Determine alcohol by chemistry

TheDuke

Hi all,

Can I calculate the rough expected alcohol content on chemistry principles rather than OG & FG?

Here's my thinking -
> 100g dextrose in 5l gives you 1% alcohol (roughly)
> 100g LME will be 0.8% to 5l (roughly)
> then 100g dextros and 750g LME to 5l should produce something around 6%

Am I way off or does that maths make sense? Just wondering.

Thanks,

The Duke

Cedrus

TheDuke wrote: »

Hi all,

Can I calculate the rough expected alcohol content on chemistry principles rather than OG & FG?

Here's my thinking -
> 100g dextrose in 5l gives you 1% alcohol (roughly)
> 100g LME will be 0.8% to 5l (roughly)
> then 100g dextros and 750g LME to 5l should produce something around 6%

Am I way off or does that maths make sense? Just wondering.

Thanks,

The Duke

In theory yes, there are software packages that do this too, but in looking at the Expected alcohol content you're really just measuring the sugar not the alcohol. You need to measure at the end to find out if you even have alcohol at all.

Chimaera

TheDuke wrote: »

Hi all,

Can I calculate the rough expected alcohol content on chemistry principles rather than OG & FG?

Here's my thinking -
> 100g dextrose in 5l gives you 1% alcohol (roughly)
> 100g LME will be 0.8% to 5l (roughly)
> then 100g dextros and 750g LME to 5l should produce something around 6%

Am I way off or does that maths make sense? Just wondering.

Thanks,

The Duke

The answer is yes but it depends on how good your chemistry/biochemistry knowledge is. In general, adding more sugar will give more alcohol, but there's an upper limit on alcohol content imposed by the yeast's ability to keep working. As you get near that limit, the rate of conversion of sugar to alcohol slows.

There's some basic info on it here and here.

At the other end, there are titration techniques that allow you to determine the exact alcohol content in your final brew: http://www.outreach.canterbury.ac.nz/chemistry/documents/ethanol.pdf

TheDuke

Thanks guys,

So the ceiling is clear 6% what ever I throw at it unless my yeast packet explicity states that it can tollerate higher alcohol content. My brews so far definitley had a good alcohol content as there's a nice buzz off them. I get the idea that OG v FG is really just a sugar content base guess work but what you're telling me here is that gravity readings really is the 'lazy mans best option' and there isn't an even shorter shot cut to be had.

masseyno9

As I understand it, OG and FG are very accurate (as accurate as the eye taking the reading anyway!). This is how I understand it, someone correct me if I'm wrong!

Basically, wort or beer is a mixture of sugar syrup, water and alcohol. Given that the density is known for all of these and we know that the density of water doesn't change (it's 1), the difference in OG and FG reflects the sugar changing to alcohol. With the other known values, the alcohol content can be calculated. The readings and the formulae do all of this for you, so yes, it kind of is the lazy man's way! Although I'm often too lazy to do it anyway!

Chimaera

The reason measuring density works is that the density of a sugar solution is quite different to that of an alcohol solution. At the very start, it's quite accurate since you're only dealing with sugar. Once the yeast gets to work it gets complicated because you're now working with a mixture of sugar and alcohol. As the fermentation progresses, the balance of sugar vs alcohol changes and so the density changes. At the end, when the fermentation is done you may still have some sugar left so your final density won't tell you exactly what your alcohol content is, but it won't be too far off.

Ideally, you want to measure density regularly through the fermentation, maybe every three to four days, and plot this on a graph. Once the density has levelled off, your fermentation is over, regardless of what level it's reached. There may be issues with opening up the vessel every few days to sample it though.

You can titrate for alcohol but the procedure is involved and you'll be contacting a lab supplies company for chemicals and glassware. Hence using density as a quick and dirty method that works pretty ok.

TheDuke

Thanks - really appreciate the super clever answers... I'm just really trying to get the super lazy mans answer... my thinking at this point is to put the hydrometer (well desinfected of course) directly into the demijohn for the whole fermentation process... I haven't seen anyone do that but these are 5l batches only.... I won't be offended if I get boohs from the propper grown up home brewers... stil on a journey here )

loyatemu

TheDuke wrote: »

Thanks - really appreciate the super clever answers... I'm just really trying to get the super lazy mans answer... my thinking at this point is to put the hydrometer (well desinfected of course) directly into the demijohn for the whole fermentation process... I haven't seen anyone do that but these are 5l batches only.... I won't be offended if I get boohs from the propper grown up home brewers... stil on a journey here )

that's fine, I do that all the time. Its a little difficult to read, but I'm not that bothered about the accuracy of the reading - there'll be alcohol in it one way or the other.

Carawaystick

TheDuke wrote: »

Thanks guys,

So the ceiling is clear 6% what ever I throw at it unless my yeast packet explicity states that it can tollerate higher alcohol content.

It's very likely any alcohol tolerance %age will be a reasonable minimum for the strain of yeast

I'd say any ordinary beer yeast would be well able to hit more than 6%
Unless it specifically was bred for very low alcohol tolerance, but that would be a speciality yeast.

TheDuke

thanks guys... really appreciate the support here as I work my way through the chemistry of it all...