The baking process can be divided into three major sections: mixing, shaping and baking (with "eating" being the fourth and, arguably, the most important). Mixing has a major influence on everything that happens later, and any short comings will often show up in the final product.
In his book "The Taste of Bread," Raymond Calvel describes mixing like this: "Its primary role is to combine the individual ingredients that make up the dough and ensure the input of a sufficient amount of mechanical work (energy) to produce a smooth, cohesive, and homogeneous dough."
A "mix" has two complementary components: the mixer (the actual equipment doing the mixing), and the process or methodology (how long, what speeds, etc). But underlying it all is the chemistry responsible for the transformation of the flour and water into a cohesive dough.
At the molecular level, the action of the mixer is organizing and strengthening the gluten forming proteins gliandin and glutenin through the development of disulfide bonds between the strands of protein.
At regular points along the gluten forming proteins, sulfur-hydrogen groups stick out (see the diagram). Through the action of the mixer-hook, flour and water are combined resulting in the flour particles become fully hydrated. At the same time, oxidation of the dough begins to occur. As these protruding sulfur groups come in contact with one another in the presence of oxygen, the hydrogen molecule is released and the sulfur elements bind two protein molecules together. This has an organizing effect on the dough. The resulting characteristics of a bread dough—elasticity in proportion to extensibility—is a function of the relative ratios of gliadin and glutenin found in the flour. As the dough mixes, you can see the dough come together and strengthen—this visible change in the dough is the direct result of these disulfide bonds forming and the proteins folding and becoming organized.
Before moving on, it must be said that while this theory of dough development through the formation of disulfide bonds has been the standard understanding of how dough forms and strengthens for the last 60 years, there is recent research that this may in fact not be the mechanism. Now rather, there is evidence that the protein molecules are cross linked via a different molecule: tyrosine. But, for our purposes, it is enough to know that the protein molecules become linked and organized and, through this, become stronger, which allows flour and water to become dough.
Armed with our basic understanding of the dough-development process at a molecular level, we turn now to the actual mechanics: how we go from a flour-water mixture to a developed dough. In the beginning, dough development was done by hand. Today, of course, we utilize mixers to input the required mechanical work to develop dough. There are several types of mixers available. Planetary mixers have a fixed bowl within which a dough-hook turns in an orbital fashion. Oblique or fork mixers consist of a fixed, "lobster claw" style fork which rotates at an angle to the bowl; the bowl turns through the action of the fork on the dough, and resistance can be applied to the bowl to slow or speed up the dough's path around the bowl.
The third and perhaps most common mixer for the small to medium sized bakery nowadays is the spiral mixer. With the spiral mixer a spiral shaped dough hook is fixed near the back of the bowl. Unlike the other two mixer styles, the bowl of the spiral mixer is turned by the mixer motor. The speed of the hook relative to the speed of the bowl is critical: the hook needs to turn faster than the bowl which gives a kneading action between the hook and the bowl edge. Also, the distance between the top hook and the bottom in relation to the bowl edge needs to be the same. This allows one to mix doughs of various sizes in the same mixer (ie: a small dough will be mixed by the bottom of the hook and a larger dough will use the whole hook). The bowl design is important as well: some bowls have a flat bottom, others a convex bottom with the highpoint in the center, and some have a guide bar.
Speaking with Michael Eggebrect of Kemper Bakery Systems, he described the action of a spiral mixer as a "grab, stretch, knead, turn loose" type of motion. The hook grabs the dough, stretches the dough toward the center of the bowl and then back onto itself; then kneads the dough against the bowl edge and finally the dough is turned loose. The kneading action is critical and takes place as the hook pushes the dough against the bowl edge which is turning more slowly than the hook. Eggebrecht indicated that if the hook to bowl speed ratio is off (closer to 1:1) you can get more of a "punching" action which is not desirable.
