Bread, Cooking Knowledge, Non Recipes

Why does bread have holes in it? (it’s not the yeast)

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The holes in bread are created while mixing and kneading bread dough. They subsequently grow into larger gas cells during fermentation. The purpose of mixing bread dough is not only to develop the gluten network but also to aerate the dough. The gas cell structure created during the mixing and kneading stage affects the pore structure of the baked loaf.

Mixing bread dough serves three primary functions:

  • Homogenization
  • Air inclusion
  • Gluten development

To understand why bread is a foam we have to look at the mixing process instead of the fermentation. The yeast doesn’t create any new gas bubbles while the dough rises. Instead, the yeast produces carbon dioxide that is first dissolved in the aqueous phase of the dough. The liquid carbon dioxide then migrates to the nucleation gas bubbles created during mixing. The carbon dioxide gets absorbed into the gas bubbles and undergoes a phase transition from liquid to gas.

A baker can influence the pore structure of bread by rearranging the gas bubbles during fermentation and shaping of the bread dough. If a baker applies pressure and punches down the dough during shaping, then he is breaking up larger air bubbles into smaller units. If the baker handles the dough gently during shaping, the dough has an uneven gas bubble size distribution and the crumb of the bread will be irregular.

Shaping bread rolls with tension to oobtain an even gas bubble distribution

It is often overseen that the mixing process also has a large impact on the pore structure of the bread crumb. Aeration during mixing is one of the topics that is well researched and of high importance for the industrial production of bread.

What happens when we mix and knead bread dough?

When we mix bread dough, we impart mechanical energy into the system. This mechanical energy input induces the formation of a continuous gluten network and the initial inclusion of gas bubbles. The gluten network is formed by the gluten proteins in wheat flour.

Gluten consists of glutenins and gliadins. The glutenins form an extensive network that gets interrupted by the gliadins. Glutenins are responsible for the dough’s elasticity whereas gliadins act as plasticizers that improve the extensibility of the dough.

The other flour components, mainly starch and pentosans (arabinoxylans), are dispersed within the continuous gluten network. The mechanical energy input during kneading is transferred from the kneading element (eg. mixer, hands) to the dough.

The mechanical energy input is compromised of:

  • Tension
  • Compression
  • Shear

When we use our hands or a spiral mixer to knead bread dough at home, then we incorporate mechanical energy into the dough mainly by tension and compression. Shearing bread dough happens in industrial high-speed mixers or food processors by the rotating blades.

Kneading bread dough by hand

The higher the rotational speed of our dough hook, the more air gets incorporated into our dough. Each revolution of the dough hook incorporates air. The incorporation of air into the dough has positive and negative effects:

  • The entrapped gas bubbles are nucleation sites to which the carbon dioxide produced by the yeast can migrate too. If there would be no nucleation bubbles in the dough after mixing, then the dissolved carbon dioxide would just migrate to the dough surface and escape the dough into the surrounding air.
  • The oxygen that is incorporated into the dough during mixing degrades the gluten network. Therefore, it is beneficial to knead bread dough in oxygen-lean environments.

Factors that impact the dough development and aeration during kneading

Many factors influence the rheology of the dough after kneading:

  • Kneading time: The kneading time should neither be too long nor too short. The kneading time of bread dough is determined by the development of the gluten network. A gluten network that is well developed, and not over-sheared, allows the dough to entrap more gas. The longer the kneading time, the more nucleation gas bubbles are incorporated into the dough.
  • Dough temperature and mixing speed: A faster mixing speed increases the consistency of bread dough. However, a higher mixing speed also raises the dough temperature. This is mainly due to the higher friction between the dough, kneading element, and wall of the bowl. Higher dough temperatures can lead to irreversible protein denaturation and a decreased swelling ability of the gluten proteins. The optimum temperature of bread dough during kneading is between 15-30 °C (59-86 °F). At home, unless you live in a tropical climate, it is usually no problem to stay below 30 °C (86 °F). However, super-high-speed mixers that are used in the food industry can raise the dough temperature by 10-20 °C (50-68 °F). Isn’t that crazy? It always wows me to see that number! If you do this kind of super-high-speed mixing you need to have excellent temperature control. For the home baker, it is advisable to knead at low temperatures (slightly below 20 °C/ 68 °F) as this improves the baking performance. At low temperatures, the number and strength of hydrogen bonds within the gluten network increase. The gluten network is mainly stabilized by disulfide bonds. Noncovalent hydrogen bonds are much weaker, however, they also play a role in strengthening and stabilizing the gluten network.
Bread dough after mixing
  • Water temperature: Obviously, the water temperature has a key impact on the dough temperature during mixing. In some instances, for example for sugary pastries, the water needs to be lukewarm to dissolve all the sugar. In other instances, even boiling water is added to wheat-based doughs before kneading. What does the hot water do? It plasticizes (softens) the gluten. This is a technique that is typically not used to bake bread. We don’t want to weaken the gluten network. The hot water dough method is mainly used for sweets, pastries, and pasta (noodle) products that require a soft texture.
  • Water absorption and water content: Water absorption and stickiness/ softness of the dough are related to each other. However, the water absorption capacity of flour is not the only factor influencing stickiness and dough consistency. I have discussed this in detail in my post about spelt flour. In general, the more water a flour can absorb the firmer it is and the lower its stickiness. But we also need to consider the quality of our gluten network and the ratio of gliadins to glutenins. Spelt flour can often absorb more water than wheat flour but it is stickier and softer at identical hydration levels because of the higher amount of gliadins in spelt flour that plasticize the dough.
  • The time when water is added to the dough: For high-hydration doughs, it is often beneficial to hold back some water and add it later in stages. This technique is called “bassinage” by the French. Adding water in stages during kneading enhances the water-holding capacity and gluten strength of the dough.
  • The addition of bread improvers: Organic acids like citric acid or acerola cherry powder used at a ratio of 0.3 % relative to the flour weight can improve the elasticity of the gluten network. Baking enzymes can also help to increase the dough strength and reduce stickiness. Emulsifiers tend to increase the number of gas cells incorporated during mixing and they can also help to stabilize larger gas bubbles during fermentation.

