Milling
Overview
Converting the wheat grain into its constituent parts, that is, bran and endosperm involves the following processes.
a) Storing
b) Cleaning
c) Conditioning
d) Gristing
e) Milling i.e. separation of the bran from the endosperm and the sizing of the endosperm to uniform particle size.
The essential difference between flour and semolina milling is the method and degree of grinding of the endosperm. Flour is much finer whereas the semolina is distinctly granular. The wheat types used also contribute to the product. Durum wheat is a very hard ‘vitreous’ glassy type of wheat compared to ‘common’ or breadmaking types of wheat. The vitreous wheats produce a hard granular semolina which is not easily broken down.
Objectives of milling
The objective of milling is to produce the maximum quantity of products at a given level of quality. This is done by a sequence of breaking, grinding and separating operations.
The break stage of grinding involves passing the wheat through a series of fluted grinding rolls rotating at different speeds. These progressively roll the wheat open, and break it up into a bran fraction which is removed, leaving large predominantly bran – free endosperm, and a small amount of flour. These endosperm chunks along with some small bran particles and the germ pass on for further processing. By carefully grinding, sifting and purifying, the materials are separated into different streams which can be processed according to requirements.
Durum semolina milling entails the careful grinding of the wheat to minimise flour production, together with the careful control of particle size.
To aid these, we do not use smooth rollermills as in flour milling, but sizing, or fluted, rolls to control the cutting up of the intermediate materials in the mill.
Another important difference between the two types of milling is that flour is produced after purification from the sifters. In semolina milling we produce the semolina, (which is obviously coarser than flour), from the purifiers. This uses stratification as a principle separation and is the introduction of air currents, to ‘purify’ the semolinas. This means ‘floating’ off the lower density branny stocks, leaving the higher density cleaner semolinas, either for further processing, or to come out to finished product.
The finished semolinas from each purifier are then blended together to make the final products.
The finished flour or semolinas are then redressed and stored in bulk silos. They are transferred to either bulk tankers or the bag packing plant through another redressing magnet / metal detector system.
