Using scavenged microbes for fermentation brings out the funky and sour flavors
Craft brewers are going wild. Some of the trendiest beers on the market are intentionally brewed with yeast scavenged from nature, rather than the carefully cultivated ale or lager yeast used in most commercial beers.
Matthew Bochman is in on the action. By day, he’s a biochemist at Indiana University Bloomington who studies how cells keep their DNA intact. On the side, he can be found bagging new kinds of wild yeast. When Bochman, a self-professed yeast whisperer, moved to Indiana, he made friends with many local craft brewers, including Robert Caputo.
Caputo wanted to make a beer that is 100 percent Indiana. He had state-grown hops and malt grains, and Indiana water is plentiful. “The missing ingredient was the Indiana yeast,” Bochman says. Caputo asked Bochman to help find the missing microbes. “So we went yeast hunting,” Bochman says.
He collected about 100 strains of yeast. “Whenever I was out and about I would grab something — a piece of bark, a berry — bring it back to the lab and get yeast from it.” The microbes are everywhere, he says. “It’s hard not to find yeast.”
But not just any yeast will do. For beer brewing, Bochman needed yeast that eats the sugar maltose in wort — the liquid extracted from grain mash that gets fermented into beer. Brewing yeast also has to be tolerant of hops, which make weak acids that might slow yeast growth. The yeast must be able to live in 4 to 5 percent alcohol. And, of course, the microbes have “to smell and taste at least neutral, if not good,” Bochman says. Not all yeasts pass the sniff test. Eight strains of Saccharomyces paradoxus made beer that “smelled and tasted ... of adhesive bandages,” Bochman and colleagues report online August 7 at bioRxiv.org.
But in 2015, a batch of wild beer brewed in an open vat in a vacant lot in Indianapolis by Bochman’s friends at Black Acre Brewing Co. yielded a winner. Among the four species and six strains of yeast in the beer was a Saccharomyces cerevisiae strain called YH166. S. cerevisiae is a species of yeast used to brew ales and to make bread. YH166 lends beer an aroma that is “an amazing pineapple, guava something. Like an umbrella drink,” Bochman says.
He doesn’t yet know what chemicals the yeast makes to produce the tropical fruit scent. He puts his money on one of the sweet-smelling esters yeast use to attract fruit flies (SN: 11/15/14, p. 13).
Sour beer brewers may also benefit from Bochman’s bioprospecting. Sour beers generally contain lactic acid bacteria in addition to yeast. Brewers need separate equipment for brewing sour beer because it’s difficult to get rid of all the bacteria when switching back to brewing other beers. Among 54 species of yeast Bochman and colleagues studied, five strains can make both alcohol and lactic acid to brew sour beers without bacteria. The researchers described the five sourpusses online July 28 at bioRxiv.org.
Bochman and Caputo formed the company Wild Pitch Yeast to sell these strains. The company supplied yeast isolated from cobwebs, trees and other spots to brewers making all-Indiana beers, dubbed “Bicentenni-ales,” for the state’s 200th anniversary last year. Bochman and Caputo are relying on brewers to tell them how the newfound yeasts perform in the real world. “The proof is in the brewing,” Bochman says. “You can do as many lab tests as you want, but you’re never going to know how something will act until you throw it into some wort and let it bubble away for a couple of weeks.”
K. Osburn et al. Saccharomyces cerevisiae strain YH166: a novel wild yeast for the production of tropical fruit sensory attributes in fermented beverages. bioRxiv.org. August 7, 2017. doi: 10.1101/149732.
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