Mash Temperature
Mash temperature is how you control whether your beer finishes dry or full-bodied. Lower temps (around 63–65°C) produce more fermentable sugars: lighter body, lower FG, drier finish. Higher temps (68–72°C) produce more unfermentable dextrins: fuller body, higher FG, sweeter finish.
Change the mash temperature in the recipe builder and your predicted FG and ABV update live. The default is a Kinetic model that simulates the mash enzymology. There's also a simple Linear model if you want to cross-check against another app. Here's how the two compare against real measured batches:
Every dot is a real batch with the brewer's own measured gravities. Nearly 50 of them, single-infusion mashes, normalised to a 75% base yeast so they line up on one axis. (Step mashes are in the test set too. They just don't have one mash temp to plot.) The Kinetic curve runs through the middle of them. The Linear formula sits a little high at low temps. The flat line, the one that ignores mash temp, misses the trend completely.
Worth being straight about. In the everyday range, 64 to 68°C, the dots scatter a good few points wide and all three curves run close together. So the model barely matters there. Any sensible guess lands inside the noise. The curves only pull apart at the extremes, and that's where the Kinetic model earns its keep. Across the whole set it lands about 2 gravity points off, and the very hot end (above 71°C) is where I have the least data, so trust it least.
The Enzymes That Set Fermentability
Three enzymes in the mash decide how fermentable your wort ends up. Beta-amylase (happiest around 63°C) snips small, fully-fermentable sugar off the ends of the starch chains. Alpha-amylase (happiest around 70°C) cuts the chains in the middle and leaves bigger sugars the yeast can't finish. Limit dextrinase (around 61°C) unlocks the branch points the other two can't reach. It turns out to be the single biggest driver of how fermentable the wort gets. The practical upshot: a cool rest keeps the first and third busy, so the beer comes out drier. A hot rest hands the wort to alpha-amylase, so it comes out fuller and sweeter.
Two Models
Kinetic
defaultunique to Brewing.ItA mash simulation. It steps through your actual rest schedule and tracks four sugar pools (starch → β-convertible dextrin / branched limit dextrin → fermentable sugar) as beta-amylase, alpha-amylase, and limit dextrinase work and slowly denature. Because it walks the real schedule, step mashes work right. A cool rest early on still counts even if a hotter one follows.
Long story short: it works out how much of your starch becomes sugar the yeast can eat, and how much it can't. There's one bit most calculators skip. Even the fermentable sugars aren't equal. Yeast finishes the easy ones and leaves some of the trickiest one (maltotriose) behind, and a hot mash makes more of it. So a hot mash ends up a touch sweeter than the sugar split alone would say.
On the test batches it's the most accurate of the bunch, about 2 gravity points off. The very hot end (above 71°C) is where I have the least data, so I trust it least there.
Linear
A simple straight line. Shift the yeast's attenuation about 2.25% per °C off a 67.5°C neutral point, read from your lowest saccharification rest. This is Grainfather's published formula. Pick it if you want numbers that line up with another tool. It's less accurate than the Kinetic model on real beer. A straight line runs high at very low mash temps, and it leans too hard on a single low rest.
Drop the yeast's rated attenuation by 2.25% for every °C your lowest mash rest sits above 67.5°C, and raise it below. Clamped to a 62.5–72.5°C window. That's Grainfather's published formula, and that's the whole thing. The Kinetic model doesn't get a tidy one-line equation, on purpose. It's the simulation above, not a formula.
Where This Comes From
The shape and the numbers behind the Kinetic model come from published brewing science, then get tuned against real measured final gravities:
- Brandam et al. (2003), A kinetic model for the mashing process. The sugar-species ODE structure.
- Muller (1991), J. Inst. Brewing 97:85. In-mash amylase thermal-decay rates (used instead of buffer-measured rates, which denature beta-amylase far too fast).
- Stenholm & Home (1999), J. Inst. Brewing 105:205. Limit dextrinase as the dominant fermentability driver.
- De Schepper / Laus et al. (2022), Food & Bioprocess Technology 15:2294. Isothermal mash fermentability measured in 1°C steps from 55 to 80°C, which informs the shape (peak ~65°C, gentle decline above it).
- Stewart et al. on wort sugar uptake. Maltotriose gets fermented last and least completely, which is why a hot, maltotriose-heavy wort finishes below its sugar breakdown.
- Validation set: ~50 measured-FG batches with the brewer's own gravities, from Braukaiser, Woodland, and Brulosophy experiments, plus published BYO/German step-mash recipes.
Where I'd be careful. The rate constants are tuned to measured beers, not derived from pure theory. The hot-side bend follows a real mechanism (yeast leaving maltotriose behind), but its size leans on only a handful of very-hot batches. So above 71°C is the shakiest part, and up there the yeast strain matters more than the mash anyway. Very high-gravity worts are hard too. Take the number as a good estimate, around 2 gravity points, not a promise.