It can be an overwhelming experience when you begin exploring the world of mushroom growing. In this post, I hope that I can help clear some bits up for you about Sterilising and Pasteurising Substrates and Grain Spawn. When we grow mushrooms, we want to create the perfect environment for our fungi to grow. The two most commonly practiced ways of achieving this are pasteurising and sterilising (as well as ‘super pasteurisation’ which I’ll touch on later). When growing mushrooms, certain things have to be sterile whereas other things can be pasteurised.

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The process by which amounts of microscopic competitors in a substrate are reduced. This gives the mycelium an advantage over harmful organisms, allowing it to take over the substrate and eventually produce mushrooms. Usually by achieving temperatures of 75c for a sustained period of time.

The process by which all microscopic competitors in a substrate are destroyed. This gives the mycelium a clean canvas and allows uninterrupted and uncontested colonisation of the substrate. Usually by achieving temperatures of 121c for a sustained period of time.

Super Pasteurisation
More commonly known as sterilisation or ‘atmospheric sterilisation’, is achieved by holding the substrate at as close to 100c as possible for extended periods of time. This gives the mycelium a clean canvas and allows uninterrupted and uncontested colonisation of the substrate. Though not textbook, the high temperature and long time acts almost identically to classic sterilisation as far as mushroom growing is concerned.  

These terms are often, wrongly, used interchangeably and in my early days, I was no exception. It’s important to know the difference when dealing with different substrates and grain spawn. This is because different substrates are colonised at different speeds by different species making them more susceptible to our old friends, contamination.

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The most commonly pasteurised substrate is chopped straw. The straw is chopped into 1 – 4 inch lengths to increase surface area, allow for better packing into bags and a greater uniformity during colonisation.

Cold Pasteurisation or Hydrated Lime Pasteurisation

This method works by pouring a diluted solution of hydrated lime into water, usually at a rate of around 2g of lime per litre, into a container then adding your chopped straw and weighting it down with something to keep the straw submerged for 12 – 24hrs. This method works by changing the PH of the water and straw to an alkaline solution, killing off microbial contamination and, once removed and drained, allowing the mycelium to colonise its way through the straw with reduced competition.

Heat Pasteurisation
This method works by heating the chopped straw in water at around 75c for about 1 hour. In the same way the lime kills the contaminants, the heat does too. Once heated, the straw is strained and allowed to steam dry before being inoculated and bagged.

Anaerobic Pasteurisation
This method works by submerging the straw (or more often, woodchips, see my post on Matt Smalls’ Blue Barrel Tek) in water for up to 14 days. This process deprives the contaminants of oxygen and kills them off. The contamination picked up during the time they are in the water are then killed when the substrate is drained and exposed to oxygen.

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These processes do allow certain bacteria to survive, however, the mycelium is usually tenacious enough to overpower all competition. It’s thought that some of the bacterium work in harmony with the mycelium, actually helping fend off bad bacteria. These processes, however, don’t kill all spores, so a slow growing culture may not win out if in a high competition environment.

This is why it’s more common to grow oysters on straw as their aggression and tenacity make them prime for fast colonisation. It is possible to grow all different kinds of mushrooms on straw however, but not considered to be as economically viable.

Woodchips, shavings and sawdust can too be pasteurised with much the same results, however, this is not often done commercially as there is far less control and repeatability when compared to sterilisation.

Heat pasteurisation is favoured by newcomers because it requires very little kit or knowledge and can be done in almost any environment without the need for sterility. Just a large pot, some spawn, bags and straw.

A major disadvantage of pasteurising anything is that it has to be used there and then. You have to inoculate your substrate otherwise all you’ve done is create the perfect environment for any floating spores and bacteria when compared to sterilising a substrate, which you can leave and come back to at any point due to the substrate being completely devoid of anything.

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The most commonly sterilised substrates are supplemented hardwood bags containing Sawdust, Bran, Soya Hulls, Coffee and other various nutritional supplements.

Classic sterilisation
For the purposes of mushroom growing, we almost exclusively use autoclaves and pressure cookers to achieve such high temperatures. Autoclaves generally have an element built into them whereas pressure cookers require an external heat source.

Some autoclaves work on dry heat, without water, but most used in mushroom growing operate by heating up water. By heating water in a pressure safe vessel to around 15psi, the boiling point of the water increases to about 121c, hotter than the boiling point at sea level of 100c.

By heating up the substrate to this temperature you will kill all contaminants, including spores, giving your mushrooms free reign to colonise the substrate uncontested. This method is also used for grain spawn prep.

