How to Pasteurize Straw for Growing Oyster Mushrooms

Before starting this project, check out our Practical Guide to Mushroom Gardening for an overview of the different stages of mushroom cultivation. In this post, we're doing a deep dive into how to pasteurize straw for growing oyster mushrooms.

Preparing Straw for Growing Mushrooms

Cold pasteurization is the easiest and most effective way to prepare straw for growing oyster mushrooms. The process is simple:

1. Add 1-2 cups of hydrated lime to 20-40 gallons of water
2. Submerge straw for 8-24 hours
3. Hang the straw to drain for 6-12 hours
4. Inoculate with grain spawn within 24 hours

Note: You can also pasteurize straw with steam or a hot water bath for 60-90 minutes. Some species, like Lion's Mane, prefer to grow on substrates that have been pasteurized with heat. Cold pasteurization, on the other hand, is much easier and better for growing oyster mushrooms.

Shopping List

Ready to start planning? Here's what we recommend having on hand:

• Grain Spawn
  
• Chopped Straw 
  
• pH Test Strips 
  
• Mesh Bags
  
• Hydrated Lime
• 5 Gallon Buckets, Nursery Pots, or 16" Poly Tubing
• 55-Gallon Drum or 20+ Gallon Storage Container
• 70% Isopropyl Alcohol

You may be able to find local alternatives to some of these items if they're not available in your area, but a couple things to consider when shopping for this project:

• Wheat and barley straw are the best for growing mushrooms, but rice straw works well too - just make sure you're buying straw, not hay! If you're buying a bale of straw, you'll need a chipper / shredder to cut it into 1"-4" pieces.
• Calcium Hydroxide is the primary ingredient in many garden products. If you're unable to find Hi-Yield Horticultural Hydrated Lime, check the product packaging to make sure it is at least 95% Calcium Hydroxide and less than 1% Magnesium.

Safety First: Handling Hydrated Lime

Calcium hydroxide (hydrated lime) raises the pH of water to kill microbes. This also makes it hazardous if handled incorrectly. Avoid inhalation and skin contact, and be careful not to splash the lime water solution as you mix it.

Protect Yourself:
3M Full Face Respirator and Particle Filters (Protects from inhaling lime particles)
Chemical-Resistant Gloves (To protect hands while working with lime water)

Cold Pasteurization Process

We mix hydrated lime and water in a 55 gallon drum with a giant spatula, but a clean shovel or piece of wood works in a pinch. Most issues with this method are caused by improper draining, so it's important to check your substrate for field capacity before inoculating! A large folding table works great for inoculation, but any table or workbench with a clean tarp laid on top will do.

1. Mix the Lime Solution – Add hydrated lime to water at a ratio of 1/2 cup for every 10 gallons and mix thoroughly. Water should reach pH 11.5 or higher when tested with a pH strip.
2. Submerge the Straw – Place the straw in a mesh bag or basket and soak it in the lime water for 8–24 hours.
3. Drain to Field Capacity – Drain the straw completely for 6–8 hours or until it reaches field capacity. Test for field capacity by taking some of the straw in your hands and squeezing it as hard as you can. A few drops of water may come out, but the straw will need to continue to drain if it creates a stream.

Draining Lime Water

Hydrated lime water should be drained in a safe area, preferably near alkaline-loving plants, or slowly neutralized with vinegar before disposal. Consider adding a spigot to your container for controlled drainage.

Questions or comments? I'm still working on this blog, please email me at Brandon@spawnshroomhouse.com - Your feedback is appreciated!