Water “hydrates” flour to make dough. What exactly does this mean? The two main parts of flour are starch granules—starch molecules packed together—and protein molecules.
The large protein molecules bond with water to form the network called gluten. Water molecules move among the starch and gluten, forming bonds in certain places and causing changes in the starch and gluten structure.
As early as 1820, scientists were studying water’s behavior toward wheat starch. [1]
Scientists (and bakers) knew that some flours (“strong”) produced good, big loaves of bread, while others (“weak”) did not. There were different theories on why. Some thought strength depended on how much water had been absorbed. Others thought it was the presence of acid and salt that changed the strength, not an actual property of the flour. [2]
A hydration study in 1918 used gluten from five different flours, both strong and weak.
Pieces of gluten were submerged in water and then weighed. Changes in weight measured how much water each absorbed. Data was taken with acid and salt water too. The study concluded that flour strength is an inherent property of the gluten and is related to thecolloidal nature of the proteins, discussed below.
Basics of the protein-water system
A colloid is a suspension of particles in a medium, such as water. It is somewhere between a solution of dissolved particles—like relatively teeny sugar molecules in water—and an insoluble mix—like non-polar oil molecules or relatively large grains of sand in water.
The colloid particles do not dissolve because they are too big. They can be big molecules or aggregates of molecules. Colloids are relevant to bread-making because doughis a colloid of protein molecules suspended in water. (Technically, this is asol, a colloid of a solid in a liquid.)
Colloids are stabilized by electrostatic repulsion, the repulsion due to charges on the colloid particles. Even though the solid particles may be neutral overall, there are charges on their surfaces that attract ions or polar molecules in the medium. Thus the particles end up with charged layers around them (below). These charged layers repel each other, preventing the particles from aggregating and stabilizing the colloidal system.
Water molecules are polar. This is because the oxygen atom in water has a greater ability to attract electrons than the hydrogen atoms do. The oxygen atom hogs the molecule’s electrons and thus has a slight negative charge while the hydrogen atoms have slight positive charges (below).
If the colloidal medium is water, polar water molecules orient themselves around each particle (below). This water layer is effectively a charged layer—more specifically, it is a charged double layer with a negative inner layer and a positive outer layer. The positive outer layers of the particles repel each other.
Colloids can be destroyed by heat or the addition of salt. Heat increases the energy of the particles, causing them to move about faster. They bump each other more and with greater force, so they are more likely to overcome their repulsion and aggregate. Adding salt, which dissolves into charged ions, interferes with the electrical layers stabilizing the particles and results in aggregation.
Research on water’s role in dough
In the 1920’s, research on the hydration of colloids became popular. A 1921 study classified five different types of water retention based on the effort it took to remove the water from the system. A 1924 study found seven forms of water. The general idea was that water in colloidal systems was not all held equally. A concept of “free” and “bound”
water developed. [3,4] Researchers measured percentages of bound water in colloidal systems, including dough.
In 1933, two mechanisms for water binding in protein were proposed. The first, now known as hydrogen bonding, consisted of water molecules partially sharing electrons with certain atoms on the proteins. Recall the partial charges on the atoms of the water molecule. Similar charges occur on certain atoms in proteins—partial negative charges on oxygen (O) and nitrogen (N) atoms and partial positive charges on hydrogen (H) atoms that are bonded to them. (Other atoms do not have the electron-pulling capability of oxygen and nitrogen and thus, though they can be partially charged, they do not have big enough charges to form a hydrogen bond.) Water molecules form hydrogen bonds with oxygen, nitrogen, and hydrogen atoms in the proteins as shown by the arrows in the figure below.
A second mechanism proposed for water-protein binding was electrical attraction. Certain sites on the protein may have a positive or negative charge caused by an atom missing an electron or gaining an electron, respectively. Polar water molecules are attracted to these charged sites and orient themselves around them, forming a kind of bond.
These two proposed mechanisms illustrated the point that there is not one kind of bound water. In addition to the two mechanisms, water held by each may be held with varying amounts of strength, depending on the atoms involved, the distance of the bond, and the neighboring atoms. Another complication is that hydration can change with changes in the system, such as adding acid or salt, and with changes in the protein configuration. For example, if the protein stretches out, more binding sites may become available and hydration will increase. [5]
Over the next three decades, bound water continued to be a topic of study. The 1960’s brought the advent of nuclear magnetic resonance (NMR), a technique that can identify the components of a sample based on how they respond to a magnetic field. Basically, as the strength of a magnetic field is increased, the nuclei of the different atoms in a sample in the field respond at different field strengths. This is represented by a plot showing peaks at the field strengths where there was a response. The peak height is relative to the amount of response and therefore the amount of that kind of atom present. NMR could be used to study bound water because the hydrogen atoms of the water molecules give different signals when they are bound or free.
A 1969 study used NMR to study water bound to proteins. In addition to finding evidence of bound water that existed well below the freezing point of normal, free water, the bound water percentage changed with changes in the conformation of the protein. This suggested that there were different types of bound water, such as water bound to the surface of the molecule versus water trapped in cavities that would be freed when the molecule unfolded. The energy needed to free a bound water molecule equaled the energy of a hydrogen bond, suggesting the presence of this kind of bonding. [6]
Studies throughout the 1970’s continued to use NMR to study bound water in dough. A relatively recent review in 1986 of the role of water in baking focuses mainly on NMR data. Even in this subset of the data, there is disagreement on the percentage of bound water. [7] The general theme that emerges is that, while a basic understanding of the possibilities of water binding in dough has been found, many of the details are still up for debate.
Thankfully, a basic understanding of water’s behavior is all that a home baker needs.
When dough is mixed, water hydrates protein by forming several types of bonds with it.
Some water molecules are tightly held, while others can come loose fairly easily. Some
are trapped within folds of protein and released when the protein changes shape. This protein-water system is the well-known substance gluten. More and stronger bonds with water form stronger gluten and therefore stronger dough.
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[1] Kuhlmann, A.G. and O.N. Golossowa. “Bound water in bread making.”Cereal Chemistry 13 (1936) 202-217.
[2] Gortner, R.A. and E.H. Doherty. “Hydration capacity of gluten from ‘strong’ and
‘weak’ flours.”Journal of Agricultural Research 13 (1918) 389-418.
[3] Skovholt, O. and C.H. Bailey. “Free and bound water in bread doughs.”Cereal Chemistry 12 (1935) 321-355.
[4] Kuhlmann, A.G. and O.N. Golossowa (1936).
[5] Lloyd, D.J. and H. Phillips. “Protein structure and protein hydration.”Transactions of the Faraday Society 29 (1933) 132-146.
[6] Kuntz Jr., I.D., T.S. Brassfield, G.D. Law, and G.V. Purcell. “Hydration of macromolecules.”Science 163 (1969) 1329-1331.
[7] Ablett, S., G.E. Attenburrow, and P.J. Lillford. “The significance of water in the baking process.” Chapter 3 inChemistry and Physics of Baking. London: The Royal Society of Chemistry, 1986.