If you want to take your mushroom grow to the next level you need a dependable pressure cooker.The only way to beat contamination is to reach sterilisation point or 121c, as the water boils and turns to steam at 100c the only way to reach the sterilisation point is under pressure.
A pressure cooker works by preventing steam from escaping the pot until a certain amount of pressure is built up, allowing for the temperature inside to become hotter than it otherwise could. There are many different styles, but most operate by using a small weight that sits on a vent on the lid of the cooker. As the pressure builds, the steam is eventually able to overcome the weight on the vent.
In this way, a stable pressure and an elevated temperature are maintained.
You want to look for a pressure cooker that can maintain a pressure of 15 PSI, which allows for steam temperature of 121°C – hot enough to kill pretty much any organism.
For the purpose of this article, we will refer to everything as a pressure cooker but they can have different names which may be confusing to some when first starting:
Autoclaves – These are normally used in hospitals to sterilize scalpels etc between uses. They are normally electric, easier to use with specific time and pressure setting adjustments and often reach a higher temperature than other pressure cookers, up to 30 PSI. Autoclaves are more expensive than other types but have more benefits and can be up to 150l or more in volume.
Pressure canners – Basically just a pressure cooker but these are normally more sturdy, they reach 15psi and are normally built to last many uses. Pressure canners are normally made in 20l – 40l sizes.
Pressure cooker – A less industrial version of a pressure canner, and some do not even reach 15PSI, care should be taken to find out what pressure rating they can reach before purchasing. They are normally made in sizes between 10l and 20l.
Instant cooker – These normally only reach 10psi and allow for a very small volume to be sterilized. They are not ideal for mycology but they can have their uses. They are normally cheap and accessible and even though they don’t reach 15psi they can be used for sterilisation if cooking times are increased.
If you are interested in a more in-depth description check out this video.
Can you sterilize at temperatures less than 15PSI ?
In theory yes but it is not ideal. Care should be taken if cooking for long periods of time to make sure the water does not run low.
Below is a graph of how to adjust cooking times at lower pressure.
Suggested Sterilisation times at 15PSI
- 1l Agar solution – 40 minutes
- 1l Liquid Culture bottle – 20 minutes
- 1l PF Tek Kit / 1 l Grain spawn bottle – 90 minutes
- 2.5kg Fruiting block 150 minutes
Above times can very depending on the size of your pressure cooker and the overall amount of substrate that is being sterilised.
How to use a Stove top Pressure cooker
- Find and read your pressure cooker documentation and do not vary from those instructions should they differ from those below, these should be used in preference to any further steps described in this article, MyShrooms takes no responsibility for any failures that may occur due to varying from your pressure cookers instructional manual.
- Check to make sure that the seal is in good condition and not degrading
- Do a visual inspection of your pressure cooker and make sure nothing is warping. Complete a visual inspection of the vent and make sure nothing is obstructing it.
- Make sure you fill enough water to last the full duration of the sterilisation process. You will learn how much water is enough over time – just pay close attention for the first few runs.
- Do not overfill your pressure cooker.
- Make sure that there is no plastic or anything else inside your pressure cooker that could potentially block the vent while in use.
- Once thevv contents have been added make sure the lid is correctly sealed. Some pressure canners have 6 tightening knobs on the lid, tighten each adjacent knob together or it could lead to the lid being skew.
- If you are cooking over gas make sure the gas flames do not go high up the sides of the cooker, this could cause it to warp and for the contents to get burned.
- Remove the weight (sometimes called the rocker) from the pressure cooker.
- Add heat at this stage, you want it at about medium high to get the water boiling.
- Once the water is boiling you should see steam escaping from the vent. Once this steam is escaping at a large volume stat timing for 10 minutes. This is an important step and will allow for all the air pockets inside to escape leaving only steam inside. After 10 minuted add the weight on top of the vent.
- At this stage the weight will stop steam from escaping and the pressure cooker will start building up pressure. When the weight starts rocking and releasing steam you have reached correct pressure.
- From this point start timing your cooking cycle
- You will need to pay attention to how much heat you are applying to your pressure cooker during the cooking cycle and adjust your stove/gas according. You generally would turn it down to around medium low during the cook. Your weight should be gently rocking during the cook or if you have a pressure gauge adjust heat so gauge remains at 15psi.
- Once you have reached your cooking time turn off heat and allow pressure to reduce naturally while keeping the weight on the vent.
- After use clean our your pressure cooker well removing any residue from inside and that may be attached to the seal.
A critical step to achieving proper pressure inside a pressure cooker is allowing it to VENT. What does this mean?
Venting is also “exhausting” the pressure cooker, a process of letting steam (and air) come out of the pressure cooker through the vent pipe for a period of time before beginning the pressure processing time. Air trapped in a pressure cooker lowers the working temperature.
To be safe, the USDA recommends that all types of pressure cookers be vented 10 minutes before they are pressurised. Your pressure cooker can be vented by removing the weight on the vent and once boiling and stem is escaping wait 10 minutes before returning the weight and allowing pressure to build.
Why vent?
It is STEAM, not water, that does the work in a pressure cooker .
Temperatures over 121°C are needed to kill spores. Water can get no hotter than the boiling point 100ºC, but steam can. Steam trapped in the canner increases the atmospheric pressure inside the canner causing the boiling point of water to increase to 121C
In order to reach the optimum temperature to destroy endospores, the air inside the pressure cooker must be exhausted to allow space for a pure steam environment to build. There is a vast amount of air in a pressure cooker due to the space between the water level and the lid as well as the air that escapes from inside the jars and from the water.
WHAT HAPPENS DURING VENTING?
As the water boils inside the pressure cooker, the empty spaces become a mixture of steam and air. As steam increases, it pushes the air out creating a pure steam environment. Steralisation times are based upon a pure steam environment which makes venting very important to achieve desired results.