“Cold sterilisation” is a perennial hot topic when it comes to sterilising and pasteurising mushroom substrates. This chemical-based process (usually using low-magnesium hydrated lime) seems like a “best of both worlds” approach. It is cheap, low-labor, and certainly cost-effective, even at extremely high volumes. No wonder it has many enthusiastic boosters and is standard in many commercial operations and industrial applications.

As an autoclave company that has supplied mycology laboratory equipment and mushroom autoclaves to many research organisations and cultivators, we can’t help but lead with a very important distinction: 

“Cold sterilisation” isn’t actual sterilisation.

So-called “cold sterilisation” is actually a form of pasteurisation, not sterilisation. Pasteurisation only heats the substrate between 150 and 180 degrees Fahrenheit for an hour and a half to two hours; whereas sterilisation requires heating to 250 degrees Fahrenheit under pressure. Pasteurisation will not sterilise—or remove all contaminants—the way actual sterilisation will. It really only weakens microorganisms, killing some pathogenic bacteria and reducing the level of substrate contaminants in the process.

Pasteurisation can be sufficient with some hearty mushroom varieties (e.g., most popular edible mushrooms, including all the varieties of Oyster mushrooms) that grow in non-supplemented straw substrates, like oat or wheat straw. These really only need a head-start on the other spores in the substrate.

But “cold sterilisation” is a huge mess (or entirely ineffective) with most non-straw or supplemented mushroom substrates. And, even with unsupplemented straw, it still doesn’t remove/kill/deactivate all forms of life present in the substrate.

Why Sterilise Non-Straw or Supplemented Substrates?

Pasteurisation may suffice for growing oysters—although to optimise your mushroom yield, you might still consider a sterilisation process instead (more on that below). But many other mushroom species—reishi, shiitake, lions mane, and button, to name a few—thrive best in different substrates. Different substrates require different kinds of substrate preparation—including supplements—many of which require steam sterilisation, not “cold sterilisation.”

For instance, grain is a common, nutrient-rich substrate addition. It can generate high yields because every single kernel can inoculate mushroom mycelium that can spread. But the robust growing environment grain creates is also vulnerable to contamination. Mycologist Paul Stamets points out that each gram of rye berries can carry 300,000 bacteria and 12,000 fungi—and some batches host into the millions! Water added to the mushroom substrate encourages growth—but not just mycelium growth. A bacterium or fungus uses the same food as a mushroom to flourish. Enriched substrate will quickly contaminate with mold before mushrooms can colonise unless it’s first sterilised and its sterility maintained during the inoculation process until the mushroom takes hold. But if you sterilise your mushroom substrate, your crop stands a much better chance.

Even still, sterilising grain for mushroom cultivation can be a challenge. Each grain kernel is honeycombed with air pockets and cavities that harbor unwanted spores and active colonies and trap cold air that can protect contaminants from sterilisation. Depending on the type of grain, additives, and volumes involved, grain spawn regularly takes at least an hour or two to sterilise completely, even in an autoclave. A purpose-built, programmable autoclave lets you fine-tune your sterilisation process for improved results.

Sterilising vs. Pasteurising Mushroom Substrates

Autoclaving doesn’t just outperform “cold sterilisation” in terms of removing contamination. Studies have shown sterilised substrates often outperform pasteurised substrates, in terms of total yield, yield per flush, and mushroom quality.

For example, in a 2012 paper published in International Journal of Agriculture and Biology, researchers Tajudeen Oseni and Sikhumbuzo Dlamini found “[t]here were significant effects of substrate pre-treatment methods on the average yield of oyster mushroom…[H]ighest growth vigour, yield[,] and B.E. [biological efficiency] were obtained in autoclaved sugarcane bagasse [substrate]” when compared to substrate treated using pasteurisation methods. They also noted greatly reduced time to colonise the substrate—in some cases by half—and improved mushroom quality (with mushrooms grown in autoclaved substrate having a heartier stipe).

Another study found that “There is a significant difference among yield of different sterilisation methods. The results revealed that lab autoclave (1hr) proved one of the best sterilisation method. … It was observed that the Pleurotus ostreatus [mushrooms grown in autoclaved substrate] gave the maximum yield in the first flush followed by second, third and fourth flush” when grown in autoclave-sterilised substrate.

A pressure cooker may have gotten you started with cultivation in supplemented mushroom substrates, but quickly become unwieldly, time consuming, and inconsistent as your volume expands—-leading you to consider an autoclave. A standard lab autoclave, not designed with mycology in mind, might be too restrictive for your needs. A Priorclave, however, consistently sterilises challenging loads like grain and other mushroom substrate.

Of course, every lab (and every mushroom) is different. For more than three decades, Priorclave has been building autoclaves to order and supporting labs in developing the right cycles and procedures. Contact us whenever you wish to discuss how your lab can get more with less wasted water, energy, and time.