#002: Agaricus bisporus
Agaricus bisporus accounts for about 90% of mushroom production in the United States, and 40% worldwide. bisporus is the classic grocery store mushroom. It goes by a variety of common names, including: “button mushroom,” “white mushroom,” “crimini,” and “portabella” (there are a variety of spellings for portabella). That’s right, all of these are actually the same mushroom! Crimini and portabella mushrooms come from a more flavorful brown strain of A. bisporus. Portabellas are exactly the same strain as criminis but the mushrooms have been allowed to mature. A. bisporus is commercially cultivated in large indoor facilities. It is a secondary decomposer, so the substrate it is to be grown on needs to be composted first and then purified of the primary decomposers. The growers then purchase “spawn” to inoculate the substrate with A. bisporus. The spawn consists of A.bisporus mycelium growing on cereal grain. The mycelium will grow through the substrate and eventually produce mushrooms. Once the substrate has been fully colonized a casing layer is added on top of the substrate. The casing layer has two purposes: to keep in moisture and to restrict nutrients to mycelia growing in the casing layer. Both of these encourage the fungus to switch from a vegetative to a reproductive mode of growth. Once the fungus colonizes the casing layer, mushrooms start to fruit. Mushrooms fruit in flushes, where the entire organism fruits simultaneously. These flushes occur every three to five days or so and three to five crops of mushrooms are picked before the crop is replaced with a fresh one. One benefit of producing mushrooms indoors is that the temperature, humidity, and carbon dioxide concentration can be controlled and optimized for each growth phase. I said last week that A. bisporus is not humongous, but could be considered huge. An individual’s size is limited by the size of the tray and amount of substrate. Because the substrate is sterilized before the spawn is added, the mycelium likely has more biomass than a similarly-sized individual in the wild. One of the interesting things about commercial A. bisporus is that most strains are genetically identical. There are two reasons for this. The first is that growers purchase spawn from other companies. The second is that mushrooms are very easy to clone. All you have to do is take a sample from a mushroom (that you purchased from your competitors at the grocery store) and use it to inoculate something to use as spawn.
For more than you probably ever wanted to know about Agaricus bisporus and its commercial production, see these sites: