Ask Mr. Wizard

Sorting Through Contradictory Brewing Advice


Thomas Wood — Nicasio, California asks,

Please comment on some seemingly contradictory pieces of brewing advice: 1. It is commonly advised to chill the wort as quickly as possible post-boil to avoid hot-side aeration or bacterial contamination. Yet hopstands delay chilling, and, if you stir to create a rotation, don’t you risk introducing O2? 2. I’m told that once the mash is mixed to let the grain bed settle without disturbing it to form an efficient filter bed and avoid a stuck sparge. Yet people also advise stirring the mash for batch sparges or to stir the mash halfway through the mashing period. So what happens to the filter bed?


The world of brewing is full of seemingly contradictory advice, Thomas. Thanks for asking about these two rules! 

Part One: Wort Chilling 

You are correct that conventional wisdom is to cool wort as quickly as possible after wort boiling is complete. The part of this general rule that is usually left off from casual discussion is how quick is quick enough. Like almost everything in homebrewing, this rule comes from commercial brewing.

Nearly all commercial breweries these days, even those using whole cone hops, use a whirlpool to remove trub, and for most of the breweries using pellet hops, hop material is also separated from wort in whirlpool vessels. It typically takes 10–20 minutes to transfer hot wort from the kettle to the whirlpool, wort spinning is allowed to slow over 5–20 minutes, during which time solids collect in the center of the whirlpool, and then wort cooling occurs over the next 30–60 minutes. That adds up to 45–100 minutes from the end of the boil to cooling the entire brew. A more meaningful rule for homebrewers is to target cooling wort in less than an hour after the end of the boil. This allows plenty of time for hopstands, if practiced, and cooling.

soiled immersion chiller being removed after chilling the wort
Should you chill the wort as soon as the boil is complete? The answer isn’t black and white. Photo by Marshall Schott

Regarding oxygen, when hot wort is splashed around and/or aerated, for example with a leaking pump seal that sucks air into the wort being pumped, it typically darkens in color. However, whirlpooling by gently stirring to get the wort rotating before removing the paddle or spoon does not splash the wort and is not on any list of process problems I have ever seen, including my own. For those concerned about hot-side aeration (HSA), it’s important to recall that HSA is primarily a concern during mashing and mash transfer when malt enzymes are still active.

Part Two: Mash Mixing

Mash stirring and filter bed development is a definite conundrum, even for commercial brewers using infusion mash tuns for mashing and wort separation. Let’s step back from this vessel for a moment and consider breweries with a mash mixer and a separate lauter tun. The mash mixer is used to stir hydrated mash flowing from the grist hydrator, or sometimes pumped in from a pre-mixing vessel or wet mill, during mash-in.

After mash-in is complete, the mash can be heated if step mashing is used, part of the mash can be pumped to a decoction kettle and then mixed with the rest of the mash following mash boiling for decoction processes, or the mash can be simply held at a single temperature. After the mash cycle, the contents of the mash mixer are gently mixed while the mash is pumped to the lauter tun. One very real consequence of mash mixing is improved raw material yield. And the modern combination of a mash mixer and a lauter tun is not known for poor mash filtration or slow run-off issues; in fact, this duo is known for flexibility, efficiency, and operational robustness over a wide range of brew types.

As far as the stirring question goes, there is absolutely nothing wrong with periodically mixing an infusion mash.

When mash from a mixer is pumped to a lauter tun, mash particles classify into layers based on particle density. Small, dense particles end up on the bottom of the filter bed (and usually below the false bottom), followed by husk bits, and topped with a gray-ish layer known as teig or top-dough (teig is German for dough). The difference between an infusion mash tun and a lauter tun is that the filter bed is established in an infusion mash tun soon after mash-in is complete.

In practical terms, part of an infusion mash tends to hover above the false bottom until the wort density falls sufficiently with sparging for the grains to settle upon the false bottom. This makes for easy wort collection. However, the infusion mash is not normally stirred and as such the yield is typically lower than the mash mixer + lauter tun duo. This reduced yield is exacerbated because lauter tun grist is usually finer than mash tun grist, and lauter tuns equipped with raking machines as a standard design feature can handle finer grist. This raises a practical process question: Can mash in an infusion mash tun be stirred during the mash? And should the mash be stirred?

As far as the stirring question goes, there is absolutely nothing wrong with periodically mixing an infusion mash. This is simple to do at home because our mash vessels are small. In breweries producing more than about 310 gallons or 1,200 liters per batch, stirring an infusion mash with a paddle is possible, but not so easy. For this reason, many breweries with infusion mash tuns don’t stir mashes after mash-in is complete.

However, I argue that infusion mash brewers should indeed give their mashes a few stirs during mashing, when practicable, to improve yield. Dough balls are not uncommon and are often left behind after mashing-in. These masses are easy to break apart after malt enzymes have digested biogums, proteins, and starch polymers. Stirring also helps to homogenize wort density within the mash that is a remnant of mashing-in.

Because the filter bed classifies based on particle density, stirring during the mash will not “mess up” your filter bed, provided you don’t stir right before wort collection begins. In fact, the primary reason for the vorlauf or wort recirculation step preceding wort collection is to move high-density, fine, and often starchy, particles from the bottom of the vessel to the top of the mash bed. This helps establish the filter bed as wort flow “shakes out” the bed as it begins to compress and trap these small particles.

As with any craft, there are myriad approaches that can be used to get to the finish line, and these varied paths do not always align. We all know that cold storage after packaging results in extended shelf life, and we also know that heat pasteurization after packaging extends shelf life. Two true statements with an obvious contradiction. When brewers are faced with such contradictions, the best thing to do is evaluate the pros and cons of the two, pick the best route for their situation, and move forward in a practical and deliberate manner. 

Response by Ashton Lewis.