Q: My latest home brewing session resulted in a stuck mash. I had 15 lb of grist in a Thermos picnic cooler with a false bottom. I had to scoop out most of the grist to get the flow going. I’m sure the batch will be badly oxidized. What can I do to reduce the likelihood of a stuck mash? How can I “unstick” the mash without oxidizing the wort?

A: The first thing to try with a set mash is cutting the bed. By cutting a crosshatch pattern with a paddle or spatula, you can break up the top part of the packed grain bed, leaving only the bottom few inches to act as a filter. If the bed is packed tight, that is all the filtration you need. Be aware, though, that you cannot cut the bed all the way to the bottom; if you do, the wort will run cloudy. Also be aware that a set mash will usually try to reset itself, so cutting the bed may have to be repeated regularly during the runoff and sparge.

For a really bad case — one where cutting has little or no effect — the usual remedy is to stir the mash thoroughly to break up the entire packed-down filter bed. Add some hot water if necessary to make the mash loose enough to stir. Then let the mash sit for at least 15 minutes before starting over again with your recirculation. While you wait, try to figure out what caused the set mash. Maybe you just ran the wort out too fast. Maybe your malt is crushed too fine. Or maybe you used a problematic grain in your grist; both rolled oats (not brewer’s flaked oats) and malted rye have given me trouble. In any case, your best hope for avoiding a repetition is to run the wort off slowly to avoid packing the filter bed too tight.

One equipment problem that often contributes to set mashes is having an unsuitable runoff valve on the lauter tun. Any valve can get blocked with grits and husk pieces, but some types are more prone to this malady and are harder to flush out than others. With a butterfly valve, a quick flick open then shut again is all it takes to clear the blockage. The momentary surge does not pack the grain bed appreciably. Ball valves are also easy to clear. Try one of those valve types if you have trouble getting a steady, slow runoff.