The largest tissue of the barley grain, the endosperm is usually referred to as the “starchy endosperm.” Unlike the embryo and the aleurone layer, it is non-living tissue and acts as a food storage organ for the seed embryo. About 60%–65% of the weight of barley grains is starch and this starch is in the endosperm. In brewing, the mashing process will break down these starches into wort sugars. Starch is composed of large (25 µm) and small (5 µm) starch granules. The large starch granules account for 90% of the total weight of the starch. About 10%–12% of the grain is protein and most of this protein is located in the endosperm as the storage proteins hordein (35%), glutelin (35%), albumin (10%), and globulin (20%). In the starchy endosperm the starch granules are embedded in this storage protein. This association of starch granules and protein in the starchy endosperm helps to give the endosperm its structure. For example, different degrees of compaction of starch granules and proteins can cause the starchy endosperm to be mealy (opaque) or steely (glassy). The starch granules and storage proteins of the endosperm are located in cells of different shapes. The cell walls of the endosperm cells must be broken by enzymes during malting to expose starch and protein to enzymic attack so that wort sugars can be realized. The walls of the cells of the starchy endosperm of barley contain about 70% beta- glucan, 25% pentosan, and 5% protein. Therefore, beta-glucanase enzymes play a significant role in endosperm modification and extract release during malting. The endosperms of other cereals have differing cellular structures, depending on the plant type. The higher the protein content of the starchy endosperm, the lower the corresponding content of starch and eventual brewing sugars.

See also barley, malting.