Inversely, if you decrease the pressure, you also lower the boiling point of water which is why, when you get to the edge of space (in an unprotected suit) you would die because the water in your blood would literally boil away. Science is fun!

Atmospheric Sterilisation or Super Pasteurisation
Autoclaves and pressure vessels are expensive, and it’s been found that using heat over long periods of time is almost as good as classic sterilisation. The substrate is brought up to between 90c and 100c and held there for as long as 18 hours.

These are often made of rusty old 205l drums and either heated with an electrical element or a propane burner underneath. A false floor hides the float valve and drain valve in the bottom.

This method will also kill all contaminants, including spores, giving your mushrooms free reign to colonise the substrate uncontested and can be used for grain spawn, however, most people use a pressure cooker for their grain spawn as it’s quicker and you don’t generally require as much volume.

We sterilise grain and nutrient rich substrates as the risk of contamination is higher when the canvas is blank. We want our mycelium to be the only organism growing within our substrate. To achieve this, we seal the grain and substrate in jars and bags, sterilise them and inoculate them in a sterile environment i.e. Flowhood or Still Air Box.

On a nutrient rich, sterile substrate you’ll find the yields are higher, less prone to contamination and take up less space when compared to straw logs. This makes them a more efficient choice in a commercial growing operation as your ‘yield per m3’ is higher.

In simple terms, sterilising gives you more control and more consistent results.


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There are plenty of other ways of preparing substrates and grain spawn. These involve include microwaves, Tyndallisation, Chemicals, Cold Prep and lots of other methods. However, these are mostly unused due to the simplicity and availability of modern methods and equipment as well as there being better, cheaper and more proven techniques.

At the end of the day it all boils down to why you’re doing this. As a hobbyist, quality is all that matters as a commercial grower there are dozens of factors to balance. You’ve just got to find out how your margins fall!

I know this has been a fairly text-heavy post so I appreciate you sticking it out. If you need help or have any questions please don’t hesitate to contact me, I’m always open for a chat! Cheers!

6 thoughts on “Pasteurising vs Sterilising When Mushroom Growing

  1. Adam says:

    Excellent post -nicely written and easy to understand – thank you, only just getting in to growing at home, this sums up the sterilization / pasteurisation process brilliantly -My 1st success was with pink oysters on straw (lime pasturised)

    I now have buckets and bags of different species dotted all round the house! and a pressure cooker on the way from ebay!, I will keep an eye on your site and shop – thanks!

    • Garf says:

      Hi mate!

      Thanks for the comment, you did well to grow pinks on your first go! They can be a right fiddly mushroom to grow! Any questions or problems feel free to contact me!



  2. Hamza says:

    Great write-up, very insightful .
    My questions are, can I combine both the cold pasteurizing and the lime pasteurizing methods for a commercial production?

    • Garf says:

      Thanks Hamza!

      Cold pasteurising and lime are generally used by home-growers as cheaper, low tech, low investment options. You could use them for commercial production but you’ll find the space, effort and time you spend doing these techniques will be outweighed by the initial investment of a heat-pasteurising rig! That would be my opinion! I’ve seen people use lime commercially and successfully but at quite the extra effort!
      Good luck with your veture!

      Take care!



    Hey, it’s been a good topic to talk about, Am asking if I can sterilize cotton hulls, usually I pasturize my gardens but such gardens are attacked by green molds, am thinking of making a giant Autoclave to entirely kill microorganisms in the cotton hulls substrate, won’t that hinder the growth of mycelium or it will help them grow healthy

    • Garf says:

      Hi mate,

      Anything high in nitrogen will be vulnerable to contamination, especially outside of a controlled environment. I would always suggest sterilising over pasteurising when supplementing with things like cotton hulls. There is probably an argument to be made about microorganisms, but we’re not cultivating crops and improving soil quality, we’re growing mushrooms in, mostly, controlled and regulated environments where we can measure and adjust precisely the exact hydration, nutrition, temperature, lighting cycle and humidity. This sort of control isn’t possible in conventional cereal agriculture so cultivating an environment beneficial to microorganisms is far more important, consequently, i’ve never paid much attention to the microorganisms thing in pasteurising!

      The species that have been bred for modern mushroom cultivation have not been bred symbiotically with a microorganism partnership. Maybe when dealing with more specialist species this is different, but again, not something I Have much experience with.

      I hope that helps a bit mate! Thanks for your comment!

      All the best!


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