Re: chemistry - alcohol

Howdy Tony.

This is a good idea (a formula to calculate the concentration of alcohol in 
a beverage, based upon knowledge of its ingredients).  In practice, I think 
it's pretty hard.  Let me satrt by saying I'm no expert in wine making, 
or fermntation in general.  I know a bit about beer making, but that is a 
simpler case as well (I think).  The complexity in making this 
determination arises from the complexity of the beverage.  For example, in 
the simplest case, you could assume that the only substance that is 
extracted from the grape is sugar.  If you then squeezed the grapes and 
weighed them before and after squeezing, you could determine how much sugar 
you had.  Then, once you added water and fermented the mixture, if you 
measured the volume of CO2 gas liberated, and wrote a balanced chemical 
reaction for the decomposition of the particular sugar to alcohol and CO2 
(assuming full conversion), you could solve for the mass of alcohol 
generated.

The problem is the complexity of the grape extract (several different 
sugars, for example), the difficulty in measuring the amount of CO2 
liberated, the incomplete conversion of sugars to alcohol (leaving some 
sugars unfermented and others converted to byproducts), and the different 
mechanisms for different sugar decomposition (fructose vs sucrose etc).

I don't know if you're familiar with a device called a hydrometer.  It is 
an inexpensive device available at any brewing or winemaking supply store 
(or probably via the internet) that allows the determination of a quantity 
called "specific gravity"...this is the ratio of the density of a liquid to 
that of water.  If you measure the specific gravity of an unfermented 
solution of "pre-wine", and then again after fermentation (do both at the 
same temp, as it is a temperature dependent quantity!), the difference will 
be due to the conversion of sugars to alcohol (principally).  You should be 
able to find a chart (wherever you obtaion your hydrometer...probably a few 
dollars for the hydrometer) which allows conversion of specific garvity 
units to alcohol concentration.

This is the simplest and most common way to determine alcohol content.  
Determining the level of "dryness", or lack thereof, is a bit harder, 
because you need to know something about the amount of sugars you start 
with.

A better determination of alcohol content in a beverage can be made with an 
instrument called a gas chromatograph (GC), but those are several thousands 
of dollars or more.

I hope this gives you a start on your quest.  I'd also recommend checking 
in with a brewing newsgroup, or a visit to your nearest brewing/winemaking 
shop.

Please don't hesitate to email me at:  weibel@chemistry.chem.utah.edu if I 
can be of further assistance.

Best Regards,
Mike Weibel