To be acceptable for use in the sterile filtration of beer, membranes must be demonstrably capable of consistently removing all spoilage microorganisms (wild yeasts, lactic acid bacteria, etc.). It is customary to test such membranes by filtering a suspension of spoilage microorganisms and then carrying out a microbiological analysis of the filtrate to determine whether any cells passed through the filter and, if so, how many. The problem is that the variable cell size and difficulty of obtaining high densities of common species of beer spoilage bacteria (especially those in the genus Lactobacillus) make it almost impossible to achieve the necessary reproducibility. Accordingly, in order to provide a more reliable test, the authors recommend using the bacterium Serratia marcescens, which is highly uniform in shape and size, can easily and rapidly be cultured to a high population density and prepared as a monodispersed cell suspension in degassed beer (in which it can live long enough to carry out the test), and forms visible colonies on a culture plate after only 18 hours of incubation. Moreover, it is already widely used in other industries (e.g. pharmaceuticals) to test 0.45 micron filters, and since it is smaller than any of the common beer spoilage bacteria, it provides a more rigorous test; no spoilage bacteria can pass a membrane filter which stops it.
Keywords : bacteria beer filter measurement membrane performance sterile filtration