What is the difference between pasteurizing and sterilizing mushroom substrate?
Pasteurizing mushroom substrate kills the majority of competing micro-organisms, keeping a few beneficial microbes for protection. Sterilizing kills all competing micro-organisms, including the beneficial ones. Pasteurization is better suited for agricultural waste products such as straws and manure while sterilization is necessary for dense, nutrient supplemented materials like grains or sawdust.
As a new mushroom cultivator, I struggled understanding why I might need employ one substrate cleaning technique over another. Especially with all the noise on the internet where every piece of minutia is debated. While minutia is important when considering microbes, I think the best course for beginners is to focus on basic principles and figuring out which method is easiest to deploy that gives the highest chance for success. This was I aimed for anyways. We don’t need to focus so much new methods, gadgets or even understanding each step it great detail yet. I believe it is best to first focus on what’s proven to work and what gets us an early win in cultivating our first mushrooms.
Cornering the Substrate Market
The goal of preparing mushroom substrate begins with first providing a hospitable environment for the mycelium to grow. This is achieved by removing the microbial competition in the substrate, so that there is little to no chance of other organisms taking the nutrient rich food supply. Essentially, we are cornering the substrate market by creating an unfair advantage for our mushrooms to grow.
As the mushroom Mob-boss of your substrate, you have some hench-men at your disposal; Pasteurization and Sterilization. We can strong arm the competing micro-organisms and clear a path for our mycelium to grow. When deciding which method to employ, we need to understand the advantages and disadvantages of each. It is important to understand which method makes the most sense for your specific grow so that you can ensure the best biological and cost efficiency.
Which method should I use to Clean Mushroom Substrate
Whether you use pasteurization or sterilization is ultimately dependent on which mushroom species you are working with. Your mushroom species, substrate material and available tools all play a role your substrate preparation method.
Because different mushrooms feed on different materials, we shouldn’t take the same approach to clean all of our substrate batches. This will not provide an equal chance of success for all the mushroom varieties we are trying to grow. As a rule of thumb, the denser a material and the more supplementation added, the greater need for sterilization. When we introduce more nutrients into the substrate, we also introduce more opportunities for mold and competing organisms to grow if not treated properly.
What is Pasteurization?
Pasteurization is a process that kills most but not all of the competing micro-organisms in the mushroom substrate. Because the substrate typically does not exceed 180F, it leaves about 20% of the microbial population in the substrate. These heat-resistant organisms that remain, serve to protect the substrate by attacking contaminants. It’s an extra buffer of protection for our mushrooms.
Pasteurization is most commonly achieved with heat or chemicals through a variety of different methods. This is a good option when you’re working outdoors with a more resilient species, like an Oyster mushroom. Another benefit to pasteurization is that it’s usually a simple, cost effect method of cleaning your substrate. There are a number of different chemical treatments that require less energy input and these are fairly straight forward methods to execute that anyone can achieve in their home.
The most common method to pasteurize is through heat and this usually occurs between 140 and 180F (60 and 82C) at atmospheric pressure. A few traditional pasteurization methods include…
- Hot Water
- Steaming
- Chemical Treatments; Lime Bath, Bleach Bath, Chlorine, Hydrogen Peroxide, Detergents
- Yeast Fermentation
- Vinegar
- Pelletizing
Which substrates can I pasteurize?
Pasteurization can be used to prepare many different agricultural, carbon rich waste products. This is generally done with a large batch, outdoors but can also be scaled back for small-batch indoor settings. It really depends on the type of mushroom species you are growing. Some common materials used in pasteurization include…
- Straw
- Manure
- Corn cobs
- Non-supplemented sawdust
Which mushroom species thrive in pasteurized substrates?
While there are a few mushroom varieties that thrive in pasteurized substrates, there are many specific species that we cannot grow using these methods. This includes a lot of popular gourmet and medicinal mushrooms such as Lion’s Mane and Reishi. Some species are more particular when it comes to substrate choice.
Some good mushroom varieties for growing in pasteurized substrates are…
- Oyster Varieties
- Button
- Portabello
Pros
- Simplicity & Reliability
- Cost Effective
- Requires Less equipment
- Lower energy input
- Less time to achieve
- Scalable: works well with small scale and large scale grows.
Cons
- Limited by type of substrates used
- Limits varieties of mushrooms available to grow
What is Sterilization?
Sterilization is the process of killing all the micro-organisms in the mushroom substrate. This includes not only the bad but also the beneficial microbes that pasteurization benefits from. Sterilization requires us to bring the temperature of the substrate material above the boiling point of water so we can kill all the heat-resistant organisms that usually persist through the pasteurization process. To achieve sterilization of the mushroom substrate, it requires the use of pressure. This is commonly done by using an autoclave in an industrial setting or a pressure cooker at home. While this introduces more complexity to the substrate preparation, it ultimately gives the cultivator more control and opportunity to experiment with growing different mushrooms species. With the popularity of Instapots, many people already have pressure cooking capabilities to sterilize in their own home in small batches.
Sterilization temperature is above the boiling point of water, >250F (121C) and above atmospheric pressure.
Why do we need to sterilize versus pasteurize the substrate?
It is common for cultivators to incorporate supplementation such as bran, coffee grounds, gypsum, soy hulls, vermiculite, etc. in an effort to increase mushroom yields. When these are added, it is like open hunting season for mold or other aggressive organisms to take over the nutrient-rich substrate. More money, more problems.
Additionally, some mushroom growers sometimes employ the use of sterilization as a cost saving measure. If the substrate is sterilized, it requires less spawn per substrate batch to initiate mycelium growth. The question will then become, what is the cost of energy consumption, time to sterilize and extra supplementation? It may be that the cost savings on the lower percentage of spawn per substrate batch does not off-set these costs.
Pros
- Less spawn needed
- Allows more variety of mushrooms
Cons
- More equipment and skill required
- Longer time to sterilize
- Safety
Key Differences between Pasteurization and Sterilization
The key difference between pasteurization and sterilization is the amount of micro-organisms that are killed off when preparing the mushroom substrate. Pasteurization kills most while sterilization kills all. For the mushroom cultivator, pasteurization is usually a cheaper and easier option but limits the variety of mushrooms you can grow. Sterilization is a process that usually requires more skill and tools but can potentially allow for greater crop yields with the introduction of supplementation. It also gives the cultivator access to grow a variety of different mushrooms species and freedom to explore using new substrates.
Additional Factors to Consider when choosing between Pasteurization and Sterilization
With everything considered about the differences between pasteurization and sterilization it is also important to think about experimentation of new methods and materials. Moving beyond limitations of current methods for substrate preparation may lead you to discover superior ways for growing mushrooms. Like cooking, the best combinations are often discovered by mistake, so experimentation is encouraged. We shouldn’t be so rigid in our approach to growing mushrooms that we ignore new ideas or possibilities.