Beer’s Least Sexy Ingredient May Be Its Most Important

Dr. Frankenstein called forth lightning to bring his monster to life. Brewers call forth yeast. Perhaps more than any other ingredient, the unicellular fungus breathes life into a beer. It also plays a key role in determining the taste on your tongue. Unfortunately, it stays largely behind the scenes, ceding the spotlight — especially on beer labels — to the sexier hops and barley. But it’s the yeast that plays Merlin and makes the magic on your palate.

In ancient times, that magic was ascribed to the gods. As early as 4000 B.C., the Sumerians were worshiping Ninkasi, the goddess of beer and brewing. And until Dutch naturalist Anton van Leeuwenhoek observed yeast under a microscope in 1680, the gods took the credit. It took Louis Pasteur to link yeast and fermentation in 1857 and Emil Christian Hansen to isolate beer yeast 26 years later to pull the real rabbit out of the hat — Saccharomyces, the “super fungus.”

A Tale of Two Yeasts

When we talk about beer or brewer’s yeast, we are really talking about two types — saccharomyces cerevisiae and saccharomyces pastorianus. The first is the OG, producing the ale family of beer, including witbiers, stouts, porters, saisons and IPAs. This top-fermenting yeast loves higher temperatures, ideal in a world without refrigeration, and makes a significant impact on the flavor and mouthfeel. They also work quickly, producing ale in in just three to five weeks compared to six to eight in lagers.

Pastorianus, a bottom-fermenting yeast, produces the pilsners, lagers, märzens and bocks of the lager family. While it’s been traced back to the 1300s, it didn’t really come into its own in the mid-to-late 1800s, when lagers began to grow in popularity along with artificial refrigeration. It loves temperatures between 48°F and 58°F and works slowly and subtly, influencing the flavor less than in ales. It’s also the most common beer yeast, thanks to the continued dominance of lagers in the global market.

Whichever yeast goes into the wort — the sugary grain water drawn from the malt and hot water — the party ensues. The yeast plays Pac-Man on the sugar, and by doing so, produces the alcohol, carbonation and compounds that not only make the beer, but help determine the flavor. Here’s how.

How Yeast Impacts Flavor

A good way to think about yeast is like a key that opens a lock. When the yeast is “pitched” into the wort, the yeast ignites biotransformations that produce the beer and drive its flavor.

“There are two major categories with what yeast does with flavor,” explains Richard Preiss, founder of Escarpment Laboratories, a producer of yeast strains for brewers. “One is making its own flavors, and two is modifying or releasing flavors from other ingredients.”

As for its own flavors, yeast tends to bring the tastes we perceive as fruity, Preiss explains. That comes from ester compounds produced by the yeast. The boozy flavor also comes from the alcohol made by the yeast, as well as the carbonation. Spicy flavors tend to come through a meeting of yeast and malts. “That’s biotransformation of precursors from the malt you’ve got, something in the malt that’s normally flavorless that the yeast turns into that baking spice flavor,” explains Preiss.

In lager yeasts, the effect is less pronounced, leading to the cleaner, crisper flavor identified with them. A good way to think of it, Preiss explains, is in movie terms. On the ale spectrum, especially the Belgian side, “the yeast is the star of the show.” Lager yeast, on the other hand, is an assistant, “helping to highlight some of the flavors of the other ingredients.”

Meet the New Yeast, Same as the Old Yeast?

In the topsy-turvy world of craft beer, standing out from the rest is key. “Brewers are looking for differentiating factors, and yeast can be a way to do that,” Priess explains. Indeed, supplying brewers with the best yeast possible is the aim of Escarpment, as its work with Drekker Brewery in Fargo, North Dakota and Boxing Rock Brewery in Shelburne, Nova Scotia can attest.

This quest for unique yeasts has led other brewers to tap unusual sources, so-called “wild yeasts,” to craft a beer unlike any other. Many of these are in the natural environment. “You can find interesting yeast almost anywhere,” notes Priess, although, he adds, some environments more likely to have them — for example, fruit trees, which basically sweat sugar.

That led scientists in Patagonia to seek yeast strains in the bark of trees and create hybrids capable of producing unique flavors, with findings published in June 2024. Brewers also do their own field work. Austin, Texas-based Beerburg sources yeast from San Antonio’s Community Cultures Yeast Lab, which produces endemic yeasts swabbed from native Texas plants like ocotillo and Torrey yucca. First Magnitude Brewery in Gainesville, FL went so far as to collect yeast from live Frosted Elfin butterflies in Apalachicola National Forest to ferment a New England-style Session Pale Ale.

Shipwrecks are also other prime sources of unique yeast. Saint James Brewery in Long Island brewed Deep Ascent ale using yeast found in bottles of beer retrieved from an 1886 shipwreck. Ancient ales reach back even further. Home brewer and founder of Primer’s Yeast Dylan McDonnell won attention in the New York Times for crafting a beer with yeast extracted from an amphora used by the Philistines for brewing around 850 B.C. But perhaps the most extravagant source of beer yeast was a Rogue Ale brewer’s own beard, creating Beard Beer.

On the flip side, some breweries like Guinness don’t want to mess with a good thing. The present yeast used to make Guinness stout brewed in Dublin, Ireland, can be traced back to 1903 and occupies “a distinct group separate from other historical Irish brewing yeasts,” according to a 2024 report in the National Library of Medicine. Perhaps that’s really behind the slogan “Guinness is good for you.”