Everything was set. The yeast were happy, I had the airlocks on so that any spare CO2 would bubble out, and so that nothing would explode.
When I woke up the next morning, though, I discovered that my explosion-prevention strategy had a slight flaw.
I woke up to this:
Turns out some strawberry pulp had made its way up the necks of the growlers and lodged itself into the airlock, producing a plug of strawberry slop. Eventually, the pressure built up inside and popped the stoppers, sending sweet, sticky strawberry pulp everywhere within about a 5-foot radius of my fermentation area (read: the kitchen counter). It almost hit the ceiling, as did Sarah when we woke up at 7 am to find strawberry slurry everywhere. The cap from one of the airlocks was later found on the other side of my apartment.
It turned out that I didn’t lose too much juice, so I cleaned everything up the best I could (luckily the walls and cabinets are white, so a little bleach worked nicely) and set the airlocks back in to continue fermentation, which, not surprisingly, was proceeding vigorously. Check out the bubblage in the video (you may have to tilt your head 90 degrees left to get the full effect). I sound a bit stoned, but I’m really just in awe of science.
By the way, fermentation comes from the Latin fervere, meaning “to boil”. Looking at a fermentation in progress, it’s not surprising that the Romans would think that’s what was going on. And I guess that technically, CO2 is escaping the surface tension of the liquid, so I guess in a way it is boiling… but I digress.
When Jean-Claude Antoine Chaptal discovered the relationship between sugar and CO2 in a fermentation, he solved a very big problem in Champagne. Knowing the exact amount of sugar to add to a secondary fermentation saved many, many bottles from exploding. So how much CO2 is generated from the sugar in a fermenation? Remember high school chemistry? No? Well buckle up because you’re going back to Mr. Burcik’s class.
OK, remember our master chemical reaction of fermentation?
sugar (C6H12O6) –> 2 CO2 + 2 alcohol (C2H5OH)
That means for every molecule of sugar turned into alcohol, 2 molecules of CO2 form and escape from the liquid in the form of carbon dioxide gas. Let’s say that I had been really dumb and sealed the vessels completely instead of allowing for the CO2 to bubble out. Also, for simplicity, let’s neglect the CO2 dissolving in the wine in the container (that gets complicated), and assume that CO2 is an ideal gas (not a great assumption but we’re not teaching thermodynamics here…yet). Some of these values are what we engineers call “engineering estimates”.
***Warning: Chemistry and Math ahead. If you just want to know if the thing will explode or not, skip to the bottom***
Anyway, our must is at 23 Brix, which means that in one growler, there is about 345 g of sugar (1.5 kg of must).
345 g sugar is 1.9 moles (glucose is 180 g/mol), so we will produce 3.8 moles of CO2 in the growler. Not so bad right?
Remember the ideal gas law? At standard temperature and pressure, 1 mole of gas takes up 22.4 L of space. Well, inside this vessel, the pressure will be anything but standard, and the temperature is 25C, not 0C. A little PV = nRT and we find that this gas wants to take up about 96 L when it is confined to about 2 L of growler. That means the pressure in there will go up to about 48 atmospheres. That’s about 20 times more pressure than in car tires.
If the container were at 48 atm and suddenly failed, the energy contained in the resulting adiabatic explosion would be V*ΔP = 2 L * (48 atm – 1 atm [atmospheric pressure]) = 9.5 kJ, which is the equivalent of 2.2 grams of TNT. That may not sound like much, but after searching some sites that in retrospect are only quasi-legal, I found out what 2 grams of flash powder (more or less equivalent to TNT in explosive energy, Vermeij et al., “Morphology and composition of pyrotechnic residues formed at different levels of confinement”, Forensic Science International, 2009) can do.
(Note: I am not dumb enough to have actually done this. There are lots of explosion videos on YouTube, though, and it’s not really that suprising.)
The bottom line is be careful with fermentations! I’m not saying don’t try this at home, (in fact, I hope I’m encouraging it!) but don’t seal off your fermentation, whatever you do! The growler would probably have broken around 4-5 atm anyway, so I wouldn’t have to worry about an M-80 (~2.5 g TNT equivalent) going off in the kitchen. In either case, though, I’d have a much bigger mess.
Extra credit: Using the above calculations, how much pressure does a fermenting champagne bottle get up to? Hint: secondary fermentations in 750 mL bottles are usually dosed with about 20 g of sugar.