Microbial Removal in F&B and Pharmaceutical Industry
F&B and pharmaceutical customers frequently use microfiltration membranes to remove microbial contamination in liquid or gas process streams. Microbial contaminants can be any of a variety of species of bacteria and yeast of varying size and concentration. Removing these organisms is essential to prevent product spoilage or food borne illness in the F&B industry, and contamination within a sterile parenteral drug (injectable) can lead to significant side effects such as infection, fever or death in pharmaceuticals. Therefore, filter manufacturers serving these industries must conduct microbial retention tests with the filters to demonstrate the efficacy of microbial removal.
Filters that have been validated for microbial removal exhibit much higher removal efficiencies than absolute rated particle filters, such as a pleated polypropylene filter. As a comparison, an “absolute-rated” Beta 5000 particle removal filter exhibits a removal efficiency of 99.98% of particles, while a sterilizing grade liquid filter exhibits a removal efficiency of >99.99999999% of microorganisms. The removal efficiency of a membrane filter is expressed as the log reduction value (LRV) of target microbes, which are selected by virtue of their approximate size and relevance to the application.
Microbiological quality is likely the most common food safety objective achieved by filtration.
Because many pharmaceuticals are damaged by heat, there is no process to sterilize these products in their final container. Thus it is critical that the process stream yield sterile product and that containers are filled and sealed in an extremely high-purity environment. In order to obtain sterile effluent in the most critical applications, double layered membrane filters are typically used.
The European Medicines Agency (EMEA) recommends 10 CFU (colony forming units – a count of viable bacteria) as the maximum acceptable bioburden prior to a sterile filter. For higher levels, a bacteria retentive prefilter should be used to reduce the bioburden. Multi-filter arrangements are routinely used in the industry to manufacture sterile products. PDA Technical report no. 26 (2008), Sterilizing Filtration of Liquids recommends a multi-filter approach to manufacture sterile products.
Sterile Filtration versus Bioburden Reduction
By definition, a sterilizing grade filter is to have a rated pore size of 0.2 micron or smaller and should be validated for the reproducible removal of viable microorganisms from a process stream, resulting in sterile effluent under worst-case production conditions when liquid challenged with 107/cm2 of surface area of a specific test organisms. This challenge protocol was originally established by the Health Industry Manufacturer’s Association (HIMA) and is often referred to as the HIMA test. The challenge level is orders of magnitude higher than would be expected in a sterilizing filtration process, thus providing a high degree of assurance of filter performance.
While the target organism for the biopharmaceutical industry is Brevundimonas diminuta, the test methodology can be applied to any micron rated membrane filter regardless of application. For example, a 0.45 micron rated membrane filter will not provide microbial free effluent when challenged with Brevundimonas diminuta, but may when challenged with Serratia marcescens. Neither of these have any significance for the F&B market, therefore more relevant organisms, such as Psuedomonas aruginosa (water borne contaminant) for 0.2 micron membranes and Lactobacillus brevis (found in dairy, wine and other food) for 0.45 micron membranes are chosen as test organisms.
In F&B applications, the criteria for effluent is not as rigorous as in the biopharmaceutical market since the goal is to achieve a low microbial load to downstream equipment or the product has microbial-static characteristics (i.e. alcohol in wine). In this case, the filters need to achieve only bioburden reduction, or significant removal of the microbial contamination. The filter is challenged at 107 /cm2 of surface area of a specific test organism, which is equivalent to 1011 organisms for a typical 7 ft2 membrane filter. The quantity of viable organisms in the effluent is determined and subtracted from the 1011 value to determine the log reduction value. For example, if there were 1000 (103) in the effluent, you would get a LRV of 8 (1011- 103 = 108). Typically, an LRV value of 5 – 7 is deemed acceptable for food/beverage applications.
In order for filter manufacturers to document the retentive potential of a filter, non destructive test procedures have been developed, including bubble point and diffusional flow. These tests can be correlated to bacterial retention utilizing the ASTM 838-05 methodology, and are designed to give you statistical confidence in the retentivity of the filter. In the process, integrity testing of the filter(s) is generally performed prior to use as well as post-use to validate retention. Biopharmaceutical companies are required by the FDA to demonstrate efficacy of their processes and maintain documentation of each process. This validation is costly in terms of time and material and thus once locked-in, there is little incentive to change. F&B producers typically do not require this level of governmental documentation and therefore generally only evaluate to a set of internal standards.