Sterilizationof the substrate is one of the critical steps when it comes to mushroomcultivation. Without a good sterilized substrate, the level of contaminationwithin the substrate is too high for the mycelium to grow. This, therefore,leads to low yield or even worse to a total loss.
While some mushroom farmers using a pressure cooker or autoclave, some don’t or even have one, especially in developed countries. This raises the question, “How to sterilize mushroom substrate without a pressure cooker?”
If you wantto sterilize your substrate, but you don’t have a pressure cooker at hand, thenthere are several methods from which you can choose to sterilize your substrateproperly. I will describe each one of them in the next sections.
- Composting
- Chemical
- Coldwater pasteurization
- Hot water immersion (Scalding)
- Pasteurization
- Tyndallization
How to use COMPOSTING to sterilize your Substrate
Compostingis typically used for growing button mushrooms (Agaricus). But it also foundits way into growing oyster mushrooms (Pleurotus spp.).
Compostingis a two-step process:
- Phase 1, which is typically done outside (Figure 1), is a biological and chemical process and the first step to decompose the mixed raw materials. During this time this phase, the substrate itself heats up to 80°C.
Figure 1:View over piles of compost[1].
- Phase 2, which is done inside (Figure 2 and Figure 3), is a biological process to finalize the decomposing. While in phase 1, the temperature is uncontrolled, in phase 2, the temperature is strictly controlled for a certain amount of time. In the first part of this phase – the pasteurization phase – the temperature is set to 56-60°C for 8 hours. After that, the temperature is dropped to 45°C for up to 7 days – the so-called conditioning period. The conditioning period ends after the volatile ammonium has been cleared form the process air.
Figure 2:Sketch of a tunnel[2]
Figure 3:View into a tunnel[3].
COMPOSTING PROCESS
How does the process look like[4]?
Step1– Substrate Preparation
Pile your substrate up and create a heap.
Step 2–Moisturizing
Water your substrate and mix thoroughly. Themoisture content should be around 75%.
Step 3–Composting
During the next 7 days, water and mix regularlyyour substrate. This prevents from creating hot spots inside the pile. The pilewill heat up during the composting process to 60 to 70°C.
Step 4–Filling the tunnels
After 7 days, fill the substrate in a tunnel.
Step 5–Pasteurization
Close the tunnel and keep the temperature at65°C for 18 hours. For this process, you need aerated steam inside the tunnels.
Step 6–Conditioning
Reduce the heat to about 48°C and keep thetemperature for 48 hours.
Step 7–Cooldown
Stop the heating process and let the substratecool down to 25°C.
Step 8–Inoculation
Inoculate with a spawn rate of about 5%.
Step 9–Bagging
Fill your bags with the inoculated substrateand put them into your grow chamber.
RESULTS with Composting
Figure 4 shows several tests with the above-described process.
Figure 4: Mushroom yield of the investigated production series. The yield was determined as a percentage of wet weights of kilogram fresh mushrooms per 100-kilogram substrate block. Bars indicate standard deviation for individual production series, based on yield values of different mushroom production houses. The thick lines show the mean ± standard deviation range for the entire dataset[5],[6].
To putthese findings into context, we come back to the definition of biologicalefficiency (BE). The BE is defined as 1 kg of fresh mushrooms for 1 kg drysubstrate or 1 kg of fresh mushroom for 4 kg wet substrate (75% water content).As the author watered to a moisture content of 75%, we will use the seconddefinition.
Example:7-118, ~ 15% yield
1 kg offresh mushrooms per 4 kg of substrate equals BE 100%
15 kg offresh mushrooms per 100 kg of substrate equals BE 60%
If we repeat this calculation for all other values, we were able to better compare the findings of the author with other papers (Figure 5). The mean BE is around 95%, with a mean ± standard variation of 75% for the lower band and 114% for the upper band. As we now see, 5 out of 11 production series or 45% are below 100% BE.
Figure 5: Estimatedbiological efficiency (BE) based on figure 20[7]
The use of CHEMICAL sterilization
Chemicalsterilization is typically used because they are inexpensive. During myresearch, I found many scientific studies in which different chemicals are usedto sterilize the substrate. Table 1 gives an overview of these chemicals andtheir classification accordingly to their risks.
