The Fresh Loaf

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Sourdough Hydration Theory

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JERSK's picture

Sourdough Hydration Theory

  I've been wondering about the actual hydration of sourdough starter. When I make my bread, I usually feed my starter in increments. Assume it's 100% hydration. I take it it out of refrigeration. Remove half. Feed that half equal amounts water and flour. I come back when it is active and it ways slightly less. In subsequent feedings it's also less. I thought it was my scales They're not digital., but I've used digital scales and it also happens. So I assume a certain percentage has escaped as gas. The microorganisms take water and flour, eat them, and turn it into other compounds. Some of this is CO2. Some of this must also be the cellular structure that makes up the organisms. It seems when I make different kinds of bread using different flours and/or different ratios of various flours the dough has different levels of "stickiness". As the dough develops it becomes less sticky. I've always assumed that it's because of different flours, gluten development, absorbtion, humidity, etc. However, now I've been thinking that maybe it's partly because of the makeup of the organisms alive in the fermenting dough. I mean the cells aren't likely equal part solids and water. This would change the hydration level of the dough. Maybe it's insignificant. Maybe it changes with different types of flours, fermentation times, temperature or whatever conditions happens to exist. Does anybody else have any thoughts/opinions/theory/facts on this?

JERSK's picture

  O.K. so now I'm answering my own thread. If the bacteria and fungii in sourdough convert the compounds in flour and water into other compounds, does this change the protein and vitamin content of the bread? I have read the the carbohydrates and minerals of fermented grains are more digestible. Does the actual nutritional value of the bread change with fermentation?

suave's picture

I've been reading lately about soudough microbiology, and it is incredibly complicated.  There's a ton of specialized (and largely almost inaccessible) literature and there's a ton of factors that go into play.  Bacterial makeup, LAB/yeast ratio, hydration, temperature, acidity, salinity, flour.  But to answer your original question the only way for the sourdough to lose weight would be by evaporation of water and alcohol and by CO2 formation, but I've never seen anything of the sort working on half-pound scale.  How much weight do you lose?

JERSK's picture

    In say, a 3lb. batch of bread dough, I might only lose about a 1/2 oz. or so. It's not much, but it's always a little less. Never is it a little more. I thought it must be scale inaccuracies, but in that case it would occasionally weigh a little more. So it must be from evaporation or gas release. The O2 in CO2 comes from the water and the C must come from the flour. Also, some carbon, hydroggen, etc, must go into the formation of the microorganisms. They are carbon based life forms after all. So, all this must change the actual hydration levels somewhat. I realize these are probably irrelevant questions. Bread rises, bakes and usually works out O.K. All I'm getting at is threre is something going on at a microbial or molecular level that isn't being accounted for.  This may be the reason why one batch of bread may turn out just slightly different then other when all other factors seem to be equal. Just a bread nerd idea.

suave's picture

Oh, you have spring scale - that could be the source of your error.  Spring tension is supposed to be linear, and it almost is but not quite, so as you increase weight the error will accumulate, and it's not gonna be random.  On the other hand CO2 comes entirely from flour - yeast will convert one monosaccharide into two molecules of alcohol and two molecules of CO2.  A book I have says direct yeast fermentation gives 0.8% of alcohol.  For 3# it is just under half-ounce, but much of it must be entrapped in the dough and can't just evaporate, especially not from a covered bowl.  If you're interested in what exactly happens during fermentation I think Floyd has a recommendation for a book that  goes into details.

ivrib's picture


The loss in weight you witness in your sourdough is definitely no glitch in the scale. I have the same thing. It's the result of the good work done by the helpful wild yeast and bacteria which make up the microbial population of the sourdough starter.

What goes on in a sourdough culture is very complex. First of all in terms of the population of microorganisms - it's varied - probably several strains of yeast and lactic bacteria. Second of all in terms of the metabolism of each microorganism, and each strain. These different microbes produce many kinds of substances - organic acids (some of which give the tang to sourdough) and other organic compounds, gases such as carbon dioxide and water.

The main thing is this - as long as these byproducts remail solid or liquid and do not evaporate, the weight of you sourdough starter should remain the same - it will only change in composition - such and such compounds will turn into other compounds.

But if some of the byproducts are gas at room temperature they will be lost, and will contribute to loss of weight from the starter.

The main chemical reaction that goes on in the yeast cells in you starter is the following:

C6H12O6 + 6O2 → 6CO2 + 6H2O 

Meaning that 1 molecule of glucose (sugar) turns into 6 molecules of CO2 (carbon dioxide) and 6 molecules of H2O (water). CO2 is a gas at room temperature so it will either be trapped in the bubbles you see in the starter or escape into the air. Water is liquid at room temp so it will stay in the starter. It will also raise the hydration level of the starter.

At some point or another the populations of yeast and bacteria grow and becomes very dense and in the process they use up all the oxygen that's dissolved or trapped in the started as a result of mixing. Then the yeast and bacteria switch from aerobic respiration to fermentation. There are thousands of types of fermentation in nature. One of the most famous is alcoholic fermentation. In the absence of oxygen yeast will use sugar (specifically glucose) and turn it into ethyl alcohol by the following chemical reaction:

C6H12O6 → 2 C2H5OH + 2 CO2  (one molecule of glucose turns into 2 molecules of ehtyl alcohol and 2 molecules of CO2). Again this CO2 is lost from the starter.

So when there's oxygen more than half the weight of used up sugar is lost (because CO2 weighs more than water, and when there is no oxygen, less than half is lost because ethyl alcohol weighs more than CO2.)

I'm sure this isn't the only process that causes loss of weight but it's the main one.


dolfs's picture

The water in the culture/dough evaporates too. Even though it is a liquid, at room temperature there is slow evaporation. In fact, most doughs have an internal temperature of more like 80-85F while fermenting/proofing, so a little warmer than room temperature. This is why you cover your doughs to prevent them from drying out. I would not be surprised if the overall weight loss due to water and alcohol (evaporates even easier than water) is larger than what little CO2 escapes from the dough.


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