The finished mash ...notice how clear the liquid in the return tube has become. The grain bed acts as a filter in this process.
Starting the recirculation in an infusion mash process. The "wort" is thick and cloudy before conversions begin.
Mash time and temperature determine the amounts of fermentable and nonfermentable sugars (dextrines). First the mash goes through a protein rest at around 120degF in order to allow the amino acids that will later convert starches into sugars to develop. Next, a beta-amylase rest at 122-148degF will activate the proteins that break starch polypeptide chains down into fermentable sugars that will later be converted to alcohol. An alpha-amylase rest between 149 and 164degF will yield unfermentable dextrinous sugars that give the beer sweetness, body and other desirable characteristics. Longer rests/higher temperatures cause more conversion to take place, yielding more converted sugars. Conversion is stopped by "mashing out" at 165degF.
Infusion mashing is one of several ways to mash grain. In this process, the grain is covered by hot liquor, which flows through the grain, and then through a screen that the grain bed rests on, to the bottom of the mash tun. The hot liquor is pulled from the bottom of the mash tun and recirculated by a pump back into the top of the mash tun. (In the photo, the return flows gently through the flexible tubing, which rests on top of the grain bed.)
Wort...pronounced "wert". The malty sweet liquid that is the result of the mash process.
Malted barley (malt) can be purchased from homebrew supply stores, or online. Malt is one of the basic ingredients in beer, but must go through the mashing process in order to become the wort that makes beer.
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The grain must be milled so that the husks are cracked open, not crushed. This allows hot liquor to react with the proteins and starches in the malt, and for the husks to create a "grain bed" that acts like a filter for the wort.
The Mash Process...

from malted grain to wort.