We're all foodies now, obsessed with farmers' markets and fashionable delis. But the gastroscenti have moved on. They buy their Parmesan, prosecco and pastries from shops where the metropolitan middle-classes once feared to tread. Caroline Stacey casts prejudice aside to reveal the gourmet secrets lurking in the aisles of Lidl, Aldi and (whisper it) Netto

The last time I was cornered at a party and regaled with details of the bargain stuffed olives, cherry jam and honey-roast ham my fellow guest had bought at the supermarket, I didn't react as you might expect. I didn't take on an expression as glazed as the £1.79 apricot tart he was describing, nor look round in panic for a means of escape as soon as I'd downed the glass of warm prosecco from a bottle that cost just £2.99.

Instead, I joined in. Because we weren't comparing notes on our weekly shop at Waitrose or Sainsbury's, we were bonding over the thrillingly cheap-as-chips but surprisingly edible discoveries we'd made at Lidl. Boasting about the "how do they do it for the price" finds and sharing the unexpected gourmet treats tucked behind the stacks of dog food at the German discount store will not get you thrown out of the party for being a crashing bore.

The fact that the stock can be capricious, and that you'll never know whether there will be a mountain made of Orangensaft cartons, or a ceiling-high Rachel Whiteread-type construction of bags of sugar, from one week to the next only adds to the thrill of shopping at Lidl. Nor is it just the novelty of the Germany groceries; what the trade calls the non-food stock lines are hilariously random. One week everyone was talking about the jodhpurs and riding equipment that allegedly appeared in every store from Penzance to Carlisle.

Every one of our supermarket chains has a history, and almost a personality. OK, I admit I'm struggling with Somerfield, although maybe they know better in its Bristol hometown. But there's the philanthropic Sainsbury family and their doggedly middle-class shops. Waitrose is posh, sells a lot of British produce made on a small scale, and operates online in the form of Ocado. From its beginnings as a market stall in Bradford in 1899, Morrisons grew big and bold enough to gobble up the dreary American Safeway.

Tesco, started by Jack "pile it high, sell it cheap" Cohen (whose daughter Shirley Porter was involved in Westminster council's homes-for-votes scandal), is all things to all people, terrifying in its vastness and power, and pioneer of the idea of Finest - more expensive lines of supposedly better-quality food. Even Asda was once the Leeds-based diminutive of Associated Dairies until the great maw of Wal-Mart gulped it down like a two-litre economy bottle of cola.

But what do we know about Lidl, and the two other "value retailers" Aldi and Netto? Only that the first two, whose names seem to consist of the same letters in a different order, are German. Netto is Danish.

Our supermarkets copy each other shamelessly. If one has a lemon tart, so do the rest. They all do hummus with extra flavours, like coriander and lemon and peppers, and we know we'll find Marks & Spencer's terrific Moroccan chickpea-topped hummus in all our friends' fridges. They run the gamut of the British diet: Indian ready meals, stir-fry sauces, Thai rice, Mexican wraps and Moroccan couscous.

Aldi and Lidl, which both have more of their own products than Netto, are by comparison quaintly European. It's as if our appetite for long-haul holidays and global food never happened, so 1970s retro are the products they stock. To most of us, they're relatively unexplored food territory. Indeed, the Hackney branch of Lidl, where junkies, professionals, East Europeans and a complete multiracial range of the local population wander between the walls of wacky breakfast cereals and continental biscuits, is like coming across Hansel and Gretel's house in the woods.

Unusually, all the check-out operators are men in blue button-down Oxford shirts, calmly managing the queues that build up when there are only three of them on at a time and one woman is paying for a stack of 60 mini fromage frais pots. And in this magic kingdom of mysterious packets with who knows what inside them, you can let your imagination run riot and speculate about the masters of this parallel universe. Are there glamorous scions of German grocery dynasties to be seen in Hello!, partying in the family schloss? Or living quietly in London, like the loaded Swedes who made their money from Tetra Pak?

When you think about it, it's really rather exotic. There's no cachet in giving your hostess a box of Tesco Finest Belgian chocolates, but hand over a 125g bar of Lidl's 70 per cent-cocoa Ecuadorean chocolate, which tastes better and only costs 89p, and her curiosity may well be piqued. Especially in London, where these stores have a smaller share of the market - only 2.5 per cent, compared to 8 per cent in the north of England - we're learning about Lidl by word of mouth. The Snacky Cracky butter cheese sticks are getting everyone talking.

