Semolina Production

Durum is the hardest of the wheats, and its endosperm fractures into angular, granular pieces rather than crumbling to powder. Semolina milling exists to exploit that behaviour: to liberate those granules from the bran with as little further reduction as possible, and to deliver them clean. The requirement is simultaneously a particle size and a purity, and the two pull against each other — pressing harder to free more endosperm from the bran also drives more of it below semolina size.

Purification is what resolves the tension, and it is the step that gives the process its name in practice. Sieving alone classifies by size, and a bran-bearing particle can be the same size as a free endosperm granule. But it is not the same density, because bran is lighter than endosperm. Passing the stock across an inclined sieve while drawing air up through it lets the two properties act at once: the air lifts and carries the bran-bearing particles while the denser free endosperm stays on the deck and travels through. Size sorts the stock into fractions; density sorts each fraction into product and not-product.

The mill yields semolina as its intended product and durum flour alongside it. Durum flour is a genuine co-product with its own market rather than a failure of the process: some reduction below semolina size is unavoidable when the endosperm is being worked free of the bran, and the flour that results is sold in its own right. Where a mill sits on the trade-off between the two streams is a commercial decision, and moving it moves both streams at once.

Semolina

Purified coarse endosperm. The primary product, specified by particle size, protein, ash, and the count of visible bran specks.

Durum flour

Endosperm reduced below semolina size, together with purifier fractions not meeting the semolina specification. A co-product with established food uses.

Bran and offal streams

The separated outer layers and the material carrying them. Routed to feed and food-ingredient markets in the same way as the bran streams of any wheat mill.

The two faults that matter in semolina are visible and compositional. Bran specks are the visible one: fragments the purifier failed to lift, which show as dark points in a pale product and carry through into anything made from it. The compositional one is the yellow pigment that gives semolina its colour, which arrives with the grain and can be lost during milling but never added by it. As always, the mill cannot improve on the wheat it is given — durum’s protein, vitreousness, and pigment are set in the field.

The characteristic loss is subtler than in a flour mill. It is not only endosperm leaving on the bran, but endosperm leaving as the wrong product: every granule reduced past semolina size has moved from the primary stream to the co-product stream. That is a downgrade rather than a disappearance, and it is why over-reduction is the fault a durum mill works hardest to avoid.

• Bran specks carried into semolina where purification is incomplete
• Endosperm reduced below semolina size, moving value from the primary stream to the co-product stream
• Endosperm retained on bran and leaving with the offal streams
• Pigment loss during milling, which colour specifications are set against
• Dust and spillage across transfers

A durum mill is energy-demanding and moves a great deal of air, since purification depends on air current and the mill is aspirated throughout. Its residual streams are almost entirely utilised: bran and offal go to animal feed, and the flour stream is food. In material terms the mill is close to fully valorised, and the air it moves is both a process requirement and the basis of its dust control.