Urban Spore

How To Load A Pressure Cooker

It’s common practice for mushroom cultivators to use pressure cookers or canners to sterilise grain for spawn or substrate for producing fruiting blocks.

In this guide we will discuss the method we use to load a pressure canner. There are many viable methods however this is what works for us.

It’s important to be aware that using a pressure canner to sterilise mushroom substrates may void the manufacturer’s warranty.


lions mane mushroom

Always Follow The Safety Data

When using a pressure canner it is important to always follow the recommended safe operating practices as indicated by the manufacturer. Deviations from these practices may cause harm. 

Pressure canners should not be left unattended.

Melting Mushroom Grow Bags

Grow bags are designed to withstand high temperatures however prolonged direct contact with the base of the canner can result in melted or brittle bags.

Line the base and lower sides of the pressure cooker with a wet cloth to avoid melting. 

  • Heavy-duty aluminium
  • Space for 20 pint jars
  • Suitable for gas & electric cooktops
  • Induction compatible model available

Saturated & Soggy Mushroom Grow Bags

To avoid saturated bags we fold bags as indicated in our grow bag folding method. 

We use the Type A fold on bags that sit semi-submerged with the filter patch in the upwards position in water & Type B fold on bags that sit above water level.

The Type A fold is a concertina fold that leaves the unsealed opening of the bag above water level. This allows the bag to seal & avoid the pullback of liquid as the substrate cools.

The Type B fold is simplified as it’s used on bags that sit above water level.

Water is added to the pressure cooker to sit just below the filter patch of the first layer of bags.  

Type a mushroom grow bag fold

Temperature Control

Temperature should be managed as to not excessively overheat the pressure canner. Patience is key when waiting for the canner to build up heat as this process can take up to an hour depending on your heat source.

Overheating the pressure cooker can cause the base to warp. 

Let It Cool Completely

Do not release the pressure from the canner once finished. The canner must cool down and slowly depressurise. Any sudden change in pressure will cause the grow bags to rupture and burst.

Using An Instant Pot

In most cases an instant pot won’t reach 15PSI however this is usually sufficient for producing smaller bags or jars. Cooking times can be extended by 30 minutes if in doubt.

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