There are two kinds of malting barley: six-rowed winter barley and two-rowed summer barley. In Belgium, summer barley is used almost exclusively, as its growing period is a hundred and fifty days and it is ready for harvesting in July or August. 

Barley malt is to beer as grapes are to wine. It is ideally suited to brewing for many reasons. Malted barley has a high complement of enzymes for converting its starch supply into simple sugars and contains protein, which is needed for yeast nutrition. Of course, one important element is its flavor. There are two types of barley: six-row and two-row. 

Six-row Barley Malt 

Generally, six-row barley has a higher enzyme content, more protein, less starch, and a thicker husk than two-row barley. The higher level of diastatic enzymes makes six-row barley desirable for conversion of adjunct starches (those that lack enzymes) during mashing. On the down side, the higher protein content can result in greater break material (hot and cold), as well as possibly increased problems with haze in the finished beer. The husk is high in polyphenols (tannins) that results not only haze, but also imparts an astringent taste. 

Two-row Barley Malt 

Generally, two-row barley has a lower enzyme content, less protein, more starch, and a thinner husk than six-row barley. Of the first two of these characteristics, the protein content of two-row barley depends greatly on the barley strain, and enzyme content depends very much on the strain and degree of kilning. American two-row barley has greater enzyme potential than most European two-row barley. The protein content of U.S. two-row barley is comparable to that of continental Europe, while barley grown in the U.K. is generally lower in protein. In comparison to six-row barley, two-row has a higher starch content – the principal contributor to extract. The thinner husk associated with two-row barley makes for mellower (less astringent) beers due to lower levels of polyphenols. 

Generally, six-row barley has a higher enzyme content, more protein, less starch, and a thicker husk than two-row barley. The higher level of diastatic enzymes makes six-row barley desirable for conversion of adjunct starches (those that lack enzymes) during mashing. On the down side, the higher protein content can result in greater break material (hot and cold), as well as possibly increased problems with haze in the finished beer. The husk is high in polyphenols (tannins) that results not only haze, but also imparts an astringent taste. 

Barley Malt Identification 

The number of rows of kernels makes for easy identification of two- and six-row varieties. In six-row varieties, two-thirds of the kernels are twisted in appearance because of insufficient space for symmetrical development. Since they must overlap, they twist as they grow. In two-row barley there are no lateral kernels; all kernels are straight and symmetrical. The kernels of two-row barley are broader than the central kernels of six-row barley and do not taper as sharply. 

From The Brewers’ Handbook