Large Guide: Filter
Now weâre really deep in it.
Filtration is the only viable method for sterilizing our preparation. You must handle the preparation with the utmost care to ensure that sterilization does actually happen.
I highly recommend that you filter the preparation directly into sterile and capped 100mL vials. This ensures that no unsterile air can touch the preparation as it goes through the filtration process.
If you are sufficiently advanced you may choose bottle top filtration as it can be a more hands off approach. The downside to bottle top filtration is that you will be dealing with an open container of sterile preparation, so your aseptic technique will need to be stellar. This is 100% a skill issue. And an equipment quality issue also. Donât do this unless youâve been doing it the other ways and youâre confident and ready to upgrade your technique.
Pre-Filtration
The USP recommends doing a pre-filtration step. This is a new-ish recommendation to me and as of May 1, 2025 it has not been fully written into this website. A pre-filter does not need to be sterile, but it does need to be PTFE or nylon. A pre-filter is ideally 0.45um.
The idea is that you use the pre-filters to catch larger debris first, and then when itâs time to use your sterile filters they can work more efficiently and effectively. This might feel excessive but itâs all in service of the CSP actually becoming sterile with a high level of confidence.
Method
- Filter using the 0.45um filters into sterile 100mL vials.
- Second pass filter using sterile 0.22um filters into NEW sterile 100mL vials.
To do this right you need double the 100mL vials as your target volume.
Filtration Options
Hand Filtration
Pros: cheap
Cons: slow. Requires your constant attention. Your thumbs will be sad. Can only do one at a time.
Caulk Gun
Pros: pretty cheap. Requires less attention than hand filtration. Easier on the thumbs. Can probably do multiple at once.
Cons: still requires a lot of attention and youâll still be giving it that attention for hours at a time. much easier to blow the filter.
Syringe Pump
This is my recommended option.
Pros: Do multiple at once if you get the right model. If you program it right you can literally just walk away for an hour. Once you get the programming right youâre pretty much good to go.
Cons: Costs a few hundred dollars. Still pretty slow. Have to change out the syringes still. Easy to blow a filter if you program it wrong.
Bottle Top Filtration
Pros: Do the entire batch in one go, completely hands off.
Cons: Somewhat expensive filters. Unclear how to verify if the filter maintained integrity during filtration. Needs vacuum pump. Requires A+ aseptic technique to process correctly, which most hobbyist brewers likely do not possess.
Donât do this unless you have a flow hood youâre confident works correctly and your aseptic technique is A+.
Vacuum Syringe Filtration
There are instructions floating around online about how to rig a syringe filter up to a vacuum pump for the filtration process. While itâs very ingenuitive, it is likely not a safe practice. Syringe filters are designed to be pushed through, not pulled through. In my own test of this tek I found there to be too many unknown variables to adequately consider it safe.
Skip the MacGyvering and just buy a syringe pump.
Choose Your Path
You can choose one of the above teks for filtration. You should end with your sterile preparation in a series of sealed 100mL vials, or in the case of the bottle top tek, possibly in a large sterile beaker with a cap.
I believe the syringe pump is the best option here as we can filter directly into sealed, sterile containers. We can also verify the integrity of the filters through a simple bubble point test.
Always Perform Bubble Point Testing
While this many not be possible for bottle top filtration, bubble point tests need to be preformed for all syringe filters that are used in this process. If any syringe fails the bubble point test you need to refilter or discard all of the preparation that went through it.