How to choose the right sterilizer-autoclave?

7 min readFeb 26, 2021


Growing mushrooms come with several challenges since they are slow growers and may require special treatment of substrates to stimulate their growth. In most cases, the substrate needs to be sterilized to take care of any fast-growing bacterial or fungal species that may impede the mushrooms’ growth. To do this, growers need to use a sterilizer or autoclave. The sterilizer helps keep the substrate-free of fungal or bacterial spores that may have settled on the substrate from the air or while handling the equipment. It also comes in handy to keep the inoculating space free from any contamination.

Since sterilization is one of the most important aspects of growing mushrooms, all mushroom farmers must get it right or spend a significant amount of money in vain. While it is common practice for mushroom growers to use home-styled pressure cookers since they are handy and affordable, there is a need to understand the principle behind this activity and why it must be perfect.

Although pressure cookers can perform excellently as sterilizers, they have limited spaces that may give room for incomplete sterilization, thus increasing contamination chances. They are only perfect for sterilizing small-to-medium batches of grain and substrates.

Inadequate sterilization of substrate has been identified as one of mushroom growers’ major challenges. These growers often settle for a makeshift sterilizer that is a vertical drum with a heavy lid heated by fire or gas.

As the need for mushrooms increases, more farmers are interested in knowing important updates about growing them. Questions like what makes a good sterilizer, the factors to consider before choosing a sterilizer or sterilizing your substrate, and many more are frequently asked questions about cultivating mushrooms. If you belong to this category, then this is your lucky day. This article will discuss the factors to consider when choosing the right sterilizer for your mushroom substrate.

Factors to consider when choosing the right sterilizer

a. Carrying capacity:

The common sources of contamination while growing mushrooms include but are not limited to equipment, substrate, and handler. While the last can be handled by practicing good personal hygiene and inoculating the substrate aseptically, the first two need to pass through the autoclave for complete sterilization. To do this, you need an autoclave with enough space that can handle your substrate weight and the equipment itself. Before purchasing an autoclave, make sure it has the right capacity to support your level of farming. You can always ask Meavien team experts for recommendations based on your intended size of the substrate. Autoclaving your substrates at once tends to save you time and energy costs.

For perfect sterilization, it is important to make sure your autoclave is not densely packed with items. Make sure you leave spaces where the steam can penetrate each item and sterilize them. The ample spaces in your autoclaves allow for the even penetration of steam. It also shortens the sterilization cycle.

b. Type of sterilizer

Sterilizers come in different shapes, designs and sizes. Please note that the pressured steam autoclave is excellent for commercial production. These autoclaves are perfect for sterilizing bulk substrates. They are horizontal and have two doors.

Hospital autoclaves can also serve an important purpose in sterilizing mushroom substrates. These autoclaves are made of stainless steel and are adequately equipped with a pressurized steam jacket. The only downside is the fact that these autoclaves are smaller and may not serve commercial purposes. Unlike the pressured steam autoclave, the hospital autoclave comes with a door with a pressure rating engineered to operate at 100 psi. This, however, exceeds the needs of most mushroom growers but may be perfect for sterilizing pieces of equipment.

When writing this article, the cost-effective vessels developed for canning industries remain the best sterilizers for mushroom growers. These retorts are designed with a steel pipe with a door fitted at both ends. They have the right pressure settings that don’t destroy your substrate.

c. Space requirements:

How large is your preparation room? This is one important factor that will determine the size of the autoclave to purchase. There is no need to purchase a sterilizer you can’t install in the right place. The autoclave should be placed under the ventilation/fans to allow for the easy escape of steam after sterilization.

d. Design/build of the autoclave

It is recommended that the best autoclave should have 2 doors (one at each end). Autoclaves with double doors are easier to operate. They also provide the best sterilization since the user can have enough space to arrange his items. For effectiveness, it is recommended these doors are placed at both ends of the autoclave. It should also have 2 redundant gauges for pressure/temperature, pressure/vacuum gauge, electricity safety interlocks, and warning lights. Overall, the sterilizer should be coated with heat-resistant, anti-corrosive paint since the autoclave will be exposed to water.

The combination of these parts is what makes an autoclave awesome equipment for mushroom growers. You can always consult with your expert mushroom cultivator or company representatives from the Meavien team for information and recommendations and the best type of sterilizers that will suit your needs.

e. Installation and maintenance

The ease of installation and maintenance of autoclaves must be taken into consideration when making a decision. There are 4 main parameters to take into consideration when installing autoclave. First, the electricity — you should have a minimum of three-phase electric power. Second, piping — depending on your autoclave size you will need access for filling and draining water from the tank sometimes ranging from 5 gallons — 25 gallons. The third parameter is ventilation — since your autoclave produces a significant amount of steam and heat (boiling temperature of course) — you will need fans installed to remove scent and heat. Last and not least — autoclave doors need to be surrounded by clean filtered air. Clean air is required so no contaminants will get inside the spawn bags with the substrate. If you are just starting, it is important to settle for simple table autoclaves to gain an idea of how they work. You can always upgrade to sophisticated autoclaves when you understand how they work.

Any precautions?

Your sterilization should be flawless if you can observe the following;

a. Do not exceed the water carrying capacity:

Adding too much water to your substrate will discourage mycelium growth while stimulating bacterial blooms and anaerobic activities. To address this, it is important to note that your substrate is wetted to 60–65% water. You can also test the water capacity by squeezing the substrate by hand to see if water droplets fall out as a stream. If this happens, it shows that your substrate is too wet. It should have 3–5 drops ideally.

b. Identify the right pressure and time of sterilization for each item.

The ideal pressure for sterilizing substrates is different from the one used in sterilizing pieces of equipment. Research-grade laboratory autoclaves are programmed to sterilize supplies and equipment in 15 to 20 minutes. This is enough to kill microbial spores on the supplies and equipment; however, substrate preparation may need longer processing. Depending on volume, bulk nutrient/substrate preparation may take from 90 to 120 minutes before achieving complete sterilization.

Sterilizing substrates is also trickier since they may have different compositions. For instance, sterilizing hardwood sawdust in small quantities may require 20 minutes of autoclaving, while sawdust in blocks or large bags may take up to 2.5 hours to be completely sterilized. Jars of grain spawn can take between 45 minutes to 2 hours before achieving complete sterilization.

Fresh hardwood sawdust may require 2–3 hours of sterilization at 15 psi. If the sawdust is supplemented with rice bran, then the sterilization time will increase to 4–5 hours. To understand how best to determine the right time, there is a simple rule of thumb that states that “as the percentage of nitrogen-supplements increases relative to the base substrate, the likelihood of contamination increases.”

On the safer side, you must develop the right substrate sterilization protocol for different substrates, preparations, and equipment.

What to do during inoculation

Apart from the precautions mentioned above to be observed while using an autoclave or growing mushrooms, the following should also be taken into due consideration:

a. Allow the substrate to cool down before inoculation with the mycelium. It is best to keep the inoculums at room temperature before inoculating the mycelium.

b. Make sure the mycelium has fully colonized the substrate before starting the fruiting stage.

c. Keep all materials needed for the inoculum within reach. Too much movement during inoculation can increase the chance of contamination.


Sterilization is very important in handling possible sources of contamination in substrate preparation. It also gives the growers a chance to adequately monitor their mushrooms’ growing progress without the fear of contamination. Please note that all processes leading to the cultivation of mushrooms must be done in a sterile environment for maximum results.

Let us know if you have questions about how to cultivate mushrooms or feel free to purchase our sterilized spawn bags in bulk. We will love to hear from you soon.

By Paul S.

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