How to Make Leak-Proof Mason Jar Lids with Gas Exchange Filters (Plus Optional Injection Ports)

Hey folks, Bobby here from South Middle Mushrooms. I’ve been doing this for years—dialing in grain spawn and brewing up liquid culture concoctions in mason jars, and I want to share a simple but essential DIY project that’ll save you time, contamination headaches, and money: making your own leak-proof mason jar lids fitted with gas exchange filters and optional injection ports.

Whether you’re prepping grain jars or mixing up a nice LC, these modified lids hold up to pressure canning and keep your cultures clean while letting them breathe.

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Step 1: Choose the Right Leak-Proof Lids

You'll need leak-proof one-piece lids in both wide mouth and regular mouth sizes. Here’s the breakdown:

• • Wide Mouth Lids – Great for grain spawn. Easier to clean and load jars. Fits 1-quart and ½-gallon wide mouth jars.
  • Regular Mouth Lids – I like these for smaller jars and liquid culture. Less surface area = less chance for excess condensation to drip down.

🛠 Pro Tip: these leak-proof lids are plastic-coated and BPA-free, and they hold up better than the two-piece metal rings over time—especially after repeat canning cycles.

Make sure you're using mason jars with matching mouths (wide or regular). It sounds simple, but trust me, I've made that mistake when in a hurry.

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Step 2: Drilling Holes for Filters and Injection Ports

You’ll need:

• Drill and drill bits (I use a ¼-inch and a ½-inch bit)

• Your leak-proof lids

• Optional: Deburring tool or sandpaper

• 0.22-micron syringe filters

• Optional: Self-healing injection ports (red or grey silicone ones from mushroom supply stores)

Drilling:

1. Mark your lid with a Sharpie where you want the holes. I typically go with:

   • One ½" hole for the gas exchange filter

   • One ¼" hole for the injection port (optional)

2. Drill slow and steady. Clamp the lid down or hold it over a scrap block of wood. Watch your fingers.

3. Deburr the holes with a deburring tool or fine sandpaper so they’re smooth and don’t tear your filter or sealant.

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Step 3: Install the Gas Exchange Filter

I like to use 0.22-micron syringe filters—they’re made for this kind of thing and do a great job at keeping contaminants out while letting CO₂ escape.

Here’s how to attach them:

1. Apply a small bead of DAP high-temp 100% silicone sealant (the kind used for kitchens and aquariums—it’s safe and withstands heat).

   • You can also use JB Weld High Heat Epoxy or Red RTV gasket maker—just make sure it’s non-toxic and can handle 15 PSI from pressure canning.

2. Press the syringe filter firmly into the hole while the sealant is wet, sealing all the way around it. I like to give it a little spin to seat it properly.

3. Let it cure for 24 hours minimum before using or pressure cooking.

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Step 4: Install the Injection Port (Optional)

If you’re making lids for liquid culture, having a self-healing injection port is a game-changer.

1. Push the silicone port into the ¼" hole (they’re made to fit tight).

2. Seal around it with the same silicone or epoxy.

3. Let it cure fully.

This way, you can inoculate or pull LC through the port using a sterile syringe without ever opening the jar.

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Final Tips

• Label your lids with a Sharpie once they’re done (LC, rye, millet, etc.)

• If you’re doing large batches, set up a little jig or template so all the holes are in the same spot.

• After pressure cooking, always double-check your filters and ports to make sure they haven’t shifted or come loose.

These modified lids have made my workflow way smoother—especially for grain spawn expansion and long LC storage. I keep a batch of each size ready to go.

Hope this helps you dial in your own setup. If you’ve got questions or want to share pics of your build, hit me up!

Stay sterile out there 🍄
—Bobby
South Middle Mushrooms