MadSci Network: Chemistry Query:

Interesting... Cornell University and Mac Farms have developed a carbonated milk-based beverage called e-Moo. And a company in Florida has one called Crazy Cow. But, based on the way these beverages are described, they are not made of pure milk but probably a diluted version.

My first reaction would have been that skim milk would fizz and the bubbles should last longer because they would be coated in proteins to keep them from popping too quickly. Creation of a stable foam is what happens when you make an ice cream float (or spider for those in Australia). But that isn't what is happening here so let's think on...

My second reaction about carbonating milk is that the carbonation effect won't last long and the milk will curdle. When carbon dioxide (CO2) comes in contact with water, it creates carbonic acid. When milk proteins are exposed to acid, which you want when you are making yogurt, the proteins denature.. curdle.. start to curl back on themselves, making them less soluble in water so they precipitate out of solution. Carbonating milk will have this same effect on the proteins, though to a lesser degree. Still not a desired effect for a beverage. If you want to see this process happen quickly, just add some orange juice to a glass of milk.

But back to your question of fizz... we already know that CO2, water and carbonic acid will try to reach an equilibrium in solution. You may also know that milk is naturally a slight acid, with a pH of ~6.6, but it contains a significant number of proteins that have basic side-groups. Acids and bases will react to create water and salt when mixed together. So, when exposing CO2 to milk, the CO2 & water reaction is going drive towards carbonic acid, which will react immediately with the basic compounds in the milk... causing even more CO2 to become carbonic acid. The CO2 will continue to be used up in this way until the bases are all used up and the milk becomes slightly more acidic... at which point, the proteins will start to curdle and you will end up with a very funky looking beverage indeed.

Very interesting series of reactions! Thanks for giving me an opportunity to figure it out. :)

BTW, other things to take into consideration when trying to carbonate a liquid: Carbon dioxide is not very soluble in water (see Solubility of Gases vs. Temperature). As a result, soda companies have to decrease the temperature and increase the pressure of the CO2 in order to get it into their sodas.

Good luck with your sodastream and thanks again for your question.

Kieran

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