Fossil records show that humans have been foraging and eating mushrooms since the stone ages. Mushroom spores were discovered in the teeth of human skeletons as far back as the late paleolithic era 19,000 years ago. As cultivators, however, we are rather new to mushroom growing, which has proven to be inherently different, and more challenging, than other forms of agriculture.
The exact origins of mushroom cultivation are murky, and historians are hesitant to name a person or place as the official beginning of mushroom farming. While many believe that the earliest attempts at cultivating mushrooms started with growing Shiitake on logs, recent research has shown that the first mushroom to be cultivated was actually Wood Ear (Auricularia auricula-judae) during the Tang Dynasty in the 7th century. The method was simply to put steamed bran on logs, cover with straw, and – almost magically – the Wood Ear would grow.
It wasn’t until around 1650 that modern mushroom cultivation took root with the button mushroom – Agaricus bisporus.
Early growers observed that button mushrooms emerged in places where they poured water that had been used to wash mushrooms. Scientists later discovered that fungi reproduce by spores dropped from mushroom gills, and thus the washing water “seeded” the fungi onto a new substrate.
These astute growers also noticed that button mushrooms were naturally popping up on spent manure and straw in melon beds. They capitalized on those discoveries and began to grow mushrooms with relative success on composted horse manure and straw in abandoned limestone quarries.
Ultimately they realized that the fungal mycelium feeding on the substrate could be used to inoculate new beds with even greater success. The endeavor came with challenges, however, as the yields were unreliable, and the risk of contamination high.
The Japanese applied their expanding knowledge of fungal ecology to develop large-scale cultivation of the revered shiitake mushroom outdoors on logs.
Aware that shiitake grows wild on dead Shii trees in the forests of Asia, aspiring Japanese growers placed freshly cut logs near wild fruiting Shiitake logs in the hopes that they would be naturally colonized through spore dispersal. More success was later achieved by directly inoculating logs with spores collected from shiitake mushrooms.
Later they discovered that by stacking freshly cut logs between colonized logs, the fungal mycelium would grow directly into new logs. While this technique offered a reliable success rate, farmers remained at the mercy of the environment – losing crops to slugs, bugs, droughts and downpours.
To master the game, growers had to find a way to bring cultivation inside where they could to control environmental conditions and maximize yield. But environmental conditions necessary for mushroom growing are the very same conditions that promote the growth of mold, mildew, bacteria, algae, and other fungi, and so required the development of sterile techniques.
The first industrial sterile method of growing oyster mushroom was patented in 1966. The substrate was treated with steam in autoclaves at elevated pressure and temperature over 120 ℃. However this technology was highly expensive and not widely used until the 90s when autoclaves were manufactured more broadly for the sterilization of bulk materials.
Substrate sterilization was a huge leap in giving gourmet mushroom cultures an advantage, but there remained the challenge to keep airborne contaminants from getting hold during incubation. No amount of cleaning and sterilizing surfaces will eliminate contaminants floating in the air, and with one cubic meter of indoor air loaded with up to fifteen million bacterial and fungal spores, airborne contamination was an ongoing risk.
Indisputably one of the biggest game changers in the industry was the invention of the polypropylene filter patch bag. Research on using bags to grow in the late 1980s revolutionized Shiitake mushroom growing in Asia almost overnight. Shiitake production in China increased over 135 times between 1983 and 2003.
These bags made of polypropylene can withstand the extreme heat and pressure necessary to sterilize mushroom growing substrate in an autoclave. HEPA-filtered air in a flow hood now allowed the sterile transfer of pure mushroom cultures into the bag of cooled sterile substrate and contamination sealed out.
The specialized filter patch on the bag allows for air exchange necessary for growth of fungal mycelium, while continuously filtering out contaminants.
There is no industry standard yet for rating filter patches. Some companies provide an air exchange rating, whereas Unicorn Bags rates their patches by pore size. For example, Unicorn’s T filter is rated as 90% effective at filtering out contaminants that are greater than 0.2 microns, and is a highly effective barrier to common pathogens such as the dreaded Trichoderma harzianum whose spores generally range between 3 and 5 microns.
Humans have a long history of growing mushrooms, but it wasn’t until the development of sterile techniques and technology over the past 60 years that allowed the prolific wide-scale mushroom cultivation that we see today.
Autoclaves, flow hoods, and filter patch bags have dramatically changed the landscape of commercial mushroom growing, and now are a mainstay of mushroom farms around the globe. These widely accessible technologies provide small and large scale growers an economical means to maximize crop yield by minimizing contamination while optimizing growing conditions.
These days there are over 10,000,000 tonnes of mushrooms produced globally each year in an industry worth over $40 billion yearly. Commercial mushroom farmers are no longer at the whims of mother nature, and new technology is bound to up the game in the future.