Why Should You Toast or Bloom Spices?

You toast or bloom spices because heat unlocks flavour that raw spices keep locked away. Dry toasting triggers browning reactions that create new nutty, roasted aromas and dries seeds so they grind easily. Blooming in hot oil dissolves the many fat-soluble flavour compounds into the fat, which then carries them into every bite of the dish instead of leaving them stuck in the spice.

What's actually inside a spice?

Spices are dried seeds, bark, roots, buds and fruits, and most of their flavour lives in tiny pockets of aromatic oil. Each spice has its own signature molecules: cuminaldehyde gives cumin its warm, earthy smell, linalool makes coriander seed floral and citrusy, anethole gives fennel its liquorice note, eugenol makes cloves smell like cloves, and cinnamaldehyde is the heart of cinnamon.

Two facts about these molecules explain almost everything about cooking with spices:

1. They're volatile. They evaporate easily, which is how they reach your nose and why a jar of ground spice slowly loses its punch.
2. Most are fat-soluble. They dissolve much better in oil than in water. A spice simmered only in water releases some of its flavour, but a lot stays trapped.

Raw spices also tend to taste a little dusty, green or harsh. Heat changes that in two different ways, depending on whether you use a dry pan or a pan of oil.

What happens when you dry-toast spices?

When you heat whole seeds in a dry pan, several things happen at once.

Moisture leaves. Seeds hold a small amount of water. Driving it off makes them crisp and brittle, so they grind into a fine, even powder instead of a gummy paste.

New flavours form. Once the surface of the seed climbs well above the boiling point of water, browning reactions kick in. The Maillard reaction between sugars and amino acids in the seed creates compounds such as pyrazines, which smell nutty, roasted and toasty. These flavours simply don't exist in the raw spice. Toasted cumin tastes deeper and rounder; toasted coriander becomes warmer and almost sweet.

Some aroma escapes. That wonderful smell filling your kitchen is volatile oil leaving the spice. A little loss is the price of new flavour, but too much heat for too long means you're perfuming the room instead of the food. That's why the cue to stop is "fragrant", not "smoking".

Water-based cooking can't do this. Boiling caps the temperature at 100°C (212°F) at sea level, and browning reactions only really get going well above that. A dry pan or hot oil is the only way to reach roasting temperatures.

What does blooming spices in oil do?

Blooming, also called tempering, frying or making a tadka, means cooking spices briefly in hot fat. It does two jobs at once.

First, hot oil reaches temperatures far above boiling, so whole seeds toast and develop roasted flavours just as they would in a dry pan, only faster and more evenly, because the oil surrounds every seed.

Second, and more importantly, the fat dissolves the spice's fat-soluble flavour compounds. Instead of the flavour staying inside tiny bits of spice, it ends up in the oil, and the oil coats everything in the pot. That's why a dish built on bloomed spices tastes seasoned all the way through rather than speckled with occasional bursts.

Colour works the same way. The red pigments in paprika and chilli powder and the yellow curcumin in turmeric are all fat-soluble, which is why bloomed paprika turns oil a glowing brick red. Capsaicin from chillies is fat-soluble too, so blooming dried chillies spreads their heat evenly; there's more on that in why chilli peppers burn.

How does an Indian tadka work?

A tadka (also called chhonk or tempering) is one of the most elegant uses of blooming. Spices are fried in hot ghee or oil, often at the end of cooking, and the sizzling fat is poured straight over a dal, raita or vegetable dish. The order matters, because each ingredient needs a different amount of time:

1. Mustard seeds first. They need the most heat. Wait until they pop and crackle, usually within 30 seconds.
2. Cumin seeds next. They sizzle and darken a shade in 20 to 30 seconds.
3. Dried red chillies and curry leaves. A few seconds. Stand back, because curry leaves spit.
4. Asafoetida (hing) or ground spices last. Just 5 to 10 seconds, or they scorch.

Ghee is a favourite because it tolerates high heat well and adds its own nutty richness. Any oil you use should be comfortable at frying temperatures; see what is an oil's smoke point for how to choose.

Chef Fizz says: Whole spices go in first and ground spices go in last. The tiniest bits burn the fastest.

Should you toast whole or ground spices?

