Not all bubbles are equal: bread texture and the science of baking

Abstract

Bogdan Dobraszczyk explores the science behind bread-making, focusing on how the formation and development of gas bubbles during mixing, proofing, and baking influence the texture, volume, and quality of the final product, with insights into the role of dough rheology, flour quality, and baking techniques.

Bread is a staple food that was consumed by 80% of the world's population during the year 2022, and a principal source of carbohydrate and protein. It comes in various shapes, flavours and textures according to various culinary preferences in different countries, from the typical white and brown bread consumed in the UK and USA, baguettes and brioche in France, rye breads in Eastern and Northern Europe to the flatbreads of North Africa, the Near East and India.

Bread is strongly associated with the development of agriculture and a move away from a hunter gatherer lifestyle. It was central to the formation of early human societies. From the Middle East, where wheat and other cereals were first domesticated, cultivation spread north and west, to Europe and North Africa, and east toward East Asia.

Bread is mainly composed of air. During mixing, bubbles are formed within the dough, and as the dough proves (ferments), these bubbles expand and grow. The size, distribution, and development of these bubbles play a crucial role in shaping the bread's final quality. They determine its texture, mouthfeel, and overall volume, making the process of bubble formation one of the key factors in creating the perfect loaf.

The images below show (Figure 2, 1) a digital reconstruction of bread texture, and (Figure 2b) an electron microscope picture showing the typical open foam structure of white bread. It is clear that not all bubbles are equal – there is quite a large distribution of shapes and sizes, which will have an impact on the texture of the bread. The higher magnification electron microscope picture of bubbles in white bread shows that the bubbles are interconnected, which indicates that a dynamic process of bubble expansion occurs during the baking process where bubbles interact with each other.

Several different ingredients are used by commercial bakers with the aim of improving the bread. This generally means increased bubble stability in the dough to allow for the mechanical shocks generated during moving the dough around in an industrial process and the rapid proofing times required for fast throughput. An artisan baker is less likely to use improvers as they can easily adapt their processing conditions such as proof time and temperature to suit their particular recipe. A basic bread recipe usually only contains flour (which is mainly starch and gluten), salt, yeast and water, but commercial bakers will add:

• Emulsifiers in bread create a stable and strong dough that retains more gas. The result of this is a light and soft crumb. Emulsifiers also slow down starch retrogradation, which is the drying out process when bread turns stale.

- Monoglycerides (mono) and diglycerides of fatty acids. Mono and diglycerides of fatty acids are the most common emulsifiers in bread and food production. They are also surfactants, which are thought to stabilise the bubble/air interface.

• Enzymes (like amylases and proteases).

- Amylases increase the rate at which complex starches are broken into simpler sugars, thereby increasing fermentation rate.

- Proteases improve extensibility of the dough by degrading some of the gluten.

• Ascorbic acid strengthens the gluten network allowing more expansion during baking.

A large number of tests are used in the industry to assess bread texture, from the traditional hand squeeze test to instrumental compression and indentation tests. Image analysis of bread texture is increasingly being used to image the bubble structure. This method can give additional information such as bubble size distribution, cell wall thickness distribution, orientation of bubbles and overall density of the product. This can easily be done by scanning a bread slice using a flatbed scanner and the appropriate software.

Many different textures can be produced in bread, depending on the recipe and baking conditions. Figure 7 shows different types of bread texture: (1) commercial sliced white loaf showing fine texture with many small bubbles;

(2) French baguette with more open texture with many larger bubbles;

(3) sourdough with open texture and thick cell walls;

(4) German Pumpernickel made from coarsely ground rye giving a very dense texture.

The mouthfeel of these different types of bread will differ according to the bubble sizes and cell wall thickness. Non-wheat breads are generally much denser with little bubble expansion, as wheat gluten allows bubbles to expand due to its unique rheological properties.