Project Results

About This Project

Mixed cultures create sour beers, but many brewers don't know which microbes are in their cultures. We will identify the species in 21 or more samples from pro/homebrewers using deep sequencing. We will also combine the cultures and follow the evolution of that population over the course of fermentation to determine which species are dominant. This work will highlight the similarities and differences between souring cultures, guiding the development of custom cultures for sour beer production.

Ask the Scientists

What is the context of this research?

"Sour" is perhaps the original beer style because the microbes living on malted grains used for brewing and the wooden vessels used for fermentation naturally create lactic acid (perceived as sour or tart) and other funky byproducts that yield complex, flavorful beers. This type of spontaneous fermentation by local microbes has since largely been replaced by fermentation with defined cultures of, typically, a single yeast species to help with predictability and repeatability for fermentation on the industrial scale. However, mixed culture fermentation lived on in many enclaves in Europe and has been growing as a category during the current craft beer boom around the world. Some work has been done to ID the microbes in these sour cultures, but they remain mostly ill defined.

What is the significance of this project?

Many brewers, at both the professional and homebrew levels, start making sour beers by using bottle dregs (i.e., the microbial slurry found in bottle conditioned beers) to inoculate their wort. Others mix defined cultures of Saccharomyces, Brettanomyces, Lactobacillus, and/or Pediococcus to create a "house" mixed culture, but these blends will evolve over time with repeated use. The evolution of these cultures affects the beer - and not always in a positive way. If we knew what common bacterial and yeast species (and their relative abundances) were found in "good" souring cultures, we could build the mixed cultures from isolated species to create house cultures or replace souring cultures that have evolved away from their desired sensory output.

What are the goals of the project?

Our goals are threefold. First, we want backers to submit at least 21 samples of mixed cultures used for fermentation. Then, we will harvest the microbes in the samples, extract their DNA, identify the species using next-generation sequencing, and analyze the similarities and differences between samples. We will also combine all of the samples into one large, diverse microbial community and use it to ferment a golden sour beer base. We will know the identity of the bugs going into the beer, and we want to follow the community as it evolves over the course of fermentation to determine the dominant species at various time points (Mixed Culture Madness).