Slack Malt is finished malt that has been allowed to pick up moisture during storage.
At the end of the malting process, the finished malt is dried in a kiln, achieving average moisture content, by weight, of approximately 3%–5%. Malt quality stability during storage is critically dependent on its moisture content. Over time, moisture levels above normal specifications can cause two principal problems: a decline of enzymatic power and the toughening of the grain husk. The enzyme degradation erodes the malt’s ability to effect conversion of starch into sugars during the mash. The grain hull, having gone from brittle to leathery, doesn’t break properly when milled, and this leads to various problems during lautering. For both of these reasons, brewhouse extract will be compromised and subsequent processing may be problematic.
Malt is hygroscopic, meaning that it tends to soak up moisture from its surroundings. It must be protected from any opportunity to pick up moisture after it leaves the kiln—which it will do from air under normal ambient conditions. Before the industrialization of the brewing industry in the 18th century, both malting and brewing were largely cottage industries. In those days, brewers and maltsters could protect a heap of malt by storing it under a layer of dry hay that would absorb moisture from ambient damp air. Modern methods involve tight seals—sacks for malt should be lined with plastic, international shipping containers similarly should have the malt contained in a plastic liner, and silos and conveyors need to be weather-tight. Trucks for road transport will often have roll tops, which need to be inspected before each filling to ensure their integrity.
In storage, malt should be kept separately from any raw barley being stored, as raw barley, having a moisture content of 13%–15%, will exude its moisture into the surrounding air, and the malt will absorb it, resulting in slack malt.