Andy Mahoney realised his habit was getting out of hand a few months ago, during a visit to an old coastal fort on the Hampshire coast. "I saw white mould on the wall, so I put some in my mouth to see if it was any of the moulds I was expecting," he says. "It struck me that maybe I'd gone a bit crazy."

Mahoney's brain had turned, as it often does, to cheese. He has attempted to make every imaginable type of cheese, with all kinds of mould. In the course of his experiments, he has even invented a few franken-cheddars of his own. (He's also poisoned himself a couple of times, but that hasn't stopped him yet.)

Mahoney belongs to a rare sub-species of gastronome: the obsessive. These individuals seek to master one particular foodstuff; they are in pursuit of an elusive Platonic form and will go to extreme lengths to get it. Seldom are they professional chefs – the catering industry lacks the time and nerve for such tunnel vision. In some cases the obsession has led to a relevant job or startup; in others it bunks awkwardly with a pre-existing life. (Mahoney, when he is not cutting curds or nibbling moulds, works as a city trader.) Either way, it's never done for the money. It's done because it has to be done.

Take the case of Andy Forbes, a former typesetter and computer programmer. Over the years bread has pushed everything else out. His Camberwell flat is a shrine: industrial-sized sacks of flour in every room, a homemade mill in his bedroom. Next to his computer sit a pair of bubbling starters that have been around since Margaret Thatcher was in No 10. In the kitchen is an antique 1940s Artofex dough mixer that he won't sell "for any sum".

Then there are the wheats: kernels of 70 heritage strains with names such as Golden Drop, Rouge d'Ecosse, Lambeth Latin, stored in hundreds of Tupperware containers. Forbes, 57, is a "wheat chaser", focused on saving ancient varieties from extinction and, where possible, bringing them back into circulation. "Orange Devon Blue Rough Chaff," he says, pointing to one of the boxes. "We got to that one just in time." Three neat rows of long grass in his garden are purple free-threshing spelt, grown from the "one handful of the seed in the world".

Food obsessives are fond of big talk. Rachna Dheer, chef-owner of Babu, in Glasgow, claims "nobody in Scotland or the north of England" goes to the same effort to make authentic Bombay cuisine. Those efforts grew out of anger at how her home flavours had been "bastardised", at how there wasn't a decent pav bhaji for hundreds of miles. After years perfecting her mother's recipes and travelling the country in search of obscure ingredients, her interest expanded into the restaurant last year.

Yet there's no bragging. Many seem embarrassed by how much time they devote to their calling. Others just want to get on with it. Patrick Quinn, the proprietor of Quinn's Irish pub in Kentish Town, north London, said he was too busy to talk. Nor would he divulge his age, a topic of intense speculation among regulars, though his son Vincent concedes 90 is close to the mark. (The regulars say 100 is closer to the mark, but they would.) Quinn senior still works every day. Beer is what keeps him going – not the consumption of it, but the sourcing and stocking, the careful husbandry of a great variety of pilsners, stouts and ales. Long before the craft beer craze, Quinn was here with his "continental selection". Doesn't he ever think about retirement? "He can't," says Vincent. "He wants the i's dotted. Many is the time he's climbed the stairs at three in the morning."

Rachna Dheer … keen acolyte of Bombay cuisine Photograph: Martin Hunter for the Guardian

Such devotion has its price. "I seem to have sacrificed everything else in my life for it," Melody Razak says of baking. The 38-year-old founder of Treacle & Co in Hove talks of months eating nothing but lentils in order to afford cakes from Ladurée, of making the same recipes over and over in her kitchen, year and year. "Rather than seeing it as repetition," she says, "it's about trying to make it more efficient, faster, better." The sentiment echoes the Japanese principle of kaizen, described in the 2012 documentary Jiro Dreams of Sushi: to perform one's duties each day better than the last.

Back to Forbes, who knows this state of mind. He has made only one type of bread, a light rye sourdough, for 25 years. But excellence and obsession are not always happy bedfellows. He looks uncomfortable telling the story of Claude Bosi, head chef at the two Michelin-starred restaurant Hibiscus, who tasted his bread in a farmer's market and asked him to be the restaurant's supplier. The problem: Bosi wanted to see the bakery.

"I tried to put him off," says Forbes. "I was in a squatted house that was falling down, with spiders everywhere. At that time I milled in the kitchen and sifted in the front room and baked in a mobile oven in the garden."