Abig disclaimer: The shown information represents the view, and the opinion ofmine and are just for education and information purpose only and should not beconstrued as legal advice or as an offer to perform legal services. Theprovided information contains general information and may not reflect currentlegal developments or information. I do not make any representation orwarranties concerning the accuracy, applicability, fitness, or completeness ofthe information. The information is not intended to substitute for professionaladvice. Please inform yourself by reading the instruction manual, materialsafety data sheet carefully, and ask your local specialist before using any ofthese mentioned chemicals. I, therefore, disclaim any and all liability to any partyfor any direct, indirect, implied, punitive, special, incidental or otherconsequential damages arising directly or indirectly from this information,which is provided as is, and without warranties.
Table 1: Overview of chemicals used in science papers for the sterilization ofmushroom substrate[8]
With thatsaid, I do not recommend any of them, because the risks of using them are forme too high. They are also poison, toxic, or hazardous to handle.
How to use COLD WATER LIME PASTEURIZATION to sterilize your Substrate
This one is somehow an exception. Attention: It is corrosive and harmful, which means you should carefully read the material safety data sheet, contact an expert and going back to what I wrote in the disclaimer section, before using it, but it has low toxicity and is widely used in the food industry (E526)[9].
The limeyou want to use should be low in magnesium (< 2%) and high in calcium.Therefore, not all lime is created equally. If your lime contains more than 2%,the mycelium growth will be inhibited[10].
If you areadding lime to water, it increases the pH level. As many contaminations preferan acidity regime, they will get therefore killed off.
How much do you need? Add 355 g hydrated lime to 200 liters of water.
COLD WATER LIME PROCESS
How does the process look like?
Step 1–Substrate preparation
Chop your substrate (typically straw) into 1-6cm long pieces. Chopping the substrate helps the breakdown and leads to higherbiological efficiency (Figure 6).
Figure 6:Unshredded straw vs Shredded straw biological efficiency comparison[11].
Step2– Prepare the water
Add about 7 grams of hydrated lime for every 4liters of water.
Step 3–Add your substrate and soak
After adding your substrate mix thoroughly.
Soak for 12-24 hours.
Step 4–Drain
Remove the substrate from the water by placingthe substrate onto a metal rack and let it drain for a maximum of 2 hours. Thelonger you dried the substrate, the more likely it is to get contaminatedagain.
Step 5–Inoculate
Inoculate the dried substrate with roughly 17%or 1 pound of spawn per 6 pound bag[12].
Step 6 – Bagging
Fill yourbags with the inoculated substrate and put them into your grow chamber.
RESULTS with COLD WATER LIME
Figure 7compares the different treatments. As indicated, lime produced in this test,the highest biological efficiency.
Figure 7:Regular straw vs. Shredded straw treatment averages[13]
Again, please take care and wear gloves, safety glasses, and a mask (read the disclaimer). If you are getting it on your skin, it can cause rashes and burns. Breathing it into your lungs can cause respiratory issues, and getting it in your eyes can be especially dangerous.
How to use HOT WATER IMMERSION (SCALDING) to sterilize your Substrate
Forscalding the substrate is typically immersed for 1 to 1,5 hours in hot water(Figure 8).
Figure 8: Example of hot water immersion[14]
SCALDING PROCESS
How does the process look like[15]?
Step1– Substrate Preparation
Chop your substrate (typically straw) into 1-6cm long pieces. Chopping the substrate helps the breakdown.
Step 2–Prepare the water
Heat the water up to about 80°C.
Step 3–Add your substrate
After adding your substrate mix thoroughly.
Let the process run for 1 hour.
Step 4–Drain
Remove the substrate from the water by placingthe substrate onto a metal rack and let it drain for a maximum of 2 hours. Thelonger you dried the substrate, the more likely it is to get contaminatedagain.
Step 5–Inoculate
Inoculate the dried substrate with 5% on a w/wwet weight basis.
Step 6–Bagging
Fill your bags with the inoculated substrateand put them into your grow chamber.
RESULTS with SCALDING
As the results in figure 9, indicating a highertemperature doesn’t always lead to a higher yield. The best outcome for thistrail was achieved at 80°c and 1 hour
.
Figure 9: Influence of different disinfection methods on biological efficiency[16]
How to use PASTEURIZATION to sterilize your Substrate
Pasteurizationat 60-80°C, up to 5 days, and 0 Psi.
Super-Pasteurizationat 80-100°C for around 15 hours and 0 Psi.
Pasteurizationand especially Super-Pasteurization, which was introduced by Paul Stamets, is awidely used method when it comes to sterilizing the substrate.
