Sterilization Methods and Key Operation Points for Edible Mushroom Stock Spawn

Edible mushroom stock spawn is the intermediate spawn obtained by expanding and propagating the mother culture, used for transferring to commercial spawn; it mostly uses wheat grains, corn kernels, sawdust or cottonseed hulls as the growing substrate, which is sterilized after being filled into bottles or bags. Sterilization of stock spawn is a key step in the spawn production process; only by thoroughly killing the bacteria, molds and their spores in the substrate can a pure environment be provided for mycelial growth, reducing the risk of contamination at the source.

Two methods are commonly used in production: high-pressure steam sterilization and atmospheric-pressure steam sterilization. Grain substrates such as wheat grains and corn kernels are rich in nutrients and carry a high microbial load, so high-pressure sterilization is generally used, maintaining the specified pressure and temperature for a sufficient time so that even the core of the substrate is thoroughly sterilized. Formulas based on sawdust and cottonseed hulls can be sterilized either by high pressure or by atmospheric-pressure steam sterilization; the atmospheric-pressure method requires a suitably extended heating time and can equally meet the sterilization requirements of stock spawn production.

During operation, the cold air in the sterilization pot should first be completely expelled, otherwise false pressure easily occurs, causing the actual temperature to fall short of the requirement. The substrate should not be packed too tightly or too full, and the cotton plugs or lids at the bottle mouths and bag openings should be given waterproof protection to prevent moisture from encouraging contaminant growth. After sterilization is finished, the pressure should be released slowly to lower the temperature, avoiding breakage of bottles and bags caused by a sudden pressure change, and then they should be moved while still hot into a cooling room or other clean area for later use.

If sterilization is incomplete, bacterial contamination or infection by contaminants such as Trichoderma and Neurospora is likely to occur during the incubation of stock spawn, and in severe cases an entire batch of substrate may be ruined. Therefore, after each sterilization, indicator testing and retained-sample observation can be combined to check the results, and only together with strict inoculation operations and environmental hygiene management can high-quality stock spawn be reliably cultivated.