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Grain Sterilization for Mushroom Cultivation

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Grain sterilization is vital to mushroom cultivation. Grain isn’t just a good high-nutrient addition to bulk substrates like sawdust or wood pellets. It’s also an ideal base for producing mushroom spawn. Grains like wheat and rye berries are an almost ideal medium for germinated mushroom spores. Allowing mycelium to flourish in the controlled environment of grain spawn lays the necessary groundwork for them to reliably outcompete any other microorganisms in their bulk substrate.

Grain is great for mushroom spawn not simply because it’s nutrient-rich. Each individual kernel of grain can act as its own inoculation site for mycelium. As such, a single liter of rye berries offers roughly 25,000 individual sites from which mycelium can ultimately spread. Each cubic centimeter of inoculated and colonized grain spawn then becomes a potent block of growth waiting to spring out into whatever bulk fruiting substrate it’s offered.

That said, contamination has ever been the bane of mushroom cultivators and grain is a fantastic source of contamination. For example, according to mycologist Paul Stamets, each gram of rye berries carries cell counts of around 300,000 bacteria and 12,000 fungi. Some batches of grain can host extremely high bacteria and fungus counts, up into the millions. Adding water is a necessary step in both preparing grain spawn and bulk mushroom substrate. But that water further supports bacterial and fungal growth. It can also inadvertently make the grains more susceptible to more complete colonization by unwanted microorganisms.

Bacteria are both the most numerous biological contaminants and the most troublesome. Moist, room-temperature bacteria double every 20 minutes or so. In theory, a lone surviving bacterium in a heat-sealed grain bag will spawn more than a million offspring in just 10 hours. That one bacterium can recolonize an entire batch of substrate in a few days.

Grain Sterilization Challenges

Complete sterilization of mushroom grain bags and any grain to be added to bulk substrate is vital to successful mushroom cultivation. But creating a sterile grain bag for mushroom cultivation is a challenge. Methods that are suitable for preparing straw and other bulk substrates for agricultural purposes—including both heat-based “warm-water” pasteurization and “cold” pasteurization methods relying on hydrated lime, peroxide, etc.—are not true sterilization. They leave far too many viable microorganisms in the grain spawn. This leads to mushroom crops failing at extremely high rates.

Even by laboratory standards—where handling deadly pathogens can be a daily task—sterilizing grain for mushroom cultivation is challenging. Most common lab liquids and glassware can be sterilized in 30 minutes or less using an autoclave (which pressurizes steam in order to reach higher sterilizing temperatures than “stovetop” boiling). A lab can sterilize metal instruments in an autoclave at 135ºC in less than 5 minutes.

But grain and grain bags to be used for mushroom spawn are not nearly so cooperative. That’s because each grain kernel is honeycombed with air pockets and structural cavities. These both harbor unwanted spores and active colonies, and trap cold air that can protect these contaminants from sterilization. Depending on the type of grain, additives, and volumes involved, grain spawn regularly takes at last an hour or two to sterilize completely, even in an autoclave.  

A High-Performance Autoclave for Mushroom Growers 

Priorclave’s EH320 (with vacuum drying option) has been attracting mycologists and mushroom growers specifically because it consistently sterilizes challenging loads like grain. Many standard lab autoclaves restrict what programs and cycles their units can run. This is especially annoying when your work falls outside of what the designers imagined, as is generally the case for mushroom cultivators.  

The roomy, programmable front-loading EH320 accommodates bagged or jarred grain spawn. And it makes it easy to run the cycles you’ve determined are best for your work. Every Priorclave, regardless of size or other features, comes with the same fully programmable control system with multi-program memory and a range of load and chamber monitoring options. With a Priorclave, you can precisely dial in your ideal cycle and store it in memory for quick recall and One-Button start.

Want to learn more about how Priorclave can help you fine tune your mushroom cultivation processes? Talk with our experts about what works for other mushroom growers and mycologists.