Nearly all mills will have large scale storage facilities on site for their grain to ensure continuity of supply for running. It is not unusual to have a month or more of storage capacity on site.
This buffer stock is stored in flat storage sheds, or silos, prior to transfer to the mill. There are many ways that wheat arrives at the mill. Each country or region will have its' own methods that overcome the logistical obstacles that present themselves.
The location of the mill itself will also present its' own challenges. The merits and techniques of road, train or direct ship deliveries, or indeed whether silos should be steel, concrete or flat storage etc. are complete topics in themselves, and as such will not be dealt with here.
Whichever method of delivery is used, there will be impurities in the grain that need to be removed prior to storage. These impurities picked up during harvest, intermediate "elevator" storage or during transport, can range from rocks and boulders, vermin, chaff, straw, other grains such as oats, barley and rye to rogue varieties of milling wheats.
At point of intake it is first essential to verify the grade of wheat and its' quality data. Typically tests will be done to determine specific weight, moisture content, protein content, enzyme activity and impurity admixture. These tests are relatively quick to do, and are used to verify the product against a purchase contract. Claims may be made at this stage against the supplier if agreed specifications have not been met and a revised price negotiated. DNA testing or "electrophoresis" testing can be used to determine specific varieties or levels of contamination by other varieties if necessary, but this is a time consuming and expensive exercise and is not normal practice.
The next step is to remove any impurities that may damage equipment or pose a safety risk to the plant. A coarse sieving and some form of dust aspiration is usually employed to render the product operationally safe. Large impurities may damage transport elements or machinery, and grain dust is highly explosive if certain concentrations are exposed to an ignition source.
When stored under the correct conditions grain can be kept for years with no adverse affects on quality. However, it is important to remember that grain is a living organism, and as such needs to breathe. As part of this process the grain takes in oxygen and gives out carbon dioxide and a small amount of heat. The amount of respiration increases with the moisture content of the grain; The accepted safe limit of moisture content for storage of grain is 15% - above this level the rate of respiration and heat generation is such that germination of the grain begins, as enzymes are activated in the grain. This not only renders the grain almost useless for breadmaking, but also creates hot spots in the grain which can lead to spontaneous combustion.