One technique accounts for much of the aroma of everyday Indian food, and it is tadka. Known regionally as chhonk or baghar, it consists of frying whole spices, and often aromatics such as garlic, curry leaves or dried chillies, in very hot oil or ghee for a short time and then combining the fragrant fat, spices included, with a dish. The step lasts well under a minute. Its effect is out of proportion to its length, because it supplies a large part of the smell and top flavour of dals, steamed gram-flour cakes and certain chaat chutneys. This reference explains first what the fat does to the spices, then where in a recipe the step belongs, then how to judge heat and sequence, and finally how to avoid the errors that produce bitterness.
What tempering does to a spice
Many of the compounds responsible for the taste and smell of whole spices dissolve in fat much more readily than in water, so they escape far more completely into hot oil than into a pot of simmering liquid. A cumin seed stirred raw into dal contributes only a faint, chalky note. The same seed dropped into oil hot enough to sizzle and darken within seconds releases its aromatic oils at once and yields a toasted, nutty and much more concentrated version of the flavour. The logic of the whole method follows from this. The spice is cooked in fat first, so that its flavour is held in a medium that will carry it through the dish, and the spice is not left to seep slowly into mostly watery liquid.
Two positions in a recipe
The step can open a dish or close it, and the two positions do different work. A tempering made at the start, with cumin or mustard seeds cooked in oil before onion, ginger or garlic is added, forms the flavour foundation, and it is standard in dal, vegetable dishes and many gravies. A tempering poured over the food just before serving gives a bright top note that has not had time to blend into the rest. Dal makhani, some raitas and dhokla are finished in this way, the steamed cake receiving mustard seeds and curry leaves in hot oil at the last moment. Most dishes use one position only. A few use both, taking depth from the opening step and lift from the closing one.
Judging heat by eye and ear
The step is too quick for a thermometer to be useful, so cooks judge it by sight and sound. The fat is ready when a single mustard or cumin seed placed in it sizzles immediately and steadily, without smoke. In a mixed tempering, mustard seeds go in first because they need slightly more heat to pop. Once the popping subsides, the flame can be lowered a little before the more delicate ingredients are added, namely curry leaves, dried chillies and hing, all of which scorch quickly and turn bitter instead of fragrant if the fat is too hot. A useful habit is to keep a lid or a splatter guard within reach, because moisture on curry leaves makes the fat spit.
Sequence matters more than most recipes state
A typical mixed tempering follows an order that runs roughly from the hardest ingredient to the most fragile. First come mustard seeds, which tolerate the most heat and time. Second is cumin. Third is a pinch of asafoetida. Fourth are curry leaves, which spatter and should be added with the face turned away. Fifth are dried red chillies. Last come ground spices such as turmeric or chilli powder, which are added off the heat or on the lowest flame because they burn almost instantly and turn bitter. Garlic, if it is part of the mixture, goes in after the whole spices and before the ground ones, since it needs enough time to turn pale gold but must be removed from the heat before it darkens further. Following this order avoids the situation in which one ingredient is raw while another is already burnt.
Choosing the fat
Each fat gives a different result. Ghee brings its own nutty, lightly caramelised flavour, which suits dal, khichdi and festive dishes. A neutral oil leaves the spices with less competition and is often preferred for snacks such as dhokla or khandvi, where the taste of mustard seed and curry leaf should be clean and not layered under a buttery note. Mustard oil, used in parts of northern and eastern India, is sharp on its own and is traditionally heated until it stops smoking before the spices are added, so that its raw pungency is softened. The choice should follow the dish: richer fats for slow-cooked, comforting food, and lighter fats where the fresh aroma of the spices is the point.
Common errors
The most frequent error is tempering in fat that is not hot enough, which leaves the spices tasting raw and faintly soapy instead of toasted. The reverse error is equally common: fat that is too hot, or spices left in a few seconds too long, turns curry leaves and ground spices bitter. A burnt tempering cannot be repaired, and it is better discarded and made again than poured into a finished dish. A subtler error is to begin the step before the rest of the dish is ready. The interval between bloomed and burnt can be only five to ten seconds for delicate ingredients such as hing and curry leaves, so everything else should be measured and within reach before the pan goes on the flame. A final error is overloading the pan, which cools the fat and prevents the quick sizzle that defines the method.
Tempering beyond curries and dal
Although the technique is associated with simmered food, it appears throughout the snack and chaat repertoire. Dhokla and khandvi are both finished with a pouring of mustard seed and curry leaf in hot oil, which provides their final burst of aroma and prevents them from tasting flat. Some versions of misal and other sev-topped curries begin with a tempered base before anything else enters the pot. Certain chutneys also benefit from a quick tempering stirred in at the end, since blending whole spices raw into a chutney does not release their oils in the way that hot fat does. In each case the principle is unchanged: the aroma is created in the fat and then distributed through the food.
Practising the timing
Because the sequence moves quickly, it is learned better by repetition than by reading. A simple exercise is to heat two tablespoons of oil, add a quarter teaspoon of mustard seeds and watch and listen until the popping stops, which usually takes ten to fifteen seconds. The pan is then removed from the heat. Observing how the residual warmth of the oil continues to darken the seeds even off the flame shows how short the margin is. One or two rehearsals of this kind, done without a recipe depending on the result, make the real sequence of four or five additions in quick succession considerably less stressful the first time it is attempted for an actual meal.