How to sterilize mushroom substrate without a pressure cooker

Rouven

5 years ago

How to sterilize without a pressure cooker

Sterilization of the substrate is one of the critical steps when it comes to mushroom cultivation. Without a good sterilized substrate, the level of contamination within 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 want to sterilize your substrate, but you don’t have a pressure cooker at hand, then there are several methods from which you can choose to sterilize your substrate properly. 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

Composting is typically used for growing button mushrooms (Agaricus). But it also found its way into growing oyster mushrooms (Pleurotus spp.).

Composting is 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]?

Step 1 – Substrate Preparation

Pile your substrate up and create a heap.

Step 2 – Moisturizing

Water your substrate and mix thoroughly. The moisture content should be around 75%.

Step 3 – Composting

During the next 7 days, water and mix regularly your substrate. This prevents from creating hot spots inside the pile. The pile will 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 at 65°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 the temperature for 48 hours.

Step 7 – Cooldown

Stop the heating process and let the substrate cool down to 25°C.

Step 8 – Inoculation

Inoculate with a spawn rate of about 5%.

Step 9 – Bagging

Fill your bags with the inoculated substrate and 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 put these findings into context, we come back to the definition of biological efficiency (BE). The BE is defined as 1 kg of fresh mushrooms for 1 kg dry substrate 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 second definition.

Example: 7-118, ~ 15% yield

1 kg of fresh mushrooms per 4 kg of substrate equals BE 100%

15 kg of fresh 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: Estimated biological efficiency (BE) based on figure 20[7]

The use of CHEMICAL sterilization

Chemical sterilization is typically used because they are inexpensive. During my research, I found many scientific studies in which different chemicals are used to sterilize the substrate. Table 1 gives an overview of these chemicals and their classification accordingly to their risks.

A big disclaimer: The shown information represents the view, and the opinion of mine and are just for education and information purpose only and should not be construed as legal advice or as an offer to perform legal services. The provided information contains general information and may not reflect current legal developments or information. I do not make any representation or warranties concerning the accuracy, applicability, fitness, or completeness of the information. The information is not intended to substitute for professional advice. Please inform yourself by reading the instruction manual, material safety data sheet carefully, and ask your local specialist before using any of these mentioned chemicals. I, therefore, disclaim any and all liability to any party for any direct, indirect, implied, punitive, special, incidental or other consequential 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 of mushroom substrate[8]

With that said, I do not recommend any of them, because the risks of using them are for me 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 lime you 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 are adding lime to water, it increases the pH level. As many contaminations prefer an 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 1-6 cm long pieces. Chopping the substrate helps the breakdown and leads to higher biological efficiency (Figure 6).

Figure 6: Unshredded straw vs Shredded straw biological efficiency comparison[11].

Step 2 – Prepare the water

Add about 7 grams of hydrated lime for every 4 liters 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 placing the substrate onto a metal rack and let it drain for a maximum of 2 hours. The longer you dried the substrate, the more likely it is to get contaminated again.

Step 5 – Inoculate

Inoculate the dried substrate with roughly 17% or 1 pound of spawn per 6 pound bag[12].

Step 6 – Bagging

Fill your bags with the inoculated substrate and put them into your grow chamber.

RESULTS with COLD WATER LIME

Figure 7 compares 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

For scalding 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]?

Step 1 – Substrate Preparation

Chop your substrate (typically straw) into 1-6 cm 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

Step 5 – Inoculate

Inoculate the dried substrate with 5% on a w/w wet weight basis.

Step 6 – Bagging

Fill your bags with the inoculated substrate and put them into your grow chamber.

RESULTS with SCALDING

As the results in figure 9, indicating a higher temperature doesn’t always lead to a higher yield. The best outcome for this trail 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

Pasteurization at 60-80°C, up to 5 days, and 0 Psi.

Super-Pasteurization at 80-100°C for around 15 hours and 0 Psi.

Pasteurization and especially Super-Pasteurization, which was introduced by Paul Stamets, is a widely used method when it comes to sterilizing the substrate.

PASTEURIZATION PROCESS

How does the process look like[17]?

Step 1 – Substrate Preparation

Chop your substrate (typically straw) into 1-6 cm 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 higher the 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 tank into the barrel (e.g., at the bottom).

Let the process run for 12 to 15 hours.

During the process, make sure you have enough water in your tank.

Step 5 – Drain

Remove the bags and let them cool down for at least 24 hours. The time depends on the density of your substrate. The temperature inside the bags should be below 28°C. Otherwise, the heat will kill the mycelium.

Step 6 – Inoculate

Inoculate the dried substrate with 5% on a w/w dry weight basis[18] and put them into your grow chamber.

RESULTS with PASTEURIZATION

In figure 10, several different sterilization methods were compared. As you can see, the pasteurization method comes in second.

Figure 10: Influence of the sterilization methods on the biological efficiency for different 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 be aware that these results are only for demonstration purposes. Your results may vary depending on the substrate you are using in comparison to the substrate used in the tests and many other factors.

With that said, you should still use them as a benchmark and see if you are getting better 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.