Cooking History, Cooking Knowledge, Non Recipes

What is strong flour? An in-depth discussion

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Strong flour is wheat flour of superior quality that is suitable to bake airy loaves of bread. Doughs made from strong flour show an increased gas-holding and water-absorption capacity. Strong flour is produced from hard wheat varieties with high protein contents and excellent gluten quality.

This question has been answered many times on the internet. So why do I write a blog post about it? Well, the answer is not as obvious and simple as it seems.

A quick note before we start: I often have the feeling that many baking blogs just copy from each other and don’t do any primary research. This is kind of the tragedy of the internet. Blogs coping from blogs. Please don’t copy anything blindly from my blog. Read the primary literature, mainly scholastic research. I am not free of faults and misunderstandings. Maybe you come to different conclusions than me. I can only offer you my understanding and my viewpoint. Don’t copy me, learn with me!

What is strong flour (according to Google)?

When I search for the term “What is strong flour?” then Google provides me with the following answer:

what is strong flour google search result

From this definition, we learn that strong flour has a higher gluten content and this higher gluten content gives the dough a better elasticity. Next question: what is the gluten content of strong flour?

wikipedia definition of strong flour

We can see that Wikipedia also provides a simple answer to our question. Strong flour is defined by its high gluten content between 12 to 14 % that makes the dough elastic.

I want to challenge this definition today. As I will discuss in detail later, it’s not wrong but very inaccurate to say that gluten makes the dough elastic. Gluten is not one homogenous molecule. It consists of heterogeneous subunits so that we need to have a basic understanding of gluten structure and quality to understand how gluten influences the dough structure and rheology. Spelt flour contains more gluten than wheat flour but it produces softer doughs with less elasticity.

I want to take a deep-dive discussing gluten quality and its impact on the strength of wheat flour. I will also briefly list other factors besides the wheat proteins that contribute to the quality of strong wheat flour.

Protein quantity and its impact on the quality of wheat flour

If you’re an experienced baker then you might’ve often heard the advice: The higher the protein content in wheat flour, the better it is suited to bake bread. North Americans classify their wheat flour according to the protein content which implies that the protein quantity is what defines the quality of wheat flour.

There is indeed a linear correlation between the loaf volume and the protein content of wheat flour. Take a look at the two graphs below from a study of American scientists about the quality characteristics and breadmaking functionality of hard red winter and hard red spring wheat. The black triangles represent the hard red winter wheat whereas the white circles represent the hard red spring wheat. The more protein is in the flour, the bigger the loaf volume.

Loaf volume and specific loaf volume in dependency of the protein content

We can see that the authors gave us two regression lines. One for hard red winter and one for hard red spring wheat. If protein quantity would be the only factor influencing the loaf volume, then these two lines should be identical. But we can see that even if hard red winter wheat possesses the same protein content as hard red spring wheat, it tends to produce a loaf with a smaller volume.

Furthermore, we can see how much the values fluctuate even within the same wheat cultivar.

  • If we focus on the hard red winter wheat (black triangles) then we can see that, at a protein content of roughly 13.8 %, there was one flour that performed particularly well whereas the other flours produced a loaf with a smaller volume (marked in red in the graph below).
  • There was also a batch of flour with roughly 12.8 % protein that produced a dense loaf as would be expected for protein contents of 11.5 % or lower (marked in blue).
Loaf volume in dependancy of the protein content

The coefficients of determination of 0.7 and 0.73 are indicators that the gluten quantity is an important factor when it comes to producing light and airy loaves of bread. But these graphs also tell us that the gluten quantity is not the only factor influencing the loaf volume.

My personal observations with strong flour

I have experienced this phenomenon first-hand at home. Flour with a higher protein content didn’t necessarily produce lighter loaves of bread. I have used flours with a lower protein content that outperformed flours with a higher protein content when it comes to loaf volume.

The standard German Type 550 soft wheat flour that I use has a comparatively low protein content of 9.8 %. That is partly because it is an organic flour and because it is produced solely from locally grown wheat. But it can handle hydrations up to 75 % without too many problems. I bake baguettes with it. It doesn’t perform worse in comparison to cheap and mass-produced non-organic supermarket flours that are advertised as strong flours (“backstarkes Mehl”, “extra backstark”) with a protein content of 11-12 %.

Strong flour from a Germany with a protein contetn of 12 %.

