Stone Milling

Stone milling exists to reduce the whole kernel to flour in one operation while keeping all of it in the product. Where a roller mill is asked to decide what stays and what goes, a stone mill is asked only to reduce — the composition of what comes out is fixed by the composition of what went in.

The mechanism is shear rather than the sequenced breaking and scraping of paired rolls. Grain enters near the centre of the runner stone and is worked outward across the dressed furrows and lands of the two faces. The gap narrows toward the rim, so reduction is progressive across the radius of the stone rather than across a series of separate machines. A single pass therefore does the work that a roller mill distributes over many passages, and it does it to the whole kernel at once.

A stone mill yields wholemeal flour and nothing else. This is not a shortcoming of the method but its definition: the absence of a bran stream and a germ stream is the direct consequence of grinding the kernel intact. Where a stone-milled flour is later sieved to remove the coarsest bran, that sieving is a separate finishing step and not part of the grinding itself.

Wholemeal flour

The whole kernel reduced to flour, with bran and germ distributed through it. The only product stream of the process.

Germ oil in the flour

The germ is crushed rather than removed, so its oil remains in the product. This is what shortens the keeping quality of stone-milled flour relative to a flour the germ has been taken out of.

Bran particles

Present throughout as fine particles rather than as a separated co-product. They carry the kernel’s fibre into the flour rather than into a feed stream.

Because nothing is separated, stone milling cannot change the composition of the grain it is given: the flour’s protein, ash, and colour follow the kernel. What the mill does decide is particle size, the heat the meal picks up from the working faces, and the condition of the germ oil dispersed through the product. Stone-milled flour is the whole kernel in a different physical form.

The distinctive loss point is not a fraction going out in the wrong stream — there is no other stream for it to go out in. It is keeping quality. Germ oil spread through a flour and exposed to air across a large particle surface is what limits how long the product holds, and heat from grinding bears on the same outcome.

• Shortened keeping quality from germ oil dispersed through the flour
• Heat pickup at the grinding faces, which bears on the condition of that oil
• Stone wear carrying fine mineral particles into the product, which is why dressing and stone condition are monitored
• Dust and spillage at feed and discharge

A stone mill has no residual stream to place: the whole kernel leaves as product, so there is no bran or germ to route to feed. That makes the material flow simple, but it is a difference in configuration rather than evidence that one method is environmentally preferable to the other — the two produce different products, and comparing them on energy or residue alone compares things that are not equivalent.