I don't think what we're talking about here is curdling. When something curdles, it comes out of emulsion and forms lumps. With dairy products, weak acids start to denature the casein proteins and this causes curdling. We don't have the casein protein in orgeat, so the only way to make curdling happen would be for some other effect to cause the fat to come out of emulsion.
One thing we can note is that it's only homemade oregeat that seems to curdle. Commercial orgeat is made by emulsifying sugar, water and almond oils together, and this doesn't curdle. Even if we hypothesize that homemade orgeat has a much higher fat content compared to commercial orgeat, consider that heavy cream at around 36% fat doesn't curdle. It is unlikely that homemade orgeat approaches the fat content of heavy cream. Curdling also typically produces a characteristic "gritty" or "lumpy" texture, as the flocculated particles form lumps that are discernable to the tongue. Again, typically homemade orgeat is either already gritty or it never gets that way.
Rather, what I think is happening here results from the fact that homemade orgeat contains lots of suspended nut solids. These particles are suspended, but they aren't emulsified. This is because the typical hardware available to the homemade orgeat maker is not sufficient to reduce the particles down to a size at which they could form a colloidal suspension. For this, we would need something like a rotor-stator homogenizer or colloid mill. Because the nut particles are not emulsified in a colloidal suspension, gravity causes them to eventually settle out and fall out of suspension. This is why homemade orgeat typically separates and has to be shaken up before use. When we shake it up, we are re-suspending the nut particles. We can add things to the homemade orgeat (I have found a combination of gum arabic and xanthan gum to be particularly useful in this regard) to help keep the nut particles in suspension and slow down separation, but eventually gravity will have its way and the nut particles will settle out.
So what happens to cause this curdle-like effect? I think what happens is that the orgeat (along with its nut particles) is diluted when it is mixed with all the other liquids, and the suspended nut particles begin to settle out in the glass. If you let the drink sit for a while, of if it is a crushed ice drink so you have millions of tiny little pockets of water melting into the drink and not quite mixing with the nut particles that are coming out of suspension, it will create a mottled "curdle-like" appearance. The reason bostonapothecary's centrifuging technique works, I believe, is primarily because it removes the nut particles. I've always wanted to see what would happen if we took the other path and made an orgeat using a homogenizer to reduce the nut particles to the size of colloids.
Edited by slkinsey, 09 April 2013 - 07:05 AM.