Sterilisation versus pasteurisation

When diving into the realm of mushroom cultivation, one quickly discovers the necessity of understanding 
and differentiating between two processes: sterilisation and pasteurisation.

In this post we hope to make things clearer so you can decide the correct method for what you are trying to achieve.

When we grow mushrooms, we aim 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 we will touch on later). When growing mushrooms, certain things have to be sterile whereas other things can be pasteurized.

Pasteurisation 

The process by which competitor organisms in a substrate are reduced. This gives the mycelium an advantage over competitor organisms, allowing it to take over the substrate and eventually produce mushrooms. Usually pasteurisation is achieved by heating the substrate to temperatures of 75c for a sustained period.

Sterilisation

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 acheiving temperatures of 121c for a sustained period.

Super Pasteurisation

More commonly known as 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. 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 contamination.

Pasteurisation Methods 

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 Pasteurization or Hydrated Lime Pasteurization

This method works by pouring a diluted solution of hydrated lime into water, usually at a rate of around 2g of lime per liter, 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.

Sterilization Methods

The most commonly sterilized substrates are supplemented hardwood bags containing Sawdust, Bran, Soya Hulls, Coffee and other various nutritional supplements. Agar plates,  Liquid Culture and grain spawn also need to be sterilised.

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 – 24 hours.

These are often made of 205l drums and either heated with an electrical element or a propane burner underneath. A false floor hides the float valve and drain valve at 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 helium 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 grow area’ is higher.

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

Pros and Cons

Advantages of Sterilisation 

A sterile environment means there’s no competition. It can be especially beneficial for cultivating mushroom strains that require specific conditions or are sensitive to competing organisms.

Limitations of Sterilisation 

The main drawback is the energy and time required. Additionally, if contamination does occur post-sterilisation, the substrate can quickly become overrun.

Benefits of Pasteurisation 

With beneficial microbes present, there’s a natural defense against harmful pathogens. It also requires less energy than sterilisation.

Limitations of Pasteurisation 

There is always a chance that harmful microbes will survive the process, which could result in substrate contamination if beneficial organisms do not outcompete them.

Choosing the Right Method for You

Deciding between sterilisation and pasteurisation largely depends on the mushroom strain, scale of cultivation, budget, and personal preferences.

It’s crucial to research and understand the specific needs of the mushroom species you aim to cultivate.

Below is a general guide:

Substrates for pasteurisation

  • Straw for growing Oyster Mushrooms
  • Bulk substrate for our magical friends

Substrates for Sterilisation :

  • Agar Plates
  • Liquid Culture
  • Grain Spawn
  • PF Tek Kits
  • Fruiting blocks for wood loving mushrooms

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