The secret life of gas bubbles and their role in bread doughs
Bread has been baked and eaten for thousands of years. Its appealing aspects include its light aerated structure and unique texture which have helped make it a staple food of many cultures throughout history. The aeration or 'leavening' of bread dough is usually achieved by adding yeast, a single celled fungus that ferments sugar molecules, generating carbon dioxide and alcohol as by-products. The CO2 inflates gas bubbles causing the dough to rise. It has been suggested that humans used yeast to produce food and beverages before they 'domesticated' any plant or animal, which would make yeast the oldest 'cultivated' organism.
Although bakers have long mastered the art of coaxing the best bread out of the available ingredients and equipment, scientific understanding of the structure of aerated bread doughs is surprisingly scanty. In fact, scientifically, very little is known about the physical changes occurring in proving dough, aside from the rather obvious fact that dough density decreases as the yeast produces more carbon dioxide and the dough expands.
Answers to questions such as the following ones would improve our understanding of breadmaking: "How does fermentation affect the elastic and viscous properties of dough?" and "How does a dough react to the physical forces applied to it during the production steps of moulding, dividing and tinning?".
Knowing the answers to such questions would be useful both in the practical management of bakery operations and in developing a better theoretical understanding of the physical and chemical processes that occur in dough during production. To find out how fermentation affects dough properties we can, of course, conduct baking studies, since this is the ultimate application of our knowledge. However, to learn about the actual elastic and viscous properties of the fermenting dough requires direct rheological measurements. This research bulletin explains some recent studies on the effect of yeast fermentation on the physical properties - particularly the rheological properties - of bread doughs.
Yeasted dough rheology
The standard definition of rheology, coined in 1929 is "the study of the deformation and flow of matter". Elasticity and viscosity are the most familiar rheological concepts and significant areas of rheology are concerned with measuring these material properties. The baking and milling industries make extensive use of rheological measurements, particularly for flour specifications. Dough rheological instruments, such as the farinograph, mixograph, alveograph and extensigraph, are used to measure the physical response of doughs to external forces applied by mixing arms, mixing pins, air pressure or a curved hook (respectively). These empirical instruments provide adequate information for day-to-day flour milling and baking operations. However, it is very difficult, if not impossible, to extract fundamental rheological information from them. This is because the measurement of fundamental rheological data requires instruments designed so that the forces acting on doughs (deformations) are rigidly defined and controlled. Armed with such devices, called rheometers, we can determine the fundamental stress and strain properties of doughs and quantify their viscous and elastic properties.
It might seem obvious that if we want to understand how yeast affects the fundamental rheological properties of yeasted doughs we have to make measurements on yeasted doughs. However, all the empirical rheological instruments employed in the baking industry are used to measure non-yeasted doughs and this focus is also reflected in the vast majority of dough rheology research. The aims of this research were to learn how yeast affects the physical properties of bread doughs during fermentation and to address the lack of rheological information on yeasted doughs. Therefore, we measured the rheological properties of yeasted doughs after varying periods of fermentation.
Figure 2. Scanning electron microscope images of yeasted doughs show the presence of bubbles embedded in the dough structure. At a higher magnification, the gluten protein network and starch granules making up the dough sub-structure are clearly visible (images courtesy of Virginia Humphrey-Taylor).
Rheological properties of yeasted doughs
Measuring the rheological properties of yeasted doughs is not easy. One major difficulty is that yeast continues to ferment during the measuring process. Since we are interested in changes in dough properties resulting from fermentation during proving, we do not want these effects to be masked by fermentation during the measurements. To prevent this, yeasted doughs were proved for various periods and then subjected to a freezing and thawing procedure to inactivate the yeast. Although they were also affected by freezing and thawing, changes in the rheological properties of dough due to fermentation were discernible.
The rheological properties of the yeasted doughs were measured under two different flow regimes: elongational and shear flow. Elongational flow was measured by extending a disc of dough vertically to create a dough filament. This elongates the dough in one direction (uniaxial). As dough is elongated the stress increases until it reaches a maximum value, at which point the elongated dough filaments break. Elongational properties of doughs are particularly relevant to breadmaking since the inflating gas bubbles in proving and baking doughs cause elongational flow in the surrounding dough.