I don’t want to say with this that all strong flours in Germany suck. There are many strong flours of good quality available in Germany and they can surely outperform a flour with less than 10 % protein. My point is that quality matters. Two different brands of wheat flour, both advertised with 11 % protein, might give you very different results. If you are dissatisfied with your current flour, then it might be worth looking for another brand or flour mill even though the protein content in the flour might be lower.

Which factors besides the protein content influence the volume of bread?

If protein quantity is not the only factor influencing the loaf volume, then what are the other factors?

  • The skill of the baker: Of course, in the study that I have shown you above the methodology was standardized and always the same. But it’s undoubtful that a skilled baker can handle bread dough better than a novice baker. Don’t underestimate this factor. It is the most important. Learning to bake bread takes time and can be frustrating at times.
  • The quality of the wheat proteins: This is the main point that I want to discuss in this article.
  • The enzyme activity of the flour: There is an optimum level of enzyme activity in flour. If it is too high or low, the bread won’t turn out well. Enzymes are also proteins but I don’t want to discuss them in detail today. Enzymes are so complex, they require a separate post.
  • The hardness of the wheat grain: Wheat can be classified as soft, semi-hard, and hard. The harder the endosperm of wheat grains, the more energy and time is required to mill them into a fine flour. A higher energy input during milling leads to a higher amount of damaged starch granules. For bread-making, a small amount of damaged starch granules is desirable because damaged starch can absorb up to four times the amount of water than native starch and the damaged starch granules also act as food for the yeast so that the dough rises faster. Typically, between 5-12 % of the starch granules get damaged during milling. If the amount of damaged starch gets too high, the bread crumb turns out wet and sticky (“klitschige Krume” in German) and the bread has a lower volume. The challenge for a miller is to produce flour with an optimum amount of damaged starch.

What is the composition of wheat proteins?

If we talk about the quality of wheat proteins, then we mainly refer to the composition of them. First, we need to recognize that wheat proteins can be broken down into two categories:

  • Nongluten proteins (15-20 %)
  • Gluten proteins (80-85 %)

Some of the nongluten proteins (mainly enzymes) play a minor role when it comes to breadmaking. The gluten proteins are far more important than the nongluten proteins because they can form a continuous network that allows bread dough to entrap air bubbles. The gluten proteins consist of two subunits:

  • Gliadin
  • Glutenin

Breadmaking requires an adequate balance of viscosity and elasticity/ strength of the dough. Responsible for the elasticity/ strength of bread dough are the glutenin subunits. Gliadins, on the other hand, act as plasticizers that weaken interactions between the crosslinked glutenin chains.

Why is wheat flour stronger than spelt or einkorn flour?

An important quality characteristic of flour is the ratio of gliadins to glutenins. In the table below, you can see the approximate ratio of gliadin to glutenin in different wheat species.

Wheat speciesGliadin to glutenin ratio
Bread wheat0.75 – 2.5 to 1
Spelt3 to 1
Durum wheat4 to 1
Emmer4.5 to 1
Einkorn6 to 1

The lower the gliadin to glutenin ratio, the better a grain is suited for baking bread. By looking at the table above, it becomes obvious why bread wheat and spelt are by far the two most popular wheat species to bake bread with. Thanks to their low gliadin content, they form strong gluten networks that can entrap large air bubbles.

Emmer is an ancient wheat variety

The table above also explains why spelt, durum, emmer, or einkorn bread tends to be flatter than bread made from common wheat. The gliadins plasticize (soften) the dough by weakening the glutenin interactions. This is great if you want to roll out the dough thinly because it shows less resistance. Try making strudel dough from spelt flour. It requires less force to pull the dough than if you prepare the dough with wheat flour.

But the higher stability of doughs made from bread wheat flour also has advantages:

  • The dough can entrap and hold larger air bubbles.
  • Common wheat bread can easily be baked without a loaf pan. It has the stability to rise upwards rather than run flat.

The gliadin to glutenin ratio explains to us why common wheat protein has a superior quality to spelt protein. However, differences in the gliadin to glutenin ratio in between the modern bread wheat cultivars are minor. The differences in the quality of wheat protein are believed to be mostly determined by differences in the quality of the glutenin subunit. So we have to take a closer look at that.

