St. Louis – Behind the locked doors of the thick brick building, past the security cameras and up the secure stairwells and another two locked doors there is a freezer that sits in a laboratory-like office.

My thoughts slip to the movie “Jurassic Park” as Kendra Bowen, manager of the corporate culture yeast center at Anheuser-Busch removes a key from her pocket, dons protective gear – gloves, heavy apron, face mask – and unlocks a cryogenic freezer that releases a subzero fog onto the floor around her feet. A chamber rises and inside are time vials filled with yeast samples. Many date back microbial generations and although not from the age of dinosaurs are descendents of beers that were once imbibed by members of the greatest generation when they were still in their prime.

The most important ingredient in brewing was the last one discovered, because yeast is a single-celled organism that is invisible to the naked eye. Still, brewers have long known that some unseen agent turned a sweet liquid into beer. Long ago, the action of yeast was such a blessing, yet so mysterious, that English brewers called it “Godisgood.”

Over the centuries brewers have been able to cultivate yeast strains to get them to a consistency that is most suitable for beer, says Chris White, president of California’s White Laboratories.

Anheuser-Busch InBev is the largest brewing company in the world and as such needs a lot of yeast to make the beers they produce. One millileter of yeast is removed and is scaled up to be shipped around corner or around the world to the company’s other breweries. In all, the facility is cultivating about 1,000 barrels of yeast each week. While Anheuser-Busch InBev has hundreds of yeast strands on hand, they currently use about 19 for their portfolio of beers, that’s up from just three in 2008. Controlling the yeast helps brewers keep a uniformity to the beers they produce, no matter the location it’s made.

Different strains of yeast behave differently, so that it’s possible to divide the world of beer according to the yeast. The sixty or more defined beer styles in the world can all be sorted by their yeast into two broad families: the ale family and the lager family.

Beers in the ale family (Saccharomyces cerevisiae )are produced by yeast strains that operate better at warmer temperatures. Ales are ready to drink in days rather than weeks, and the yeasts produce extra flavors in addition to creating alcohol: fruity, spicy, or earthy flavors are not unusual. Ales are the traditional beers of England and of Belgium.

Beers in the lager family (Saccharomyces pastorianus also known as Saccharomyces carlsbergensis ) are fermented by yeast strains that operate better at cooler temperatures. These beers need to be conditioned or cellared (“lager” is German for a storage place) for several weeks or more to reach peak drinkability. The lager beers are the traditional beers of Germany, the Czech Republic and central Europe.

The action of yeast can generate a range of interesting beer flavors and aromas as varied as apple, pepper or apricot. Some, such as banana or clove, are the typical flavors of particular beer styles; others, such as butterscotch, may be considered defects.

How does yeast work? When it is added to a sugar-rich solution, it immediately begins to consume the sugars and create more yeast. But from the brewer’s point of view, the important thing is not the growth of more yeast, but the waste products of yeast metabolism: alcohol and carbon dioxide, that gives beer its fizz.

Drinkers owe this bit of information to a French inventor and scientist named Cagniard de Latour who, around 1840, discovered that it was in fact yeast that was adding carbonation to beer. A few decades later Louis Pasteur released a book that studied fermentation and beer and, of course, pasteurization methods.

Brewer’s yeast does not produce a phenolic flavor, where as most other strains will. Brewer’s yeast does keep some other “enjoyable flavors” that White said were selected over the years as brewers worked to perfect batches.

The brewer’s yeast also ferments the right kinds of sugars, where as some strains, White said, will consume all sugars. The yeast also settles to the bottom, which is unique and does not happen in nature. That last point is also a bonus for bottled conditioned beer.

“They were domesticated by brewers, which is unusual for a microorganism,” he said.

Yeast may be invisible, but without it, there would be no beer.