Sterilizing Rye Grain for Spawn in a Pressure Cooker


Dry rye grain

Gram scale or measuring cups and spoons

Mixing bucket

Mixing spoon or paddle

Gypsum (pelletized or powder)

Large cooking pot

Colander, muslin bags or cheese cloth

Mason jars with filtered lids and rings or filtered grain bags

Aluminum foil

Pressure cooker







Measure the desired amount of grain.


 90 g dry grain per US pint jar

½ cup dry grain = 90 g

 As a general rule of thumb, rye grain will double in mass after the soak and boil.


Place grains in the bucket and add hot water, filling about an inch (~2.5 cm) above top of grain.

Some of this water will be absorbed, it isn’t going to hurt to have more than an inch above the grain.


Add 2% (by weight of dry grain) gypsum to the bucket and mix.





 1 teaspoon of gypsum powder weighs about 3.4 g.

 Gypsum powder helps establish a more basic pH of the grain spawn and helps prevent grains from sticking to each other.


Let the grains soak for 12-24 hours. 


Soaking allows the shell of the grain to soften and absorb more water later, when boiling.  Soaking also provides the perfect environment for mesophilic bacteria to thrive and blossom, allowing them to be subsequently killed off during the boil.


Dump the grain and water into the cooking pot (no need to change out water). Add more water if necessary to raise the water level 1 inch (~2.5 cm) above the top of the grain.

Changing out the water will wash off the gypsum powder and affect pH.


Turn the stove to medium-high and bring to a gentle boil, then reduce heat and simmer for 10 minutes.  Stir once or twice to prevent grain from sticking to the bottom.

Boiling helps the grains absorb water, and starts the process of killing off some competing organisms prior to drying and sterilization.


Drain the grains into the colander and shake to drain. If you don’t have a colander, pour grains into a muslin bag or cheese cloth and hang to drain.  If using a muslin bag or cheese cloth, use a spatula or large spoon to gently move grain around from the outside of the bag.  

Stops the soaking process and prevents grain from expanding and breaking.


Spread the grain out on a pan or solid surface and allow to air dry for about an hour.  The grain is ready when the outside just starts to lose its shine.

While the grains are dry on the outside, the insides have absorbed the perfect amount of water to support mycelium colonization later on. Over/under drying will likely result in poor mycelial growth, or encourage other organisms to grow first. 


Add dried grains to your mason jars or filter bag.  For mason jars, fill to about ½ to ¾ full.  For grain bags, fill about 5 inches (13 cm) of the bag. 

Don’t overfill on this step.  You’ll need space in the jar to allow for shaking or mixing later on during the colonization process.


Install filtered jar lids and cover with tinfoil, or fold the bag down with the filter inside the folds.


Filtered lids allow for gas exchange later on during colonization, but prevents contamination from entering the grain spawn.

 Aluminum foil prevents the grain from absorbing more water during the sterilization procedure.


Place a trivet in the bottom of the pressure cooker.

Jars can break if directly on the bottom of the pressure cooker, and bags can melt.


Load jars or bags into the pressure cooker. 



Add water to just below the bottom of the jars or bags. 

If the water level is too high, it could cause the bags to melt.  If using jars, this is not as important.


If using bags, place a weight on top of the bags.

This prevents the bags from swelling during sterilization and blocking the vent/safety valve and pressure gauge ports on the pressure cooker lid.


Install the pressure cooker lid and rocker weight.

The rocker weight will allow the pressure cooker to reach 15 psig.


Turn heat source on high until pressure reaches about 13 PSI, then lower the heat source temperature to maintain 15 PSI. Maintain 15 PSI for at least 90 min, starting your timer when the pressure reaches 15 PSIG. 

Turing the heat down before reaching full pressure minimizes water and energy loss through the vent.


Allow the pressure cooker to cool on its own, do not attempt to force cool the cooker.

Force cooling the pressure cooker can cause damage to the pressure cooker or crack jars.


Remove the jars or bags from the pressure cooker and proceed to inoculation when jars have cooled to room temperature.