stone flour mill in Winchester UK

Where Your Flour Comes From – Roller & Stone Milling Explained

The smooth soft white, or slightly brown powder is one of the most common staples in stores around the world. With a little help from water and a few other ingredients, you can transform it into cakes, bread, muffins, cookies and lots of other based goods.

Annually, we grow more than 750 million tonnes of wheat a year around the world. Wheat takes up more acreage than any other crop we grow. But, most of us will never buy a whole grain. Instead, we buy grain in the form of finished products such as bread or we buy flour to transform into something else ourselves.

After visiting the City Mill in Winchester (UK) as well as the Mill City museum in Minneapolis (MN, USA) we got curious to learn more though. How does all that wheat get transformed into some of the most common staples around us?

Wheat kernels

The journey of wheat flour starts in a stalk of wheat. During harvest specialized equipment separates these kernels from the rest of the stalk. Still, some stones and other undesirable pieces might end up in the mix of kernels. That’s why any further processing starts with cleaning those kernels.

Before looking at the rest of the process though, it is worthwhile to look into the kernel in some more detail. The natural role of a wheat kernel is to serve as seed for a new wheat plant. The germ within the kernel is what will develop in a new plant if the seed gets a chance to germinate and grow. The endosperm provides the food for the germ to grow. As such, it is full of starch. Surrounding the kernel is a protective bran layer. The bran is quite hard, meant to protect the inner endosperm and germ to increase the chances of it becoming an actual plant.

wheat kernel from city Mill in Winchester UK
Illustration of a wheat kernel, this photo was made at the City Mill in Winchester, UK

Different types of flour

So how does a kernel transform into flour? This is done through the milling process, which we’ll discuss in greater detail further down. Milling is nothing more than breaking down these kernels into small fine particles. No major chemical reactions occur, it is quite a simple process really.

Different grains

Nevertheless, it is possible to create a wide range of different flours. Some of these flours are different because they’re being made of different types of grains. Wheat is the most common grain, but grains like rye, barley and many more can be made into flours. Within the wheat category there are different types of wheat kernels as well. Some are harder than others and their composition can change. We discuss these differences in more detail elsewhere on this website.

Using only part of the kernel

But even if you have just one type of wheat grain available you can make different types of flour. Millers do this by including different fractions from the kernel in the final product. Whole wheat flour for instance is made from the whole kernel. The bran, endosperm and all other components end up in the flour. In white flour however, the bran is removed. White flour is only made from the endosperm and germ. There are various in-between blends as well.

Fineness

Lastly, millers can change how finely they grind the flour. Larger particles make a very different flour then a finely milled flour. An example of a flour with large particles is semolina, whereas cake flour is very finely milled. We discuss those differences elsewhere in more detail as well.

whole wheat pastry, all purpose and cake flour
Different types of flour which we tested for functionality

Milling

Humans have been milling wheat kernels into flour for thousands of years already. In its most basic form, it doesn’t require any advanced technology. Rubbing two stones over one another with wheat in between is how it likely started. But quickly humans figured out more efficient ways to mill grains. Using rotating grinding stones has been done for thousands of years.

The first automation

What started out as a manual and very labor intensive process quickly transformed into a more automated version. Over two thousand years ago the Romans started using animal power to turn the wheels and mill flour. Not long after, in what is now Turkey (Asia Minor) water mills were invented. The power of flowing water could be used to move the grinding stones. Such grinding flour is so time and energy consuming, these inventions were likely life changing for communities at the time.

All around the world you’ll still be able to find examples of mills that used natural powers to operate. The City Mill in Winchester (UK) is one of many that is driven by a flowing river. The Netherlands of course is known for its use of wind to drive mills.

stone flour mill in Winchester UK
A stone flour mill in the City Mill of Winchester UK). The round tub on the bottom right encases the two stones.

Mill stones

Despite using different sources for their energy, the basic process for milling didn’t differ that much. Up until the late 19th century, most millers used mill stones to mill the wheat into flour. Mill stones always come in a pair. Both are large round stones, with a flat top and bottom and are incredibly heavy.

The bottom stone, the bed stone, stands still during the milling process. The top stone, or roller stone, rotates on top of the bed stone. The top stone has a hole in the center. The miller feeds grain through this hole to get in between the two stones.

