ABV and sucrose to ethanol conversion question

I have a question about how sucrose (table sugar) is converted to ethanol. I know that the basic equation for alcohol fermentation is C6H12O6 (aq) -->(with yeast and enzymes) 2CH3CH2OH (ethanol) + 2CO2(g).

That being stated, when i do the calculation on how much sugar to add to my various brews to end up around a certain ABV I get that approximately 16-17 grams of sugar is needed per litre of brew for each 1 percent alcohol by volume desired.

This calculation is for glucose though, not sucrose- so my question is how does sucrose get converted to ethanol? I know the yeast breaks it up into glucose and fructose(same chemical formula C6H12O6, different structure) , but are each of these then used up in the conversion to ethanol? it seems to me that if that were true the moles of ethanol produced would be doubled. I have always read that around 17-20 g of sugar per litre of brew is needed to produce 1% abv for sucrose, I am wondering how this is true if I get this number from doing the calculation for glucose.

After much cognitive processing and deliberation, I have your answer. This one stumped me, too. Formula weight for glucose and fructose is 180.16 g/mol, for sucrose it's 342.3 g/mol...for the purposes of this discussion, let's assume sucrose is twice the formula weight of glucose/fructose (since it's a disaccharide of these two sugars).

Now, let's say you have 180.6 g of each sugar. You would have one mole of glucose, but you would only have 1/2 mole of sucrose. That is, it takes twice as many grams of sucrose to make one mole.

Now let's talk about ethanol production. Because sucrose is comprised of glucose and fructose, you actually get four moles of ethanol.

If we obtained complete fermentation of the glucose, we would get two moles of ethanol. If we obtained complete fermentation of the sucrose, we would get 1/2 of four moles, or two moles.

End story...highly fermentable simple mono- and disaccharides such as these give the same ethanol production volume per gram.

Yeast cells do not posess the biochemistry (or enzymes with binding sites) to ferment fructose; sucrose only consists of one glucose molecule out of two total molecules and therefore can only be converted half as much as pure glucose. IF we could cut a sucrose molecule in half and throw away the fructose molecule, we see that we only have approx 1/2 our original sugar mass and volume; we can say with confidence that density of sucrose is approx. half that of glucose and therefore requires twice as much sucrose to get to equaling amounts of glucose.

Jbrewman said:

I... when i do the calculation on how much sugar to add to my various brews to end up around a certain ABV I get that approximately 16-17 grams of sugar is needed per litre of brew for each 1 percent alcohol by volume desired.

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Balling found many years back that each 2.0665 grams of 'extract' produced 1.00 grams of alcohol, 0.11 grams of yeast biomass and 0.9565 grams of CO2. It doesn't really matter whether the 'extract' is glucose, fructose, sucrose, maltose or any other fermentable sugar. Obviously one mole of hexose produces a certain amount of ethanol, yeast and CO2. A monosaccharide contains an equal number of moles of hexose per gram as a dissacharide and so on. Thus the finding that it is the weigh of extract that predicts.

Jbrewman said:

This calculation is for glucose though, not sucrose- so my question is how does sucrose get converted to ethanol? I know the yeast breaks it up into glucose and fructose(same chemical formula C6H12O6, different structure) , but are each of these then used up in the conversion to ethanol?

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Yes. After inversion glucose is phosorylated twice to become fructose 1,6 biphosphate. Aldolase then splits the F1,6B into Glyceraldehyde 3 Phosphate and Dihydroxyacetone phosphate. G3P gets phosphorylated again and down the rest of the pathway it goes to eventually emerge as Pyruvate which is decarboxylated to acetaldehyde which is reduced to ethanol.

The fructose half of the sucrose molecule gets phosphorylated (Frucktokinase) and that gets split into the 3 carbon G3P and DAP by the enzyme Fructose 1 phosphate aldolase. At this point its fate has been the same as that of the glucose molecule and the G3P get phosphorylated and goes down the resp of the EMP pathway to emerge as pyruvate as before.

Jbrewman said:

...it seems to me that if that were true the moles of ethanol produced would be doubled.

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That is the case but as noted above sucrose has the same number of moles of hexose (fructose and glucose) as an equal weight of either fructose or glucose. One mole (342 grams) of sucrose contains 1 mole each of glucose and fructose. 342 grams (2 moles) of glucose contains approximately 2 (we are ignoring the water that ties sucrose together) moles of glucose and 342 grams of fructose contains approximately 2 moles of fructose.

Jbrewman said:

I have always read that around 17-20 g of sugar per litre of brew is needed to produce 1% abv for sucrose, I am wondering how this is true if I get this number from doing the calculation for glucose.

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Those 17 - 20 grams of sucrose would, according to Balling, produce 8.22 - 9.68 grams of alcohol. In a liter of beer that would result in approximately 0.822 - 0.968% ABW which is 1.04 - 1.2% ABV. Sounds about right.

What ajdelange said is pretty much correct.

Both glucose and fructose can be metabolized and yeast will turn both into ethanol.

If you want a pure mole:mole ratio you get more alcohol from sucrose (double per mole), BUT since sucrose becomes 1 glucose and 1 fructose the amount per mass is pretty close. If you look at the masses it takes just a bit over 360 g of glucose to get the same alcohol as 342 g of sucrose (about 5% less).

If you want to explore the difference find a priming calculator (e.g. mrmalty) and compare the amount of glucose (corn sugar) needed vs sucrose (table sugar) and you will see the relationship.

I'm pretty sure that yeast ferments both glucose and fructose, otherwise my wine would be left sweet.