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Mycology Laboratory Equipment and Techniques: Pasteurizing Mushroom Substrates via “Cold Sterilization”

“Cold sterilization” is a perennial hot topic when it comes to sterilizing and pasteurizing 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 organizations and cultivators, we can’t help but lead with a very important distinction: 

“Cold sterilization” isn’t actual sterilization.

This is a form of pasteurization and thus really only weakens microorganisms, killing some pathogenic bacteria in the process. That can be sufficient with hearty mushroom varieties (e.g., most popular edibles, including all the varieties of Oyster mushrooms) that grow in non-supplemented straw substrates. These really only need a head-start on the other spores in the substrate. 

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

Sterilizing vs. Pasteurizing Mushroom Substrates

Autoclaving doesn’t just outperform “cold sterilization” in terms of removing contamination. Studies have shown sterilized substrates often outperform pasteurized 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 pasteurization methods. They also noted greatly reduced time to colonize 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 sterilization methods. The results revealed that lab autoclave (1hr) proved one of the best sterilization 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-sterilized substrate.

impact of sterilizing/pasteurizing mushroom substrates on harvest yield

The impact of sterilization/pasteurization on mushroom yields [source]

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.