The yogurt culture in the Middle East is so extensive that it would take many years to fully describe it. Rare is the town in the area that does not use yogurt profusely in hundreds of recipes, including starters, first and second courses as well as drinks. Interestingly, unlike what has happened in the West, the use of yogurt in desserts is much less, if not zero, and sweetening it with sugar still sounds to Oriental people little less than heresy.
Related to Turkish ayran, Pakistani lassi and other drinks made from yogurt and water, doogh is typical of Iran. But there the similarities end.
To prepare doogh we need a natural yogurt without sugar that we must beat to make it creamy (or we save work by going directly to a "Greek" type yogurt).
Then we add water while continuing to beat until we get a consistency similar to that of thick milk. If you want - and of course the Persians almost always want it - you can mix very chopped dry mint, salt (half a teaspoon per liter of liquid) and some powdered black pepper.
And now comes the distinctive touch of this drink because we have to carbonate it naturally. To do this, we will leave the mixture without refrigerating for about 2-3 days.
In this way we achieve that the bacteria in the yogurt generate carbon dioxide in the same way that the yeast sponges the bread crumbs. These bubbles are very fine, similar to those obtained naturally in various wine products.
Unfortunately, this momentous step is often overlooked to simply mix yogurt and carbonated water. It is not the same, in fact it is not even doogh, although it can give us a general idea about what the "real" drink is like. If you do this little heresy please use at least one type of naturally carbonated water as the bubbles in artificially carbonated water are too big.
It is served cold or very cold.