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Abstract
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.