Membrane Filter Integrity Testing

Typically an Absolute Membrane Filter

By: Thomas J. Payette, Winemaking Consultant

The previous article in The Grapevine Magazine addressed steaming the bottling line.  Following steaming, the winemaker will want to perform a check to determine whether or not the process has been successful and that the filter has not been damaged by the steaming process or other handling. 

  The following process is a way to check the membrane filter’s integrity prior to wine bottling use.  During proper steaming, sterility should have been achieved from the cartridge filter downstream to the filling spouts.  Please keep this in mind as we follow this procedure to insure the sterile conditions will not be compromised during the testing process.


  The objective of this procedure will be to test a pre-wetted membrane filter with air or nitrogen to determine if the filter will hold a certain level of pressure.  The surface tension of clean water on this filter matrix will determine the amount of pressure the filter will hold.  After steaming and cooled the filter cartridge is considered pre-wetted.  Do not remove to wet the cartridge after steaming.  That would violate the “sterile conditions”.


1.     Review the information that came with your filter and contact your supplier representative to see if they have any tips on the procedure or process about to be performed.  They may have helpful recommendations and data for you about that specific filter.

2.     After the steaming operation, described in the previous article, allow the assembly to cool to room temperature.  {If this is not done properly the results will not be accurate because the temperature change inside the closed system will show a pressure drop due to cooling and contraction.} The cooling process may take some time.

3.     Attach a source of compressed air or nitrogen (not Carbon dioxide) with a regulator to the top valve or to any sealed up stream orifice of the filter assembly.  Close all of the upstream valves with the exception of the one you would like to use to pressurize the filter system.  Be aware that any downstream orifice could be a contamination point. 

4.     Make sure all the valves are in perfect shape, will not leak and will have the ability to withstand the pressure that will be applied to the filter.

5.     Slowly turn the pressurized air source on and allow it to flow into the upstream side of the membrane housing. [Note: use a clean source of air that does not have any wine residue on the tip of the hose or any possible chance of introducing yeast or microorganisms.  Being the upstream side of the filter this should not be a problem but remember we are about to bottle a wine in a sterile environment.  Try not to introduce any micro-organism: Think Cross Contamination!]

6.     Using the regulator adjustment, allow the pressure to slowly increase up to the designated pressure for the micron rating of the filter.  (Typically 18 PSI is sufficient for a 0.45 micron rated membrane that would hold 20 PSI wetted.  Do double check this number with your supplier in the event this rating changes since publishing of this article. )  The test pressure will be in the literature of the filter package or it can be obtained from your supplier’s technical department.  Be aware, some filters have the same hold pressure even though their micron rating may be different.  Be certain not to “slam” the filter with immediate pressure.  That action may rupture the filter media and that filter may not pass the test or perform the filtering function as designed and desired.

7.     Allow the pressure to rise slowly while monitoring both the pressure gauge on the filter housing and the gauge on the regulator supplying the compressed air or nitrogen.  There should be little or no discrepancies between them.  This also indicates the gas is flowing into the filter housing.  One may see a slight amount of water come through the down stream side of the unit though a bleed valve.   This is normal since some water may “push” off the outside of the pre-wetted filter. Do not disassemble the down stream side of the set up because it will compromise the sterility of the bottling.  [Do make sure an outlet for air is open on the downstream side of the filter so the indication of a pressure, on the up-steam pressure gauge, is not a false one caused by back pressure from a closed valve]

8.     Once the proper pressure has been achieved and both pressure gauges agree – turn the valve supply of the gas into the housing to the off position.  Record the pressure gauge and the time of day.  One may disconnect the gas supply at this time since it should not be needed anymore for this test.  Allow the filter, without any downstream back pressure, to hold the upstream pressure with only the dampened filter holding back the gas.  If the filter holds this pressure for the length of time obtained from the literature in the filter box for that cartridge or from the technical department for that filter, the filter passes the test!  [For clarification : It is the surface tension of the water in the matrix of the filter that is holding back the gas].

9.     Time the holding pressure for the designated time for that filter.

10.   At this time, double check to see that the pressure does show the proper pressure; then slowly open a back stream valve.  Make sure to listen to hear that indeed pressure is coming off the filter housing set up and that the gauge was not stuck at the desired pressure.  Do this slowly so the filter does not go through an abrupt change in pressure that may damage the filter media just proven to be appropriate for the function of sterile filtration.[ If it passed ]

11.   Record any data that may be required by the bottling department or winemaker showing the filter was tested and checked out ready for use.

12.   Double check that all the downstream areas are still attached and that their sterility has not been compromised.

13.   Start the flow of wine for the day’s bottling run

14.   Pull samples at different times of the day and test them under the microscope ( if equipped and your winery has the expertise ) to insure the designated function did its job and continues to the job.  Numbering pallets as you bottle is a smart operation in the vent you find a filter failure during a days bottling run.  { Not a norm typically by the way }.

15.   Some wineries, after the day’s bottling, will re-wet the membrane with water and follow the testing procedure again to confirm the integrity was not lost on the filter during the day’s run.   This give “back end assurance” as performed as expected and desired.

  The above test should be performed each time a new or stored filter is installed into the filter housing and each time you bottle.  In many instances winemakers are able to get 10,000 cases or more through their cartridge filters before compromising the sterile bottling conditions.  Your supplier will be able to guide you with knowledge on how many cases or steamings your cartridge filter will be able to withstand. Typically I become most concerned of the steaming so I will discard a filter after a certain number of steaming or after a certain amount of time under steam.

Supplemental Notes:

•     Perform this procedure, for the first time, on a day you do not plan to bottle or on a day you have plenty of time to think the process through – not being rushed.

•     Check with the cartridge supplier to determine if the filter purchased has a “steamable life span”.  If so be sure to record the amount of time each cartridge has been steamed and discard the filter when appropriate.

•     Make sure that only water is on the filter during the testing of the filter as other “contaminants” may give a false reading of passing the test.

•     Setting the filter housing up with a male quick disconnect at the top port will greatly improve the ease of attaching the source of the desired gas.

•     The author prefers nitrogen since some compressed air has oils or odors that may interfere with the wine or the testing process.

•     Many cartridge filters were designed for the pharmaceutical industry and they are made to very strict standards.  Handle them with care!

•     Wineries now have the luxury of purchasing a machine to perform this function; however they are not inexpensive and this process, when mastered, does not take long.  The results are inexpensive and easy to obtain.  What is the machine fails ?  Will you have the expertise / knowledge know?


•   Contact your supplier to review the Hold Test operation with them.

•   Make sure the filter assembly is cooled to room temperature before testing

•   Use Nitrogen to pressurize the unit.

•   Make sure the pressure reading is not caused or influenced by a downstream obstruction.

•   Be cautious of downstream Cross-Contamination.

A Word About the Hold Test:

  The hold test should be performed in the clean environment of the bottling room under strict standards and precise conditions.  Keeping a keen eye on the process for cross-contamination possibilities, potential sources of error and other out of the norm conditions will lead to the winemaker’s ultimate success each and every time this is done.  A sterile bottling will be achieved providing the consumer with fresh and consistent wine each and every time they relax with one of your products.  One can not express the importance of doing this procedure correctly.  The winery’s success depends on the proper execution of sterile bottling and that process rest heavily on testing the membrane before bottling and overall proper steaming of the complete bottling line.

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