Grow mushrooms the EZ way.

Do Grain Jars Need Gas Exchange?

100% colonized grain jar

Yes, colonizing grain spawn needs a gas exchange from the jar. The Mycelium will suffocate, stall or die without it. Alongside this, gas exchange will prevent certain forms of bacteria and mold from colonizing inside the jar. Gas exchange should not be mistaken for fresh air exchange as these are two different things.

To ensure proper gas exchange all that is needed is three or four 1mm holes in the top of the jars which are covered by one of the following:

Holes larger than 1mm could result in fresh air exchange instead of gas exchange which may induce early mycelium pinning, this is not normally desirable. 

Synthetic filter disks or Polyfill (such as pillow stuffing) are often touted as the ideal filter as these can be reused many times without hassle or breakdown of the material. However I prefer to use stiffened felt as I discuss below.

In contrast, micropore tape and Tyvek can create issues when it comes to removing the lids during grain to grain transfer (which often requires a steady hand).

Alongside this, micropore tape can stick to the lids and edges of the jars leaving an adhesive which becomes difficult to remove. 

Here is a post if you are interested in learning how to make mushroom lids.

How do I use each filter?

Each of these filters should be applied before sterilization in a pressure cooker.

  • Micropore Tape – Place two layers of tape over each gas exchange hole and underneath the ring of your jars.
  • Polyfill – this is pulled through the gas exchange holes leaving some sticking through both sides of the holes but not touching the grain.
  • Synthetic Disk/Tyvek/Stiffened felt – place a disk over the metal lid with holes in place the mason jar ring on top. Ensure the ring is turned on tight to secure the jar.

Which filter would you recommend?

I have been converted to using EZ-felt filters recently, I have used Polyfill for a long time but once that ran out I moved over to using EZ-felt filters as they seem to provide a lot better protection.

To create an EZ-felt filter lid you will need:

Stiffened Felt (Ez-felt)

Permatex 81160 High-Temp Red RTV Silicone Gasket, 3 oz

  • Put holes in lid
  • Cut small square piece of felt which covers the holes
  • Place the felt on top of the hole
  • Seal edges with RTV silicone

Alternatively you can cut the felt to size and place it under the lid. It will be secured when you screw the lid on.

i have a more detailed discussion on making mushroom lids.

Can I inject through the filters?

Here is an outline for how to use each of your filter medium:

  • Polyfill – Yes you can inject through this filter medium.
  • Tyvek/Synthetic disks/Stiffened Felt – No, it is recommended to remove the lids and inoculate the jars. However you can inject straight through these mediums if you cover the holes up with micropore or scotch tape.
  • Micropore Tape – Yes, but make sure to cover up the hole with a new piece of micropore tape swiftly after innoculation. I recommend having your second piece of micropore tape already attached to the jar and flipping it down the tape while removing the syringe.

Where can I buy filters?

  • Synthetic Disks – Ebay, amazon or an online mycology supplies website.
  • Tyvek Filters – Ebay, it is also possible to buy a tyvek coverall and then cut disks to size.
  • Micropore tape – 3M micro tape can be found here, this is the most recommended brand online but I have used generic pharmacy brands without any problems.
  • Polyfill – also known as pillow stuffing can be bought at most convenience stores, hardware stores or online here.
  • Stiffened Felt – Craft stores, Walmart.

What is the difference between gas exchange and fresh air exchanges?

Gas exchange is the ability for the mycelium to release gases which have built up inside your jars due to the myceliums metabolic activity. This gives the mycelium a chance to exchange small amounts of CO2 with the air outside the jar, without completely removing all the gases built up in the jar.

Fresh Air Exchange (FAE) is the introduction of completely new gases by completely removing the pressure built inside the jar. This will result in a larger exchange of gases than that described as “gas exchange”. This would result in older stale air inside the growing environment being replaced with fresh air.

Why is gas exchange necessary?

Gas Exchange allows the stale air inside the jars, which will often contain high levels of CO2, to be exchanged for room filtered air which has other gases such as O2. Mycelium will then continue to consume the oxygen and the exchange CO2 just like humans!

On top of this, stale air is more likely to produce mold, trich, in particular, enjoys growing in stale, moist air and will often die out when introduced to fresh air.