Limau kasturi, or calamansi: the lime that is not quite a lime

Limau kasturi is calamansi, and calamansi is not a lime, although it does a lime's job so often that nobody much minds the distinction. You notice it when you run out and reach for an ordinary lime instead, and the dish comes out sharper, plainer and very slightly wrong.

Here is what it is, where it belongs, how to make the juice, and what the words on a bottle of it mean.

What calamansi actually is

A small round citrus, roughly two to four centimetres across, with a thin glossy peel, usually green when sold and orange when fully ripe. Cut one open and the flesh is already orange under a green skin, the first sign that this is not a lime. The second is the seeds: a great many, for a fruit this size.

Botanists usually file it as Citrus × microcarpa, and you will also see ×Citrofortunella microcarpa. It is generally described as a cross between a mandarin and a kumquat, and it tastes like one: the juice is properly sour, while the peel is sweet and fragrant in a way that lime peel is not.

It also has more names than most fruit its size. Limau kasturi in Malay, sometimes spelt kesturi. Calamansi or kalamansi in the Philippines, where it is close to a national condiment. Jeruk kasturi or jeruk kalamansi in Indonesia. Chinese speakers in Malaysia and Singapore commonly call it 酸柑, which translates, fairly accurately, as sour mandarin. And in British and American garden centres it is sold as calamondin, a pot plant more often admired than eaten.

Look it up online and a good share of what comes back is about its health benefits. We will leave those claims where we found them.

How it differs from lime and lemon

Limau nipis, the ordinary lime, is a little larger, pale green inside, and sharper and cleaner on the tongue. Kasturi is sour too, but rounder, and its peel brings a perfumed, faintly mandarin note that is most of what you notice when you squeeze one over a plate.

Lemon is the outsider: softer, with a different perfume, and in a sambal it tastes faintly of somewhere else.

None of them is a straight swap. Kasturi in place of nipis makes a dish rounder and more fragrant; nipis in place of kasturi makes it sharper and plainer. Both work. Neither is quite the same dish. Abroad, a common stand-in is lime juice with a little mandarin or orange juice, which gets nearer than you might expect and still misses the peel.

Like every citrus, its aroma does not survive heat. Squeezed into the pot, kasturi gives sourness. Squeezed over the plate, it gives sourness and perfume, and the perfume is the point. The piece on asam and the other souring agents sets out how it sits beside tamarind and asam keping, and balancing the four tastes covers why the last squeeze does so much.

Where it belongs

On the side of the plate, mostly. Mee goreng mamak, mee rebus and mee siam all arrive with a half or two, and it comes separately for a reason: it cuts richness at the last moment, in whatever amount you choose.

In sambal belacan, squeezed in after pounding or over the top at the table, where its perfume does as much as its acid against the toasted belacan. With ikan bakar, over the fish and into the dip. In Singapore it does the same job beside sambal stingray and fried Hokkien mee.

In Chinese Malaysian kitchens it often sharpens the chilli sauce that comes with chicken rice. In the Philippines it is stirred into soy sauce to make toyomansi, an everyday dip for grilled and fried food, and squeezed over pancit.

Calamansi juice

Jus limau kasturi, or 酸柑水, is about as simple as a drink gets: juice, sugar, water and ice. Limau asam boi, or kasturi asam boi depending on who wrote the menu, adds a preserved sour plum whose salt turns a sweet drink into a far more interesting one. It sits on the same drinks board as the coffee in the kopitiam breakfast, and siew dai works on it just as well.

Extract, puree, concentrate: reading the bottle

Bottled and frozen calamansi vary far more than their labels suggest, and the words on the front are not used consistently. Roughly, this is what each one means.

Juice, or extract. Pressed juice, ideally with nothing else in it. On a bottle, "calamansi extract" usually means exactly this rather than anything concentrated, and the frozen ones are often the closest thing to fresh you can buy. Shelf-stable bottles have usually been heat-treated, and heat is not kind to citrus aroma.

Concentrate. Juice with much of its water removed, to be diluted at whatever ratio the label gives. Unsweetened, it is a convenience. Sweetened, it is a cordial by another name, and should be priced and used as one.

Puree. Juice with the pulp left in, so it has more body, and often more of the fruit's aroma, than juice alone. Cafés and drinks makers like it because a standardised puree tastes the same on Tuesday as it did on Monday, and a crate of fruit does not.

Syrup or cordial. Sweetened, sometimes heavily. If sugar is the first ingredient, you are buying sugar with some calamansi in it.

Powder. Convenient and long-keeping, and the aroma is what suffers most.

Whatever the front says, the ingredient list decides. Ingredients are listed by weight, as reading a Malaysian food label explains, so if water comes first you are buying a drink, and if sugar comes first you are buying a syrup. Check for added water, sugar and preservatives, and for the percentage of juice where one is declared.

If you are buying it by the litre

For a café or a drinks business, consistency matters more than any single tasting. Ask a supplier for the specification: Brix, which measures dissolved solids and is mostly a proxy for sugar, along with acidity, pulp content, storage temperature and shelf life once opened. Then ask whether those numbers hold from batch to batch. A drink that is right in March and thin in July is a supplier problem that your customers will blame on you.

Ask for the halal certificate as well, and check that this product is on it, not just the company's name. If the drink goes into anything you certify yourself, every ingredient needs current halal evidence, which is the part of halal certification that catches out many first applications. The suppliers page has the questions worth putting to any manufacturer, and a short list of some, each labelled as paid or not.

Buying and storing

Choose fruit that is firm, glossy and heavy for its size, which usually means juicy. Pass over any that are shrivelled, soft in patches or mouldy at the stem. Greener fruit is sharper. Yellow to orange fruit is riper and softer, with less bite.

Whole fruit keeps a few days at room temperature and a week or two in the fridge, bagged so it does not dry out.

Supply and price move with the weather and the season, and fruit that was cheap last month may not be this month. A tray of frozen juice cubes is the hedge, and a better one than anything in a powder.