Dextrose vs Table Sugar for ABV

[allanmb]

I know roughly what the differences are between dextrose and table sugar but I expected them to be equivalent in the final ABV when used in a recipe. I have a 23L recipe which has 4.5kg of LME and 500g of table sugar which takes the ABV to 7.3% according to Beer Engine (there is 250g of Caraf III too but that is negligible I think). When I swap out the table sugar for the exact same weight of dextrose, the final ABV is only 6.7%.

Is this correct? Why is there a difference?

I thought both would be fully utilised by the yeast? I know the yeast breaks down the disaccharide into two monosaccharides, but how would that create double the ethanol from the same weight of sugar?

[allanmb]

So I spoke to my wife who used to be a lab scientist working with yeast (not saccharomyces cerivisiae). She wasn't 100% sure of the answer but she reckons that during the process of splitting the disaccharide into two monosaccharides, the yeast reduces it's effectiveness and therefore cannot convert all of the monosaccharides into alcohol. This theory seems to be backed up by Beer Engine as 500g of Dextrose gives a final gravity of 1.017 (+0.002 over no additional dextrose/sugar) whereas 500g of sugar gives a final gravity of 1.013 (-0.002 over no additional dextrose/sugar).

With this in mind it looks like if you want to increase the ABV of your beer without adding any extra colour/malt flavours (which is my goal for my current brew) then there is a difference between using dextrose and table sugar. Adding dextrose will have the desired effect of adding ABV but not adding colour, however it may also add a little sweetness to the brew. Adding table sugar instead will also have the desired effect but will also dry the brew in the process.

[Wonkydonkey]

Dextrose is glucose, sucrose ( sugar) is glucose-fructose bonded together,
Mrs yeast chomps away at sugar, it takes time and energy chomping away at the bond, and you can get other flavours if you make them chomp on to much.

Candi, ie sugar that has been treated with heat and an acid so Mrs yeast does not waste time and energy trying to chomp at the bond.

That's how I see it.

[allanmb]

I think I got mildly confused, but I still dont understand why there is a difference in output ABV. Here are my understandings do far:

Table sugar = sucrose which is a disaccharide
Dextrose = glucose which is a monosaccharide

Monosaccharide + Yeast -> Alcohol
Disaccharide + Yesast -> 2 monosaccharides -> Alcohol

When used in a beer calculator (as mentioned in my OP), table sugar seems to give approximately twice the amount of alcohol per weight of fermentable. Example

500g sucrose: OG: 1.069, FG: 1.013, ABV 7.3%
500g dextrose: OG 1.068, FG: 1.017, ABV 6.7%

So, if I want to get a higher ABV beer, I could use less sucrose than dextrose to get the equivalent ABV. Also, the end result would be a drier beer which may or may not be desirable. I have read some posts suggesting that sucrose adds an off taste, however I have also read contradictory posts that say that it doesnt impart any different flavour.

So, the point of my post is. Sucrose seems to give twice as much ABV as Dextrose per gram, why?

[Wonkydonkey]

I just came back to this, while reading a few older posts of mine.

First of all, I'm no expert, I can get the grasp of most things in brewing but never fully understand all the ins and outs.

So now I've said that, I think you have answered your own question. And it's late so I could be reading it wrong. But also while I think about it, so depending on what you eat eating there is energy used in digesting it. Same goes for yeast.

But anyway, from what I understand about table sugar giving off flavours, think of it like most flavours, ie. there is a perceived taste, and a flaour threshold, so if you can use a little ( what ever a little is) and cannot taste it, then great but others might taste it when you don't . But as in those trappeast type beers it would be noticed as what ever flavour because they use x amount.,. The point is you can hide a bit of sugar and a bit more in other type beers. But at some point you notice it.

[Pegasus]

Both glucose and fructose are fermentable by yeasts to produce alcohol. Wiki has some good info.

Can't comment on flavours but Graham Wheeler's book on British real ale recipes has a number with sugar included so should be ok to use.