The main purpose of the milling process is to produce a consistent, quality product which meets customer requirements and is free from harmful contaminants or foreign material.  Small amounts of foreign material, such as seeds of cereals other than the desired species, straw, stones or small pieces of metal, may be present in wheat delivered to mills. 

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Modern UK flour mills include a host of machinery that cleans grain before milling into flour. These machines remove anything other than wheat grains by separating material on the basis of size, weight, colour and magnetism. By subjecting grain to a barrage of these cleaning steps, millers ensure the wheat is free from physical contaminants when it is milled into flour.