Hey guys? Help me brainstorm how to make a new drink I'm kicking around.

[xiphias]

It's called the "Boba Libre".

See, the "Cuba Libre" is the official bartender name for a rum and coke. "Boba" are those tapioca pearls that you get in bubble tea.

And it's not quite working yet. I can't get the texture of the boba right. I may just need to practice with normal boba for a while.

So, you boil the tapioca pearls for, like, 25 minutes, then you let them cool for half an hour, and they'll supposedly stay appropriately chewy for several hours after that.

I've been doing the boiling in rum instead of water, and then putting the rum-soaked boba into a glass and pouring Coke over it.

It's almost working. But the texture is just a little off.

Any thoughts? The pot the boba boil in is sealed tightly to keep any alcohol vapors from escaping, and to keep oxygen out, so that the whole thing doesn't just catch on fire -- and it's a glass lid, so I can watch the alcohol condense on the top of the pot and drip back down in.

Should I be boiling the boba longer, assuming that the mixture is boiling at, say, 200 degrees F (93 C), instead of 212 F / 100 C? (Alcohol has a boiling point of 173 F/ 78.3 C, and distilled liquors are usually 40% alcohol, 60% water, so would that mean that it's boiling at about 200 degrees? Or would it mean that the alcohol is floating around as a vapor, and that then the water boils normally?)

Does anyone know anything about the specific chemistry of tapioca?

Comments

[brooksmoses]

Alcohol has a boiling point of 173 F/ 78.3 C, and distilled liquors are usually 40% alcohol, 60% water, so would that mean that it's boiling at about 200 degrees? Or would it mean that the alcohol is floating around as a vapor, and that then the water boils normally?

Mixtures are considerably more interesting than either of those two options. :)

The canonical graph looks like this, if I'm remembering my materials science correctly: Take a horizontal axis of mixture fraction, from 100% water to 100% alcohol, and a vertical axis of temperature. Put dots at 100% water and 212 F, and 100% alcohol and 173F. Then, draw two curved lines between the dots -- one line is concave upwards, one is concave downwards.

Normally, both lines will have a monotonic slope from one side to the other -- that is, when going from the "water" point to the "alcohol" point, they're both always sloping downwards -- but IIRC this isn't always true; there are cases where mixtures can boil when either compound alone won't. (Also, IIRC, alcohol and water do this slightly, which is why you can't get pure alcohol by distillation alone.)

Now, for stuff in thermodynamic equilibrium, anything below the bottom line is liquid, and anything above the top line is vapor. Anything between the two lines does not exist.

So, if you've got a mixture that's partly liquid and partly vapor at, say, 200F, to figure out what's in it you draw a horizontal line across the graph at 200F. The horizontal position where that intersects the top line gives you the composition of the vapor; the position where it intersects the bottom line gives you the composition of the liquid.

Alternately, if you want to find the boiling point of your distilled liquor, you can draw a vertical line at that concentration, and where it intersects the bottom line gives you the boiling temperature of that mixture. Draw a horizontal line through that intersection, and where that intersects the top line then tells you the concentration ratio of alcohol and water in the vapors that are boiling off. (That ratio will have more alcohol than water, which is why the mixture will loose alcohol faster than water when it's boiling in open air, but it is losing some of both even though the boiling point is notably less than that for pure water.)

Since the line is concave-upwards, that means the boiling point is less than 200F; I have no idea how much less.

I have no idea what this means for the boba, but it probably will need to boil at least somewhat longer.