Stuck Mash, a dreaded occurrence in breweries large and small, sh occurs when wort will not filter correctly in a mash or lauter tun and leads to a slow or negligible run-off.

Ideally, the wort produced by the mashing process drains through a filter bed of husk particles, leaving solids behind and becoming clarified. In some cases this range of digestion is incomplete and the mash fails to drain easily from the mash or lautering vessel (in British brewing systems, both the mash and the run-off are typically done in a single vessel), leaving a stuck mash. Typically the filter bed becomes clogged with a matrix of semi-digested protein and cell wall gums and cannot filter the wort. Stuck mashes can be a major problem in a brewery and lead to rejected brews and considerable brewer irritation.

A stuck mash is typically caused by poorly modified malt, malt with too high a protein level, high beta-glucans, or the addition of too much adjunct material. Adjuncts typically lack enzymes and their overuse can result in a mash enzyme level too low to achieve proper conversion. See adjuncts. Occasionally a mistake in temperature control may lead to too hot a mash, denaturing the enzymes and rendering them inactive. Overfilling a mash vessel may also cause a stuck mash by pressing down and compressing the mash bed. If malt is milled too finely, the fractured husk material may be too small to filter the wort properly, again leading to the stuck mash. A stuck mash can also occur if the brewer has attempted to run off the mash too quickly, drawing smaller particles down to the bottom of the mash and even forcing the mash down upon the plates of the vessel.

A simple solution is to stir the mash carefully in the hope that a resettlement of the filter bed will be more open and restore filtration. Underletting by pushing fresh hot water through the plates may achieve this with less disturbance. See underletting. If this does not work, brewers with mechanical rakes in their lautering vessels may stir the mash in with hot water, once again hoping for a successful resettling of the mash. As a last resort, the addition of enzymes or possibly fresh malt with high enzyme activity may be necessary.

For many brewers, the phrase “stuck mash” conjures up images of late nights in the brewhouse, dinners gone cold, and attempts at Zen-like calm in the face of great frustration.

See also mash and mashing.