When the malt is dried it will be milled to a coarse mix of flour and husks which is called grist. That grist is filled into a big mash tun where it is mixed with hot water to dissolve the sugars.
The mashing process consists of three runs.
First run: The grist is mixed with 60-65°C hot water to dissolve the easier soluble sugars. The sweet water or sparge is drained and filled in tank no. 1.
Second run: The remaining grist in the mash tun is now mixed with hotter waters, 70-75°C, to dissolve the longer chain sugars that couldn’t be extracted in the first run. The resulting sparge from this run is also drained into tank no. 1.
Third run: The last run is essentially squeezing the last bits of sugar out of the grist so nothing will be left behind or wasted. The leftover grist is sprayed with 90-95°C hot water and the sparge is filled into tank no. 2. It will be used as the first water for the next mashing run.
The sugary, brown water inside tank no. 1 is now called wort and is ready for fermentation. The leftover residues, or draff, are taken out of the mash tun and sold to local farmers as cattle feed. Again: Nothing is going to waste.
How does it work?
When you look at the process itself it seems straight forward and relatively simple. Water dissolves the sugars, that’s it. The chemistry behind it is far more complex and interesting.
Starch is present in two forms in malted barley grist: amylose, a straight, long chained, straight molecule, and amylopectin, long but with branches spreading out. So being a multi-chain sugar, those long chains have to be split into smaller units, sugars that can be used for fermentation.
When starch is exposed to 64°C hot water it activates two amylase enzymes that “cut” the long chains into smaller units. The most important ones for distillers are glucose, fructose, sucrose, maltose and maltotriose. These sugars are made from either one sugar unit (glucose, fructose), two units (sucrose , maltose) or three (maltotriose).
And to form those fermentable sugars from the long, complex chains of sugar, water serves as a catalyst. So aside from dissolving and soaking up the sugars, water helps cutting starch down into fermentable sugars and makes them accessible for alcohol production.