2 October 2019
The article describes how a mill works, in particular the stone grinding. We have already explained the grain selection and the grain cleaning (you can read the dedicated article here) and illustrated how the cylinder grinding takes place (you can read the dedicated article here).
The history of grinding
From the earliest times the ancient farmers crushed the grains by means of mortars and then between two stones. Until, in the year 1000 b.C. approximately, it succeeded in giving the millstones a rational shape.
The use of hydraulic force, wind and steam propulsion represented the revolution of this art. At the end of the 18th century the millstones were powered by “batteries” by hydraulic turbines or steam engines. Subsequently also the auxiliary machines were “motorized”, so the dampeners, the elevators and the plansichter appeared.
The refinement of the plansichter revealed the limit of the stone grinding. That is to treat all the grain fractions in the same way, regardless of the different resistance to grinding. So a few years later, roll mills began to take hold.
The functioning of the stone grinding
Stone grinding technology is based on the knowledge from the late 18th century, shortly before the evolution of cylindrical grinding.
A millstone consists of two horizontal wheels in relative motion one on the other. One of the two wheels is fixed, the other is mobile.
The grain is loaded by means of a hopper to be milled. The grain passes into the space that remains between the two grinding wheels. To facilitate the entry of grain between the two grinding wheels, these have grooves. They also reduce the working surface, increase the air passage and reduce overheating of the product.
The millstone exerts a strong pressure and rubbing action in a single solution. Then the miller adjusts the fineness of the product by approaching or distancing the surface of the two wheels. The flour obtained from stone milling contains the 100% of the grain of wheat. It exceeds the maximum ash limit. Therefore, a selection step is necessary, which takes place in a plansichter. The results are the whole flours and type “2” and type “1” flours.
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