We've been drinking beer for centuries. But only now are scientists learning the secrets of the perfect pint. Sanjida O'Connell learns the recipe for success from the master brewers

It's a drink that is almost as old as civilisation: beer was originally made from emmer, an early form of wheat, by the Babylonians who offered it to their gods. In this country, it was made mainly by women, known as beer witches; the last witch was burnt alive in 1591 for her bad brew. But until Louis Pasteur discovered that micro-organisms cause fermentation in 1860, no one understood what turned a gloopy slop of grain into a potent pint: brewers used to add beer from the previous batch and call it 'godisgoode'.

"It's a magical process," says Adrian Tierney-Jones, writer and beer expert. "Brewers never fail to be fascinated at how a beer turns from being lifeless to full of life." Alex Bell, head brewer for O'Hanlons, will explain to the Royal Institute tonight how scientists have finally got a handle on the magical alchemy of beer brewing. As Bell says, "The key thing in the brewing process is making alcohol." We now know it's the yeast that's responsible: it's a type of single-celled fungus, which produces the fizz and intoxication – alcohol and carbon dioxide. It takes 34 million of these cells to produce just one pint of beer.

The starting point for most beer is barley. The grains are soaked and then left to dry until they begin to germinate. To prevent them turning into mini-barley plants, the grains are heated and crushed. This malted barley smells of lightly toasted granary bread but the grain turns darker and richer at higher temperatures: crystal is a mixture that has toffee and biscuit flavours and roasted barley, used to make stout, looks and smells like ground coffee.

"What malted barley adds to beers are roasted and smoky aromas and coffee, chocolate, bready, biscuity, sweetcorn, hay, toffee, caramel and butterscotch flavours,"says Tierney-Jones.

The malted barley is heated with water to release enzymes that break the complex starches into simple sugars like glucose and maltose. Other enzymes chop the proteins in the grain into shorter segments. This wort, as it's called, is drained out and the remaining mashed barley is fed to the local cows. The wort is pumped into a brewing kettle where it's boiled with hops.

It was Belgian monks who first added hops to beer approximately 500 years ago. Before that a range of ingredients were used to impart flavour such as juniper berries and bog myrtle and even henbane, which is a hallucinogen. "The monks were allowed up to five litres of beer a day when they were fasting," says Bell, "so they took beer seriously."

The proper name for hops is Humulus lupulus; lupulus is derived from the Latin for wolf. A wild climbing vine, they're now grown by training them up 10ft vines. It's the flower, the "cone", that contains most of the unusual and complex chemicals that impart bitterness, aroma, antioxidants, enhance foam production and act as an anti-bacterial. Normally two types of hops are used – ones that add bitterness from humulone, an astringent acid – are boiled with the wort for at least an hour, whereas aroma hops are added towards the end of boiling so that they release their aromatic oils without the oils being evaporated. O'Hanlons uses dried hops from America, the UK and the former Czechoslovakia, which smell of Thai spice – of lemongrass, galangal and coriander. Tierney-Jones says, "Hops add herbal, grassy and lavender aromas as well as bitterness, spiciness and citrus, and tropical fruit flavours." The mixture is then left to ferment with yeast for up to five days. "After 24 hours, the yeast foam looks like a lemon meringue pie, the peaks are astonishing," says Bell. "What yeast adds to beers are the flavours and aromas of soft fruit," says Tierney-Jones. There are two main species of yeast: Saccharomyces cerevisiae, which works at warmer temperatures and is used to make bitter and ale, and Saccharomyces carlsbergensis, which needs cooler temperatures and creates lager. There are more than 500 strains of these two main species, often particular to a brewery. The larger brewers send their yeast to be stored in labs so that if anything goes wrong, they can recultivate their own strain.

Yeast converts the sugars from the barley into carbon dioxide, which gives beer its fizz, and alcohol. It's called the glycolytic pathway and its extremely metabolically inefficient as the yeast hardly recoups any energy from the process. "Yeast would gain more energy if it converted the sugars into carbon dioxide and water, as we do, but we're grateful that it doesn't," says Bell.

The by-products of this process are the production of other compounds that contribute to the flavour of the beer. For instance, the final step in the pathway is the conversion of glucose to the compound pyruvate. This is then converted into ethanal, which can then be turned into ethanol (alcohol); sometimes though, the ethanal gets converted to diacetyl. This chemical produces butterscotch flavours that are good in stout but not so welcome in other beers. Leaving the mixture for longer allows the yeast to turn the diacetyl into 2, 3-butanediol, which doesn't taste as strongly of butterscotch. Saccharomyces cerevisiae strains of yeast produce esters, such as isoamyl acetate, which tastes of pear drops and bananas, and ethyl hexenoate that has an appley aroma.

The beer is then put into casks or bottled and a secondary fermentation will take place that produces the carbonation. "There are so many factors that can change the finished product," says Bell, who has created a beer called Fire Fly, "the weather, the barley, which affects the range of proteins, the amount of oxygen in the beer that's added during mixing, the dynamic of the boil. The beer in your glass might not taste anything like you'd planned."

What determines whether a beer is an ale, a bitter or a larger is down to the amount and type of hops as well as the malted barley, but is primarily due to the strain of yeast. "A yeast that has been used for the same beer over a period of time will have a specific character and adds its own particular imprint to the beer. Different yeasts produce different flavours. O'Hanlons produce a beer that contains 12 per cent alcohol, lasts up to 25 years and is almost scarlet in colour when you hold it up to the light. It's named after Thomas Hardy, who wrote in The Trumpet-Major: "It was the most beautiful colour that the eye of an artist in beer could desire; full in body, yet brisk as a volcano; piquant, yet without a twang; luminous as an autumn sunset." Not quite what those Belgium monks might have expected from the action of a fungus and a wild wolf plant.