The kernel of wheat

The caryopsis of the wheat (grain) is structured as in the diagram we have published, independently of whether it is "durum" or "soft", or whether it is cultivated in the great flood plains of the Ukraine, or in the ex-prairies of North America.
Wheat feeds the vast majority of the inhabitants of the planet (along with rice and corn). It is certainly the most important of all the cereals. For this reason men of science have explored its caryopsis in the deepest and most intimate of details: they know every little secret and they have analyzed each and every cell.
For millers, the caryopsis is a small masterpiece of nature made up of external layers and an internal kernel.
The layers and the kernel are shown in the drawing with their proper names. From the wheat caryopsis, the mill must obtain flour (or semolina), trying to reconcile the yield from milling with the quality of the product obtained. "Yield" simply means that the more flour (or semolina) you obtain from the grain, the greater the gain from both a productive and economic standpoint. On the other hand, "quality" refers to the fact that the flour (or semolina) must be as well suited as possible for its intended use (bread, pasta, etc.).

It is obvious that the "quality" does not depend on (or does not only depend on) the "yield", but mainly on the chemical-physical and original intrinsic characteristics of the milled wheat in addition to the technology and the machines used for this purpose.
The external layers and the internal kernel of the grain have their own specific chemical and morphological characteristics. In the external layers of the caryopsis, the chemical characteristics are given by the concentration of cellulose (fibre), minerals, and protein. In the internal kernel, the presence of starches is dominant. There is a third, distinct part of the caryopsis, the "germ". It is the embryo destined to create a new plant (let's not forget that the caryopsis of the wheat is a seed …).
The wheat germ is, from a chemical and nutritional standpoint, an absolute masterpiece. Unfortunately, however, it is rich in fats that can easily go rancid. For this reason, during milling, it must be removed from each grain so that its fats are not lost in the flour (or in the semolina) making preservation precarious.
The "strategic" objective of modern wheat milling (for both soft and durum wheat) is the internal kernel of the grain, a concentration of starches that, in addition to another basic chemical component, protein, make up the nutritional (and technological) nucleus of the wheat and the products obtained from milling wheat.
The phases of wheat milling are summarized in the published diagram, based on a durum wheat mill (production of semolina). For the most part, these phases can also be considered for the milling of soft wheat and for the production of flour.

Chemical composition of soft wheat and durum wheat

The external part of the grain (pericarp and perisperm) is formed by different layers of intercrossing cellulose. The chemical composition of the grain is made up of cellulose, minerals (bran) and protein with a high biological value. However, due to the low sifting rates of the flour (and the semolina), the protein is practically lost.
The external part of the wheat caryopsis is largely made up of indigestible and irritating parts (cellulose and lignin) as well as minerals (ashes) that can interact, creating undesirable compositions during the technological production processes of both dried and fresh pasta. From a technological standpoint, the low sifting of the flour and the semolina determines a "rational" sacrifice of the biodynamic components present in the external layers of the caryopsis (the protein in the aleuronic layer). A good percentage of these components are, however, present in more highly sifted flour (and semolina).
In modern mills (high milling) the sifting rate determines the type of flour and semolina.
The relationship between the principal chemical characteristics of the flour and the sifting rate are very close, even if these characteristics always depend on the intrinsic characteristics of the grains that are milled.

That is, with a 50% sifting rate, the parts farthest from the floury kernel or the layers nearest to the cortical part of the grain will be excluded from the flour. More than for the flours subjected to a 72% or 80% sifting rate, in which the percentage of the parts closest to the external layers of the caryopsis is obviously greater.
To clarify this basic but important concept, see table 1, the cross-section of the wheat caryopsis indicating the concentration of the chemical elements that most interest the flour and the semolina intended for use in making pasta.