"Among the chattering classes," says Chris Longbottom, a director of the market analysts TNS Worldpanel, "these stores are less familiar, and because they're so strong in the Continent, off the back of their distribution networks they stock things that are interesting for us." Pumpernickel, all sorts of fruit juices, jams, any number of biscuits like the waffel-schoko-röllchen, cured hams, pickled gherkins - they've got them.

The usual supermarkets have become so predictable, and the choices so bewildering, that the shopper risks option stress. Do we really need to choose from 40 different types of "buttery taste" spread, or face an entire aisle of sugary breakfast cereals? The "discounters" don't have half as much choice; picking what's on offer that week off the pallet - they don't bother to stack the shelves, the stuff shifts itself - is part of the fun. It's like crossing the Channel to shop at Mammouth without enduring a ferry crossing and messing about with the currency converter.

Overall, these three stores account for only 5 per cent of our supermarket shopping, though their share has grown steadily since they arrived here 15 or so years ago. A quarter of us shop in them at least once a month. And although Lidl, Netto and Aldi have a cult following among students, it's not just the hard-up that use them.

"Rising shoppers" - halfway between the thriving group at one extreme and the striving at the other - are professionals, urbanites and better-off executives, often young singles and pre-family couples. Should they need to make savings, they would probably visit the "hard discounters" rather than, say, Asda, according to research by the Institute of Grocery Distribution (IGD).

TNS Worldpanel's figures show all social classes to be almost equally represented in the shoppers who visit the stores. If anything, Aldi attracts slightly more ABs than C1s, and they're not hugely outnumbered by the most impoverished Es. Those who can afford to pay more for food if they choose to - and let's face it, cheese straws, antipasti, madeleines and Spanish charcuterie are not essential if you're feeding a family on a budget - compete to show who's the smartest shopper.

There's no shame in owning up to the humble origins of the smoked salmon and prawns you bought from Aldi. The socially confident flourish their cut-price purchases with pride. They have the confidence and knowledge to make their own judgements. They don't need the reassurance of a trusted brand. The IGD survey's summary is that shoppers are wising up to the idea of value.

Because of the competition, all the supermarkets are moving away from the more expensive branded foods. Asda has opened a store in Northampton that's 95 per cent own-label. Aldi, Netto and Lidl, full of cans and jars and packets with names we've never heard of - Choc Me, Jolly Boy, Celebrity, OceanSea, Holly Lane, Ann Forshaw's - clearly have them rattled.

In Germany, Netto, Aldi and Lidl combined take half the market. In Denmark, these "hard discounters" pocket 36 per cent of supermarket shoppers' krone. But in the UK, they are competing against what Longbottom calls "world-class supermarkets" such as Tesco. And when the Europeans arrived in the early 1990s, our supermarkets retaliated with Value and Basic and Economy ranges. The British shopper is spoiled for rock-bottom prices.

In the Supermarket Own Brand Guide (Mitchell Beazley), Martin Isark tried 250 types of food, comparing the familiar names with other versions from 10 leading supermarkets. He included Aldi, although he admits that "most people probably don't admit to shopping there. There's a bit of a stigma about it."

I can't think why. I have more difficulty finding food I want to eat in Asda than I did in my nearest branch of the "discounter". The pesto, 99p for 190g, which comes in several varieties (pistachio and fennel, parsley and ginger, and classic Genovese) is made in Liguria; the tear-and-share brioche comes from the Vendée in France.

Aping Tesco and the rest with their Finest, So Good, The Best and Taste the Difference labels, Aldi has introduced a Specially Selected range. There are gastropub-style lamb shanks and frozen sushi sets. This week's special offer is organic tea bags (80 for £1.29) and fusilli (39p).

Aldi is spreading its wings, and claims to be unique among the discounters in having a buying team for the UK. It denies that the products are no-frills, or basic; that's just the style of the business model. Even the store, not one of their showcase ones, was appealingly cool and strangely underpopulated, and bright and easy to navigate. Really, it's almost enough to convert a sceptic who avoids supermarkets at all costs.

In Martin Isark's tastings, Aldi's products come out top in 32 categories. That includes baby beetroot in sweet vinegar, which unfortunately on closer inspection contains sweeteners. Its grissini with black olives for 69p was his favourite. And if you were thinking of buying tinned fruit cocktail, buy Aldi's; it's better than the M&S, Waitrose, Morrisons and Del Monte versions.