The structure of glutenin

Wheat glutenin is not one uniform molecule. The glutenins are a heterogeneous mixture of proteins. We can roughly categorize them as:

  • High molecular weight glutenins (HMW glutenins)
  • Low molecular weight glutenins (LMW glutenins)

The weight ratio of HMW glutenins to LMW glutenins in wheat flour can range from 0.18 to 0.74. The LMW glutenins account for 55 to 85 % of the total weight of glutenins found in wheat flour.

To understand the structure of glutenins, we have to look at how these proteins are stabilized. It has been known for a long time that disulfide bonds stabilize the glutenins. When we knead bread dough we constantly break and form new disulfide bonds. The dough gets firmer.

Kneading wheat flour leads to the breakage and formation of new disulfide bonds

By adding reducing agents (eg. cysteine) to bread dough, we break those disulfide bonds and thus weaken the dough. If we break disulfide bonds, we reduce the molecular weight (size) of the glutenin polymers. The same principle also applies in the opposite direction. Oxidizing agents (eg. ascorbic acid) strengthen the gluten network because they encourage the formation of disulfide bonds.

In general, we can assume that the glutenin networks in bread dough are a mixture of bonded HMW glutenins and LMW glutenins. There is evidence that networks of only HMW or only LMW glutenins can be formed but these appear to be more seldom in bread dough. Glutenin networks most likely show an ordered structure that scientists can’t accurately describe until this day.

What defines the glutenin quality and how can it be improved to produce strong flour?

If we talk about the glutenin quality then we talk about the degree of aggregation. In simple words, how strong and well connected the glutenin network is. The less free, unaggregated glutenin molecules can be extracted from bread dough, the better the baking performance. Thus it is widely believed that flour with a large amount of high molecular weight glutenins is better suited for bread-making than flour with a large amount of low molecular weight glutenins. A high ratio of HMW glutenins to LMW glutenins is correlated with an increased dough strength.

Unfortunately, things are not as easy as they seem. HMW glutenins are a heterogeneous mixture of proteins. There are HMW glutenins that are associated with high quality and there are HMW glutenins that are associated with low quality. Wheat breeders and millers also need to consider that the flour should ideally contain as many of the high-quality HMW glutenins and as little of the low-quality HMW glutenins as possible.

Pizza dough is best made from strong flour

How can breeders achieve a flour with high glutenin quality (a high ratio of HMW glutenins to LMW glutenins)?

  • Selective breeding or genetic manipulation of wheat plants to create new cultivars with better breadmaking properties.
  • High levels of nitrogen fertilization.
  • Growing wheat on sulfur-deprived soil.
  • Prevention of heat shocks during maturation of the wheat grains.

In Germany, there is huge resistance against genetically modifying wheat or any other crop. Many people here claim that wheat has become less healthy because of selective breeding and modern agriculture. And I have to admit that I prefer local organic flour over mass-produced stuff. But that doesn’t have anything to do with the fact that I am against genetic modifications when they obviously make sense. I buy local and organic because of the lower use of pesticides, to support small and local growers, and because of the sustainability aspect.

The not so glorious history of bread wheat cultivation in Western Europe

In the 1830s, wheat was brought to the United States by Russian Mennonites. European settlers quickly discovered that wheat grew extremely well in the Great Plains and parts of Canada. An efficient rail network and good connections to Europe enabled the US to export large parts of their excessive wheat production for low prices to Europe.

The American wheat had much higher protein levels and excellent baking qualities compared to most European wheat varieties. Nearly all wheat used for breadmaking in Northern and Western European countries was imported from the New World at that time. The “American Wheat Imperium” supplied 75 % of the bread wheat that was processed in Western Europe until the 1950s.

American farmers working on the wheat fields

This was not an amusing situation for European countries. Western Europe was dependent on the US. Hitler needed to expand to Eastern Europe as quickly as possible because Germany was running short on grains. The first goal was to conquer Ukraine with all the wheat fields there. Hitler’s goal of expanding the empire to the East was not just ideologically driven but also a necessity. The empire needed access to Eastern European wheat to feed its population.

Starting in the early 1900s, European breeders intensified their efforts to create a European bread wheat cultivar that could match the qualities of American wheat. At first, they focused on increasing the yield, disease resistance, and tolerance to climatic stress factors. Then they started to, step-by-step, increase the protein content in European wheat. Until today, European wheat is lower in protein than American wheat but the gluten content of present-day German wheat is high enough to bake tasty bread. I am happy with 10 % protein!