Whole spices are the better choice for toasting, and the reason is surface area.

A whole cumin seed keeps its aromatic oils sealed inside its own tough little package. Grinding ruptures that package and exposes a huge amount of surface to air, so ground spices lose aroma far faster. Whole spices kept in a sealed jar away from heat and light stay vibrant for a couple of years. Ground spices are at their best for roughly six months to a year, then fade into dull, dusty versions of themselves.

The same surface area makes ground spices burn quickly in a hot pan. A seed has a protective coat and a bit of bulk to absorb heat; a pinch of powder has neither.

The best routine for most seeds: toast whole, let them cool for a minute, then grind with a mortar and pestle or a spice grinder. You get roasted flavour and the freshest possible aroma.

How do you know when spices are done, not burnt?

Use your nose first, then your eyes and ears.

• Smell: Toasting spices go from faint to warm and nutty. The moment the smell turns sharp, acrid or smoky, you've gone too far.
• Colour: Seeds should darken by about one shade. Black flecks or a deep brown means burnt.
• Sound: Seeds start to crackle and some jump. Mustard seeds pop like tiny popcorn.
• Smoke: A few thin wisps from whole seeds are fine. Steady smoke means pull the pan off the heat now.

Burnt spices taste bitter and harsh, and there's no rescuing them. Because the pan stays hot after you turn off the burner, tip the spices onto a cold plate the moment they're ready so they don't keep cooking.

Which spices shouldn't you toast?

Not every spice benefits from a hot, dry pan.

• Paprika, chilli powder and ground turmeric scorch very easily and turn bitter. Bloom them briefly in oil over low heat, or add them with onions or liquid so they don't sit on a hot surface alone.
• Saffron gives the most colour and flavour when its threads are crumbled and steeped in warm water, stock or milk, not toasted.
• Garam masala and similar finishing blends are often made from already toasted spices. Stirring them in near the end keeps their delicate top notes.
• Dried leafy herbs like oregano, thyme and dried mint don't gain much from dry toasting. A few seconds in warm oil, or simply rubbing them between your fingers, wakes them up.

Garlic and fresh ginger often join spices in hot oil, but they burn quickly and turn bitter, so add them after the whole seeds and keep the heat moderate. For more on why garlic changes so much depending on how you prep it, see why crushed garlic tastes stronger.

Want to practise this without wasting a jar of cumin? The Cooking Scientist app has quick lessons on toasting spices, blooming in oil, whole versus ground and tadka tempering, each with a mini game.

Try it at home

The cumin side-by-side, about ten minutes.

1. Measure two teaspoons of whole cumin seeds.
2. Put one teaspoon in a dry frying pan over medium heat. Shake the pan every few seconds for 1 to 2 minutes, until the seeds smell warm and nutty and turn a shade darker. Tip them onto a cold plate.
3. Leave the other teaspoon raw.
4. Once cool, crush each batch separately in a mortar and pestle or with the bottom of a heavy mug.
5. Smell each one. Then stir each into its own small bowl of plain yogurt with a pinch of salt, wait five minutes, and taste.

The raw cumin will smell sharp, a little dusty and grassy. The toasted cumin will smell deeper, rounder and nuttier, with a roasted note the raw one simply doesn't have. In the yogurt, the toasted version tastes more savoury and more integrated. That difference is browning chemistry you made in two minutes.

Common mistakes

• Walking away from the pan. Spices go from perfect to burnt in seconds. Stay at the stove and keep the pan moving the whole time.
• Leaving toasted spices in the hot pan. The pan keeps cooking them after the burner is off. Tip them onto a cold plate as soon as they're fragrant.
• Adding ground spices first. Powders burn before whole seeds are ready. Start with whole spices and add ground ones in the final seconds, or after the onions.
• Using oil that's too cool. If seeds don't sizzle when they hit the fat, they soak up oil and stew instead of blooming. Test with one seed; it should bubble immediately.
• Toasting paprika on high heat. It scorches quickly and turns bitter. Bloom it gently in oil, off the heat or over low heat.
• Keeping spices for years. Ground spices fade within a year, and toasting can't bring back aroma that's already gone. Buy whole where you can, in small amounts, and grind as you need it.