Isark, a professional taster, also recommends the German supermarkets generally for jam: "They're superb, put the British ones to shame. Germany always had a relationship with Poland, which grows great fruit."

Given the speed at which new branches of Lidl are going up - 40 or so have opened in the past year - he'll include them and Netto in his next set of taste tests. "All three have wonderful bargains - and some horrors," Isark says. He must be thinking of Aldi's peeled plum tomatoes for 15p. "Leave them, and let somebody else ruin their pasta," he says in the book. Or the corned-beef pasties that came eight places below Ginsters, or the Ocean Rise tuna chunks in brine - worse than anyone else's. Isark is the first to admit that own labels can be hit and miss, and it's more pleasurable to shop in Waitrose, "but if it's a matter of getting a product at a great price, these stores do a great job".

On my first, and probably not last, visit to Aldi, I did what you're not supposed to do; I bought on impulse because there was so much that looked tempting. Fat green olives stuffed with cream cheese (99p); a jar of roast peppers (£1.29); luxury fudge (£1.29); Spanish charcuterie (£1.19); hot smoked-salmon fillets (£1.29); Italian stone-baked pizza (£1.69) with proper crusty dough; and an astonishingly juicy and fruity continental plum tart with a crumble topping (£2.49) - all went in the trolley. Most of these don't feature in Isark's assessments, either because there's no comparable brand or they've been added recently to help take Aldi upmarket. Aldi's TV advertisements feature its extra virgin olive oil and pinot grigio.

The company used to be considered secretive, possibly less as policy than because the charms of shopping there were somewhat fugitive. But it is spending £500m on its ambition to have 500 stores in the UK by 2010. The TV commercials have been running for a year, and it has a PR agency.

Lidl may have a better-established cult following but, like Aldi, it doesn't release the phone numbers of local branches, and also makes it impossible to call head office. Like charging for plastic bags, it's all about keeping overheads down so that prices stay that way too.

But Lidl has no need to be reticent. I shopped with a list of recommendations: the Black Forest air-dried smoked ham; 74 per cent cocoa content continental chocolate; multivitamin juice; gorgonzola; Parmigiano-Reggiano; Italian espresso ground coffee; balsamic-pickled roast onions; vanilla ice-cream with specks of real vanilla; and, putting the prosecco in the shade, the 1998 Gran Reserva Tarragona from Spain - as good as a Rioja twice the price.

The Hackney store was more soothing than my local Sainsbury's - where you also have to put in a £1 coin to get a trolley. Until a couple of years ago, these discounters only accepted cash. Now, they will take debit cards.

In Netto, I had to shop without radar. A Polish couple, entirely representative of the demographic, helped me to unlock the trolley. I tried and failed to do a family shop and instead filled the trolley mostly with crisps and loo roll, garnished with Serrano ham, muesli, juices, a 45p pack of kippers, a 100g wedge of Parmesan for £1.29, and so on. It looked a lot for £16, but I came home with nothing to eat for supper except organic carrots (from Spain, unfortunately), Dutch tomatoes and pears (ripe, mind you).

Now, this economy food shopping is all very well, but it will earn you only contempt in some quarters. For, just as one lot of cognoscenti is loading up with cut-price continental antipasti and cured meats that are vague about their country of origin, the hardcore gastrocenti want to know the provenance of their food and feel nothing but loathing for supermarkets.

There are many charges against them. Their suppliers, farmers and manufacturers suffer the consequences as the chains compete with each other to offer the lowest prices. Even the global food groups are complaining. Supermarkets kill off specialist local shops, needlessly transport food in lorries up and down the country, sell out-of-season vegetables flown in from the other side of the world and fruit that never ripens and build superfluous stores on greenfield sites. Recently, some have been resisting the Food Standards Agency's proposals for healthy-eating labels.

In Bad Food Britain, Joanna Blythman berates us (yes, all of us; her book made even me, a farmers' market, Waitrose, butcher, baker, fishmonger and street-market shopper, feel defensive about my occasional forays into supermarkets) for our shopping habits and diet. "We are a nation obsessed with cheap food," she says.

