Thandai is a cold spiced milk drink whose body comes largely from a paste of nuts and seeds. That construction distinguishes it from drinks that rely on dairy fat alone.
Ground nuts form an emulsion in the milk
Almonds, cashews and melon seeds are soaked and ground into a fine paste, which disperses through the milk as tiny particles carrying their own oil.
Those particles thicken the liquid and hold flavor compounds that would otherwise sit on the surface, so the drink tastes fuller without added cream.
The finer the grind, the more stable the suspension, which is why the paste is worked to smoothness and the finished drink is usually strained.
Seeds contribute more than the nuts do
Melon and poppy seeds are traditional and carry a substantial proportion of the thickening role, since they grind to a very fine paste and swell in liquid.
They also bring a mild, slightly grassy flavor that keeps the drink from tasting purely of almond, which would be cloying at this concentration.
Fennel and peppercorn are ground in alongside them, providing aromatic weight and a faint heat that reads against the cold milk.
The spice mixture is designed for a cold drink
Cold suppresses aroma perception, so a drink served chilled needs more aromatic material than the same drink served warm would.
Thandai answers that with a long spice list at high concentration, including rose, cardamom, saffron and pepper, which would be overwhelming in a hot drink.
The pepper in particular is doing sensory work rather than heating work, since it registers as a lift on a palate dulled by cold.
The paste is made ahead and the drink assembled late
The soaked and ground mixture keeps for a short time chilled, so households prepare it in advance and combine it with milk close to serving.
Mixed too early, the nut particles settle and the ground spices continue extracting into the milk, which pushes the flavor toward bitterness.
Commercial pastes and syrups exist for exactly this reason, trading some freshness for the ability to make one glass at a time.
It is a seasonal and occasional drink
Thandai is associated with hot weather and with spring festivals, and its cooling reputation comes from the seeds and the serving temperature together.
The concentration of nuts makes it substantial enough to sit closer to a light meal than to a beverage, which shapes the portion it is served in.
Cooks in the United States can source every component from an Indian grocery, and melon seeds are the one item unlikely to appear in a general supermarket.