Water & Steam

Mash Out

Raising the mash to a higher temperature at the end to stop enzyme action and ease lautering.

Mash out is the brewing step where the grain mash is raised to around 76C at the end of conversion to halt enzyme activity and lock in the sugar profile. The higher temperature also thins the sugary liquid so it drains and rinses more freely during lautering. It is an optional but useful step for efficient extraction in all-grain brewing.

Mash-out is the deliberate final temperature raise of an all-grain mash, typically from the saccharification rest (148–158 °F / 64–70 °C) up to around 170 °F / 77 °C, held briefly before lautering begins. The name suggests the goal is to 'stop' enzymatic conversion, and it does sharply curtail β-amylase activity, but the more practical effect — and the reason most brewers perform it — is mechanical. A mash held near 170 °F thins out noticeably as the wort itself becomes more fluid at higher temperature and some proteins denature, making the grain bed easier to drain and the runoff clearer.

The technique is widely taught in homebrewing literature but quietly skipped in a lot of professional practice. Macro brewers and many seasoned homebrewers argue that with well-modified malts, a properly thin mash thickness, and a well-designed false bottom or manifold, the viscosity reduction is barely perceptible — and the extra 10–20 minutes of brew day is not always worth it. The case for mash-out is strongest when grain bills are heavy (high-gravity beers, large percentages of adjuncts like flaked oats or wheat), when lauter tun design is marginal, or when the brewer wants to lock in fermentability before stepping away from the tun.

There is also a tannin caution that often goes unmentioned: above roughly 170 °F, polyphenols in the grain husks become more extractable, and at elevated mash pH (above ~5.8) the resulting wort can take on an astringent, tea-like harshness. Mash-out is best done quickly, with the pH already dialed into the 5.2–5.6 range, and the temperature not pushed past 172 °F.

Difficulty: Medium

Types & varieties

Infusion mash-out

The most common homebrewer method: stir in a calculated volume of near-boiling water to step the mash up to ~170 °F.

Direct-heat mash-out

Applied on stovetop or burner-equipped mash tuns; the tun is heated gradually while stirring constantly to avoid scorching the grain bed.

HERMS (Heat Exchange Recirculating Mash System)

Wort is pumped from the mash tun through a coil immersed in the hot liquor tank and back, with the HLT transferring heat into the recirculating wort to ramp the mash.

RIMS (Recirculating Infusion Mash System)

Mash is pumped through an external electric heating element controlled by a PID, allowing precise ramp-and-hold temperature control.

Decoction mash-out

A portion of the thick mash is pulled out, heated (often to a boil), and returned to raise the main mash — labor-intensive and largely of historical interest for this step specifically.

How to do it

1
Confirm conversion is complete
Before raising temperature, verify with an iodine test (no blue-black color) or check the refractometer reading against expected gravity. Mash-out locks in the sugar profile, so conversion must be finished first.
2
Calculate and prepare the infusion
Determine the volume of boiling or near-boiling water needed to step the mash from its current rest temperature up to ~170 °F / 77 °C. Pre-boil or pre-heat this water so it is ready at hand before you begin adding.
3
Raise the mash temperature
Slowly add the hot water to the mash tun while stirring continuously with a mash paddle or large spoon. For direct-heat setups, raise the burner in small increments with constant stirring. For HERMS or RIMS systems, set the controller to ramp to 170 °F.
4
Hold for 10 minutes
Once the entire mash reads 168–172 °F throughout, hold for about 10 minutes to allow the viscosity to drop and enzymes to deactivate. Do not exceed 172 °F.
5
Begin vorlauf and lautering
After the hold, re-circulate the wort from the lauter outlet back into the tun until it runs clear, then begin sparging and collecting runnings as normal.

When mash-out earns its place

The clearest case for mash-out is mechanical: when the mash is physically hard to lauter. Brewers working with high percentages of wheat, oats, rye, or flaked adjuncts often find that the beta-glucans and proteins from those grains make the mash thick and slow to drain. A mash-out noticeably thins this bed and reduces the risk of a stuck lauter. The same is true for very high-gravity mashes where grain-to-water ratios are pushed beyond roughly 1.3 qt/lb. Conversely, brewers using well-modified two-row or pilsner malts at conventional thicknesses with a well-designed false bottom or manifold frequently report that skipping mash-out makes no practical difference to their runoff time or clarity.

Heavy adjunct bills (wheat, oats, rye, flaked grains): beta-glucans and proteins raise viscosity — mash-out helps.
High-gravity mashes (grain-to-water under ~1.3 qt/lb): physical thickness is the problem, not enzyme activity.
Marginal lauter tun design (small false-bottom slots, shallow bed depth): viscosity reduction pays off.
Well-modified malt, thin mash, good manifold: mash-out is largely cosmetic — skip it and save fifteen minutes.

