The Mexican Chilli Heat Scale, Translated into Chillies You Can Buy in the UK

Scoville is a range, not a number. How Mexican chillies map onto the fresh ones in British shops, and how to rebuild a recipe that asks for serranos today.

Our dried chilli guide handles anchos, guajillos, pasillas and the rest: the ones that arrive in a bag, get toasted and soaked, and form the base of a sauce. This article is about the other half of the problem, which is fresh chillies, and about the scale itself. A Mexican recipe asks for two serranos. You are standing in a British supermarket in front of a plastic tray labelled simply red chillies. What do you do?

Scoville is a range, and anyone quoting you a single number is rounding

The Scoville scale measures capsaicin concentration, and capsaicin concentration in a living plant varies with everything: the variety, the growing conditions, the amount of water the plant got, how much sun it had, how ripe the fruit was when picked, and which fruit on the plant you happened to pick. Two jalapenos from the same bush can differ several-fold. Published figures are averages across a sample, which is why every honest source gives a band rather than a figure.

What follows, therefore, is bands. A jalapeno sits in the low thousands of Scoville units, a habanero in the low hundreds of thousands, and the gap between them is two orders of magnitude rather than a precise multiple. Treat it as a map, not a measurement. The practical upshot is a rule we would put above everything else on this page: taste a sliver of the raw chilli before you commit it to the pot. It takes four seconds and it is more accurate than any chart ever printed.

Why heat is not evenly spread inside the fruit

Most people believe the heat lives in the seeds. It does not. Capsaicin is produced in the placental tissue, the pale pithy membrane the seeds are attached to, and it concentrates there and in the surrounding veins. The seeds pick up some by contact. This is why deseeding a chilli reduces the heat, and why scraping out the white pith reduces it a great deal more. The flesh of the wall carries flavour and comparatively little burn, which is the whole technique behind a roasted poblano.

So a recipe asking for a chilli with the veins removed is not being fussy, it is asking you to separate flavour from heat. If your chilli turns out hotter than the recipe assumed, the first move is not to use less of it. It is to strip the pith and use the same amount.

The map

Mexican chilli	Heat band	Nearest fresh chilli in British shops	What changes
Poblano	Very mild	A sweet green pepper for bulk, with a single green finger chilli added for character	You lose the poblano's thick smoky flesh. Worth seeking out the real thing at a Latin or Turkish greengrocer for chiles rellenos.
Padron and similar frying chillies	Mild, with occasional hot ones	Padron peppers, now stocked by most large supermarkets in season	Nothing. This is one of the few direct matches on the shelf.
Jalapeno	Mild to medium	Large mild green chillies, sold loose or on a tray	British trays are inconsistent. Taste first. Jarred sliced jalapenos are pickled, so they bring vinegar as well as heat.
Serrano	Medium	Green finger chillies from an Indian grocer, or a bird's eye used at half the quantity	Serranos are grassy and bright and you can eat several. Bird's eye are sharper and much hotter.
Chile de arbol, fresh	Medium to hot	Thin red finger chillies, or a single bird's eye	Close enough in practice. The dried version is far more common in Mexican cooking anyway.
Manzano	Hot, fruity	Nothing on a British shelf. Scotch bonnet at reduced quantity is the closest in character	Manzano is a different species with black seeds and an apple-like flesh. A true gap.
Habanero	Very hot	Scotch bonnet, from any Caribbean grocer and most large supermarkets	Same species, similar heat, different aromatic: scotch bonnet leans sweeter and more tropical. Usable in almost every habanero recipe.
Chiltepin	Very hot, sharp and fast	Bird's eye chillies, dried or fresh	Chiltepin heat arrives and leaves quickly. Bird's eye lingers longer.

The problem with the tray labelled red chillies

British supermarkets sell an unnamed generic chilli, usually long, red and tapering, sometimes green, almost always sold as simply chilli or red chilli. It is typically a cayenne-type cultivar, and its heat lands somewhere around the jalapeno band or a little above, which is to say it is useful. The problem is not its heat, it is the lack of labelling: you cannot plan a dish around an ingredient whose identity changes between deliveries.

Two habits fix this. Buy from a greengrocer or an Indian, Caribbean or Turkish grocer where the chillies are named and sold loose, so you can pick the size you want. And when you do use the supermarket tray, taste one before cooking and adjust the count, not the recipe.

How to rebuild a recipe that asks for serranos when you only have bird's eye

Work by weight and by function, not by count. Four serranos in a salsa verde might weigh fifty grams and contribute both heat and a green vegetal flavour that is part of the sauce. Four bird's eye chillies weigh perhaps eight grams and will make it hotter than the original while contributing almost nothing to the body.

So split the job. Use one bird's eye, pith removed, for the heat. Make up the missing fifty grams with something green and mild: a green pepper, two large mild green chillies, or the white part of a spring onion if that is what is in the fridge. You are reconstructing the chilli as two ingredients, which is what the swap actually requires. The same logic runs in reverse: if a recipe calls for one scotch bonnet and you have a tray of mild supermarket chillies, no quantity of them will get you there, and you should use a few drops of a habanero hot sauce for the heat instead.

Heat you have already added, and what to do about it

Capsaicin is not water soluble, so water, beer and lager spread it rather than removing it. Fat and casein do bind it, which is why soured cream, crema, cheese, avocado and whole milk genuinely help at the table. In the pot, the levers are dilution with something starchy or fatty, acidity to distract the palate, and a little sugar to round the edge. Adding more tomato to an over-hot salsa works better than adding water, because the sugars and the pulp both do something.

There is no way to remove heat once it is in. Plan for it: hold back half the chilli, finish the dish, taste, then add the rest. Every cook who has made a salsa for a table of eight has learned this the hard way once.

Fresh, dried and smoked are three different ingredients

A fresh jalapeno, a dried chipotle and a tinned chipotle in adobo are the same fruit at three stages, and they are not interchangeable. Drying concentrates sugars and develops dried-fruit and tobacco notes; smoking adds a whole dimension that the fresh fruit has none of. A recipe asking for chipotle wants the smoke. A fresh jalapeno will not give you it, and a pinch of smoked paprika plus a fresh chilli is a better approximation than either alone.

British shops are much better supplied with the tinned chipotles in adobo than with any fresh Mexican chilli, which quietly shapes what is easy to cook here. A tin keeps for weeks in the fridge once opened, freezes well in single-chilli portions, and solves the smoke problem for a dozen recipes.

Growing your own is more realistic than it sounds

Serranos, jalapenos and chiltepines are all grown successfully in British conditions, in a greenhouse, a conservatory or a sunny windowsill, and seed is easy to obtain. Our piece on growing chilli plants in a British garden covers the practicalities. One productive jalapeno plant removes the supermarket tray problem entirely for a season, and a frozen bag of your own chillies behaves perfectly well in any cooked sauce.

Common questions

Is a scotch bonnet really the same heat as a habanero?

Broadly yes; they are the same species and both sit in the very hot band. The flavours differ, and most people find scotch bonnet slightly sweeter. For Mexican cooking it is the single most useful substitution available in Britain.

Does removing the seeds make a chilli mild?

It helps a little. Removing the pale pith the seeds attach to helps a lot more, because that is where the capsaicin is made.

Why was my salsa hotter the next day?

Capsaicin keeps distributing through the liquid as it stands, so a resting salsa reads hotter and more even than a freshly blended one. Season for the day you plan to eat it.