Sour Notes: The Science Behind the Sour Beer Craze

In this new bimonthly ­column, Benjamin Wolfe, a microbiologist at Tufts University, and his husband, Scott Jones, the chef de cuisine at Menton, reveal the science behind what we eat and drink.

sour beer

Sour Beer Photo by Bruce Peterson. Microscopic photographs by Benjamin Wolfe.

Brettanomyces (above) is a type of yeast that gives sour beers their farmhouse funk. Winemakers often use sulfites to combat brett’s effects.

Lactobacillus (above) turns milk into yogurt and dough into sourdough. Together, Lactobacillus and Pediococcus are found in naturally aged pickles, and make their way into fermented foods via fruit skins, grains, and even human hands.


The craft-beer craze has gone sour—from a taste perspective, that is. Distinctly tart brews—Belgian lambics and gueuzes, German Goses and Berliner Weisses, and American farmhouse and wild ales—are dominating tap lists citywide, offering mouth-puckering alternatives to the ubiquitous IPAs and lagers.

So what’s the biology behind their uniquely tart, funky flavors? Culprit number one: a type of yeast called Brettanomyces. Mention “brett” to a brewer in casual conversation and you’ll likely elicit a shudder—the yeast is widely regarded as a contaminant, because in large quantities it produces compounds with an off-putting bouquet. But under controlled conditions, beer makers can balance the flavors brett produces, yielding wild, “farm-fresh” results (sours and wild ales are often described by devotees as having an eau de “barnyard” or “horse blanket”). Some beer makers add the yeast themselves; others use wooden barrels that are already contaminated; still others rely on “spontaneous fermentation,” a method of brewing using open-air tanks to encourage brett production.

While brett brings the funk, two bacteria strains bring the tang: Lactobacillus and Pediococcus turn sugars into lactic acid, adding sourness and also producing diacetyl, a chemical compound that adds a not-always-desired buttery flavor. All together, though, these bacteria and yeast are responsible for turning malted grains into the rich, complex flavors we’ve grown to love.

Drink This Now

For sour-beer-imbibing advice, we turned to J. C. Tetreault, owner of Fort Point’s Trillium Brewing Company. His go-to bottles, below.


All beers available at Craft Beer Cellar, various locations (call first to check availability).