How the flour mills work?

Delivery and storage of grain
Each load of wheat is tested against a contract specification for variety, moisture content, specific weight, impurities, enzyme activity associated with sprouting, protein content and quality. It passes through a preliminary cleaning process to remove coarse impurities, such as nails and stones, and may be dried before being stored in silos according to quality.

Assessing the wheat quality
The ‘Hagberg Falling Number’ measures the time, in seconds, a plunger takes to descend through a heated mixture of ground grain and water. The test indicates the alpha-amylase activity in the grain. This natural enzyme converts starch to smaller sugar units that would be used by the seed to fuel its growth.

If there is little enzyme activity, the mixture will remain viscous. The plunger will take a long time to descend and a high Hagberg Falling Number will be recorded. Too much enzyme activity and the reverse will be true. High enzyme activity impairs bread quality, producing a very weak and sticky crumb mixture.

Cleaning and conditioning
Cleaning and conditioningWhen wheat is drawn from the silos prior to milling, it is thoroughly cleaned. Powerful magnets extract any remaining ferrous metal objects.

Machines, which separate by shape, remove barley, oats and small seeds. Gravity separation removes stones and, throughout the cleaning process, air currents lift off dust and chaff.

The wheat is then conditioned to a suitable moisture content by tempering it with water and leaving it in conditioning bins for up to 24 hours. This conditioning softens the bran and enhances the release of the inner white endosperm during milling.

Cleaned and conditioned wheat is then blended in a process known as gristing. This combines different wheats to produce a mix capable of yielding the required quality of flour.
flour mill production line
The grist is passed through a series of ‘break’ rolls rotating at different speeds. These rolls do not crush the wheat but split it open, separating the white, inner portion from the outer skins.

The various fragments of wheat grain are separated by a complex arrangement of sieves. White endosperm particles, known as semolina, are channelled into a series of smooth ‘reduction’ rolls for final milling into white flour.

Bran and wheatgerm are streamed into this flour to make brown or wholemeal flour. Wholemeal flour contains all the parts of the grain (endosperm, germ and bran); brown flour will contain less bran and may or may not include wheat germ.

Baking powder (raising agent) will be added to make self-raising flour at this stage. The nutrients calcium, iron and the B vitamins (niacin and thiamin), which are legally required in all white and brown flours, are also added. (Wholemeal flour already contains these nutrients, although it is lower in calcium.)

The whitest flours are produced from the early reduction rolls, with the flour getting less white on later rolls as the proportion of bran particles increases. Brown flour is a mixture of white flour and a portion of the other streams. To produce wholemeal flour, all the streams must be blended back together in their original proportions.

White flour produced in the UK and elsewhere in Europe is not bleached. This was sometimes done in the past but the process was phased out in the early 1990s, though it does still take place in other parts of the world, for example in North America.

In a typical mill, there may be up to four break rolls and 12 reduction rolls, leading to 16 flour streams, a bran stream, a germ stream and a bran/flour/germ wheat feed stream.

Finally, the flour is sifted before being automatically packed into bags ready for delivery to shops or supermarkets.

Bran and wheat feed left over from producing white and brown flours is sometimes used in breakfast cereals or animal feed

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