How the food industry bakes bread: the Chorleywood process

At home, we usually don’t have to worry about aerating bread dough during mixing. It just happens if we mix the dough ingredients under atmospheric pressure. It’s impossible to not or excessively aerate bread dough in a home kitchen.

But not all bread is produced at home. In fact, most of us regularly purchase bread produced by the food industry. The food industry approaches breadmaking very differently than I do on this blog.

Sandwich bread (toast)

You might’ve heard of the Chorleywood process. This is the most common and famous way to industrially produce sandwich bread. In the Chorleywood process, the bread dough gets mixed intensely for a short amount of time, no more than 2-5 minutes.

The intense mixing process can lead to excessive aeration of the dough during mixing. To solve this problem, the dough is first briefly mixed above atmospheric pressure (up to 2.5 bar) to aerate it. Then the pressure is reduced to vacuum pressure (down to 0.35 bar) so that the gas content in the dough doesn’t get excessive.

But why don’t manufacturers just knead the dough more gently for a prolonged period of time? After all, that would fix the problem of excessive aeration.

The advantage of the Chorleywood bread process is that it combines mixing and bulk fermentation in one step. After mixing, the dough is ready to be proofed and baked. This saves manufacturers time.

The final bread crumb cell structure is entirely generated during mixing. Bread dough that is mixed gently for a prolonged amount of time at atmospheric pressure possesses an uneven gas bubble size and distribution after mixing. However, when the baker shapes the dough after bulk fermentation, he can redistribute and equalize the gas bubble size and distribution.

The Chorleywood process was invented to produce sandwich bread. Sandwich bread should have an even pore size distribution. Thus a high energy input during mixing is required to obtain a small and even gas bubble distribution within the dough. There is no step in between mixing and baking with which the gas bubble distribution (number, size, regularity) gets modified. The final crumb structure is an expanded version of the gas cell structure after mixing.

All this information probably does only have little impact on how you approach breadmaking. For the home baker, aeration during the mixing of bread dough is of little relevance. But hopefully, you learned something new and interesting today!

Resources and further reading

Technology of Breadmaking

The kneading process: A systematic review of the effects on dough rheology and resulting bread characteristics, including improvement strategies

Effect of Temperature on the Solubility of CO2 in Bread Dough

The Impact of Water Content and Mixing Time on the Linear and Non-Linear Rheology of Wheat Flour Dough

Molecular Oxygen and Reactive Oxygen Species in Bread-making Processes: Scarce, but Nevertheless Important

Aeration of bread dough during mixing: Effect of mixing dough at reduced pressure

Aeration During Bread Dough Mixing: I. Effect of Direction and Size of a Pressure Step-change During Mixing on the Turnover of Gas

Aeration During Bread Dough Mixing: II. A Population Balance Model of Aeration

Proving of Bread Dough II: Measurement of Gas Production and Retention

Understanding the effect of emulsifiers on bread aeration during breadmaking

Dough aeration and rheology: Part 2. Effects of flour type, mixing speed and total work input on aeration and rheology of bread dough

Aeration of bread dough influenced by different way of processing

Effect of sugar on bread dough aeration during mixing

Measurement of Dynamic Dough Density and Effect of Surfactants and Flour Type on Aeration During Mixing and Gas Retention During Proofing

Aeration of bread dough influenced by different way of processing

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