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Three Shortcomings of ‘Cold Sterilization’ for Mushroom Substrate

A close up of maitake (Hen of the Woods) mushrooms. The mushrooms are brown and numerous, halfway between trumpet-bells and brackets. The image is public domain, courtesy of the US Dept of Agriculture:

“Cold sterilization” of mushroom substrate is popular with early stage commercial mushroom growing operations. It’s low-tech, DIY, and can be cost effective. But cold sterilization isn’t always effective, and many operations outpace its capabilities.

 “Cold Sterilization” Defined

So-called “cold sterilization” doesn’t meet the technical definition of sterilization at all. Instead, it’s a term used for a collection of pasteurization strategies.

By definition, sterilization aims to completely destroy or deactivate all forms of life and biological agents present in the substrate (e.g., fungi, bacteria, viruses, spores, unicellular eukaryotic organisms, and so on). Pasteurization, meanwhile, will only weaken microorganisms, kill pathogenic bacteria, and reduce the level of biological contaminants in your substrate.

When people refer to “cold sterilization” of mushroom growing substrate, they usually mean one of two things: “heat pasteurization” or “chemical pasteurization.” (Yes, this is confusing! You are not alone.)

Heat pasteurization uses heat, but at temperatures well below those required for true sterilization (hence the term “cold sterilization”). Sterilization requires heating the substrate above boiling—usually as high as 121ºC—whereas heat pasteurization doesn’t exceed 60-80ºC. Growers can compensate for using these lower temperatures by holding the mushroom substrate at that temperature for a longer period.

Chemical pasteurization is a chemical-based process that requires no heat. Mushroom growers instead soak their substrate in water treated with magnesium hydrated lime for 12 hours or more. This caustic soak raises the pH of the substrate, making the environment too hostile for bacterial or fungal spores to germinate and thrive.

In many contexts, pasteurization is sufficient, and might even be preferable. For example, with foodstuffs where true sterilization might degrade flavor or other aesthetic qualities, heat pasteurization is often preferred. Likewise, mushroom growers of some common varieties might choose to start out with heat or chemical pasteurization to prepare their substrate. Chemical pasteurization is especially popular, as the method can be used to process high volumes of substrate at lower cost and without special equipment.

Three Downsides to Pasteurization

But there are three major shortcomings to pasteurization of mushroom substrate.

Limited Applications

Broadly speaking, pasteurization is effective for cultivating certain common edible mushroom varieties. These include all the varieties of oyster mushrooms and most types of Agaricus (which includes both portobello mushrooms and the “white button” mushrooms in most grocery stores). These are fast-growing mushrooms with very hearty mycelia. They grow in the types of substrate that pasteurize most easily (like unsupplemented straw). Their strong mycelia can take hold in caustic substrates (like those resulting from cold pasteurization in hydrated lime) and shift its pH back to a more hospitable range. This is part of the reason these mushrooms are so common: They are easy to cultivate using low-effort methods.

Now consider maitake (also called “hen-of-the-woods”). Maitake grows in oak, which is too dense to be pasteurized by any method. Additionally, maitake mycelium grow much more slowly than Pleurotus ostreatus (oyster) or Agaricus varieties, giving them little opportunity to gain a foothold and outcompete other microbes.

Resource Usage

Pasteurization is low effort, but comes at the cost of high resource consumption. Both heat and chemical pasteurization require fully immersing all your substrate. Heat pasteurization is additionally fairly energy intensive, as it demands holding water temperature at 60-80°C (just a tad below simmering) for up to five days.

This brings us to the other major resource pasteurization consumes: Time. As noted, heat pasteurization takes nearly a week. Chemical pasteurization takes roughly 12 hours.


Chemical pasteurization results in large volumes of caustic wastewater. This is rarely an issue at a smaller production scale, but as cultivators increase the scale of their operation, they may run afoul of local water authorities (microbreweries run into similar issues once their production exceeds a certain threshold, and their effluent begins to have a noticeable effect on water treatment plants).

Similarly, past a certain threshold, simmering hundreds or thousands of gallons of water for a week results in a problematic carbon footprint.

Optimize Cultivation Substrate with Autoclave Sterilization

An increasing number of mushroom growers and mycology labs are working with Priorclave to minimize contamination and maximize their yield. An autoclave can prepare any substrate (from straw to large hardwood blocks) in a matter of hours, rather than waiting days for heat pasteurization. In addition to saving time, autoclaves use fewer resources and produce less waste: Instead of producing hundreds or thousands of gallons of caustic wastewater, each autoclave cycle uses just a few gallons total. A lab-quality autoclave can be used for many applications; it is as effective for equipment sterilization and preparing agar plates as it is for preparing growing substrate.

Priorclave Can Help Your Lab Reach its Goals

Every Priorclave machine is built to your specification, optimized for minimal resource consumption, and is backed by our free lifetime technical support. Contact us today to discuss the challenges and requirements specific to your needs.