Retention of Beer Spoilage Microorganisms by Polyvinylidene Fluoride Microporous Membranes with Various Retention Ratings. K. L. Roche, P. M. Meier, and R. V. Levy, Process Division, Millipore Corporation, Bedford, MA 01730. J. Am. Soc. Brew. Chem. 51:0004, 1993.

Polyvinylidene fluoride microporous membrane disks rated at 1.0 µm, 0.65 µm (with and without a contiguous upstream cellulose-ester prefilter layer), 0.45 µm (with and without a similar prefilter layer), and 0.22 µm were evaluated for the retention of three spoilage microorganisms suspended in beer: Saccharomyces diastaticus, Pediococcus damnosus, and Lactobacillus brevis. Pore-size ratings were assigned by an integrity test measurement called the "bubble-point pressure." Membranes were challenged with concentrations of microorganisms greater than or equal to 10⁶ colony-forming units per square centimeter of surface area. A challenge differential pressure of 50 psi (3.45 bars) was selected to represent worst-case processing conditions. A correlation was found between microorganism retention and the bubble-point pressure. Complete retention of P. damnosus and L. brevis, the lactic-acid bacteria, occurred only with membranes rated at 0.45 µm and 0.22 µm, whereas S. diastaticus, the yeast, was completely retained by all membranes. To be assured of a microbially stable product, one should consider only 0.45 µm (or "tighter") membranes for which the performance has been verified by microbial retention testing and validated by an integrity test method.

Keywords: Aseptic packaging, Beer filtration, Bubble-point pressure, Membrane filtration, Spoilage microorganisms