How To Load A Pressure Cooker

Learn To Sterilise Spawn Jars & Filter Patch bags

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Using A Pressure Cooker

A pressure cooker is an indispensable tool of the home mushroom cultivator. A pressure cooker is used to sterilised substrate for the production of spawn, or for sterilising bulk substrates which have a high percentage of supplementation, such as masters’ mix. 

Sterilising media will eliminate all micro-organisms rendering it susceptible to a wide range of contamination. For this reason sterilised media should be inoculated with the aid of a laminar flow hood or cabinet. However, many home cultivators undertake inoculation in an enclosed space while following aseptic technique with varied results.

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Can I Use An Instapot?

An instapot or an electric bench-top pressure cooker can be used to sterilise media for mushroom cultivation. Although instapots will often only reach an approximate 7PSI they will provide sufficient heat for preparing grain spawn, bulk substrate, agar and liquid culture.

When sterilising jars in an instapot it may be required to extend cooking times by 30 minutes.

Cool Before Opening

Whether using an instapot or a pressure canner it’s important to allow the pot to depressurise and cool completely before opening. 

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How To Sterilise Spawn Jars

Grain spawn jars can be packed into the pressure canner or instapot with a towel placed beneath to act as a cushion between the glass jars and the metal rack. 

Water can be filled to half way up the jars with rags inserted into spaced to keep everything sitting upright.

Spawn jars are usually sterilised for 90 minutes however this may vary with the method used.

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How To Sterilise Agar & Liquid Culture

Agar and liquid culture both require much less time for sterilisation. Pack the jars containing agar or liquid culture broth ensuring that they remain upright by filling spaced with pieces of towel.

Sterilise for approximately 15 – 20 minutes in an instapot or pressure canner.

How To Sterilise Mushroom Grow Bags

Mushroom grow bags, or filter patch bags can be challenging to sterilise in a pressure canner. It’s not uncommon for beginning mushroom growers to waste many bags while learning how to master the pressure cooker.

Stretched & Burst Bags

Filter patch bags will often warp, stretch and burst due to the extreme changes in temperature and pressure within the pressure cooker. This can be avoided by reducing the amount of vacant space within the chamber.

Once the cooking cycle has finished, vacant spaces will cool quickly resulting in rapid expansion of the grow bags.

Fill vacant spaces with damp rags and towelling, this will prevent the bags from being able to expand and reduce the rapid change in temperature. 

Always place a plate or tile on top of the grow bags to prevent the valve from being blocked be moving bags.

Brittle & Melted Bags

Brittle and melted bags are a result of mushroom grow bags making direct contact with the super-heated metal wall of the pressure cooker.

Always use the pressure cooker with the canning rack placed in the bottom with a layer of towel on top. This will ensure that no bags make contact with the base. 

Use towel to keep grow bags from touching the walls of the chamber – especially important for bags in the lower half of the pressure canner. 

Once the pressure cooker has achieved pressure it is recommended to reduce the stove temperature to sustain pressure with minimal steam release. Sustaining the cooker at unnecessarily high temperatures can cause permanent damage to the pressure cooker.

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Folding Mushroom Grow Bags

Mushroom grow bags will self seal during sterilisation if folded following the correct technique. This will allow the changes in pressure to remove excess air from the grow bags before sealing the ends under a vacuum. This will ensure that no contaminants can enter the bag.

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