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Options for Mushroom Substrate Preparation: Pasteurization vs. Sterilization

Options for Mushroom Substrate Preparation: Pasteurization vs. Sterilization
November 05 , 2021
Options for Mushroom Substrate Preparation: Pasteurization vs. Sterilization

You have many options when preparing your mushroom substrate. These tend to break into two broad categories:

1. true sterilization (e.g., Steam Autoclave, mushroom autoclave, mushroom sterilizer)

2.pasteurization (e.g., “traditional” heat-based pasteurization, or “cold pasteurization” methods like lime bath, peroxide bath, or cold-water fermentation)

An important distinction here is that pasteurization seeks only to weaken microorganisms and kill pathogenic bacteria present in the substrate. Pasteurization does not actually remove/kill/deactivate all forms of life present in the substrate (e.g., fungi, bacteria, viruses, spores, unicellular eukaryotic organisms, and so on). Sterilization, on the other hand, aims at complete destruction of all microorganisms and their spores.

Pasteurization is favored by many edible mushroom growers, because it scales easily and demands no specialized equipment. Provided you’re working non-supplemented straw substrate, it’s both effective and forgiving.  But supplemented substrates (and especially denser ones, like hardwood sawdust) cannot be prepared using pasteurization. They harbor mold spores that easily survive pasteurization, and will take over the substrate long before most mushrooms have a chance to gain a foothold.

Supplemented substrates—and even plain straw substrate, in some applications—need to be fully sterilized.

Heat-based Substrate Sterilization for Mushroom Cultivation (High Pressure Steam sterilization For mushroom cultivation)

But not all sterilization methods are created equal. There is an array of laboratory sterilization options, each with its strengths and weaknesses. Mushroom labs almost invariably opt for heat-based sterilization. With respect to microorganisms and macromolecules sheltered in soils and cultivation substrates, heat is much more destructive than other strategies, including gaseous/liquid chemical sterilization (e.g., ethylene oxide or hydrogen peroxide) and gamma radiation.

Steam sterilization is, by far, the most common approach used in mushroom labs. (Some fungus cultivators do use dry heat for their non-straw mushroom substrate, but that’s only in special cases; in general sterilizing substrate with dry heat is counter-productive, as you will then need to rehydrate the substrate to cultivate fungus). While some mushroom cultivators rely on something as simple as a kitchen pressure cooker or bulk atmospheric sterilizer unit, for most lab applications, autoclave-based steam sterilization will prove to be the most cost-effective, consistent, and quick strategy (relative to other solutions). It’s also the simplest to validate (which is important, as undesirable resting spores can be so resilient in mushroom substrate).

Steam Sterilization in the Mushroom Lab

Purpose-built laboratory autoclaves flood a sealed chamber with high-pressure steam under constant monitoring. These programmable units make most lab sterilization tasks extremely easy. For common tasks (like sterilizing glassware or preparing agar), it’s as simple as loading a dishwasher and hitting “START.”

Most research-grade lab autoclaves will sterilize equipment and supplies (scalpels, beakers, etc.) in just 15 to 20 minutes. Growth media preparation is another common task, especially if your autoclave has special liquid processing or “media prep” cycles. With a programmable lab autoclave, preparing agar, bulk nutrient/growth media, or Petri dishes takes between 15 and 45 minutes (depending on volume).

That said, substrate sterilization is trickier, because of the variety of substrates, and because the sterilization time varies wildly (and not necessarily linearly) by substrate volume. For example, hardwood sawdust in small volumes can be sterilized in under 20 minutes, while the same sawdust in blocks or large bags could take upwards of 2.5 hours. Jars of grain spawn can likewise take from 45 minutes to two hours.

Regardless of your sterilization task, every lab needs to develop several substrate sterilization protocols (for the various substrates, sizes, and preparations). Each of these will need to be validated carefully with a quality biological indicator (such as Self-Contained Biological Indicators (SCBI), Mini Self-Contained Biological Indicators (MSCBI), or self-contained spore ampoules).

We are mainly produced the Sterilizer Autoclave by high pressure steam sterilization method for Mushroom Substrate.

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