I ordered a lassi at a place in a part of the country I did not know well and received something thin, salted and full of curry leaves. It was excellent and it was not what I expected, and it started me paying attention to a category I had treated as one thing.
The two ends of the range
At one end, a thick sweet drink, closer to a dessert, sometimes with fruit, sometimes topped with cream. Eaten with a spoon as much as drunk.
At the other, something thin and savoury — yoghurt let down with a lot of water, salted, spiced, sometimes tempered with mustard seeds and curry leaves.
These have different names in different places and the names overlap confusingly across regions. The functional difference is dilution and seasoning, and everything else follows from that.
What each is for
The distinction that made the category make sense to me.
The thick sweet version is a treat, or a meal in itself, or something to line the stomach before a long day. It is filling and it is not thirst-quenching, because the fat and the sugar work against that.
The thin savoury one is for heat and for digestion after eating. The salt matters — it replaces what you lose sweating, which is the whole point in a hot climate, and a sweet drink does not do that job.
Which is why the savoury version is what appears at the end of a heavy meal, and the sweet one at the start of a day or on its own.
The ratio is the whole recipe
Everything else is seasoning. The ratio of yoghurt to water determines what you are making.
Roughly equal parts gives you something in the middle — drinkable, with body, versatile.
Yoghurt with a splash of water gives the thick end.
One part yoghurt to two or three of water gives the thin savoury end, which looks alarmingly dilute if you are used to the other and is correct.
Getting this ratio right for what you want is more important than any of the additions, and it is the thing recipes are vaguest about.
The yoghurt itself matters enormously
More than in almost any other application, because there is nothing to hide behind.
Sour yoghurt makes a sour drink. Some traditions want that and consider a fresh mild yoghurt wrong for the purpose.
Fat content changes the mouthfeel completely. A drink made from full fat yoghurt is a different substance from one made with low fat, and the low fat version tends to taste thin and slightly chalky.
Set yoghurt versus stirred yoghurt affects how it blends. Set yoghurt whisked properly gives a smoother result than stirred yoghurt, which sounds backwards and is consistent in my kitchen.
Temperature and the ice question
A small argument I have had repeatedly.
Blending with ice is convenient and it dilutes as it melts, which means the drink changes over the ten minutes you spend drinking it.
Using chilled yoghurt and chilled water gives a stable drink at the ratio you intended.
Traditional practice in most places is the latter, partly because ice was not available and partly because it gives a better result. I have come round to it entirely, which required buying a jug and planning half an hour ahead.
The savoury version's seasoning
Since this is the one most people outside the region have not had.
Salt, obviously, and more than you would expect. Roasted cumin, ground, which is the dominant aroma. Sometimes black salt, which brings a sulphurous note that is either the best part or an acquired taste.
Fresh coriander, chopped fine. Green chilli, sometimes. Ginger, occasionally.
And in some regions a tempering — mustard seeds, curry leaves and asafoetida bloomed in a little oil and poured in, which sounds strange in a cold drink and is remarkable.
That tempered version was what I was served in the place I did not know, and it is now what I make most often.
What I would suggest trying
If you have only had the sweet version, make the thin salted one on a hot day and drink it with a meal rather than instead of one.
The proportion will feel wrong while you are making it. Trust it, add the salt more generously than instinct suggests, and use roasted cumin rather than raw, because the difference between the two is larger than anything else on the list.
Setting your own yoghurt
Since the yoghurt is most of the drink, it is worth a line.
Home-set yoghurt is straightforward — milk boiled, cooled to the point where it is warm rather than hot, a spoon of existing yoghurt stirred in, left somewhere warm for several hours.
The result is milder and looser than most commercial yoghurt, which is exactly what these drinks want.
The variables are the temperature when you add the culture, which if too hot kills it, and how long you leave it, which determines sourness. Both are learned in two or three attempts and neither is difficult.