Dough Development
Dough development is a relatively undefined term. Among other things, it addresses a number of complex changes in bread ingredients that are set in motion when the ingredients first become mixed. The changes are associated with first the formation of gluten, which requires both the hydration of the proteins in the flour and applied energy. The role of energy in the formation of gluten is not always fully appreciated. It is often erroneously associated with particular breadmaking processes, especially those which employ higher speed mixers.
Initially, gluten is formed when flour and water are mixed together. The proteins in the flour, glutenin and gliadin cross link, using the water as a vehicle to form gluten. Enhancing this gluten structure is important relative to developing a gas retaining structure in the bread. (Corriher) Energy is provided through the process of kneading. Simply put, gluten does not form spontaneously in that energy must be provided for its formation. There is no spontaneous combustion…at least not in breadmaking.
Cell Creation and Control Thereof in the Dough
The production of a defined cellular structure in the baked bread depends entirely on the creation and retention of gas bubbles in the dough. After mixing has been completed, the only 'new' gas which becomes available is the carbon dioxide gas generated by the yeast fermentation. Carbon dioxide gas has many special properties. At this point we are concerned with two: its high solubility and its relative inability to form gas bubbles. As the yeast produces carbon dioxide gas, the latter goes into solution in the aqueous phase within the dough.
If the carbon dioxide does not form its own gas bubbles how then does expansion of the dough through gas retention occur? Two other gases are available in significant quantities within the dough as a result of mixing. These are oxygen and nitrogen, both of which are derived from any quantities of air trapped within the dough matrix as it forms. In the case of oxygen, its residence time in the dough is relatively short since it is quickly used up by the yeast cells within the dough Indeed so successful is yeast at scavenging oxygen that in some breadmaking processes no oxygen remains in the dough by the end of the mixing cycle. Thus, the bread fermentation process is referred to as an anaerobic, alcoholic fermentation brought about by fermenting agents present in the dough, The rapid loss of oxygen from mechanically developed doughs has been illustrated previously for a wide range of nitrogen to oxygen ratios
With the removal of oxygen from the dough, the only gas that remains entrapped is nitrogen. Nitrogen plays a major role by providing bubble nuclei into which the carbon dioxide gas can diffuse as the latter comes out of solution. The number and sizes of gas bubbles available in the dough at the end of mixing will be strongly influenced by the mechanism of dough formation and the mixing conditions in a particular machine. The effects of mixer design are very important, but this is not within the scope of this presentation. At this stage it is only necessary to register the significant role that mixing will play in the creation, and/or manipulation of dough bubble structures.
Osmotic Pressure
The osmotic properties of a yeast cell are due to selective permeability of the cell wall with regard to solutions. This selectivity plays an important role in controlling the movement of nutrients into a cell. Nutrients are present in a medium in the form of ions, sugar, and amino acids. The permeability of the cell wall also permits the release of alcohol and carbon dioxide from the cell during fermentation.
High concentrations of sugars, inorganic salts, and other solubles inhibit yeast fermentation as a result of effects produced by high osmotic pressures. Basically, all fermentable sugars begin to exert an inhibiting effect on yeast when their concentration exceeds about 5% in the dough, with the degree of inhibition becoming progressively greater as the concentration of the sugar rises. This inhibitory effect is more pronounced with such sugars as sucrose, glucose and fructose than with maltose. The last sugar is a disaccharide that persists as such in the fermenting medium, and therefore exerts a lower osmotic pressure than the monosaccharides and the readily hydrolyzed sucrose, The sensitivity of yeast to osmotic pressure varies with different yeast strains, with some being better suited than others for fermenting sweet doughs with their high sugar contents.
Salt exerts a similar osmotic effect, except that some fermentation inhibition appears to set in at concentrations below the normal 2.0% level. A decrease in gas production occurred over a four (4) hour period when the concentration of sodium chloride was increased from 1.5 to 2.5% in a straight dough. One percent (1%) salt, based on flour, exerts an osmotic effect that is equivalent to that of 6% glucose.