The flat touching surfaces of the stones aren’t smooth. If they would, the grains would get stuck in between them. Instead, the stones are both engraved on the inside. Different manufacturers use different patterns, which also vary by region. Wheat wasn’t the only product milled with these types of mill stones. Spices as well as oils could be made with mill stones. Each product would have their own engraving pattern on the stones.

After the grains are fed into the center they move to the outside of the two stones, carried along by the patterns. Along the way they are crushed and thus milled. By changing the distance between the two milling stones and the amount of kernels in between the mill at one time, the fineness of the flour can be changed.

various mill stone designs
From the Mill museum in Minneapolis (MN, USA)

Whole wheat flour

If you’d add your kernels and collect all of the flour that comes out of the mill you have whole wheat flour. This flour contains all the different parts of the kernel, including the bran. Whole wheat flour contains all nutrients of the flour, however, because of the presence of fats in the fat, it does spoil a bit more quickly than a white flour.

Separating fractions

Instead of packaging the whole wheat flour, millers can decide to remove part of the wheat kernel from the flour. By using cloths, filters and other separation systems they can separate the lighter bran particles from the rest of the flour. This is what transforms the flour into white flour.

In older mills with just one grinding stone this was often as advanced as they could get however. Millers that have access to more than one stone can pass some of these resulting fractions through another stone mill. That way, the finer could be made into a finer grade.

Roller mills

Nowadays, stone mills are still used by mostly smaller millers. As of the early 20th century the majority of millers uses roller mills nowadays. These are more efficient and easier to handle.

Instead of using two round stones that (almost) touch with their flat sides, roller mills using two parallel cylinders. The cylinders align next to each other and the wheat passes through the two rolls. Just as stone mills are engraved, so are roller mills. These patterns help feed the grains through for optimal milling.

A big difference though is for how long the wheat is in contact with the stones vs. rollers. In a stone mill the wheat has to pass through the two stones, starting at the center and coming out at the outer edge of the circle. However, this is not the case for roller mills. The contact area between these two rollers is a lot smaller. As a result, they pass through one set of these rollers quite quickly. This is why roller mills always consist of a series of these roller pairs after one another.

stone vs roller mill
Graphic visualization of roller vs stone mills.

Conditioning

Wheat kernels can be fed into a stone mill as is. They do not need any special pre-treatments. However, in the case of roller mills the grains are generally pre-treated in a tempering/conditioning step. Millers will moisten the kernels and leave them in this humid environment for several hours. The duration as well as the amount of moisture that’s added depends on the type of grain that a miller wants to mill.

Doing this makes it easier for the miller to separate the bran from the rest of the kernel. As you will see further down, in roller milling it is important to remove that bran early on in the process (even if you’re making whole wheat flour). It makes the mill run more effectively.

Breaking

The first set of rollers of a roller mill serve merely to break the outside bran. This hard layer is tricky to break down effectively which is why it’s a separate step in the process.

After breaking the bran a few additional machines are used to then separate that bran from the rest of the flour.

Milling white flours

Once the bran has been removed you have the ingredients needed to make a white flour. The endosperm passes through several sets of rollers to get the flour to the required size. After each roller mill split off those fractions that are milled enough. The parts that are still too large are fed into another set of rolls.

A large scale mill is mostly made up of a series of roller mills as well as a lot of purifiers and sieves to split the flours into all its different types. Every factory is organized slightly different and for each product a slightly different optimal process exists.

After separating and roller milling, the miller might have to blend various fractions of flour back together to create the desired product. Once the flour has all the required properties it’s packed and ready to be shipped.

old roller mill
This roller mill stands in the Mill City museum. It is one of the earlier and older mills. Nowadays wood isn’t used in facilities like this anymore and equipment is from stainless steel instead!

From there on, flour is shipped to stores and factories all over the world. Wheat is global commodity. Whether you eat chapati in India, whole wheat bread in the Netherlands or a donut in the USA. That flour has at some point passed through mills!

References

Edward Bradfield, Wheat And The Flour Mill : A Handbook For Practical Flour Millers, 2011, link

Stanley P. Cauvain, Linda S. Young, Technology of bread making, 2013, link

de Zaanse Schans, Discover their wind mills, link

Mill City museum in Minneapolis (MN, USA), link

Knight Neftel, Report of flour milling processes, chapter: Flour milling process, 1884, link ; this is quite an old book and gives a fascinating insight into the milling knowledge at the time!

The City Mill museum in Winchester (UK), link

The art and science of flour milling (YouTube), link

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