Kemper Bakery Systems, a bakery equipment manufacturer, describes the process as it takes place in their spiral mixers as a "Three Zone Mixing Principle". In this system the mixing bowl is divided into three zones. In a spiral mixer the dough forms a continuous ring around the bowl with the guide bar in the center. The first zone is around the dough hook: the mixing zone, and is where the actual mechanical work on the dough takes place. If the mixer is sized properly to the amount of dough in the bowl, all of the dough will run through this zone. The second area is the resting zone where the dough is allowed to relax before re-entering the mixing zone. The third and final area is what Kemper refers to as the "air zone" or the hole in the center of the dough ring. The guide bar maintains this by peeling the dough off the hook as the dough leaves the mixing zone. This hole in the dough ring serves as an air exchanger allowing the warm air of the dough to escape and cooler air to enter, keeping the dough cooler, longer.
Of course, the process described here exists in any high-quality spiral mixer but is most evident in spiral mixers employing a guide bar and a properly set "bowl to hook" speed ratio.
Now that we have combined our understanding of the chemical transformation that must take place with a mixer, let's look at a fairly typical mixing method.
Standard Baking is a small retail/wholesale bakery on the waterfront in Portland, Maine. I spent a few hours there one afternoon while Ellen Lyford, one of their bakers, went through the mix of their "French" dough. For the purpose of our article, a basic white flour dough, such as Standard Baking's "French dough" (from which they produce baguettes among other breads) is a useful dough for examining the mixing process.
As we have discussed, the role of mixing is to incorporate the ingredients, flour, water, salt and yeast, plus the preferment, and then work the dough sufficiently to develop it into a dough that exhibits the proper balance of elasticity and extensibility. Mixing comes at a cost, however, as the action of the mixer heats and oxidizes the dough. As we have seen, oxidation of the dough is necessary, however, in too great an amount it is not good. While we need oxidation of the dough to facilitate the formation of the disulfide bonds, the downside is oxidation also destroys the delicate carotenoid pigments which can leave your dough pale and overworked and lacking in taste. Thus a balance must be struck and Standard's mixing process is a good example of how to accomplish this.
In this example, we are going to look at mixing times for 153 pounds of their French dough as mixed in a Kemper spiral mixer which has a rated capacity for 275 pounds of dough. It must be noted that there are many ways to satisfactorily mix a dough, and this is just one method, albeit a good one.
At Standard, the first step is introducing the weighed dry ingredients into the mixer bowl. At this stage they leave out the salt and yeast and preferments. The salt has a tightening effect on the dough which is not desired at this point of the mix. Once the ingredients are in the bowl the water is added and mixed on first speed for 4 minutes. At a point they switch to reverse for a short period of time in order to fully incorporate the ingredients. At this stage there is no development of the dough occurring (just incorporation and the wetting of the flour particles), so the reverse will have no deleterious affect on the dough.
After the 4 minutes of incorporating mix on first speed, the dough is given an autolyse or rest period for 20 minutes. During this time the dough fully hydrates and develops which results in improved extensibility of the dough benefiting the mix once it resumes. The net effect of the autolyse can reduce your overall mix times, thus reducing the deleterious effects of oxidation from over-mixing.
After the autolyse, the salt, yeast and preferment are added to the mixer bowl and mixed for an additional 12 minutes on first speed followed by 3 1/2 minutes on second speed. For larger doughs everything up to second speed is left the same, however the time on second speed increases proportionately to the size of the dough (ie. for their 253-pound mix they use 5 minutes on second speed). Lyford says Standard Baking is always looking for ways to reduce their mix times while still developing the dough fully.
Determining the end point of the mix can be difficult. Care must be taken to not mix your dough to full development. In addition to the mechanical development that takes place in the mixer, further development occurs during the handling of the dough throughout the remainder of the baking process. There is also "chemical" development (or maturation) of the dough that takes place primarily as a result of the increasing acidification of the dough as the dough ferments. With this in mind, one must find a point that is short of full development at which to stop the mixing of the dough, knowing that throughout the remainder of the bake, the dough will reach full development just prior to reaching the oven.
The above mixing times reflect a level of development that fits Standard's process, which includes a fold at one hour, dividing, shaping and retarding (overnight for most of the dough). Depending on your complete process you may have shorter or longer mixing times.
Mixing is such a fascinating process,and here we've just had space for the briefest overview. Regardless of your equipment, a good dough can be mixed if you seek to minimize mix times, which reduces oxidation of your dough and holds temperatures down. There are other techniques to achieve this goal but those outlined here should give you a good starting point.