The enzyme question, honestly

The 'stop enzyme action' framing is partly true and partly misleading. β-amylase, the enzyme that produces maltose and is most responsible for fermentability, denatures rapidly in the 158–163 °F / 70–73 °C range — well below typical mash-out temperatures. α-amylase, which chops long starch chains into shorter ones, is more heat-tolerant and survives up to roughly 176 °F / 80 °C, so it can continue limited activity in the kettle until the boil begins. So when brewers say mash-out 'locks in' the sugar profile, what they really mean is that β-amylase is dead by the time the mash reaches 170 °F, the longer-chain sugars are settled, and a stable gravity is what comes out of the lauter. The real and undisputed benefit is mechanical: thinner mash, faster runoff, cleaner wort.

β-amylase: dies around 158–163 °F / 70–73 °C; mash-out is overkill for stopping it.
α-amylase: survives to ~176 °F / 80 °C; can still work briefly in the kettle before boiling.
Practical effect of mash-out on fermentability: small at best; gravity is essentially set by the prior saccharification rest.
Practical effect on lautering: large; this is the real reason to do it.

Calculating the infusion

For a typical infusion mash-out, the rule of thumb is to add roughly 10–20 % of total mash water as boiling (or near-boiling) water. The exact figure depends on the starting mash temperature, the temperature of the infusion water, the mass of grain in the tun, and the thermal mass of the equipment. Brewing software (BeerSmith, Brewfather, Grainfather, Promash) handles this precisely, but the underlying equation is a simple weighted average: account for the heat capacity of the grain (≈0.4 cal/g·°C), the water (1.0 cal/g·°C), and the tun. A practical shortcut: heating about 0.5 qt of water per pound of grain to a rolling boil gets most mashes into the 168–170 °F window without much fuss.

Pre-boil or pre-heat the infusion water; adding water that has cooled in transit is the most common reason mash-outs fall short.
Add slowly and stir constantly — a slug of boiling water into a thick mash creates local hot spots that gelatinize starch and clump the bed.
Account for the thermal mass of the mash tun: a stainless steel insulated cooler loses and gains heat very differently from a thin aluminum pot.
Re-measure mash temperature at multiple points (top, middle, bottom) before declaring the mash-out complete.

Common uses

Lock in a desired fermentability profile at the end of conversion, preventing over-attenuation from a runaway temperature.Thin a thick, porridge-like mash so lautering and sparging flow faster.Improve wort clarity by helping the grain bed settle and filter cleanly.Recover stuck or sluggish lautering by lowering viscosity before runoff.Stabilize the mash before a long or interrupted vorlauf and lauter.Buffer against late enzymatic activity in high-adjunct mashes that retain α-amylase heat tolerance.

Tips & pitfalls

Use brewing software (BeerSmith, Brewfather, etc.) to calculate the exact volume of boiling water needed to hit 170 °F — eyeballed additions routinely miss the target by 5–10 degrees.
Stir the mash vigorously and continuously while adding hot water to avoid dough balls and temperature stratification; a mash paddle or long stainless spoon works well.
Keep the mash pH in the 5.2–5.6 range before ramping up. Higher pH combined with high temperature extracts harsh, tea-like tannins from the husks.
Don't overshoot 172 °F / 78 °C. The starches begin to swell and set, which can actually clog the grain bed and undo the viscosity benefit you are after.
Pre-boil or pre-heat the infusion water so it does not drop your mash temperature below target mid-addition — this is the most common cause of a failed mash-out.
If using direct heat, use a low flame and constant stirring. Scorched grain on the tun bottom is a common source of acrid off-flavors in the finished beer.
Skip the step if your mash is already thin, conversion is complete, and you have a stable grain bed — the time cost is not always repaid.
Always re-circulate (vorlauf) after mash-out. The bed has been disturbed and the runoff will be cloudy for the first quart or two before it clears.

Good to know

Target temperature: 168–172 °F / 76–78 °C; 170 °F / 77 °C is the canonical target
Hold time: 10 minutes typical; 5–15 minutes is the commonly cited range
Position in brew day: After the primary saccharification rest, immediately before lautering
Primary purpose: Reduce mash viscosity for easier runoff; halting enzymatic conversion is secondary
Enzyme behavior at target: β-amylase denatures around 158–163 °F (70–73 °C); α-amylase denatures around 168–176 °F (76–80 °C) — mash-out largely kills β-amylase and slows but does not stop α-amylase
Tannin threshold: Mash pH above ~5.8 combined with temperatures above ~170 °F (77 °C) noticeably increases tannin/polyphenol extraction from grain husks
Infusion water volume: Roughly 10–20 % of total mash water, depending on starting mash thickness and target temperature
Skipped by: Most industrial brewers and many experienced homebrewers with thin mashes and well-designed lauter tuns