One of the earliest US patent issued relating to the cultivation of mushrooms in plastic bags, in 1958 by Mr. Guiochon of France. Described is a method of mushroom spawn production by incubation of inoculated substrate in plastic bags. It relates the sterilization of the substrate, inoculating of mushroom culture, venting of the bag with cotton, and incubation of the bag until use. Based on this principle, improvements on plastic bag construction, materials and sophisticated venting means on plastic bags, methods of sealing and introduction of inoculum, have continued until today. The "collar and cotton wool" method is still profusely used by many parts of the world, especially countries with low labor cost.
The general term "plastic bags" can be better specified in two grades of polyolefin, namely Low density polyethylene (PE) and polypropylene (PP). These two materials are still the least expensive, and the most produced in the world.
PE has a low melting point, and PP has a higher melting point. In general PE becomes useless at temperatures above 90 degrees, while PP can withstand up to 130°C. A variation of PE is HDPE, or high density polyethylene. This material has a higher melting point than regular PE and can withstand up to 108°C. HDPE has temperature advantage over PE, but it has the disadvantage of having a heavy haze and does not allow as clear viewing properties as PE, therefore most growers dislike using this material, as contamination is not easily identified.
Synthetic log production
Sawdust is the base ingredient used in the formulations of “synthetic substrate” for producing many gourmet and medicinal saprophytic mushrooms. Other base ingredients may include straw, corncobs, or combinations of these.
Starch-based supplements such as wheat bran, rice bran, millet, rye, and maize are sometimes added to the mix. These supplements serve as nutrients to create an optimum growing medium. Other supplements, added in lesser quantities, include chalk, gypsum, and sugar.
Water is added to raise the moisture content of the mix to about 60 percent. Bags that are made of heat-resistant PP, sometimes containing a special breather patch or fitted with collar and cotton filter, are used to sterilize. Bags made of PE may only be pasteurized.
The micropores in the filter allow the substrate to “exhale” carbon dioxide while not “inhaling” any bacteriological or fungal contaminants. The filled bags are stacked on racks, loaded into an industrial-sized autoclave and sterilized for 2-4 hours at 121°C or pasteurized for 8-12 hours, cooled in a clean room, and then inoculated with spawn.
An alternative method of substrate processing and spawning is to heat-treat, cool, inoculate, and then aseptically bag the substrate.
If through-mixing of the spawn into the sterilized substrate is the method used, an 18-60 day spawn run at 21-23°C is all that is required to ensure optimum growth. With this method, the bags are sliced open and removed after the completion of the spawn run, leaving “blocks” or “logs” that are held together by tiny tendrils of mycelium that have pervaded the substrate.
The production cycle for synthetic-medium cultivation takes approximately 2 to 6 months from inoculation to cleanup. Biological efficiency for this method averages from 75 to 125 percent. In contrast, the natural-log cultivation cycle usually takes about 2 to 6 years with a maximum biological efficiency of around 33 percent. The total time required for production on synthetic substrate is about 6 percent of the natural system cycle time, with about three times the yield efficiency. | 
