Yogurt-based drinks are widespread across India in both sweet and salted forms. They are not simply milk with flavouring, and the differences begin with what fermentation did to the milk.
Fermentation restructures the protein
Bacteria in yogurt convert milk sugar into lactic acid, which lowers the acidity until casein proteins lose their mutual repulsion and form a loose gel.
That gel traps water, which is why yogurt is thick while the milk it came from was liquid despite containing almost the same components.
Whisking or churning breaks the gel into fragments, producing a pourable liquid whose viscosity depends entirely on how thoroughly it was broken.
Why the drink separates if it stands
Once broken, the protein fragments no longer hold water in a network, so they gradually settle and clear whey rises to the top.
Shaking or whisking again redistributes them, but the drink will separate once more because the original structure cannot be rebuilt.
This is why these drinks are made close to serving and stirred before drinking, rather than prepared long in advance.
Acidity does more than add sourness
The lactic acid gives the drink a clean tartness that plain milk lacks, and that sourness is what makes it work alongside rich or heavily spiced food.
It also affects how other flavourings register, sharpening salt and cumin in the savoury versions and cutting through sugar in the sweet ones.
Because the drink is already acidic, adding fruit or citrus rarely causes the curdling that the same additions would cause in fresh milk.
Salted and sweet versions do different jobs
The salted version is thinner, often churned with water until the fat separates out, and is seasoned with cumin, salt and sometimes green chilli or herbs.
The sweet version keeps more of the fat and is thickened rather than thinned, sometimes to the point of being eaten with a spoon.
Both are traditionally drunk with or after a meal, and their prevalence is greatest in the hottest and driest regions.
Why churning changes the result
Churning yogurt with water separates butterfat, which was historically the point, since the butter was collected and clarified into ghee.
What remains is the buttermilk, thin and tangy with most of the fat removed, and it became a drink in its own right rather than a by-product.
Commercial versions sold as buttermilk are often yogurt diluted directly without any churning, which gives a similar taste but a noticeably different body.