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Flour Milling

To the average person, processing wheat into flour (milling) would seem quite simple. After all, you only need to crush it to turn it into powder! You don't need to be an expert, however, to understand that this has little chance of producing the white flour that we've come to expect [1].

The miller's role is to separate the wheat's husk (bran) from the floury kernel (endosperm) (Figure 1). If we were dealing with rice, removing the husk would be simple. It can be rubbed against an abrasive surface until it disappears. Nature has given wheat a crease, which has a tendency to make things difficult.

Figure 1: The various parts of a grain of wheat

For many years, wheat was ground between two stone wheels. This was, therefore, whole-wheat milling (because it kept both the bran and germ of the wheat). Millers then put methods in place (sifting and sieving) which allowed them to separate the flour into several parts, depending on their grain size... and their colour. The whiter parts, being intended for the wealthier, were even called "fleur de farine," or the "flower of the flour." Meanwhile, the poor ate their "black" bread, so called not because it was over-baked, but because it was made with very dark flour.

The industry was revolutionized in Hungary in the mid-19th century with the advent of cylinder machines (Figure 2). By allowing a much more progressive and controlled milling, these machines made white flour, and the breads that come from it, accessible to all.

Figure 2: Cylinder machines in a mill

How does a mill work today?

Remember that these are real factories which are very precise and inevitably quite different from one another. Nevertheless, they must all follow the main principles which we'll present here:

  • Receiving and storing wheat

It might seem obvious, but when the wheat arrives at the mill, it needs to be stored somewhere. It must be protected from weather, humidity, rodents and insects. The growth of mold must be prevented and overheating must be avoided since this can impact quality. The more varieties of wheat that need to be stored, the more storage silos the mill requires. The management of incoming wheat and the mixing of wheat varieties are important elements in the final quality of flour.

  • Cleaning the wheat*

When the wheat arrives, it is often quite dirty. The problem is multi-faceted: on the one hand, nothing extra can end up in the finished flour (for example, ergot, a fungus that is toxic in high doses, the content of which is regulated in cereals, in Europe). Additionally, the equipment must be protected (by avoiding the grinding of stones, for example). And lastly, the presence of foreign matter may disturb the proper preparation of the wheat. So essentially, these impurities must be removed. This can have significant economic consequences, since it is purchased at the price of wheat but is discarded (and therefore decreases the overall value significantly).

  • Blends of wheat

In reality, flours are systematically made from wheat mixtures. A bit like a fine wine, it is the balance between the characteristics of each wheat variety that give the final flour its characteristics. Therefore, one of the miller's great arts is to find the optimal blend of wheat. This is done using the laboratory's rheological test data.

  • Wheat preparation*

Separating the floury kernel from the bran can only be achieved if the wheat is properly prepared for milling. By preparation, we mean the process of tempering the wheat to slightly increase its moisture (to around 17%), followed by a rest period which allows the water time to penetrate the kernel. Good millers know how to prepare the wheat perfectly. They know that if they don't prepare it (dry milling), the main consequence will be a very significant increase in the ash content (i.e. mineral materials mainly found in bran) due to poor separation of the bran from the kernel. If they are tempered too much, difficulties will be encountered during processing (clogged sieves, for example...).

  • Milling

Milling is a succession of reductions in grain size, achieved by passing the grain between cylinders, followed by sieving to sort the particles, and sending it back for another pass, etc., until the maximum amount of flour has been extracted. Without going into the details of the milling diagrams, it should be noted that there is also a steadfast rule here: the milling of standard wheat is done using both fluted rollers AND smooth rollers (Figure 3). The purpose of these two types of cylinders is different and their impact on flour quality (damaged starch in particular) is completely different.

 Figure 3: Grooved and smooth rolls

  • Mixing the flour.

Once the flour has been produced and its quality is controlled*, it may be mixed before sending it to the end user.

                Obviously, this article only touches on the main parts of a process that is much more complex than it first appears. The idea here is to highlight the guidelines that apply to all milling, because these rules also govern laboratory milling. This will be the subject of another article.

 

[1] Clearly, the current trend is toward less refined flours. However, the author has chosen to focus here on white flours, which are still the most widely produced.

* See the following articles:

"Preparation of wheat for milling"

"Quality control part 1: Quality"

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