For a while I was looking for a method to get my pistachio syrup stable, be it cloudy or clear, without using metres of muslin cloth. (pistachio syrup with xanthan). I actually solved this a little while back and after making it a few times have pretty much nailed down an easy method.
The recipe I was using had a splash of unexplained vinegar in it. For most of the time I have been making it I assumed it must have been included as a preservative measure, as with shrubs, as there was no mention of it in the recipes method. One day however, as I was melting sugar into my pistachio milk on the stove, I noticed a thick skin forming on the top of the liquid, and I began to skim it off, as you might while clarifying butter. So many things clicked in my brain at the same time (citric acid in cheesemaking, alternative milks like soy and almond curdling in hot coffee) and that unexplained splash of vinegar suddenly made sense.
Now I bring that stuff to the boil, stand over it at a simmer with a large flat spoon, and skim as much of that gunk from the surface as I can, and then it’s just one pass through few layers of muslin for a perfectly clear syrup. I wouldn’t necessarily use vinegar in every orgeat recipe I made. In this recipe it works because the pistachio syrup is used in combination with a shrub that also contains the same type of vinegar, so the distinctive flavour is balanced in the final drink. It’s definitely worth looking into how small an amount of acid you can add to encourage it to curdle, to reduce the chances of altering the flavour.