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How are mushrooms grown?

Updated: Aug 26, 2019


Golden Oyster Mushroom (Pleurotus citrinopileatus)

For the first blog post we will be exploring the fascinating world of mushrooms and how they grow. There are a number of steps but we'll focus on our commercial operation and how we do it.


To understand this article let's get some understanding of the terminology.

Mycelium (Source: Wikimedia Commons)

Mycelium

This is the main part of the living organism that grows the mushrooms we see in the wild or in the market. Mycelium can grow underground or in dead/living trees depending on the species. It features a massive network of tiny threads that excrete certain enzymes to help the organism digest the substrate (wood, compost, vegetation, etc).


Spawn

In nature mushrooms are the reproductive structure of the living fungus. They release microscopic spores that must find another compatible spore to produce more of the fungus. Producing mushrooms from spores is possible but not always the best option. The quickest and best option for growing a mushroom is by utilizing a culture (spawn) of mycelium.


We utilize commercial spawn that's grown on bird seed and a proprietary nutrient mixture. The bird seed not only provides nutrients but also allows for ease of use in the next step.


Substrate

There's a vast number of substrates (growing medium) that mycelium digests to further it's expansion and production of mushrooms. White Button, Cremini and Portobello (all are Agaricus bisporus) producers utilize a compost based on manure and straw. With the Oyster (Pleurotous sp.), Lions Mane (Hericium erinaceus) and others grow mainly on tree so we utilize a hard wood based substrate. We use a mixture of oak sawdust and soy bean hulls. Oak trees Soy bean hulls are utilized for it's high nitrogen content resulting in massive mushrooms.


In commercial production of gourmet mushrooms we use biodegradable plastic bags to hold 10-12 lbs of the substrate and spawn mixture. This allows us to be able to sterilize the substrate, colonize and grow mushrooms from each bag. Bags are used for their ease and mobility.


Let's Grow Mushrooms!


Sterilization

The first step in successfully growing mushrooms is to ensure that the only living thing growing in the substrate is our fungus. Contamination from Trichoderma, cobweb molds and others are a serious threat to successfully growing mushrooms. These contaminants usually already exist in the substrate and must be eliminated. There's several methods to achieving this for our purposes.

  • Sterilization - this can be fully achieved utilizing either a pressure cooker or atmospheric steam sterilizer. Sterilization is highly effective with wood based substrates. Pressure cookers are quick and ensure sterilization due to heating water over 212 degrees. With atmospheric steam sterilization you're limited to a temp of 212 degrees but can run for far longer periods (12-16 hours). This is the method we use in our commercial production due to the much larger capacity.

  • Pasteurization - this method involves heating the substrate to 160-180 degrees for over an hour. This method is really only effective with straw based substrates. This kills most contaminants but not everything.

  • Cold Pasteurization - this involves soaking straw in water and hydrated lime mixture for 12-24 hours. It's a very low tech way of killing enough contaminants to ensure the fungus gets a good foothold. The downsides are waste water disposal, contamination risks and it's only effective on straw substrates.

Inoculation and Colonizing

Once the substrate is ready, we inoculate it with our fungus of choice. This involves taking the previously sterilized substrate and mixing in some of the spawn (myceliated bird seed). You must keep in mind that the substrate was sterilized and therefore precautions need to be take to ensure it stays that way. This is why mushroom growers utilize HEPA Flow Hoods to keep the substrate sterile. A Flow Hood is simply a large HEPA filter that provides laminar (non-turbulent) air flow. These filters are effective at reducing about 99.999% of air contaminants.


Here's a brief overview of the inoculation process

  • Thoroughly clean and disinfect work area and tools.

  • Break up the spawn. Mycelium grows together and can create a large mass.

  • Open the substrate bag within the clean and active flow hood.

  • Pour in the appropriate amount of spawn in to the substrate bag. This amount can range widely from 5% to 15% depending on species and desired colonization speed. The percentages are based on the overall weight of the substrate to the weight of the spawn.

  • Seal the substrate bag with an impulse sealer to lock out any potential contaminants.

  • Shake the bag to mix up the substrate and spawn until it's consistently distributed.

Once the substrate is mixed with the spawn and properly sealed the only thing to do now is wait. This part of the process is called "colonization". It's simply a point where we wait for the mycelium to expand and consume the substrate. Most colonization takes about 2 weeks to consume the entire bag of substrate.


Blue Oyster (Pleurotus ostreatus) pinning

Fruiting

We use the term "fruiting" to describe the actual growing of mushrooms themselves. One thing to keep in mind here is while these bags are colonizing they're producing a lot of CO2. We can trigger the growth of mushrooms but cutting into the bag. This introduces oxygen to the mycelium and makes it think it's found the surface of the soil or wood. Since mycelium lives to consume and reproduce, oxygen sends a signal that it's should start growing mushrooms.


A few days after cutting open the bag you will begin to see tiny bumps (pins) that will rapidly turn into small mushrooms. Some species of mushrooms take just 5 days to completely grow, while others may take 2 weeks.


Harvesting Blue Oyster (Pleurotus ostreatus)

Harvesting and Storage

A few days after cutting open the bag you will begin to see tiny bumps (pins) that will rapidly turn into small mushrooms. Some species of mushrooms take just 5 days to completely grow, while others may take 2 weeks.


Each species of mushroom has a varying indication of maturity. Most oyster mushrooms will begin flattening out towards maturity. This is the perfect time to harvest. While others like Italian Oysters (Pleurotus pulmonarius) will begin to curl up and become wavy. Harvesting of Italian Oysters is somewhat tricky even for experienced growers. You always want to allow enough time for maturity but you also don't want them to become overly mature. Mushrooms grow very rapidly and timing the harvest right can be challenging.


Once the mushrooms are ready, the best way to harvest is by grabbing the base of the mushrooms and tilting away from the bag. This allows you to pull all of the mushrooms off the bag in one motion.


To ensure the freshness of the your mushrooms we recommend eating them as soon as possible. It is, however, possible to store Oyster and Lions Mane mushrooms for about 5 days in the your fridge. Mushrooms will stay fresh for about 3 days in a paper bag. If you need to store them longer you can loosely wrap them in wax paper and place them in a paper bag in your fridge. This should get you a few more days of storage before they become dry.


We will be posting more blog entries like this. Stay tuned!