Mash pH: measuring it right and bringing it to 5.2–5.6

Why mash pH matters, when to measure, why the sample must be cooled, and how to adjust with lactic, phosphoric or acidulated malt.

25 Sept 2026 · 7 min read

Optimal mash pH is 5.2–5.6 (measured at room temperature, 20–25°C); most recipes aim for 5.3–5.4 — Castle Malting uses 5.3 in every sample recipe. In this window enzymes work best, husks release less astringent tannin, wort runs clearer and the finished beer tastes crisp and clean.

Measuring pH correctly

Wait 10–15 minutes after mashing in so the malt–water buffer settles — earlier readings are unreliable.
Draw a small sample and cool it to 20–25°C before measuring. At 65°C the reading is about 0.3 units lower than cold, and the probe ages quickly.
Calibrate with pH 4.01 and 7.00 buffers before each brew day; store the probe in KCl solution, never dry.

How to lower pH

Method	Rough dose	Notes
Axit lactic 88% · Lactic acid 88%	≈ 0,3–0,5 mL/kg malt cho mỗi −0,1 pH	Rẻ, dễ mua; dùng nhiều có thể thấy vị chua sữa · May taste tart in large doses
Axit phosphoric 75% · Phosphoric acid 75%	≈ 0,3–0,5 mL/kg malt cho mỗi −0,1 pH	Gần như không vị — hợp lager, pilsner · Flavour-neutral
Malt axit (acidulated) · Acidulated malt	1% khối lượng malt ≈ −0,1 pH	Trên ~3% bắt đầu cảm nhận được vị chua nhẹ · Tart above ~3%
Muối canxi (gypsum, CaCl₂) · Calcium salts	Hạ nhẹ, khoảng 0,05–0,1	Ca²⁺ phản ứng với phosphate trong malt · Ca²⁺ reacts with malt phosphate

The acid doses above are only starting points — the real amount depends on your water's alkalinity and the share of coloured malts. Estimate first with software (Brewfather, Bru'n Water), then add small amounts, stir, wait 5 minutes and re-measure until you hit the target.

When pH is too low

Roast-heavy mashes (stout, porter) with RO water can fall below 5.2 and taste harsh and sour. Add baking soda (NaHCO₃) or chalk (CaCO₃) a little at a time, or add the roasted malt late in the mash instead of at the start.