A bowl of plain curd sits alongside most Indian home meals, unseasoned and unsweetened. It functions as a structural part of the plate rather than as a dish in its own right.
It sets the sour note in a spiced meal
Fermentation converts milk sugar to lactic acid, which is what makes curd tart. That acidity is the counterweight to the fat and spice in the rest of the meal.
Indian plates are built around contrast, and sourness is one of the axes. A meal without an acidic element reads as heavy however well the gravy is made.
This is also why the curd is served plain. Anything added to it would compete with the job it has been given at the table.
Fat carries chile heat away from the tongue
Capsaicin dissolves in fat rather than water, so dairy lifts it off the receptors it would otherwise keep stimulating. Water moves it around without removing it.
Curd works better here than milk in practice, because its thickness keeps it in contact with the palate long enough to do the job.
Cooks rely on this when a dish comes out hotter than planned. A spoon of curd on the side rescues a meal that some at the table could not otherwise finish.
It changes the texture of a plate of rice
Curd mixed into rice at the end of a meal binds loose grains and cools the mouth after spiced courses. In several regions this closing course is fixed practice.
The starch and the dairy together give a soft, cohesive mouthful that neither delivers alone. It is a texture with no equivalent earlier in the meal.
Leftover rice suits it better than fresh, since cooled grains have firmed up and hold their shape instead of breaking down into the curd as they are mixed.
The live culture is the point of setting it at home
Home-set curd is made by adding a spoon of an existing batch to warm milk and holding it warm while the bacteria multiply and acidify the milk.
Each batch seeds the next, so a household culture persists indefinitely. The strain mix drifts with the kitchen environment, which is why homemade curd tastes local.
Commercial yogurt varies in whether cultures remain live and in how much thickener and sweetener has been added, so labels are worth reading in a US supermarket.
Not every claim about it holds up
Fermented dairy has been studied extensively, and findings vary by product, strain and population. Broad health promises attached to curd usually outrun what the evidence supports.
Anyone with a dairy intolerance or a specific medical concern should raise it with a clinician rather than treating a traditional practice as a remedy.