PASTEURIZATION PROCESS
How does the process look like[17]?
Step1– Substrate Preparation
Chop your substrate (typically straw) into 1-6cm long pieces. Chopping the substrate helps the breakdown.
Step 2–Bagging
Fill your bags with the chopped substrate.
Step 3–Prepare the water
Heat the water up to about 100°C. The higherthe temperature, the shorter the processing time.
Step 4–Fill the barrel
Add your bags to a barrel with a pressure relief valve (0 Psi).
Introduce the hot steam from your water tankinto the barrel (e.g., at the bottom).
Let the process run for 12 to 15 hours.
During the process, make sure you have enoughwater in your tank.
Step 5–Drain
Remove the bags and let them cool down for atleast 24 hours. The time depends on the density of your substrate. Thetemperature inside the bags should be below 28°C. Otherwise, the heat will killthe mycelium.
Step 6–Inoculate
Inoculate the dried substrate with 5% on a w/wdry weight basis[18] and put them into yourgrow chamber.
RESULTS with PASTEURIZATION
In figure10, several different sterilization methods were compared. As you can see, thepasteurization method comes in second.
Figure 10:Influence of the sterilization methods on the biological efficiency fordifferent substrates[19]
Tyndallization – The forgotten sterilization method
Sterilization method developed by John Tyndall (1820-1873).
With this method, the product is treated in cycles. Each cycle contains the following steps.
- 30 min, 100°C
- 12 hours, 37°C
- 30 min, 100°C
- Repeat Step 2 and 3 until 72 hours are reached
During the germination phase (step 2), the bacteria start growing. In the sterilization phase (step 1), these bacteria will then get killed.
During these three days, there will be a total of 5 to 6 cycles.
https://www.umsl.edu/microbes/files/pdfs/tyndallization.pdf
Alternative process
- Prepare your bags/bottles/jars
- 60 min, 100°C
- 24 hours, RT
- Repeat steps 2 and 3 3 times.
SUMMARY
As you could see, you don’t need a pressure cooker to sterilize your substrate. If you don’t have one, don’t worry, the results are quite promising, while at the same time, the processes are not that complicated.
But beaware that these results are only for demonstration purposes. Your results mayvary depending on the substrate you are using in comparison to the substrateused in the tests and many other factors.
With thatsaid, you should still use them as a benchmark and see if you are gettingbetter results or not.
The links to the used articles and books can be found in the literature section.
Now I want to hear from you:
Which sterilization method from this article are you most excited to try?
And which method are you using today?
Let me know by leaving a quick comment.
RECOMMENDED READINGS
How your Sterilization Method can Impact Your Mushroom Yield
This article goes even deeper into the aspect of sterilization. Some parts are overlapping with this article but it describes each sterilization method in more detail. In addition, I will show way more results of the different sterilization processes on the mushroom yield.
How Your Substrate can Influence Your Mushroom Yield
Starting with the basics, I then will talk especially about the composition of your substrate and how they correlate with the growth rate and the yield. I will address the influence of the particle size and how different types of substrate will affect your mushroom yield. And finally, I will go over the impact of supplementation on the yield.
How your Inoculation Method can Impact Your Mushroom Yield
Whileinoculation seams straight forward it isn’t. While going over several aspectsof inoculation, I then will introduce a six-step process to reduce contamination.At the same time, this six-step process improves the process of inoculationitself.
LITERATURE
Stamets1983 The Mushroom Cultivator
Atila 2016
Oseni 2012
Vajna 2010
Alexander1994 The Best Growing Edge
Kashangura2008 http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1021.4118&rep=rep1&type=pdf
Jones 1964
https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1965.tb05378.x
Kumar 2018
https://scialert.net/fulltextmobile/?doi=ppj.2018.19.24
Shahid 2006
http://www.fspublishers.org/published_papers/62199_..pdf
[1] Source
[2] Paul Stamets (1983)
[3] Source
[4] Vajna 2010
[5] Baláz Vajan (2010)
[6] The first number indicates the yearof production, while the second number indicates the production series
[7] Own calculation
[8] Own table based on different sources
[9] Source
[10] Alexander 1994
[11] Holliday 2012
[12] Holliday 2012
[13] Holliday 2012
[14] Source
[15] Atila 2016
[16] Own figure based on Atila 2016
[17] Stamets 1983
[18] Oseni 2012
[19] Own figure based on Oseni (2012)
