Indirect detection of microbial contamination in beer by chemical fingerprints

Analytical Session
Jennifer Koob, Research Center Weihenstephan for Brewing- and Food-Quality, TU Muenchen, Freising, Germany
Co-author(s): Robert Riedl, Mathias Hutzler, Mehmet Coelhan, and Fritz Jacob, Research Center Weihenstephan for Brewing- and Food-Quality, TU Muenchen, Freising, Germany

ABSTRACT: A few bacteria, especially species of the genera Lactobacillus and Pediococcus, are able to spoil beer. Procedures to detect and identify these bacteria are numerous, but nearly all of them are based on the presence of living or augmentable cells. This study attempts to detect beer spoiling microorganisms indirectly using chemical fingerprints. Nine different, as determined by real time-PCR (polymerase chain reaction) and DNA-sequencing, strains of beer spoiling bacteria were inoculated in bottled beer and incubated for a period of at least 2 weeks. As reference samples, uncontaminated blank beers were processed and analyzed in the same way as the contaminated samples. After the formation of turbidity by the beer spoiling bacteria the sample was chemically analyzed. Several chromatographic methods were applied to detect the differences between the contaminated media and the blank samples. The accumulation or degradation of compounds had to be caused by microbial growth. The analyses performed were, for example, the determination of the fatty acid content and the concentrations of aging-relevant substances by GC and flame ionization detector (FID) after steam distillation. In addition, the contents of amino acids, dimethyl sulfide, organic acids, and fermentation by-products were determined by chromatographic methods. The major differences (±60% of the initial values of the blank samples) were collected and transferred to a profile that is specific for every bacteria strain used in this study. The results for L. brevis and L. backi differed particularly with regard to organic acid, fatty acid, and aging-relevant compound contents. The profiles or chemical fingerprints can be regarded as a novel tool to identify a certain beer spoiling microorganism by recovering them in contaminated beer samples that don’t have to contain living cells. This work establishes the basic experimental setup and analysis for further studies on the chemical detection of microbial infections in unfiltered and filtered sections in breweries.

Jennifer Koob was born in Bad Kissingen, Germany, in 1986. From 2005 until 2010 she studied brewing science and beverage technology at TU München and graduated with a Dipl.-Ing. degree. After her studies she worked as an assistant manager for the Lang Brewery, Waltershausen. Since October 2011 she has been a scientific assistant at the Research Center Weihenstephan for Brewing and Food Quality. Her main research topics are beer spoiling microorganisms and their chemical identification.