Increase in the wheat yields in Europe since World War 2

Getting to where we are today in Europe was not an easy path. The European Community (EC), the predecessor of the European Union (EU), was the first organization to require imported wheat to be sold at an artificially higher price in its member countries. Nowadays, people would be enraged. Don’t we want free markets? No, not in this case.

I am deeply thankful that we have developed high-quality wheat cultivars in Western Europe and that I can buy German bread flour. Today, it seems unimaginable that all European bread flour is grown in America. Modern wheat that has been selectively bred for high protein content is a blessing for Europe. I don’t want the old days back like some Germans in their nostalgic world where grain shortages exist and bread flour is predominantly grown in the New World.

How to determine the gluten quality of wheat flour?

After this little history digression, let’s get back to the technology. We learned that the protein content is not always indicative of the flour quality. The reasons for this are:

  • Variable growing conditions
  • Yearly growing variations
  • Wheat varietal variations
  • Heat damage
  • Bug damage
  • Variable enzyme activity/ enzyme addition

Flour mills compensate for seasonal variations in the gluten content and quality of wheat flour by blending different batches of flour to specified protein contents. It’s very simple to calculate. Let’s say you want to achieve a protein content of 11 %. You have a low protein flour with 9.5 % protein and a high protein flour with 13.6 % protein.

11 = 9.5x + 13.6(1-x)

4.1x = 2.6

x = 0.63

So we have to blend 63 % of the low protein flour with 37 % of the high protein flour to achieve a flour with a protein content of 11 %. If you want to blend more than two flours, the number of possible blending ratios increases and the calculation gets a bit more complex. But that is a topic for another time.

Blending flour in a mill to produce a standardized strong flour

Now that we have blended our flour to our desired protein content we still need to consider that the gluten quantity doesn’t tell us anything about the gluten quality.

The most reliable way to judge the gluten quality is to perform a baking experiment. However, this takes a lot of effort to perform. We can also observe the gluten directly. I have shown you a simple method to judge the gluten quality in my post about bakery-style German bread rolls.

You need to separate the gluten network from the flour to observe it. This can be done by washing bread dough in a bowl of salted water. The starch will get removed and what you will end up with is pure gluten. You can measure its weight to determine the wet gluten content but you can also stretch the gluten network and observe it visually. Try it yourself at home. Do you observe differences between different brands of flour? I did!

A popular method in the food industry is to centrifuge the washed gluten. A coarse sieve gets inserted into the centrifuge. Small gluten subunits that are not bound strongly to the gluten network will fall through the sieve. The gluten that remains within the sieve gets weighed and divided by the total wet gluten content (the total weight of the washed gluten). This measurement is called the gluten index. The more gluten gets retained within the sieve, the higher the gluten index and the better the gluten strength.

Summary: What makes flour strong?

At this point, I don’t want to cover all the analytical methods in detail but instead, come to an end. My goal with this post was to show you that the strength of wheat flour is determined by multiple factors, not just the gluten quantity but also:

  • the gluten quality
  • the wheat cultivar
  • the enzyme activity of the flour
  • the milling process (starch damage, blending)
  • the growing conditions of the wheat (soil, climate, fertilization, damages by heat/ bugs)

If all these factors are optimal then we end up with a strong flour that enables us to bake airy bread with a great taste and texture.

References and further reading

Comparison of Quality Characteristics and Breadmaking Functionality of Hard Red Winter and Hard Red Spring Wheat

Quality (End-Use) Improvement in Wheat

The Effect Of Damaged Starch On The Quality Of Baked Goods

Wheat protein composition and properties of wheat glutenin in relation to breadmaking functionality

Relationship of gliadin and glutenin proteins with dough rheology, flour pasting and bread making performance of wheat varieties

Combined effect of genetic and environmental factors on the accumulation of proteins in the wheat grain and their relationship to bread-making quality

Genetic Variation in Wheat HMW Glutenin Subunits and the Molecular Basis of Bread-Making Quality

Genetic improvement of grain yield and bread-making quality of winter wheat over the past 90 years under the Pannonian Plain conditions

Comparative Study on Gluten Protein Composition of Ancient (Einkorn, Emmer and Spelt) and Modern Wheat Species (Durum and Common Wheat)

Developments in bread-making processes

WHEAT and FLOUR TESTING METHODS

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