Grain size is determined by the amount of starch deposited in the grain. High screenings mean smaller grain and less starch. Less starch is associated with lower quality. Grain size is also important in how well the grain physically processes in the malt house. A low variation in grain size improves the processing of the grain in the malt house.
Small grains are easier to wet up than large grains.
Small grains also modify more completely than larger grains. Batches containing a mixture of large and small grain have uneven overall quality because of differences in germination and modification between small and large grains. Grain colour is the most subjective of the quality specifications. In many cases grain discolouration does not affect quality, but buyers prefer to purchase bright, clean grain from an aesthetics view. If they have little choice but to buy stained grain, maltsters often will do so at a reduced price.