The first stage of mushroom cultivation involves culturing a strain of the fungus you want to grow onto a medium you can work with. Making cardboard spawn is a very simple way of doing this. However, it only works for some species and it will contain other micro organisms and so cannot be used to inoculate a sterilised substrate. Instead, we can use a sterilised medium of enriched agar to culture a fungus in a petri dish. This can then be used to inoculate grain spawn or make liquid cultures under aseptic conditions.
Agar (or Agar Agar) is a seaweed extract that is used as a gelatin substitute in the food industry. It is also used in laboratories as a solidifying agent for a nutrified media to culture bacteria, fungi and other micro organisms. The recipes of nutrient enrichment can very in complexity but for fungal growth sources of sugar, starch and b vitamins must be added to the agar. This mixture is then sterilised to remove all source of competitive micro organisms. When hot the mixture flows like a liquid but will solidify when cooled. After sterilising and before it cools it must but poured into perti dishes. Once completely cooled the perti dish can be used, under aspetic conditions, to culture a species. The mycellium will grow over the surface of this solidified agar.
To read more about how to prepare and use agar media see: