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.
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.
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.