Disclaimer: I’m not an expert on this topic. This is information from Internet and papers collected in a personal research. Be careful if you take any decision based on this post.
One of the common discussions in forums about the no-chill practice is if wort stored in a cube can be an appropriate environment for the development of Clostridium botulinum.
C. botulinum is a bacterium that produces botulinum, one of the most lethal neurotoxins known. Botulinum infection results in botulism, that causes flaccid paralysis, with muscle weakness, double vision, difficult to speak… and death.
Let’s investigate this little dangerous guy. It’s an anaerobic bacterium, meaning that it does not require oxygen for growth, like yeast; but C. botulinum is obligate anaerobic: it dies with oxygen exposure.
But wait, C. botulinum can form endospores. Endospores, you say?
Endospores show no detectable metabolism and can survive extreme physical and chemical stresses, such as high levels of UV light, gamma radiation, detergents, disinfectants, heat, freezing, pressure, and desiccation. In this dormant state, these organisms may remain viable for millions of years,
Ok, we have, then, three guys: the bacterium, its spore stage, and the toxin. The bacterium can’t survive with oxygen, so we can forget about it: when we are introducing our wort in the no-chill cube there’s no C. botulinum.
What about the toxin? Well, the toxin is destroyed at 85ºC. So boiling during five to ten minutes is enough to get rid of it. We are free of toxins in our cube too.
The endospore is the tough one. We can have some of them in our wort and we’re going to seal them in the cube.
Eating the spores is not dangerous and not the point of this discussion (eating them it’s not a no-chill problem, but a general food problem). Don’t worry: our digestive tract is too acidic for them to grow (but be careful with newborns, they have low-acid digestive systems that can be a good environment for the spore).
So we are isolating the problem here to a concrete question: can the highly resistant spore germinate in our low oxygen sealed wort and generate the neurotoxin?
The spore can’t grow in acidic conditions (PH less than 4.6), but our wort is not as low.
Neither can’t grow with the presence of oxygen, and our cube is made of plastic so it’s a little permeable; but
The small amount of oxygen permeability that plastic shows is not sufficient to let in enough oxygen to inhibit C. botulinum. Oxygen levels over 2% are required for this.
(from Beer and Wine Journal)
Bacterium can’t grow in high salt or sugar concentrated environments. Osmotic pressure can kill them. This is why C. botulinum spores can’t germinate in honey and generate the neurotoxin. Some people states that the wort sugar content is too much for the bacterium, but I’m not really sure if this is true.
Honey has more than a 50% of sugars (water is only 14-18% of the concentration). A wort with 1050 SG will be equivalent to 12,4 °Bx, meaning that there’s one 12,4 sucrose grams for every 100 grams of solutions. Is this enough?
I won’t talk about water activity induced by sugar concentration. First because I don’t really understand its thermodynamics and second because although C. botulinum can’t grow with water activity lower than 0.85, neither Saccharomyces cerevisiae.
But we have hops. Hops are antibacterial:
hop extract is useful as an antibacterial agent against the dangerous pathogens Clostridium botulinum, Clostridium difficile, and Helicobacter pylori at levels below that at which a flavor from the acids contained therein is objectionable.
(from patent Antimicrobial activity of hops extract against Clostridium botulinum, Clostridium difficile and Helicobacter pylori)
Are hops enough to prevent C. botulinum germination? Sadly, this is an open question for me.
Botulism is rare. In all USA an average of 22 cases of foodborne botulism are reported yearly. There’s no known case of no-chill related botulism in the world.
Are we overanalyzing this “new” practice? Are our kegs or bottles dangerous too? The common sense says that the alcohol concentration is high enough to prevent germination, but bad news:
Growth and neurotoxin production were delayed by an ethanol concentration of 4% ethanol and completely inhibited by a concentration of 6%
(from Effect of ethanol on the growth of Clostridium botulinum)
Is the remaining yeast in the bottle enough to compete with the bacterium? Will the 4.0ph be enough to prevent germination? It seems so, but who knows.
Chris Colby, who panicked everybody with the Storing Wort Runs the Risk of Botulism article added something in the comments
It takes at least 3 days for botulism to grow to the point where it can be a problem.
This claim is fairly debatable, but maybe can reassure ourselves.