The second flow regime, shear flow, was measured by placing yeasted dough samples between two parallel circular plates and rotating one of themxxxxDoes it result from the decrease in density as a dough is leavened? The freezing and thawing process used to inactivate the yeast was found to degas the fermented doughs, giving them uniform densities regardless of their fermentation time. So the dough density at measurement was not the sole cause of the weakening phenomenon seen in Figure 2. One possible explanation is that fermentation changes the chemical structure of doughs.
Yeasted dough contains flour starch granules enmeshed in a network of dough proteins (Figure 2). This protein network accounts for the unique breadmaking properties of wheat flour. The breadmaking quality of flour is linked to its content of the largest dough proteins, the insoluble glutenin proteins, because flours that have more insoluble glutenin bake into better quality bread than flours with less. Greater quantities of the high molecular weight insoluble glutenin proteins provide the strength needed to stabilise dough. They help prevent the collapse of gas bubbles and thus contribute to leavening during fermentation and baking. Therefore, the dough content of the insoluble glutenins was followed through fermentation using a chemical analysis technique known as size exclusion high performance liquid chromatography (SE-HPLC) (Figure 3).
SE-HPLC measurements of the yeasted doughs showed that the amount of insoluble glutenin increased during fermentation (Figure 3). This implies that fermentation helps to develop the protein structure of dough by increasing the cross-linking of the dough proteins and forming more insoluble glutenin proteins. Thus, the SEHPLC experiments provide direct evidence that fermentation contributes to the development of dough protein structure.
|Although the words stress and strain are common everyday words in the English language, they have specific meanings in the scientific field of rheology. Stress is defined as the force exerted on a body that tends to deform or strain its shape. Stress is the intensity of this force and is expressed in terms of force per unit area with the scientific unit of the Pascal (Pa) (which is used in tyre pressure gauges (kPa)). Strain is a measure of the change in form or size of an object resulting from the action of a stress or force. Strain is expressed in terms of the ratio of final deformed or stressed shape to the original shape and is therefore dimensionless.|
This increase in dough protein structure during fermentation would be expected to strengthen their dough rheological properties in line with their baking properties. However, in fact, the rheological properties of the yeasted doughs deteriorate with fermentation. What causes this apparent contradiction between the rheological and protein composition results?
What's happening? - Towards a theory
The best explanation for this apparent contradiction between the rheological and protein changes in fermenting yeasted doughs involves the inflating gas bubbles within the dough. Carbon dioxide generated by the fermenting yeast diffuses into and inflates air bubbles incorporated into the dough during mixing. As these bubbles grow in size they subject the surrounding dough regions to compression and biaxial elongation. The result of this deformation is that previously separate dough proteins are forced together, which encourages protein cross-linking. This forms larger dough proteins, including the very large insoluble glutenins measured by SE-HPLC (Figure 3).
Although the dough proteins become more cross-linked and larger, these increasingly large gas bubbles interrupt the dough protein network and weaken it. This weakening effect is quite pronounced since the gas bubbles are significantly larger than the dough proteins. This weakening behaviour still occurs in the degassed fermented doughs measured in this study since degassing the dough did not allow crosslinking.
Figure 4. Diagram showing how the small air bubbles present in the freshly mixed dough (a) are inflated by the fermenting yeast, forcing the dough proteins together and leading to the crosslinking and polymerisation of the dough proteins (b).
Gas bubbles play a highly significant role in the development of dough protein structure, especially in traditional breadmaking systems. The mystery of how dough protein structure developed in these processes may be explained by these results. In traditional breadmaking processes, dough is fermented in several stages separated by punching steps. Our results suggest that the fermentation process itself increased cross-linking between dough proteins and so helped to develop the dough protein structure.
In modern breadmaking methods fermentation only inflates and aerates a fine partially-aerated dough protein structure that is created during high-speed mixing or by chemical additives such as ascorbic acid. Our work shows that gas bubbles also determine the rheological and physical properties of fermenting doughs, proof that the secret of bread lies in the bubbles!
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