Integrity Testing for Food & Beverage Filters

Pall’s food and beverage portfolio is comprised of many integrity test devices that help the industry ensure membrane filters are integral and suitable for continued service.

Why perform an integrity test?


Integrity tests are non-destructive tests and as such, can be performed before filtration and after filtration without affecting the filter itself. At the same time, they provide confidence that the filter is intact and providing the expected performance and retention characteristics.


Filter integrity testing during validation relates to the microbial removal of final filters based on membranes used during product development. For this purpose:


  • Filters with a range of integrity test values under set conditions are challenged with microbes.
  • Then the results of the microbial challenge tests are plotted against the integrity test values. 
  • Based on the desired microbial removal level - sterilizing grade or a specific log reduction value (LRV) - an acceptable test limit for the integrity test is established. (e.g., mL/min or mbar pressure drop)

Integrity Test Values and Microbial Challenge


A correlated integrity test can confirm filters are integral and suitable for continued service, for example:


  • It confirms the filter is the right grade, has been installed correctly and is not damaged.
    • One can undertake an integrity test on an empty filter housing with closed upstream and downstream valves to confirm that all housing o-ring seals are intact.
  • It confirms that the filter meets the manufacturer’s specifications.
  • It is important for process validation, monitoring and batch record files and could be helpful during process documentation review.
  • Finally, it allows for faster identification of contamination.
    • If the filter passes the integrity test and contamination is found downstream of the filter, then the following root causes are most likely:
      • Contaminated sampling port
      • Contaminated filling heads
      • Contaminated closures
      • Contaminated bottles


At Pall, 100% of the filters are integrity tested during the manufacturing process before release. There is a production integrity test limit specific to the manufacturing site’s integrity test method and the results of the validation. Additionally, there is a customer integrity test limit suitable to use with Palltronic® integrity test devices supplied by Pall for customer use (e.g., Palltronic Compact Touch Integrity Test Device).


Recommended Integrity Tests to confirm the integrity of microbial reduction and sterilizing grade membrane filters


There are two integrity tests recommended to confirm the integrity of microbial reduction and sterilizing grade membrane filters: Forward and Pressure Decay. The forward flow value is correlated to the bacterial challenge test results. The pressure decay value is calculated from the forward flow value and the upstream volume of the housing.Flow


These tests are correlated to a set level of microbial removal or reduction during the initial filter validation. As non-destructive tests, they may be used after sanitization and at the end of a production batch to show the filter assembly is integral. These tests are based on the same physics that applies to gas flow through a wetted pore.


Integrity test and the physics of flow of gas through a pore

Forward flow test 


  • The pressure within the upstream volume of the filter housing is maintained at the filter-specific forward flow test pressure.
  • The total gas flow, made up of diffusive flow-through wet pores and bulk flow through open pores or defects, through the filter is measured by the test device upstream or downstream forward flow measures the flow required to maintain the test pressure.


Pressure decay test (also known as pressure hold)


  • The pressure within the upstream volume of the filter housing is stabilized at the filter and housing-specific pressure decay test pressure.
  • The total gas flow through the filter is measured by the test device upstream.
  • Pressure decay measures the drop in the pressure.


Water for Intrusion Test (WIT)


  • Water flow is measured through a hydrophobic filter and correlated to microbial retention.


Bacterial retention testing is the most sensitive integrity test for sterilization ability and microbial reduction, but this testing is destructive and therefore cannot be used if the filter needs to be put into service after testing as the filter would be contaminated with the test bacteria. Non-destructive integrity tests, if correlated, are an indicator of bacterial challenge retention.


Practical Considerations When Performing Integrity Tests



 Selection of Integrity Test Fluid


  • Standard "Reference" Test Fluids


  • Water: Hydrophilic & Hydrophobic Filters
    • Hydrophilic Filters by FF/PH/BP
    • Hydrophobic Filters by WIT


  • Alcohol/Water: Hydrophobic Filters
    • 60/40 IPA/Water is common
    • 25% t-butanol (Pallsol)
    • 100% alcohols (IPA, Ethanol) not recommended (elevated diffusion)


  • Process Fluids (derived values)
    • Product
    • Buffer pH Adjusters, etc.



② Selection of Gas for Pressurization


  • Acceptable Test Gases:
    • Air
    • Nitrogen
    • Argon


  • Unacceptable Test Gases:
    • Carbon Dioxide (solubility concerns)
    • Oxygen (reactivity concerns)


③ Temperature


The temperature should not vary during an integrity test by more than 5 °C. If the temperature changes significantly, the diffusion of the gas in the fluid will change. The recommended temperature range for integrity testing is 20 °C ± 5 °C (68 °F ± 9 °F). The effect of changing temperature on gas pressure is critical. A 1 °C change in temperature will cause an approximate 0.3% change in volume.


④ Upstream Volume


The upstream volume is an important consideration for Pressure Decay tests. In applications where filters are being effectively regenerated, their service lives will be dictated by their structure integrity.


The Pall Compact Touch (Integrity Test Device).