Acrospire is the sprout of a grain seed, the beginning of a new plant. In the field, after the snow has melted and the moist soil is being warmed by the spring sun, the acrospire grows in a spiral from one end of the seed—hence its name of acro“spire”—while rootlets develop at the other end. As the acrospire grows, enzymes in the grain become active, too, which breaks down nutrients so that they can be readily absorbed by the emerging plant. This process is called “modification” and involves the alteration of the molecular structure of the kernel material, which consists mostly of complex proteins, carbohydrates (starches), glucans (cellulose), and lipids (fats). See modification. In the malt house, this process is imitated through the steeping of the grain at the beginning of the malting process and its subsequent germination in an air-, moisture-, and temperature-controlled box. The size of the acrospire, therefore, is a good visual indicator of the progress of modification.

If germination continued, all the nutrients the brewer tries to preserve for beer-making would be used up by the plant for its own development. The maltster, therefore, interrupts germination when the acrospire is about 75%–100% of the length of the kernel. At this point, there is just the right balance between the resources converted by the enzymes and the resources consumed by the acrospire. The grain is then kiln-dried, and the dead acrospire and rootlets are knocked off through mechanical agitation.