Saturday, January 6, 2007

Why taking specific gravity readings are important

Today I was a negligent scientist and brewer.

A measurement used ubiquitously in brewing is specific gravity (SG). SG is used to gauge the extent of fermentation and to estimate the alcohol content.

Specific gravity is defined as the density of a substance divided by the density of water. The density of water is 1 g/mL (technically only at 4° C), so for practical purposes, SG is the density of the substance divided by 1 g/mL. Not your most interesting mathematical operation. What it essentially does is give a value without units. This value would be (and is) more accurately described as "relative density."

SG is measured using a hydrometer, a sealed glass tube that floats in the liquid. The deeper it sinks, the less dense the liquid is.

Wort (the beer before it is fermented... so, it really isn't beer) is comprised of water and sugar (as well as many other things). This has a typical SG of 1.050. As the sugars get converted to ethanol, the SG drops. This is because the ethanol is less dense than the aqueous sugar solution. Pure ethanol (100% v/v) has a density of 0.789 g/mL. The fermentation does something else of note. Before fermentation, water is the only solvent and the sugars (and other things) are dissolved solids. When the dissolved sugars gets converted to ethanol, the ethanol (a liquid completely miscible with water) becomes a co-solvent.

The bottom line of all of this is that when the fermentation is finished and all of the fermentable sugars are gone, the SG stops dropping. When a homebrewer measures the SG 3 days in a row with no change in the reading, the fermentation is finished.

So, why was I a negligent scientist and brewer? Well, I didn't take SG readings of my Fat Tire Clone #2 (FTC2). I assumed it was time to bottle. So, I went through the process of cleaning bottles and sanitizing them in the dishwasher. I went downstairs to prepare the bottling bucket and get 1 cup of malt for the bottle priming. It was then that I noticed a positive pressure in the airlock of the secondary containing the FTC2. I watched for a few seconds and, sure enough, a bubble of fresh CO2 bubbled out: a sure sign the fermentation was NOT done. The SG is currently 1.014. I'll measure again in a few days.

So, I have a dishwasher full of sanitized bottles. I'm not sure I can get away with leaving them in there until next week.


Woller's Disciples said...

I feel that a joint venture could resolve the problem of your steril bottles. Woller & Faugstad's should potentially invest in a kegging system.

Also,is it crucial that the water added to the wort in order to "top off" the 5 gallon carboy be steril?

Chemgeek said...

#1 Sterile

#2 As long as the water didn't come from the pond behind your house, it should be OK straight from the tap.

David said...

Loved the simple breakdown of Specific Gravity and the fermentation process. I involve my 11 year old daughter in the brew process because I think it will help her appreciate and understand chemistry in school.

I have bottled beers before with positive pressure in the airlock before when they are near their target FG and have not changed much in a few days. In my experience, the ambient temp of the room can increase or reduce the pressure you see in the airlock, so an airlock that bubbles occasionally could be done fermenting, just fluctuating temps.

When I did extract partial boils, I just topped off my carboy with cold tap water that had been running for a minute or so already. I've never had a bad batch or a bottle bomb in 3 years (knock on wood).

I also went all grain for $11. See my $11 mash tun and the process I used to make it here