Treatment Methods
There are three methods generally recommended for treating straw prior to inoculation. Each has its own considerations for time, cost, and available materials:
Cold Fermentation
While this method is the lowest tech, it’s also very stinky! Straw is soaked in water for as little as 5 and as many as 10 days, which essentially cultivates anaerobic bacteria and thus “cleans” the straw for the oysters. The time it takes depends on ambient temperature. During cooler months, expect longer times and during hot months, a much quicker process time. Since all that is needed is water and a container (e.g. a 5 gallon bucket or 55 gallon drum), this is by far the simplest and cheapest method. After soaking, drain the straw and then you are ready to inoculate! Contamination rates for this method are high and this is not recommended for a commercial method of production.
Lime “Pasteurization”
The concept with using lime is to quickly spike the pH of the straw to 12 or 13, killing most competitors, and then inoculate the substrate with oyster mycelium. The key is to use not just any lime, but specifically hydrated lime with a magnesium content (Mg) of LESS THAN 10%. This detail is critical to success. One brand that is available online is Hi-Yield lime, when buying lime always be sure to check the guaranteed analysis to ensure magnesium is below 10%.
Generally, a 50 lb bag costs around $10 - $15 and will be sufficient to “pasteurize” several hundred pounds of straw. A starting point is to add .35 to .5 lbs per gallon of water, but because of the wide range of compositions in lime its necessary to test the solution with pH test strips to determine the proper amount. Soak the straw for 16 - 18 hours in this solution - no longer!
After soaking, drain the straw and then you are ready to inoculate. It’s important to consider an appropriate disposal method for the wastewater, which has a high pH and would be considered toxic. Always wear gloves when handling the straw or water, and use an acid-heavy liquid like “pH balance” or “pH down” (available from hydroponic suppliers) to neutralize the water before dumping.
It should be noted that some people report success using wood ash instead of hydrated lime, but you need to add A LOT of this to get the pH to change, and this method may not prove to be commercially viable for operations.
Hot Pasteurization
The most “complex” method is still rather simple, requiring a metal container (a 55 gallon drum is most common) and a heat source (e.g. propane turkey burner, wood fire, or on-demand hot water heater). The goal is to keep the substrate at 140-160 F for at least two hours to pasteurize. The most efficient way to do this is to heat the water and straw to 180 F, shut off the heat source, and put a lid on. This will cool over time but should keep the material in the proper range for 2 hours. You can experiment with timing as some have reported getting clean straw after as little as 1 hour in this manner.
After the straw has been “cooked”, it should be drained and spread out on a cleanable surface (table, clean tarp used only for this purpose, etc), and spread with a clean tool (used only for this purpose) to cool as quickly as possible. Once the straw is able to be handled, or drops below 100 F, it is ready for inoculation. An infrared laser thermometer is helpful to maintain an accurate measure.
Inoculating and Packing
Regardless of the treatment method, the material will next be packed along with oyster spawn into bags or containers. Five gallon buckets with ⅝” holes drilled in a diamond pattern every 6 - 8” can work well and reduce waste on the farm, but need to be completely and thoroughly cleaned between uses to prevent contamination. 5lb or 10lb filter patch bags or bulk 10” plastic tubing rolls are also commonly used, but do result in waste from the process that must be disposed of.
No matter what container is used, what is most important is that the spawn is well distributed and the material is packed tight. Either grain or sawdust spawn can be used, with the grain offering a little extra nutrition, which can increase growth rates. There are generally two methods of adding spawn; one is to pour a bag and mix evenly, the other is to layer a handful of spawn every 3 - 4” as you pack the container or bag. Growers tend to develop a preference for one over the other, over time.
Check out Fungi Ally’s guidebook on oyster mushroom cultivation for more information on this method.
