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Cultivating mushrooms is a challenging task. It’s an exceedingly delicate process that takes up to four weeks, and the ones that grow slowly can take months to harvest. One detail that you need to consider is whether you should sterilize or pasteurize your casing mix. Both are useful for keeping your casing—and by extension, your mushroom mycelium—safe and free of contaminants. So, which is better? Check out our short guide below.

What Is Sterilization?

Sterilization is the process of killing all living organisms in a space. Sterilizing mushroom bulk substrate by placing it in an oven or pressure cooker will kill all microorganisms and spores in the substrate. However, this process will cause the substrate to become prone to contamination because sterilization kills off good and bad bacteria. For the best yield, use a pressure cooker or oven to sterilize at 100 degrees Celsius for one to two hours. Some grow bags have a filter patch so the substrate can properly ventilate without introducing pathogens to the mycelium.

When Should You Sterilize?

You’ll want to sterilize when you prep media like liquid culture, agar, and grain spawn. They must be free of any contaminants to expand and create healthy cultures and spawn. It would help if you also sterilized any substrate with many supplements to help mycelium grow. This is when you should sterilize your casing mix rather than pasteurize it.

What Is Pasteurization?

Pasteurization is a heat treatment that deactivates some, but not all, harmful microorganisms. This helps your substrate by reducing competing pathogens. Some of the top barriers to mushroom cultivation are competing bacteria and mold. Pasteurization won’t eliminate the problem, but it will mitigate most of the harmful microorganisms. This process is also easier for a beginner since inoculation and colonization require no extra equipment.

When Should You Pasteurize?

You’ll want to pasteurize with bulk substrates that don’t have too much supplementation. Paper and hardwood fuel pellets, straw, and sugar cane mulch are good candidates for this method. Keep in mind that the more supplementation for your substrate, the more likely contamination will develop. Since you have a lot of supplements, you should probably choose sterilization instead.

Overall, these two methods aren’t just determined by personal preference but depend on supplementation within your substrate. That isn’t to say you can’t use sterilization or pasteurization when you’re in situations that don’t call for it. One is not more beneficial than the other. You have to determine the usefulness of it for your situation.

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