Brewer Reources —

Turning Off Unwanted Flavors: How Berkeley Yeast’s Fresh Strains Create Low Diacetyl IPAs and Lagers

By Joshua M Bernstein

Like most brewers, Danny Priddy has a pretty discerning palate. The director of brewing operations at Riip Beer Company in Huntington Beach, California, is particularly sensitive to diacetyl, an organic compound that, at low concentrations, can mute hop notes in beers like Riip’s aroma-charged IPAs. At high concentrations, diacetyl can add an unwanted buttery note. 

To avoid packaging IPAs that mimic movie popcorn, Riip fastidiously checks for diacetyl. But even if the compound registers below the flavor threshold, Priddy might perceive an “oxidized sweetness” that intensifies as a beer warms up. “Something holds down the hop expression.”

Seeking extra olfactory oomph, Riip brewed a test batch of Dankster Squad, a West Coast IPA hopped with Citra and Mosaic, with a Berkeley Yeast strain called Fresh Chico that uses low-diacetyl technology. The yeast lab modified the classic Chico strain, favored for well-hopped West Coast pale ales and IPAs, to keep diacetyl levels ultra low during fermentation and packaging.

“It laid down this clean slate for building hop flavors and rollercoasters of aromatics,” Priddy says. Riip used Fresh Chico in the Dankster Squad that won gold at 2022’s Los Angeles IPA Festival, later netting silver at that year’s Great American Beer Festival. The brewery now uses Fresh Chico as its house strain for IPAs, blonde ales, and imperial stouts alike. “It’s a workhorse,” Priddy says.

Diacetyl is an inevitable byproduct of brewing. In low levels, diacetyl can be a pleasing component of Czech pilsners and British ales. Diacetyl is detrimental to most other styles of beer, especially clean-drinking lagers and fragrant IPAs. Brewers will go to extremes to avoid diacetyl.

“For the longest time we weren’t dry hopping beers,” says Brian Cendrowski, the co-founder of Fireforge Crafted Beer in Greenville, South Carolina. He fretted about hop creep, which occurs when excessive hopping rekindles fermentation and causes diacetyl to form. One ill-fated West Coast IPA became a “diacetyl bomb” after packaging, he says, forcing Fireforge to dump half the batch.

To lessen brewers’ worry, Berkeley Yeast engineered a range of ale and lager strains to produce alpha acetolactate decarboxylase (ALDC), an enzyme that stops diacetyl from forming. Diacetyl levels remain ultra low during fermentation and packaging, preventing beer from developing a buttery aroma at any point.

“I almost feel like it’s a cheat code,” says Cendrowski, who now makes his IPAs with Berkeley Yeast’s Fresh strains. And yes, he’s since resumed dry hopping. “It’s been a game-changer.”

Fresh Strains Tirelessly Work to Remove Diacetyl Without Resting

Eliminating diacetyl from a beer has historically required patience. Brewers raise a tank’s temperature a few degrees Fahrenheit during the end of fermentation, encouraging yeast to metabolize the organic compound. The diacetyl rest, as it’s called, requires several days for ales, while lagers might need a week or longer. “That increases the amount of time it takes you to make beer,” says Jeremy Roop, who leads research at Berkeley Yeast.

Using a Fresh strain lets brewers skip a diacetyl rest, freeing tanks for another batch. Fast turnaround times are important for taproom-focused breweries like Fireforge, a seven-barrel brewery that fills its 20 tap lines with IPAs, fruited sour ales, and lagers.

The brewery ferments its lagers with Berkeley Yeast’s Fresh Andechs strain, and they’re typically ready to drink in 17 to 20 days. Unlike many smaller breweries, “we can have a wide variety of lagers on at any given time,” Cendrowski says. They might range from the Game Time Decision light lager to the German-style pilsner and Novemberfest märzen, all of which medaled at the 2023 World Beer Championships

North Park Beer Company in San Diego has also won numerous medals using Berkeley Yeast’s Fresh Chico strain. At the 2022 Great American Beer Festival, North Park received a Brewery of the Year award and nabbed four medals, including a gold for NZ-Fu!, a New Zealand–style IPA. (It also grabbed gold at the 2023 World Beer Cup.)

Founder and head brewer Kelsey McNair began using Fresh Chico after the pandemic forced him to firm up his production timeline and package more beer with a mobile canner on a set weekly schedule. That left no leeway for a longer-than-expected diacetyl rest. “We needed the reliability,” McNair says. 

Regional production breweries also find that Berkeley Yeast’s Fresh strains can smooth worries. Dust Bowl Brewing in Turlock, California, about 90 miles southeast of Sacramento, makes about 22,000 barrels a year, led by Therapist imperial IPA. Brewmaster Don Oliver battled hop creep with the 100-IBU brand. “It’s 10.4 percent ABV, so you can get variability in yeast activity and diacetyl cleanup,” Oliver says. 

Dust Bowl brought in Fresh Chico and shaved up to four days off production time, creating a consistent 16-day process. The brewery also uses the Fresh Vermont strain in its hazy IPAs including the year-round staple Peace, Love & Haze. “We’re so confident that we won’t have problems that we don’t have the lab do forced-diacetyl tests” on Fresh beers, Oliver says. “After a year’s worth of data, none were popping.”

Engineering Yeast Creates Foolproof Efficacy 

Scientific research of ALDC’s role in limiting diacetyl formation dates back several decades, but the trek from academia to the brewhouse is long and vexing. “No one had tried to implement it,” says Roop of Berkeley Yeast. 

In recent years, brewing suppliers have released liquid forms of ALDC that should be added to cooled wort, prior to pitching yeast, because the enzyme is pH sensitive. The enzyme’s optimal pH level is 6.8, where it’s most active. Depending where the wort is during fermentation, the pH might be around 5.4 to 4. As the pH drops, the enzyme loses activity, Roop says. “You’ve got to remember to add it at the right time and make sure the enzyme is fresh.”

In contrast, the pH inside of the yeast cell is 6.8, so the ALDC produced by Fresh strains remains highly active. The robust yeast-produced enzyme works every time, regardless of recipe or process variation.

Resident Culture Brewing in Charlotte, North Carolina, makes numerous IPAs like its flagship hazy, Lightning Drops. Initially, head brewer and co-founder Chris Tropeano used ALDC, but “we weren’t always preventing diacetyl from spiking,” he says. Issues appeared after dry hopping. “We were spending a bunch of money on a product that wasn’t 100 percent effective.”

Once Tropeano heard about Berkeley Yeast’s Fresh strains, “it didn’t take any convincing,” he says. In early 2022, Resident Culture switched to Berkeley Yeast’s Fresh London strain for hazy IPAs, and it now uses the Fresh Chico and Fresh Augustiner strains too. “It’s saved us a ton of money on ALDC, and we’re never worried about diacetyl.”

Berkeley Yeast’s Fresh strains have the ALDC gene embedded in the yeast’s chromosome, meaning it’s stable no matter how many times a yeast replicates. “Just knowing that [ALDC is] built into the yeast is a security blanket,” McNair says. 

A truncated tank time can also minimize additional unwanted flavors. The longer hops stay in contact with beer, the higher the polyphenol content, potentially causing astringency. Waiting for diacetyl clearance delays crashing—the process of lowering tank temperature—so yeast, hop solids, and sediment fall to the bottom of a tank, improving clarity. 

“For us, it’s very important be able to crash when need be,” Riip’s Priddy explains, helping avert the overextraction of flavors and aromas. Berkeley Yeast’s Fresh strains help “make our jobs easier and make better beer for the public.”

Brewing science is in a state of constant discovery and improvement. Taking days to eliminate diacetyl is equivalent to mailing letters instead of sending emails. The wait isn’t always worth it when the faster Fresh alternative has no unforeseen speed bumps, putting profit-generating lagers and IPAs in cans and hands ASAP.

“There’s no reason to go back to the old way,” says Cendrowski’s Fireforge.