Mash pH

Mash pH is the one prediction we solve rather than calculate. There is no formula that takes a grist and a water report and hands back a pH. There is a balance that has to come out even, and we find the pH at which it does.

The balance

Every malt, on its own in distilled water, sits at a pH of its own, and resists being moved from it by an amount that can be measured, its buffering capacity. Your water pushes the mash up through its alkalinity and down through its calcium and magnesium. Acid pushes it down. At the true mash pH all of that cancels, and we bisect until it does, to within a hundredth of a pH.

Where the malt figures come from

Where a maltster publishes titration figures, the catalog carries them and the model uses them, and the ingredient’s record says so. Where none are published, the malt is classed: base malts sit near 5.7, Munich a little lower, crystal lower again on a line through the measured range, and roasted malts near 4.7. Those are averages of published measurements, and your sack is not the average. A malt with real figures on its sheet beats them, and this is the one place a catalog match matters more than anywhere else.

What the model does not know

It has not tasted your water. It assumes your alkalinity was measured the way it thinks, as bicarbonate or as calcium carbonate, whichever your report says. And it does not model the mash’s own temperature dependence: a pH meter reading a hot sample and a prediction stated at room temperature are two different numbers, and the hot one reads lower. Use the prediction to get close, then measure, and the water guide says how to read what you get.

The beer these numbers come from

Reference pale ale — 20 L into the fermenter, a 60 minute boil.

Grist	Amount	Color
Crisp Maris Otter Pale Ale Malt	4.5 kg	2.8 °L
Thomas Fawcett Caramalt	0.35 kg	9.9 °L

Hops	Amount	Alpha	Addition
Magnum	15 g	13.5 %	60 min boil
Cascade	30 g	6.8 %	10 min boil
Citra	40 g	13 %	20 min stand at 80 °C

It comes out at
Original gravity	1.0507
Final gravity	1.0094
Alcohol	5.52 %
Bitterness	35.66 IBU
Color	5.14 SRM (10.12 EBC)
Into the fermenter	20 L

The numbers

Mash pH

5.45 pH, from charge-balance-troester.

Σ Bᵢ·mᵢ·(pH_DI,ᵢ − pH) = acid_added(pH) + Ca/3.5 + Mg/7 − alkalinity(pH)
⇒ Mash pH = 5.45 pH

Input	Value	Unit
Grist in distilled water	5.689	pH
Grist buffering	201.5	mEq per pH
Mash volume	13.58	L
Thickness	2.8	L/kg
Alkalinity	77.9	ppm as CaCO₃
Residual alkalinity	-37.1	ppm as CaCO₃
Water alkalinity consumed	18.63	mEq
Calcium	29.76	mEq
Magnesium	1.44	mEq
Added acid	34.55	mEq
Acidulated malt	0	mEq
Alkalinity model	titrated-to-mash-ph	—
Iterations	22	—

What it assumes:

• Grist alone in distilled water would sit at pH 5.69.
• Converged to ±0.000001 pH in 22 bisection steps.
• 88 % of the measured alkalinity is titrated between the water pH and the mash pH; the rest is still bicarbonate at mash pH and consumes no acid.

Sources:

• Understanding Mash pH — Kai Troester (Braukaiser)
• A general approach to brewing water — Kai Troester (Braukaiser)
• Der Einfluss des Brauwassers auf den pH-Wert der Würze und des Bieres, Monatsschrift für Brauerei 6 — Paul Kolbach, 1953
• The Ionization Constant of Carbonic Acid in Water, J. Am. Chem. Soc. 65 — Herbert S. Harned & Raymond Davis Jr., 1943

Lactic acid to reach pH 5.40

0.994 mL, from mash-ph-dose-bisection.

solve amount such that predictMashPh(grist, water, amount) = target
⇒ Lactic acid to reach pH 5.40 = 0.994 mL

Input	Value	Unit
Agent	Lactic acid	—
Concentration	88	%
Baseline	5.455	pH
Target	5.4	pH
Achieved	5.4	pH
Iterations	14	—

Sources:

• Understanding Mash pH — Kai Troester (Braukaiser)
• Water: A Comprehensive Guide for Brewers — John Palmer & Colin Kaminski, Brewers Publications, 2013

Lactic acid to bring water to pH 5.50

1.89 mL, from carbonate-titration.

acid mEq = V × [Alk(pH_source) − Alk(pH_target)];  amount = mEq / (mmol/unit × f(pH))
⇒ Lactic acid to bring water to pH 5.50 = 1.89 mL

Input	Value	Unit
Volume	16.13	L
Alkalinity	77.9	ppm as CaCO₃
Source water	7.6	pH
Target	5.5	pH
Total carbonate	1.641	mmol/L
Acid	21.84	mEq
Protons	0.9776	per molecule
Concentration	88	%

Sources:

• The Ionization Constant of Carbonic Acid in Water, J. Am. Chem. Soc. 65 — Herbert S. Harned & Raymond Davis Jr., 1943
• Water: A Comprehensive Guide for Brewers — John Palmer & Colin Kaminski, Brewers Publications, 2013