In this article we continue to trace the path that takes us from a grain of wheat to our tables by telling you about stone milling, a method of milling that allows us to create special flour types for bread, pizza and some kinds of cake.
On the same theme, you can also see how the wheat is selected, the key steps of “cleaning” and how soft wheat is ground with a cylinder mill. In this in-depth analysis we focus on the stone milling technology of soft wheat. We use both stone and cylinders milling technologies at our mill.
Before starting, we would like to say that there are many articles about stone grinding online, which aim to evaluate the products of this milling technique from a nutritional point of view, often comparing them with similar products obtained from cylinder grinding in order to decide which of the two technologies is best.
We are convinced that neither technology is better than the other, so we will go into this topic no further. Instead we will describe stone grinding from a historical and technical point of view.
Stone grinding wheat: a short history
Man has been crushing and crumbling cereals since time immemorial. The idea of working one of the earth’s products by subjecting it to an “initial transformation” that would make it more usable and digestible, was one of the key points in the evolution of the human species, like the discovery of fire or the invention of wheel (here you can get more information on the properties of soft wheat).
The refinement of the art of processing wheat took millennia. Initially ancient farmers crushed the grains using mortars, then they began to crush them between two stones, until in Greece, around the year 1000 BC., they succeeded in giving millstones a more rational shape. A millstone, with a very different shape to modern ones and operated by human or animal power, was found amongst the remains of Pompeii.
In the slow but incessant evolution of this technology, known as “low” or “flat” grinding, fundamental leaps were made with the adoption of hydraulic power, wind power and, finally, steam propulsion.
At the end of the 18th century, at the height of the evolution of this technology, millstones were operated “by battery” using hydraulic turbines or steam engines. Thanks to the new and abundant availability of motive power, even the machines’ accessories were “motorized” and screw conveyors, elevators and sifters made their first appearance.
It was precisely the improvement of the sifter, one of the first machines in milling history, which highlighted the greatest limitation of stone milling, namely that of treating all the fractions of the grain in the same way regardless of the difference in resistance they offer to grinding. For this reason, a few years later, at the beginning of the 19th century, the first cylinder rolling mills began to take hold and a new production method , once defined as “high” (or “gradual”) grinding, began to be adopted.
Figure 1. The evolution of grinding.
Stone grinding: how does it work technically?
As we explained in the previous paragraph, the stone grinding technology still adopted today, is based on knowledge that reached full maturity at the end of the 18th century, shortly before cylinder grinding began to evolve. Here are some images to help you understand how a stone mill works.
Figure 2. The stone milling machine as a whole. (Italian Encyclopedia of Sciences, Letters and Arts – 1929)
The structure of the stone milling machine
As can be seen from the image, a stone milling machine (or “palmenti” mill) is made up of two horizontal grinding wheels or millstones (A and B in the figure). One of the two millstones is fixed to the frame and does not move (the sleeping millstone, indicated by the letter A in the figure), while the other is mobile (the mobile millstone, indicated by the letter B in the figure).
The grain to be ground is loaded through a hopper (D in the figure) located in the centre of the upper grinding wheel, into which an entry hole is made. The grain is forced to pass through the space that remains between the two millstones, the height of which is adjusted using a hand-wheel. To facilitate the passage of the grain between the two millstones and force it to travel towards the edge of the stone, grooves or “furrows”, are carved into the stone respecting theoretical and practical considerations. For example, for the grinding of soft wheat it is advisable to use a greater number of furrows of suitable depth, to reduce the grinding surface, increase the passage of air and, consequently, reduce any overheating of the product.
Figure 3. Diagram of the surface of a millstone. (Encyclopedia of arts and industries – Raffaele Pareto, 1878)
The difference between stone and cylinder mills
Unlike the cylinder mill where, to put it simply, the grain is first broken, releasing the endosperm which is then reground in several steps, in the stone mill strong pressure and a rubbing action is essentially applied in a single step.
The miller can adjust the fineness of the ground product by moving the surface of the two grinding wheels closer together or further apart. If the grinding wheels are too close together, the product will overheat and the bran will be crushed excessively; on the contrary, if the two millstones are too far apart, a lot of flour will remain stuck to the bran.
As we have said in other articles, “grinding is not enough to produce flour”. The flour obtained from stone grinding contains 100% of the wheat grain (in this link you can find the technical data of a product with this characteristic) and, almost always, exceeds the maximum ash limit, of 1.7%, which allows a product to be classified as “wholemeal” flour (in this link you will find more information on the ash content of a flour).
As stated above, therefore, even in the case of the stone mill, after grinding a selection is necessary, which usually takes place in a sifter. Following this further step, wholemeal flours and type “2” flours are obtained. With a little more difficulty, and by adopting a finer sieve on the sifter (or “plansichter”), type “1” flour can also be obtained.
Figure 4. Our stone milling machine
What types of flours can be obtained with stone grinding?
By stone grinding we can obtain excellent dark flours, such as type “2” and type “1” flour. These types of flour differ from similar flour ground in a cylinder mill due to their humidity values which, in the case of stone grinding, are always lower. In fact, the stone mill is only capable of grinding cereals with a lower humidity content than those that can be ground by a cylinder mill. Furthermore stone milling has a rather high impact on the product from the point of view of overheating.
For this reason, one of the quickest methods to understand if a flour is truly stone ground is to check its humidity, which is usually at least two percent lower than flour obtained from cylinder milling.
In our opinion, the most distinctive product that can be obtained with stone milling is wholemeal flour. This will have a finer and more angular grain size than similar flour obtained with the cylinder mill, and will therefore be particularly suitable for the preparation of rustic and fibre-rich products.
Figure 5. On the left, stone-ground wholemeal flour (note the finer and more angular grain size of the bran fragments). On the right, wholemeal flour milled in a cylinder mill (note the larger grain size and flakiness of the bran fragments).
Stone grinding: a summary of the salient points
- The stone mill saw its greatest evolution at the end of the 18th century;
- A stone mill treats all the fractions of the grain in the same way, regardless of the different resistance they offer to grinding. This makes a precise classification of the flour impossible and, in fact, allows us to produce only wholemeal, type “2” and type “1” flour easily;
- Stone-ground flour is less humid than cylinder-ground flour;
- Stone-ground wholemeal flour has more angular bran particles with a finer grain size.
In conclusion, some advice. Faced with the offer of type “0” or “00” “stone-ground” flour with a humidity count greater than 15%, or of a flaky, large grained wholemeal flour it would be appropriate to ask yourself a few questions! We reiterate, in fact, that stone grinding does not allow for the classification of flour in the same way as cylinder ground flour does. Stone grinding leads to flour characterized by a low moisture content and gives a fine grain size and angularity to wholemeal flours.
