Bound and free water in dough

[Danijel Gorupec]

To prepare bread dough, I mix flour and water. They say that water hydrates starch and protein molecules in the dough...

What this 'hydrating' mean at the molecular level? I am imagining that water molecules make short-lived bounds with large dough molecules (carbohydrates and proteins). If so, does it mean that those bounds (between water and large molecules) are stronger and longer-lived than bounds between water molecules themselves? What is the thickness of this bound water layer - is it one molecule thick?

There is also free (unbound / bulk) water in the dough. I imagine there is some sort of equilibrium between bound and unbound water - that is, you cannot have a dough that only has bound (and zero unbound) water? Still, I imagine that in drier doughs, larger proportion of water is bound and smaller proportion is free?

Yeast needs water - I guess it lives inside water - is this the free (unbound) water where the yeast lives in? 

Btw, I am interested in all things dough, so if you have anything else interesting/important to mention, go ahead.

[exchemist]

2 hours ago, Danijel Gorupec said:

To prepare bread dough, I mix flour and water. They say that water hydrates starch and protein molecules in the dough...

What this 'hydrating' mean at the molecular level? I am imagining that water molecules make short-lived bounds with large dough molecules (carbohydrates and proteins). If so, does it mean that those bounds (between water and large molecules) are stronger and longer-lived than bounds between water molecules themselves? What is the thickness of this bound water layer - is it one molecule thick?

There is also free (unbound / bulk) water in the dough. I imagine there is some sort of equilibrium between bound and unbound water - that is, you cannot have a dough that only has bound (and zero unbound) water? Still, I imagine that in drier doughs, larger proportion of water is bound and smaller proportion is free?

Yeast needs water - I guess it lives inside water - is this the free (unbound) water where the yeast lives in? 

Btw, I am interested in all things dough, so if you have anything else interesting/important to mention, go ahead.

I found this paper on-line about hydration of starch which goes into some detail: https://www.sciencedirect.com/science/article/pii/S0144861722004477

From this it looks as if the hydration process is the opening up of the starch structure, replacing some of the internal hydrogen bonds between the sugar units of the chains (starch is a polysaccharide) with hydrogen bonds to water. Water that is chemically bound in this way will not make the, starch or dough, "wet", as it is chemically bound in. (In inorganic chemistry you may be familiar with the fact that copper sulphate can take an "anhydrous" (white) or a hydrated (blue) form. Both are dry crystals but there is more water bound chemically into the structure in the blue form. If you heat the blue form strongly it steams, losing water and turns white.) 

Presumably something similar can happen with proteins.

So yes I expect your dough has bound water, which will alter the structure of the starch and proteins by inserting water molecules between chains, and also unbound water which makes the dry material wet to the touch, sticks the grains together in a lump and makes it feel doughy. I don't know how much water the starch will absorb, but I don't think it will be just a monomolecular layer on the outside of each grain. From the paper, I take it that it disrupts the internal cross-linking structure of the starch as well. 

 

Edited April 3 by exchemist