Membrane cartridge filtration is widely used to provide microbial stability to beverages prior to packaging. Often referred to as ‘sterile’ filtration, how do you know that the filter is capable of doing its job? Non-destructive test methods such as bubble-point and pressure hold integrity tests can give an indication that the filtration membrane is intact. However, any test is meaningless unless it can be correlated to the filters' ability to remove certain micro-organisms, which needs to be established by destructive testing of finished filters during their development.
This article explains the process by which filter manufacturers should validate the retention of their membrane cartridge filters to ensure that they are fit for purpose. It emphasizes the importance of ensuring that your supplier can demonstrate that the filters' retention characteristics match the requirement of the process and can be regularly checked with a meaningful integrity test, thereby assuring the performance of the filter, the quality of your product and protecting your brand reputation.
Figure 1
‘Sterilising’ grade membrane filters are available in various formats to suit the application. Cartridge filters such as those shown are most commonly used.
Courtesy: Parker Bioscience Filtration Division
What is validation?
Validation is the process applied during the development of the filter to ensure that all of the relevant tests and support documents are in place to make sure that the filter meets its design requirements. Components of validation may include, amongst others, qualifying that the filter is suitable for food contact, that it is robust enough to withstand its intended duty and that it is capable of removing certain contaminants from the fluid that will be passed through it.
What is a filter integrity test?
An integrity test is any method that can be used to non-destructively check a filter’s efficacy and ensure that it is capable of removing contaminants to the level it was designed for. At point of use, integrity testing is usually only applicable to microporous membrane filters that are used to remove microorganisms from the process stream. Therefore, the integrity test is only meaningful if, during the validation of the filter, the test method applied has been correlated to its ability to remove those organisms.
What is ‘sterile’ filtration?
The adopted global standard for defining sterile filtration of a liquid is the Parenteral Drug Association’s Technical Report No. 26 2008 “Sterilizing Filtration of Liquids”. This describes a method to carry out a destructive bacterial challenge test that defines a 0.2 micron sterilising grade filter as one that is capable of removing 10 million live cells of the bacterium Brevundimonas diminuta per square centimetre of filter area. Applied primarily to pharmaceutical processing, if used to filter beverages, this grade of filter could alter the products' desirable characteristics. So, in the case of beverages, the term microbial stabilisation would better suit the removal of all spoilage and pathogenic organisms. However, since the term ‘sterile’ filtration has been widely adopted in vinification and brewing, we just need to ensure that it is used in the right context.
The PDA test procedure cited above can be adapted to test the ability of other grades of membrane filter to remove alternative organisms. For example,
Serratia marcescens is widely used to define a 0.45 micron pharmaceutical membrane, but this is not representative of microorganisms which are going to lead to problems in beverage production. Instead, a better approach to developing a filter to protect your beverage is to validate it using typical micro-organisms that are pathogenic or recognised as leading to product spoilage or unwanted secondary fermentation, some examples of which are given below.
| Example organism | Typical healthy cell size range µm (various sources) | Comments |
|---|---|---|
| Brevundimonas diminuta | 0.3-0.4 x 0.6-1.0 | Used to define a 0.2µm sterilising grade filter. It can be cultured consistently to produce very small, monodispersed cells with a narrow size distribution. |
| Pseudomonas aeruginosa | 0.5-1.0 x 1-5 | Opportunistic pathogen. The smallest organism listed in most drinking water standards. |
| Serratia marcecsens | 0.5-0.8 x 0.9-2.0 | Opportunistic pathogen. Commonly used to define a 0.45µm pharcaceutical filter. |
| Acetobacter oeni | 0.8-0.9 x 3.6-5.1 | Responsible for vinegary taints in alcoholic solutions. |
| Lactobacillus brevis | 0.7-1.0 × 2.0-4.0 | Responsible for desirable malolactic fermentation but can lead to taints if present in final package. |
| Brettanomyces bruxellensis | >2.0 x >3.5 | Yeast contamination responsible for phenolic and 'horsey' taints. |
| Saccharomyces cerevisiae | >3.5 x >5.0 | Yeast used for alcoholic fermentation. Buds and stressed cells can be sub-micron. |
Further content to follow