The spoilage of lager by draught beer microbiota.

Why was the work done: To determine whether the susceptibility of lager to microbiological spoilage is determined by composition, microbiota or both. To assess beer spoilage by a consortium of yeast and bacteria from draught beer rather than pure laboratory cultures. How was the work done: Four draught beer styles - cask ale and keg lager, ale, and stout - were sampled twice in five different public houses in four different locations. The beers were forced by static incubation at 30°C for four days. 'Challenge testing' with an inoculum of heterogeneous microorganisms from the forced samples was used to assess the spoilage of ten commercial lagers after incubation by the increase in turbidity at 660 nm. The same approach was used to evaluate the role of nutrients in beer spoilage by forcing with the addition of yeast extract or vitamins (thiamine and riboflavin). What are the main findings: The ten lagers varied in susceptibility to spoilage ranging three-fold from the least to most spoilable. Average spoilage across the beers was comparable for microorganisms from lager, keg ale and stout but ca. 50% greater with microorganisms from cask ale. The ranking of spoilage of the 10 lagers was similar for microbiota from cask ale, keg ale and stout but less so from lager. Spoilage was influenced by beer composition and was inversely related to beer pH and level of free amino nitrogen. The addition of yeast extract stimulated spoilage of the least spoilable lager but the addition of vitamins B1 and B2 had little or no effect. Spoilage was extensive at 30°C, measurable at 12°C but imperceptible at 2°C. Why is the work important: The oft-quoted statement that beer is 'robust to microbiological spoilage' is a fallacy. All ten lagers were spoilt by draught beer microorganisms, but some were more spoilable than others. It is suggested that spoilage may be reduced by lowering beer pH and curbing the availability of nutrients for microbial growth. Whilst (as would be expected) beer storage at 2°C supresses microbial growth, storage at 12°C (as practiced in UK public house cellars) allows spoilage microorganisms to grow in beer. Although the threat of microbial spoilage in the brewing process is managed by good manufacturing practice, draught beer is vulnerable and requires more focus and commitment to hygienic practices to assure quality. [ABSTRACT FROM AUTHOR]