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Like most food products yeasts are labeled with nutritional values for calories, carbs, etc. When yeast is 'activated' it is said to be 'reproducing' (or making more yeast). So, my question is,

"If I start with a tablespoon of yeast in bread, does the nutritional value change as the yeast grows?" .and if so

  • Is there a good scale for knowing by how much?
  • Are the nutritional values of the other ingredients (sugars) altered by by being consumed by the yeasts?
  • Are you asking if the nutritional value of the bread changes, or the yeast itself? – GdD May 2 '17 at 7:57
  • Effectively both. If the value of the yeast changes with the growth and if that then effects the bread, or is there a balance of some sort where as sugars are consumed by the yeast and the growing yeast 'offset' each other. – Cos Callis May 2 '17 at 14:20
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Well, it's a vanishingly small amount of sugar lost to the CO2. Nevertheless, from first principles, I would say that the answer is definitely yes: the nutritional value of the yeast-plus-bread food complex will change as the yeast consumes the sugar. All metabolic activity comes at the cost of energy, and while the nutritional values of the other ingredients will not, from first principles, be altered merely by being consumed into a yeast cell, once inside the yeast cell, they will be broken down by metabolic activity; and in the case of sugars, the metabolic byproduct of sugar is CO2, the gas which causes bread to rise.

Thus, one rough measure of how much CO2 has been produced would be to measure the volume of dough at the beginning of yeast activation, before and after any punching-down of the dough, and then at the end. Once getting those measurements, you'd take the total increase in volume over the total baking process, assume that that volume is equivalent to the volume of pure CO2 produced inside the bread, and then calculate what number of moles of CO2 would be required to take up such a volume at the relevant temperatures measured. You'd then calculate the number of moles of sugar that needed to be fermented (not fully-metabolized) in order to produce that much CO2, and convert the moles likely first to dry weight and then to calories, and voila! You have your number, specifically (according to the below comments) 3.75g of sugar per liter of CO2.

One caveat about this measure (beyond merely its internal assumptions about CO2 volume) is that it assumes the yeasts are primarily using glucose/fructose as a power source. The reality is that the metabolism of unicellular organisms is complicated, and there may be other compounds other than these in there. However, these minor metabolic side-effects will be even smaller components of the nutritional loss than that of the sugar, negligible in scope.

  • 1
    I feel like you might want to lead with the fact that it's pretty much nothing. It might also be nice to have at least a rough number for how vanishingly small. I don't think the OP was expecting to be asked to do calculations. Looks like it's pretty close to 1 gram of glucose per liter of CO2 produced at 20C room temperature? – Cascabel May 11 '17 at 4:03
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    Or actually, I think 3.75 grams of sugar per liter of CO2. Previous comment said 6 mol CO2 per mol of sugar, but that's for full metabolism. Yeast is just fermenting, and only makes 2 mol CO2 per mol of sugar. (And this is true for glucose and fructose. How much of each it consumes depends on the type of yeast. I'm not sure what typical bread yeast does, but I think most yeast prefers glucose.) – Cascabel May 11 '17 at 13:46
  • @Jefromi Edited, both the lead-in you mentioned, and to include mention of fermentation. – Jacob Stai May 11 '17 at 18:20

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