The term "sterile filtration" refers to the reduction of yeast and bacteria to levels that do not result in spoilage of the beer over its planned shelf life. Viruses do not survive the brewing process.
Sterile filtration has been used as an alternative to pasteurization for many years. It has the advantage over pasteurization in that the risk of flavor damage by heat is eliminated. However, many brewers report that there is little difference in flavor stability between sterile-filtered and properly pasteurized beer (37).
The brewer should set a specification for the maximum allowable concentration of yeast and bacteria in sterile-filtered beer since not all microorganisms are removed by sterile filtration. Opinions differ about what is the critical level for contaminants, and for beer it has been set at between 4 and 10 yeast cells per 12 ounce bottle. In the case of lactobacilli, it has been reported that between 1 and 3 organisms per 12 ounce bottle is acceptable. In other words, there is no universally agreed-upon minimum safety level for Lactobacillus sp., and use of the term "sterile filtration" can be misleading.
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