. It is also the only one in the file that a roaster can verify alone, by weighing a known volume of beans from two profiles of the same color. It is the most accessible lead we have noted.

What the bean loses, and four figures that do not agree

Weight loss in roasting is the only figure on this plate that translates directly into money: a coffee bought green and sold roasted is paid for on the difference, and two points of gap on that difference change the margin on every bag. It is also, curiously, one of the places where the sources agree least.

Mass loss

The plate gives 10 to 13 percent for light, 14 to 16 for medium, 17 to 20 for dark, up to 22 for very dark. Münchow writes that light roasts lose about 15 percent and dark ones up to 20, and he draws from it a simple commercial rule: buy about 1.2 kilo of green to sell one kilo of roasted, which is 16.7 percent loss.

His figure for light therefore falls in the band that the plate reserves for medium, and his ceiling for dark is two points below that of the plate. Between 13 and 15 percent loss, there are two points of margin on each bag, which is not a nuance of wording. Nothing in the two sources says on what the figures were taken, nor on which coffees, nor at what starting moisture; yet the loss depends first of all on the water the green contained, which would be enough to explain the gap without either of them being wrong.

Residual moisture, and the gap is a factor of two

The plate gives 2.8 percent water in the roasted bean for a light roast and 1.4 for very dark. Münchow writes that specialty coffee contains between 1 and 1.5 percent water after roasting. Yet specialty coffee is mostly light: his figure for a light coffee is half that of the plate for the same coffee.

The two arithmetics nonetheless hold separately, and that is what makes the disagreement instructive rather than embarrassing. Starting from 12 percent water in the green and arriving at 1.3 gives about eleven points of water loss; arriving at 2.8 gives nine and a half. Both are compatible with a plausible total roast loss, the difference being carried over to the organic matter consumed.

The question is therefore which of the two values was measured, on what device and by what drying method, and neither source says. This is the point in the file where a measurement would settle things fastest: a moisture analyzer and ten light coffees would be enough.

The stakes are not only descriptive. Residual water governs the aging of the coffee and the speed at which it degasses, and a bean at 2.8 percent does not behave over time like a bean at 1.3.

Green coffee moisture: three targets, and above all three lines of reasoning

Münchow writes that the origin dries the coffee to about 12 percent, that specialty green should be between 9 and 11 percent on arrival, and that one should aim for 60 percent relative humidity in the storage room. Karen Foley gives the opposite rule: stay under 65 percent relative humidity, which lets the bean keep 12 percent moisture before roasting. The plate reasons in water activity and retains 0.45 to 0.55, which corresponds roughly to 10.5 to 11.5 percent.

The disagreement concerns less the number than what each is trying to protect, and that is where it becomes legible. Münchow justifies his lower limit by the integrity of the cell structure, which an overly dry green compromises. The plate justifies it by the death of the seed and by the flatness of the cup that follows. Foley gives no lower limit, simply because her article deals with the risk of mold, and from that point of view drier is always better.

Three authors, three different risks, three optima: there is no green moisture target in the absolute, there is one target per danger one wants to avert, and the danger must be named before the number.

One last point to add to the file, out of honesty: the notion of water activity, on which the plate rests this whole passage, is absent from the three books. Münchow and Foley reason in relative humidity of the room, and Rao treats it in a chapter that could not be read. The plate's material on this subject is therefore neither confirmed nor contradicted here.