Surveys have shown that, to most shoppers, price is more important than taste, quality, production and animal welfare. We spend less on food as a proportion of our income than ever: in 1980, it was 23 per cent of total spending; in 2004-05, it was 16 per cent. We spend more on motoring than on food; more on TVs, computers, leisure and holidays than on food and non-alcoholic drinks.

Our obsession with the cost of food is a particularly unhealthy one. Pandering to our meanest, cheeseparing shopping instincts doesn't encourage us to improve our diets, the quality of our food, and our cooking skills, or to buy locally and responsibly produced food. Economies have to be made to keep prices down. Cheap food is bad for the environment, bad for animals that suffer overcrowding and cruelty, and possibly bad for our health.

But let's not be too hard on ourselves. We've shown that we will pay more for fair-trade food in our supermarkets - and you won't find that in the "hard discounters". We fondly imagine that Europeans do all their shopping in patisseries and markets, when they too are lured into supermarkets in ever greater numbers. In Norway, the "value retailers" have 40 per cent of the market; in France, it's 12 per cent.

Penny-pinching is not, it seems, peculiarly British. How can it be when Aldi is the largest wine retailer in Germany, and 91 per cent of the country's shoppers buy in the discounters? The welfare standards for much of Europe's pork production are lower than those British pig farmers must meet. And Danish blue cheese is nothing to write home about. If you thought that was an exception in a country otherwise wholesome and well designed, don't go to Netto or your illusions will be shattered.

So I wouldn't buy fresh meat or poultry from any of the rock-bottom retailers. Nothing will convince me that the £1.25-a-kilo chicken pieces at Lidl are an appealing proposition. You do have to draw the line somewhere. The environmental and animal welfare arguments in favour of buying free range or organically reared British meat and poultry make buying those from Lidl, Aldi or Netto a no-no.

Even those of us who love our delicacies, and don't wince at paying double figures for a farmers' market chicken, need to offset the cost. So here's a summer tip; add Waitrose Funkin' white-peach purée to - if you can find it - Lidl prosecco to make a bellini. The prosecco isn't fit to drink on its own, but you'd never know. That, as a promiscuous food and drink shopper, is what I call a balanced diet.

Netto: Italian specialities but no baskets

The discount store originated in Copenhagen in 1981. The first store of its type to arrive in Britain, it first opened in Leeds in 1990. Since buying more than a dozen Somerfields, it now has 150 shops in this country. Netto is a private limited company that is part of a larger, Danish-owned group with discount stores and hypermarkets across Europe. It stocks more branded foods than the others, as well as its own labels. No baskets (only trolleys), no free carrier bags, no shelf-stackers. Last year, it took 0.7 per cent of the hard-discount market in the UK, a share that isn't going up.

Aldi: For the plummiest tarts

The food shop that was to become the discount chain was founded in 1946 by brothers Karl and Theo Albrecht in Essen, western Germany. Aldi now extends across Europe, and into North America and Australia. It arrived in the UK in 1990, and now has more than 300 stores, most of which have been refurbished. It is aiming for 500 by 2010. Its 2.5 per cent share of the market is 21 per cent up on the previous year. It has always been ahead of the others, but Lidl shows signs of catching up.

Buy: the pesto, which Good Housekeeping voted the best, 99p; olives stuffed with cream cheese, 99p; black-olive breadsticks, 69p; the hot smoked-salmon fillets (125g), £1.29; roasted red and yellow peppers in a jar, £1.29; black-olive grissotti (200g), 69p; stone-baked pizza, £1.69; and the Specially Selected continental plum tart, £2.49.

Lidl: From Black Forest ham to blackcurrant jam

The Lidl & Schwarz wholesale grocery opened in the Thirties. Keeping the Lidl but dropping the Schwarz - they didn't want it to be known as the schwarz (black) market - by the 1970s the shops had evolved into the Lidl we know today. It's the fifth-largest supermarket chain in Germany, and came here in 1994. Now has around 400 stores and a 2 per cent market share, up by 12 per cent on this time last year. It has more than 5,000 stores in 17 countries.

Buy: waffel-schoko-röllchen (chocolate-filled wafer rolls), 59p; wild berry sorbet, £1.29; Black Forest air-dried smoked ham, £1.39; smoked-trout fillet, £1.29; blackcurrant jam, 64p; vegetable juice, 79p; peppers stuffed with Greek cheese, £1.29; Landgut rye bread, 49p; cheese straws, 79p; Ecuador 70 per cent-cocoa chocolate, 89p.