The wheat silo is used to store the grain. It is a tall building containing a number of round concrete bins about 20 metres high, each holding about 250 tonnes of wheat.
When the grain arrives by road or ship, it is weighed and filled into the silos by mechanical elevators.
If the grain contains over 15-16% moisture, It is necessary to dry it, otherwise it can become hot and would be likely to deteriorate rapidly. Damp wheat is dangerous to store.
Wheat passes from the silo to a screen room where any impurities are removed from it. Here it passes through:
A coarse sieve which removes large particles, followed by a fine sieve which allows smaller impurities such as soil and dust to pass through.
This is a sloping vibrating table which removes stones.
Which separate other cereals, for example barley from wheat.
Modern mills use a scourer to clean the grain. This gives a better result than washing as was done in the past.
Remove any metal objects.
Moisture may need to be added to the grain, depending on the moisture content of the wheat. Stored wheat is too dry for milling and needs to be conditioned by adding water in order to make it suitable for the rollers.
Different types of wheat are combined to give the required mixture (known as grist) for milling.
The grain passes through a series of rollers and sieves during the milling process:
These are ridged rollers which revolve at high speed in opposite directions. They are carefully set, so that as the wheat passes between them, they do not crush but peel it open. In this way the floury inner endosperm is released without the outer bran layers breaking into tiny fragments.
The opened grain passes through a series of rotating sieves. There are about 10 to 12, placed one below the other. Starting on top the coarsest mesh removes the bran skins, working down to the finest nylon sieves which separate the flour. At each stage of sieving, the rough material is removed, to be passed again through rollers to further break it down.
This separates the lighter particles of the grian, such as the bran, from the heavier particles of the endosperm. This is done by passing a current of air up through the sieve so that the lighted bran ‘floats’ on top of the heavier white parts, which are shaken through the sieve.
The grains of endosperm at this stage are still quite large and need to be ground down further. This is done by passing them through a series of smooth steel ‘reducing rollers’ until a fine flour is produced.
The flour and bran are collected, each in their own channel, from a number of different machines, and finally brought either to bulk storage bins or to packing stations. From there the flour is filled into 1kg or 2kg paper bags for household use or into sacks for small bakeries etc. Flour is also dispatched to large bakeries by bulk tanker.
From the time the grain enters the mill until it is distributed to the shop or bakery, it is untouched by hand. It passes automatically from one stage to the next, carefully controlled by skilled workers. Samples of the product are tested in a laboratory at various stages to make sure the flour maintains a high standard of quality.
Milling can be regulated to produce flour of any extraction rate. The extraction rate means the percentage of the whole grain that remains in the flour after the milling process.
Wholemeal flour has a 100% extraction rate.
Wheatmeal or brown flours usually contain 80-90% of the grain.
Wheat feeds consist of the outer skin, from which the floury endosperm has been largely removed. It is used as an ingredient in animal feeds.