To prepare a substrate, wheat straw is shredded into pieces from 4 to 6 cm in length for easy handling during pasteurization and bagging. Crushing straw might be beneficial. It is soaked in water for 6 to 12 hours at room temperature, drained and then mixed with 2 to 10% (dry weight) gypsum. Several supplements containing materials that are lacking in wheat straw itself are added to provide sufficient nutrients for shiitake mycelium. As a nitrogen and oligoelements source, soybean flour added at 4 kg per ton can increase yield by 30%, but soybean flour is more likely to lower substrate selectivity.
Wheat straw must be pasteurized in order to kill possible competitor microorganisms as well as insects in the straw. Another goal of pasteurization is to propagate thermophilic microorganisms that will improve substrate selectivity by immobilizing readily available nutrients to competitors and by producing toxic or inhibitory molecules to limit rapid growth of competitors. The substrate mixture is placed in containers or directly in special rooms for pasteurization with steam at 65 ºC for 12 to 24 hours and then cooled to room temperature. Water content of substrates after pasteurization must be about 70%.
fter cooling, pasteurized substrates are mixed with spawn in a clean environment. Aseptic conditions are not necessary because the substrate is not sterilized. In order to improve competitiveness of shiitake during the first days after spawning, mycelium in spawn must be vigorous, adapted to components of the substrate and able to colonize all particles. For these reasons, Delpech and Olivier (1991) recommended limiting use of supplementation in wheat straw substrates to prevent growth of bacteria and molds. Spawn must be mixed with sterilized or pasteurized straw at 5% (w/w) and the mixture should be placed in plastic bags that are lightly perforated or equipped with a microporous filter. Incubation must be carried out at 25 ºC ± 2 with a 12-hour light and 12-hour dark cycle that is recommended at least for shiitake cultivation on wheat straw for 1–2 months depending on the strain. At the end of the incubation period, the entire surface of the substrate turns brown, indicating that mycelium is ready for fructification.
After the incubation period, plastic bags are removed and substrate blocks are sprinkled with cold water. Room temperature must be adjusted to 17 to 19 ºC. A relative humidity of 90% and a cycle of 12 hours light/12 hours dark are necessary to encourage mushroom development. After obtaining the first harvest, blocks can be rehydrated to induce a second flush by soaking them in water for 12 hours . Although it is not a general rule, soaking substrate is a method that is frequently used since mycelial growth compacts the substrate and hinders hydration of the samples. On the other hand, the dark external layer, also known as pseudosclerotium or pellicle, protects the samples from dehydration but at the same time hinders absorption of environmental humidity and causes samples to float during soaking. Generally, the first flush of mushrooms obtained from wheat straw substrate is the most abundant with 50- 70% of the total harvest, depending on strain and growing conditions.