But Bosi insisted. The chef spent several hours studying Forbes's Heath Robinson set-up – including a threshing machine made out of BMX bike rims, scooter wheels, a Chinese sewing machine, and a rubber mat used for wiping shoes outside hospitals – and announced he would buy as much bread as Forbes was willing to sell. For about a year, until Forbes and his fellow squatters were evicted, well-heeled diners at Bosi's Mayfair restaurant ate this bread with their foie gras and champagne.

Forbes is still involved in various bakeries, and educational projects such as Brockwell Bake, though the task that consumes his waking hours is his online cereal database, "a website for all the wheat in the world". He has catalogued more than 400,000 varieties on his site. "Every grain has more [genetic] diversity than we do," he says. The world revolves around wheat when you talk to Forbes. The real cold war is in wheat gene taxonomies (Russia uses a different classification system to the US). And to hear him describe the siege of Leningrad, you might think bread was the primary motive for conflict. "The Russians had one of the largest wheat gene banks in the world, but [it] got stuck in the siege," he explains. "The curators starved to death guarding all these seeds. The SS had a special unit to grab it."

Discussing the process of making bread with Forbes can be perplexing. A seemingly straightforward question about wholemeal might be answered with a discourse on the number of chromosomes in a particular strain. Things are never simple. Forbes and his ilk have reached a sort of quantum theory of food: the more they look, the more they see.

"It's never-ending," agrees Mahoney, the cheese devotee. "Trying to control all the variables – the temperature, the milk, the pH, the time you cut the curds – becomes mindboggling. Something people accept as such a basic thing they buy off their supermarket shelf is actually such a difficult problem."

Catherine Sheay … coffee obsessive. Photograph: Martin Godwin for the Guardian

Mahoney's obsession started six years ago, out of a basic curiosity for what made one cheese different from another. Why was Comté hard and Camembert creamy? What gave Stinking Bishop its distinctive scent of something freshly stepped in? Soon the spare room of his south London flat was choked with fridges, cheese presses built from scrap metal, a terrarium pond fogger ("the kind you put in a lizard enclosure"), a bain-marie, and a set of diamond scales accurate to one-thousandth of a gram.

At times his habit has pushed the boundaries of taste. To the chagrin of his wife, Mahoney recently developed an interest in moulds found on the human body. "The reason Époisses and stuff like that exists is because of monastic traditions where the cheese was handled by people who weren't very sanitary," he says. "I thought I'd make a cheese from the moulds that grow on your hands when you sweat. You'd be out in the street and you could smell it. My wife said I couldn't make that cheese again."

Catherine Seay says she has never gone too far in her search for the perfect cup of coffee, though she has flown to Norway to try an espresso, which some might consider excessive. What she seeks is known in coffee circles as "the God shot". "It's beautiful, balanced, with a creamy mouthfeel," she says in tones of rapture. "It's sweet and there's acidity and depth of flavour and it's clean, you drink it and you get the full flavour but there's no lingering aftertaste. That's the dream."

Seay, 27, is an obsessive with a business to match. Three years ago she set up Curator's Coffee in the City of London, where she spends her days texturing milk, experimenting with water pressure, and tasting as much coffee as possible without overdosing on caffeine. She met her husband through coffee (he was a customer, she was the barista) and plans her holidays according to where she'll find a good cup of it (even so, she always packs a coffee grinder, beans, a Chemex flask and a pouring kettle). Despite her years of dedication, Seay does not feel she is even close to mastering coffee. "I feel I don't know a tenth of what there is to know about it," she says.

Even with vast knowledge, the perfect form is elusive. It's the final part of obsession's spell: the sense of something beyond one's control. Religious metaphors are rife in these conversations about bread, cheese and coffee – these everyday items have been elevated to gnostic mysteries. Occasionally, ecstasy is glimpsed. Years ago, cycling in the south of France, Forbes tasted the best bread of his life. "I followed a little track and found a house with an open door," he says. "It was a dark room. In one corner was a big loaf. Eventually an old lady came out and I bought a section. I went away, and then I tasted it and realised ... It was a holy-grail moment. I'll never find that place on the map. Was it really there? Was that little old lady in the dark room with the best loaf in the world? You could search for ever and ever, you'll never find it again."