Mash acid: lactic and phosphoric dosing

Why acidify the mash

Mash enzymes work in a narrow pH range. Amylase breaks starch down best at about pH 5.2–5.6; above that the extract yield falls and the flavour suffers with it: the beer tastes flat, holds its head poorly and oxidises faster. For most styles the goal is a mash pH of 5.2–5.6, measured ten to fifteen minutes after dough-in.

Mash pH is set by two forces: malt acidity and water alkalinity. Pale malt acidifies only slightly, dark and roasted malt strongly, so dark beer often lands in range on alkaline water by itself. A pilsner or a pale ale on water with high residual alkalinity will not come out right without acidification.

Acid can go into the mash water, into the mash, or into both. Acid added to the mash water before the grain has time to spread more evenly and cannot end up in one spot next to the malt. For home brewing the easiest route is to measure the acid into the water, stir, and only then dough in.

Residual alkalinity by Kolbach

The key quantity is residual alkalinity. It shows how much alkalinity is left after the acidifying effect of calcium and magnesium is taken into account. The formula is empirical, published by Paul Kolbach in the 1930s, and it is still the brewing standard: it predicts ordinary waters well and needs nothing but a water report.

Working in milliequivalents keeps the arithmetic simple: 1 mEq/L of alkalinity equals 50 mg/L as CaCO₃. That way one language serves both alkalinity and acid: one milliequivalent of alkalinity is neutralised by one milliequivalent of acid.

The recommended range depends on wort colour. For very pale beer it is close to zero or even slightly negative, for amber a small positive value is acceptable, and for dark and black beer it is noticeably higher. When residual alkalinity already sits inside the range, no acid is needed at all: the malt sets the pH.

RA = Alk − (Ca / 3.5 + Mg / 7),   mg/L as CaCO₃
for bicarbonate, Alk ≈ HCO₃ · 0.82
RA_mEq/L = RA_mg/L / 50

Acids and their strength

Every acid has its own concentration and its own density, so “a millilitre of acid” is not a fixed quantity. It is easier to work in milliequivalents per millilitre: the backend converts percentages and density into mEq/mL, and the calculator shows both the volume and its equivalent. Below are approximate values for the solutions brewers are sold.

Lactic acid partly turns into lactate during fermentation — it is present in beer and useful in small amounts, but at around 400 mg/L it starts to read as a sour-milk note. Phosphoric acid brings no lactate, but it does bring phosphate: the yeast needs it, yet an excess is undesirable too. Phosphoric acid is noticeably stronger than lactic and harsher in concentrated form, so home brewers usually take the ten-percent one.

Acid affects beer flavour in two ways: it lowers pH, and with it the perception of bitterness and freshness changes. An over-acidified mash gives a sharp, astringent aftertaste, so it is better to undershoot than to overshoot — and always check pH with a meter.

MEQ_acid = ρ · C / M
where ρ is the solution density, g/mL; C is the acid mass fraction; M is the equivalent molar mass, g/mol (lactic 90.08; phosphoric 98.00)

How much acid is needed

First the calculator works out the excess alkalinity: the target is subtracted from the residual alkalinity. When no target is set it takes the top of the recommended range for the wort colour — a reasonable compromise at which the beer comes out well and the character of the style is not lost. If there is no excess alkalinity, no acid is needed, and the calculator says so plainly.

Then the amount of alkalinity is converted into milliequivalents over the whole volume and divided by the strength of the acid. The answer is millilitres. This is a model calculation on the nominal composition: it knows nothing about the buffering of a particular grist, nor about the part of the acid that binds to the salts in the water.

The by-product ion is worked out separately. Lactate in beer is usually small, with a taste threshold around 400 mg/L; as the dose approaches it, the calculator warns. Phosphate adds noticeably less by taste, but phosphoric acid shifts pH more, so it is easier to overshoot with it.

n = RA_excess (mEq/L) · V (L)
V_acid = n / MEQ_acid,   mL
C_by-product = n · M_ion / V,   mg/L

The pH drop is an estimate

Mash buffering capacity shows how much acid it takes to shift pH by one unit. It varies widely — roughly from 8 to 25 mEq/(L·pH) — and depends on the grist: dark and caramel malts buffer more, pale ones less, and the water-to-grain ratio and the crush matter too. So the honest answer to “how far will pH fall” is a range, not a number.

The lower bound corresponds to a lightly buffering pale grist, the upper to a dark and caramel one. If the predicted drop is large and the grist is pale, pH can undershoot the target: it is safer to add half the calculated acid, stir, wait ten minutes, measure pH and add the rest as the reading dictates.

pH is measured with a meter, not calculated from the water. The acidity and buffering of the malt, the temperature, the water-to-grain ratio and even the crush shift it more than the water composition does, and no formula without a grist model captures that. The calculator gives a sensible starting point — nothing more.

ΔpH ≈ n / (B · V)
where n is the added acidity, mEq; B is the mash buffering, 8…25 mEq/(L·pH); V is the mash volume, L

What the calculator does not do

• It does not replace a pH meter: any pH estimate without a grist composition is approximate, and pH has to be measured in the mash ten to fifteen minutes after dough-in.
• It does not select acid to taste: acid lowers pH, but over-acidification is as bad as under-acidification.
• It does not account for the buffering of a particular grist: the range 8…25 mEq/(L·pH) marks the bounds of what is reasonable, not a calculation of your recipe.
• It does not account for the fact that some of the acid binds to the salts in the water and that carbonates precipitate on boiling.
• It does not recommend acid in the sparge water beyond the calculation: sparging above pH 6 leaches tannins from the malt, but its own buffering is not calculated here.
• It does not replace a water analysis: the bicarbonates in a water utility report are averaged, and reverse osmosis changes the composition over time.

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