Salt in concentrations over 1.5% exerts an inhibitory effect on yeast activity, either by its osmotic pressure or by a specific chemical effect. For this reason, salt is generally withheld from the sponge in the sponge-and-dough process. Interestingly, it has been shown that at lower levels, rather than being detrimental, salt actually exerts a favorable influence on yeast fermentation, A series of studies have shown that the use of 0.5 to 1.0% salt in the sponge of a sponge-and-dough process resulted in reductions in the fermentation time, while at the same time producing a better quality bread than was obtained with a sponge containing 0.15% or no salt.
The Fermentation Process
Please note that the majority of scientific data available to us regarding sponge and dough fermentation is found in Baking Science and Technology by E. J. Pyler. He describes sponge and dough as follows: "In the sponge-and-dough method, the major fermentative action takes place in a preferment, called the sponge, in which normally 50 to 70% of the total dough flour is subjected to the physical, chemical, and biological actions of fermenting yeast. The sponge is subsequently combined with the rest of the dough ingredients to receive its final physical development during the dough mixing or remix stage…. Sponge consistency may vary from stiff to soft or slack, depending on the baker's over-all expectations regarding its influence on final product quality".
As bakers know, before any dough can yield a light, aerated loaf of bread, it must be fermented for a sufficiently long time to permit the yeast to convert the assimilable carbohydrates into alcohol and carbon dioxide as the principal end products.
The most apparent physical change marking the course of fermentation in a dough is the steady increase in the volume of the dough mass. The sponge expands to four to five times its original volume before it recedes, assuming at the same time a light, spongy character. The findings described in Pyler relative to the gassing power of yeast action on carbohydrates are interesting and of value to the baker. For example, if 100 lb of flour will yield approximately 180 lb of dough, the degree of expansion in the dough during fermentation and proofing can be sustained by about 3.5% of fermentable carbohydrates, based on flour. Part of these carbohydrates may be comprised of the native sugars of flour, part may result from alpha-amylase action on damaged starch, and part may comprise added sugar. Any sugar over and beyond the 3.5% level will show up as residual sugar in the finished bread.
In bread baking, fermentation occurs due to a conversion of sugars (technically, glucides or sugars, naturally present in the flour) to alcohol and carbon dioxide under the effect of commercial or naturally occurring yeast and bacteria. This is categorized as alcoholic fermentation. Figure 3 outlines some of the basic chemical reactions which occur during fermentation. Not included in this schema is the conversion of sucrose to glucose and levulose by the enzyme invertase. Glucose and levulose are then subsequently converted to carbon dioxide and ethanol by zymase in the reaction shown in the Figure 3.
Sugar Transformations (Rosada)
Simple sugars: The main simple sugars, glucose and fructose, represent about 0.5% of the flour. Yeast can directly assimilate them by penetration of the cell membrane. Simple sugars are transformed into alcohol and carbon dioxide by zymase, an enzyme naturally present in yeast cells. Because of this easy absorption, these sugars are the first ones used in the fermentation process. Their consumption takes place during the first 30 minutes or so at the beginning of the fermentation process.
Complex sugars: The two main types naturally present in flour, saccharose and maltose, represent approximately 1% of the flour. Because of their complex composition, these sugars will be used later on in the fermentation process. The lapse of approximately 30 minutes at the beginning of the fermentation period is necessary to achieve their enzymatic transformation into simple sugars. The enzymes involved are saccharase, which transforms saccharose into glucose and fructose, and maltase, which transforms maltose into glucose.
Very Complex sugars: The main very complex sugar is starch, which represents about 70% of the flour content. Two types of starch are found in flour: amylose and amylopectin. Amylose is degraded by the enzyme beta amylase into maltose, and in turn the maltose will be degraded into glucose by the maltase enzyme. Amylopectin is degraded by the alpha amylase enzyme into dextrin, after which the dextrin is degraded by the beta amylase into maltose. This maltose will them be degraded by the maltase into glucose.
The simple sugar, glucose, obtained during these transformations is used by the yeast to generate carbon dioxide and alcohol. During the fermentation process, most of the starches used are the ones damaged during the milling process. Because the particles are damaged, they can easily absorb water during the dough making process. This water contact triggers the enzymatic activity. A non-damaged particle of starch will only retain water at its periphery and not inside the particle itself.
Last Edited on: 12/25/2001 11:30:56 PM