Spontaneous explosion of a bottle containing alcohol and milk

A friend of mine gave me a bottle of home-made liquor which is a concoction of pure alcohol and sweetened milk. I accepted his gift happily and then I put the bottle in my cocktail cabinet. After an hour, we happened to hear a terrible noise of a some sort of explosion. We were indeed right - the bottle had exploded with actually no reason! Could someone please explain this strange phenomenon?

I am not sure whether this is important but he claims he had been keeping it in his fridge, although the difference of temperatures (in the fridge and in the cocktail cabinet) does not seem terribly significant to me. Also, the bottle was almost full meaning there was little room for the air in the bottle.

Please find attached the full list of ingredients:

• 0.5 litre of condensed, unsweetened milk,
• 0.5 litre of condensed, sweetened milk,
• 0.25 litre of pure alcohol,
• few sweets.
• I'm guessing that the mixture generated carbon dioxide gas, and that storing in the fridge retarded the formation of the $\ce{CO2}$. Alcohol is a weak acid, and in proximity to calcium carbonate, which may be in your milk, you could obtain $\ce{CO2}$. But I can't say this for certain, which is why I'm posting this as a comment. – user467 Mar 30 '13 at 23:51
• Please don't cross post your questions. The proper protocol is to post it one place and then ask for it to be moved if it hasn't gotten much attention. I think it's probably better off topic-wise here, to be honest. – jonsca Mar 31 '13 at 2:14
• @jonsca: Wait, are you calling it off topic here or on Physics? (IMO it's on topic on both, but better here). – ManishEarth Mar 31 '13 at 6:39
• @ManishEarth Sorry, my words blended together, it was "better off", "topic-wise". No, I think it's fine here! :) – jonsca Mar 31 '13 at 7:22
• @jonsca: Ah, that makes more sense :). I've closed the one on Physics and left a note; I'll open that one if there aren't any answers here (by the normal policy of only cross posting/migrating overlap questions when they don't get answers) – ManishEarth Mar 31 '13 at 9:44

The bottle can't have exploded for "no actual reason"; there may have been no apparent reason, but everything does indeed happen for a reason, at least at this level. But this isn't English.SE...

My bet is on a combination of things. As with many things in everyday chemistry, the substances involved are never pure enough to pinpoint one exact cause. Case in point, milk is an emulsion containing a significant percentage of fats and proteins. So, possible causes include:

• Hygroscopy and acidity of the alcohol causing the milk to curdle, by taking the water from the milk to form an alcohol solution that the complex milk proteins and fats precipitate out of due to lower solubility. This may produce a "greater than the sum" volume of product by leaving these proteins and fats in a porous gel (basically cottage cheese, though usually a stronger acid than ethanol is used in cheesemaking).
• Esterification. Milk, as stated, contains fats including fatty acids. Add alcohol, and the fatty acids react with the alcohol to produce esters (neutral fats). The byproduct is water, formed from the acid stripping the alcohol's hydroxyl group. Again, the products of this reaction can take up more volume than the reactants.
• Biological reduction. I actually do not think the concentration of alcohol in this solution would be antiseptic; if the alcohol is pure then we're only talking about a 20% solution by volume, and that would have to be a lab-distilled product like Everclear; "natural" distillation in open air only gets you to about 150 proof, and using even 100-proof vodka gives this mixture only a 10% concentration by volume of ethanol. According to at least one source, ethanol needs to be concentrated to at least a 65% solution by volume to be effective as an antiseptic. So, to a bacteria's point of view, the alcohol is an overworked police officer; it'll slow down bacterial growth, but there's way too much fat and sugar here for the alcohol to be much of a deterrent. Consumption of organic fuel by cellular metabolism, as a rule, produces CO2 gas, and in a closed container at room temperature, well, you get the picture.

I am guessing there was only a small gap between the cap and the liquid.

It was not a fermentation with that much alcohol.

It could be a question of gas being less soluble in liquid at a higher temperature (with the gas being CO2) causing a pressure rise.