If you want to try the real thing, rather than the lurid imitations in the freezer section, you'll have to travel to Florida's southern coast. Oliver Marshall tucks in

Not that vile-looking, green-coloured cream pie thing I've seen in the local supermarket's frozen-food section?

Not that vile-looking, green-coloured cream pie thing I've seen in the local supermarket's frozen-food section?

Whatever those cakes may be labelled, they're certainly not real Key lime pies. Key limes are pale yellow in colour - and the pie is most definitely not green.

So we're talking about a yellow, lime-flavoured, cream pie thing?

I'm talking about Key lime pie, a dessert that is served at pretty well every diner and restaurant in the Florida Keys, a chain of coral islets to the south of Miami. While distantly related to the bright green-skinned limes that you're thinking of, Key limes look and taste very different.

Like lemons?

Not at all. For one thing they're tiny, no more than an inch in diameter; the yellowish skin is very, very fine and the flesh is full of seeds. The trees - really more like bushes, complete with thorns - were introduced to the Keys by Spanish sailors hundreds of years ago and adapted well to the Keys' poor soils. But it's the taste that makes them distinctive. They're bracingly sour but, because they're picked at a riper stage than other limes, they also have a touch of sweetness.

So, has Key lime pie also been around for a long time?

Some people claim that the pie is an adaptation of an old recipe introduced to the Keys at the start of the 20th century by immigrants from the Bahamas. More prosaic - and more likely - is the suggestion that the pie as we know it was a 1920s invention of an American producer of condensed milk seeking to reverse the falling demand for tinned milk. With fresh milk still something of a luxury in the Keys, the simple recipe caught on as the ingredients - Key lime juice, egg yolks, sweetened condensed milk and sugar for the filling and Graham crackers (rather like digestive biscuits) for the crust - were all easily available locally. A great thing about the Key lime was that, being so acidic, the juice curdled the condensed milk and egg yolks and you could make a custard pie without needing to cook it.

Surely no one - least of all Americans - would dare to sell a product featuring uncooked eggs?

Because of worries regarding salmonella in eggs - and the possibility of lawsuits - pies are now usually baked briefly. While that's how the meringue topping came to be introduced, many feel that the appearance of whipped cream is taking a step too far from the pie's humble roots.

Are they much of a muchness?

Apart from the topping, they vary enormously. One reason for the pie's popularity is that the blend of sweetness and tangyness makes it a good dessert for most kinds of cuisine. Key lime pie connoisseurs say that the pie should "dance on the tongue"; the balance of sugar and acidity should be just right. And with the popularity in south Florida of Latin American and Caribbean fusion styles of cooking, quite a few restaurants have developed Key lime pie as a signature dish.

So where should I go to get the best?

In Key West you'll soon find your favourite slice of pie. The Key West Key Lime Pie Company at 701 Caroline Street in old Key West is a bakery that produces a well-balanced pie. Another firm favourite is the Blond Giraffe, a Brazilian-owned café at 629 Duval Street, where Key West's most popular restaurants and bars are located. They serve a prize-winning Key lime pie, especially delicious when accompanied by ice-cold maté (a South American tea). For a new take, try Louie's Backyard at 700 Waddell Avenue which has created a Key lime pie on a spicy ginger-snap crust. And should you tire of pie you can shop for Key lime cake, cookies, marmalade, ice cream, soap, shower gel, candles ...

Wow! They must go through a lot of those tiny limes

You're right. And it's a problem. Land is very expensive on the Keys and Key lime orchards are a thing of the past. No one volunteers the fact, but almost all Key lime juice used for making pies and other Key lime produce is now imported as concentrate from Mexico. A few restaurants, however, are supplied by local residents who have trees in their backyards. The suppliers remain a closely guarded secret.

Is there much else to do in the Keys?

Most people head for Key West, an utterly charming historic town that's full of Caribbean-style clapboard and Victorian gingerbread houses. Many of these have been converted into upmarket guesthouses or restaurants, but the old "Bahamian Village" retains more of an authentic, slightly dilapidated edge. Getting to Key West is a major part of the pleasure of the Keys. The Overseas Highway, which connects the Florida mainland with Key West via a succession of causeways and bridges, is one of America's most amazing roads, passing by or across expanses of beautiful tropical water. It's possible to drive the 159 miles between Miami and Key West in just three hours, but you'll want to take your time. For example, there's superb snorkelling or you can go sea kayaking to explore the back-country waters. And throughout the year there's exceptional game fishing.

Oh yes, isn't Ernest Hemingway associated with the Keys?

Attracted by Key West's isolation and deep-sea fishing, Hemingway moved to Key West in 1931 and produced some of his best work during his time there. Much is made locally of his fishing exploits, hard drinking, bar-room brawling and Key lime pie-baking skills.

He baked pies?

All right, I made that bit up, but it's certainly true that he appreciated good food. Hemingway's home - one of the most beautiful stone buildings in Key West - is today a museum dedicated to the writer and several local bars compete for ownership of the American icon.

I'm convinced. Can I find out more?

Return fares to Miami start from around £360 return with British Airways (0870 850 9 850; www.ba.com). Virgin Holidays (0870 220 2468; www.virginholidays.com) offers a seven-night fly-drive, including room-only accommodation in Key Largo, from £559 per adult and £259 per child (under 12). Hurricane season is from 1 June to 30 November. For further information call the Florida Keys and Key West Development Council (01564 794555; www.fla-keys.com).