Could endophytic bacteria Pantoea agglomerans isolated from hop cones contaminate beer?

Abstract

Gram-positive bacteria, particularly lactic acid bacteria, are generally considered the primary potential contaminants of beer. However, given recent publications on the isolation of Gram-negative bacteria from hop pellets, it is important to also consider whether endophytic bacteria found in the above-ground parts of the hop plant, such as the facultatively anaerobic bacterium Pantoea agglomerans, might pose a potential risk during so-called dry hopping of beer. To find an answer to this question, we conducted practical experiments in which we intentionally inoculated various types of alcoholic and non-alcoholic beers, as well as culture media mimicking the beer environment. We subjected the inoculated beers to a 5-day incubation at two temperatures: 7 °C in a refrigerator and 30 °C in a thermostat. Our observations revealed that the main factor preventing the development of this type of contamination is the low pH of the beer, which is unsuitable for bacteria. Other contributing negative factors that prevent contamination are alcohol content and oxygen limitation. The only theoretical risk of contamination could be associated with improper storage of non- alcoholic dry-hopped beer at high temperatures.