Milling wheat in NZ
Wheat arrives at a mill by truck after either being shipped or hauled from its point of origin. Once at the mill the incoming wheat is subjected to a series of quality control tests, then the wheat is unloaded and stored in silos. The diagram shows a simplified view of the milling process.
The Cleaning of Wheat
Before wheat can be milled the impurities that were gathered up with the wheat during harvesting must be removed.
Different mills use various makes of machinery to remove the small stones, husks, weed seeds, etc gathered with the wheat during harvesting but they all use the differences in size, weight, shape and density to isolate and remove impurities.
The Conditioning of Wheat
Water is added to the wheat in small amounts to ensure easy separation of the bran (outer coating) from the endosperm (inner part of the wheat). The water helps to toughen the outer bran layers and softens the inner portion. This makes the soft inner portion easier to remove.
Rolls and Sifters
The whole milling process can be seen as a repetition of two processes – grinding and sifting. The wheat is first passed over a series of fluted break rolls. A pair of break rolls do not turn at the same speed, the higher roll usually turns about twice the speed of the lower roll.
Wheat travelling between the break rolls is ripped apart and the white endosperm material is released. After passing through each set of break rolls the particles are sorted on a sifting machine. The flour is removed and the coarse branny material is returned to the break rolls in order to separate out any flour still attached to the bran.
Semolina, which is chunks of endosperm, is also produced in the break system and this Semolina is passed onto a new series of rolls called reduction rolls. Eventually all the wheat going through the break system is removed as either flour, semolina or coarse bran.
The reduction rolls are a series of smooth rolls which grind the semolina particles down into three products, flour, fine bran and wheatgerm. Each of these products can then be separated by repeated grinding and sifting.
The flour obtained by the various rolls and sifters is of differing quality depending upon when it was removed from the system. Mills can blend flours from the various parts of the system to obtain a flour suitable for selling. Finally the mill ends up with wheatgerm and pollard, which is fine bran and flour.
Mills can then either bag these products or send them off via bulk supplies. Mills can also add value to their product by making flour into self-raising flours, pastry flours and premixes. All these are produced by using flours from different parts of the system and in some cases adding additional ingredients.