Identification of spoilage microflora in draught beer using culture-dependent methods.

Aims: To determine whether the culture-dependent spoilage microflora found in draught beer are influenced by beer style.

Methods And Results: Four beer styles-lager, ale, stout and cask ale - were sampled twice from five different public houses (accounts) in four different locations. The microbiological quality of the dispensed beers was determined by a culture-dependent method ('forcing'), measuring the increase in turbidity after incubation at 30°C. The quality of draught beer varied from 'excellent' to 'poor' with cask beer samples having a higher Quality Index (90%) with keg ale the lowest (67.5%). With PCR amplified DNA (ITS1, ITS4, 16S rRNA primers) and blast identification of microflora, 386 colonies from agar plates were identified with 28 different micro-organisms from five genera of yeast and six of bacteria. Seven micro-organisms were found in all beer styles with Brettanomyces bruxellensis, B. anomalus and Acetobacter fabarum representing 53% of the identified micro-organisms. A subsequent, limited study using PALL multiplex PCR GeneDisc technology on forced samples (without selection on plates) suggests that draught beer microflora is qualitatively broader. It is noteworthy that the microflora of spoilt draught beer resembles that involved in the production of Belgian Lambic sour beers.

Conclusions: Draught beer was of variable quality. Culture-dependent analysis suggests that species of Brettanomyces and Acetobacter are core microflora with some micro-organisms being associated with beer style.

Significance And Impact Of The Study: The microbiological quality of draught beer is important both commercially and to the consumer. Here, we report the core and diverse microflora found in different styles of draught beer using culture-dependent methods.