Beer-makers promote the purity of the water they use, they import their authentic Saaz hops from the Czech Republic and the barley they employ is fresh off the farm. Now breweries are increasingly focused on the unsung ingredient in any good beer—the yeast that eats up the simple sugars in grains, converting them to alcohol and carbon dioxide in the essential process known as fermentation.

With the rise of craft beer has come an appreciation of the vast variety of yeast strains available to brewmasters. A Chicago company, Omega Yeast Labs, has emerged in only a few years as a leading supplier of rare, sometimes ancient strains that give beers unique flavors. Omega creates not millions, but trillions of single-celled yeast organisms within a 10,000-square-foot laboratory on Chicago's Northwest Side, in the Old Irving Park neighborhood.

The firm, which has 17 employees, was founded five years ago by Mark Schwarz, 38, a former lawyer and electrical engineer who once worked at Boeing, and Lance Shaner, 39, a microbiologist and homebrewer with a Ph.D. in molecular genetics from the University of Texas (he also has a law degree). They spent $300,000 of their law incomes to get started, plowing subsequent profits into expansions to keep ahead of sales, which have been doubling every year.

They've grown as the industry has grown: "There were 2,500 breweries in the U.S. in 2013 when we started," says Schwarz. "Now there are 6,500, with close to 3,000 more in planning. They all need yeast."

The biggest beer-makers, like Chicago-based MillerCoors, maintain their own yeast labs in-house. For a long while, the largest independent suppliers were Wyeast Laboratories in Hood River, Ore., and White Labs in San Diego. But liquid yeast is highly perishable and doesn't travel well, and independent Midwestern breweries were thirsty for a source closer to home. Of some 220 breweries in Illinois, Omega is a supplier to 180, and sells beyond here to all 50 states.

Shaner, who heads the work in the lab at Omega, gets his yeast—there are thousands of strains—from so-called yeast banks along with a variety of other sources, including an obscure Lithuanian woman, 70 years old, who has been making beer for a half-century with her own proprietary strains. Omega's products have names like Voss Kveik, imported from a Norwegian farmstead, and Saisonstein's Monster, an original Omega hybrid of yeast strains from France and Belgium. There are 15 strains alone available to make British-style ale, and another 15 for Belgian ale.

Even homebrewers are catching on to the brand: A 100-milliliter slurry of yeast cells—enough to make 5 gallons of beer—goes for $9 in retail brewing shops.

Ben Saller, head brewer and co-founder of Burnt City Brewing in Chicago's Pullman neighborhood, says he loves the wide range of Omega's products, plus free shipping in the Chicago market. "We work only with Omega at this point," he says.