Hammer Milling

Hammer milling exists to reduce grain to a specified particle size, cheaply and without regard to which part of the kernel a particle came from. In feed manufacture, particle size governs how well an animal can digest the ration and how the meal behaves in later mixing and pelleting; in food use, it decides the texture of the meal. In both cases the requirement is a size, not a composition.

The principle is impact fracture. Hammers moving at speed transfer energy to a kernel faster than it can deform, and it shatters. Fragments continue to be struck and to strike one another until they pass the screen, which means the mill is as much a classifier as a grinder — material is retained by size and released by size. Grain that resists fracture simply stays in the chamber longer.

One stream leaves a hammer mill: meal, carrying the whole kernel. Where the grain is maize, that meal is cornmeal — the whole-grain, non-degermed kind, which is a different product from the degermed cornmeal a dry-milling plant produces even though both carry the same name. Where the grain is barley or sorghum, the meal is generally destined for feed or for local food use and is not traded as a distinct product entity in this reference.

Whole-grain meal

The kernel reduced to a specified particle size with bran, germ, and endosperm all present. The single product stream of the process.

Germ retained

The germ is shattered into the meal rather than recovered. Its oil stays in the product, which is why hammer-milled whole meals do not keep as long as degermed ones.

No by-product stream

Nothing is set aside, so the mill generates no bran or germ fraction to place in another market. The whole kernel is the product.

The mill controls particle size and its distribution, and little else. Composition arrives with the grain and leaves unchanged. Screen aperture, hammer condition, and tip speed together set how fine the meal is and how wide the spread of sizes around it — and the spread matters, because a meal ground fine enough to satisfy one requirement almost always carries fines that create dust and handling problems downstream.

Losses here are not fractions leaving in the wrong stream, since there is only one stream. They are dust, heat, and the keeping quality of a product that still contains its germ oil.

• Dust generation, which is both a material loss and the mill’s headline hazard
• Heat pickup during grinding, which bears on the condition of germ oil in the meal
• Worn hammers and damaged screens widening the particle-size distribution
• Shortened keeping quality of whole meal relative to a degermed product
• Spillage at feed and discharge

Hammer milling is energy-demanding, and its energy demand rises steeply as the target particle size falls — finer meal costs disproportionately more to make than a coarse one. It produces no residual stream to place, because the whole kernel is retained. Dust extraction is simultaneously an air-quality measure, a product-recovery measure, and a safety measure.