What is The Mousy Taste in Sour Beer?

Lallemand Partners with UC Davis to Create a Method for Detecting THP

At Lallemand Brewing we often partner with academic leaders in the brewing world to dive further into fermentation research. In 2023, we partnered with the Anheuser-Busch Endowed Professor of Malting and Brewing Science at UC Davis, Dr. Glen Fox, and his team to develop a method for detecting the mousy off-flavor in sour beer. This is first of its kind research in beer – while the compounds that create the mousy off-flavor in wine have been studied extensively, in beer, a method of detection has eluded scientists and brewers for many years.

What is the mousy off-flavor?

In scientific terms, the mousy, or dried Cheerios flavor, comes from a class of compounds called tetrahydropyridines (THP). These compounds come in many forms but are in part derived from certain strains of Brettanomyces, specific heterofermentative lactic acid bacteria (LAB), and acetic acid bacteria. Naturally, these microorganisms play a role not only in wine but also in sour beer. Some sour beer brewers are unusually sensitive to THP, noting cage mice or dried Cheerio sort of flavor in these beers. Still, they have never been able to quantify the amount present in these beers analytically until now.

Another source of the specific THP compounds, 2-acetyl 3,4,5,6-tetrahydropyridine (ATHP) and 2-acetyl-2-pyrroline (APY), which is perhaps more pressing in beer than in wine, comes from the result of the degradation of certain amino acids. As malted barley contains a significant amount of amino acids, particularly proline and lysine, there is reason to believe that this mousy, dried cereal off-flavor is not only a concern for sour beer brewers.

Next Steps in Research

To begin identifying and quantifying THP in beer, UC Davis sampled several commercially available beers to establish a method of detection. This method was established last year and published in 2024 in the Journal of Agricultural and Food Chemistry, A Review of N-Heterocycles: Mousy Off-Flavor in Sour Beer, and presented at the Craft Brewers Conference in 2024 by Paulina Martusevice. This first step was accomplished. The next step is to further the research by looking at specific strains of lactic acid bacteria. Stay tuned!

Link to Paper: A Review of N-Heterocycles: Mousy Off-Flavor in Sour Beer