Phase 2 of Mushroom Composting: Pasteurization, Conditioning and Microbial Selectivity
- Vivek Sinha
- Jul 19
- 3 min read
Introduction
Phase 2 of mushroom composting plays a critical role in transforming partially decomposed raw compost into a selective substrate suitable for mushroom cultivation. This phase involves two key steps: pasteurization and conditioning. Each step is crucial in ensuring the destruction of pests and harmful microorganisms, while promoting beneficial microbial populations that favour mushroom mycelium growth.
Pasteurization: Destruction of Harmful Organisms
Even after proper fermentation in Phase 1, compost contains several harmful organisms such as nematodes, mites, fly larvae, insect eggs, and competitive moulds. These often survive in the outer layers of the compost heap. Pasteurisation aims to eliminate these threats.
Method:
Pasteurisation is achieved by maintaining the temperature of both compost and air in the tunnel at 57–58°C for 5 to 6 hours, after an initial warming-up phase.
Key Considerations:
Temperature irregularities in tunnels can lead to incomplete pasteurisation. Better insulation, air distribution, and door sealing can minimise these variations.
Both temperature and duration are vital to eliminate pests effectively.
Excessively high temperatures or long durations in low oxygen conditions can:
Harm beneficial thermophilic organisms.
Cause excessive protein breakdown, leading to ammonia formation, which is undesirable.
Optimal Approach:
Use the minimum effective temperature and time needed to kill pests.
Avoid harming beneficial microbes or altering the nutrient balance.
Indicator Moulds:
Some moulds signal poor compost conditions caused by:
Improper compost formulation.
Overheating during pasteurisation.
Faulty conditioning.
Indicator moulds include:
Olive green mould
White and brown plaster moulds
Botrytis
Trichoderma
Ink caps
These moulds may promote the secondary growth of mites and nematodes, leading to misdiagnosis of initial pasteurisation failure.
Conditioning: Creating a Selective Substrate
Pasteurised compost must undergo conditioning to become truly selective for mushroom mycelium.
Objective:
To continue microbial fermentation under controlled conditions—optimising temperature, oxygen, and humidity—to create a substrate that supports mushroom growth while suppressing competitors.
Key Functions:
Conversion of Ammonia to Microbial Protein: Thermophilic bacteria transform nitrogenous compounds into protein.
Decomposition of Simple Sugars: Prevents competitor moulds from thriving.
Important Microorganisms:
Bacteria, Actinomycetes, Moulds
These microbes are thermophilic (warm-loving) and aerobic (oxygen-dependent).
Optimal development occurs at 45–55°C with proper oxygenation and moisture.
Microbial Indicators:
Actinomycetes: Appear as white speckles on straw during early conditioning.
Humicola: Greyish mould appearing later; gives compost a sweet smell.
These organisms contribute to microbe biomass—a nutrient source rich in polysaccharides.
Compost Appearance:
Straw shows a black, scrappable layer composed of:
Humicola hyphae
Actinomycete filaments
Polysaccharide slime matrix
Importance of Thermophilic Microorganisms
Due to the environmental conditions during Phase 1 and 2, thermophilic microbes dominate.
Microbial Contributions:
Bacteria (e.g., Pseudomonas):
Produce slime rich in polysaccharides.
Serve as food for mycelium.
Actinomycetes (e.g., Streptomyces thermovulgaris):
Degrade cellulose
Synthesize vitamins (thiamine, biotin, niacin, etc.)
Produce amino acids that stimulate mushroom mycelium
Thermophilic Moulds (e.g., Humicola, Torula):
Feed on bacterial biomass
Form humic substances (brown colour), essential for selectivity
Studies show:
Mushroom mycelium grows 3x faster with actinomycetes present.
Polysaccharides from microbe biomass are 6x more effective than glucose in supporting mycelium growth.
Formation of Selective Substrate
Selective compost allows mushroom mycelium to flourish while suppressing competitive organisms.
Selectivity Factors:
Formation of humic substances (dark brown materials).
Removal of ammonia and simple sugars.
Creation of microbe biomass (protein + polysaccharides).
Possibly, antibiotics produced by microbes suppress competitors.
Non-selective Compost Risks:
Weed moulds (olive green, Trichoderma, Penicillium)
Indicator moulds due to poor fermentation
Secondary pest development (mites, nematodes)
Note: Parasitic diseases like dry bubble, wet bubble, cobweb, and viruses are unrelated to compost selectivity.
Optimal Temperature Range
Different microbes have distinct temperature tolerances.
Summary:
Temperatures >55°C harm key microbes.
Conditioning at 45–50°C is ideal.
Pasteurisation above 60°C for too long inhibits compost selectivity.
Ventilation and Gas Balance
The success of Phase 2 depends not only on temperature but also on gas composition:
Oxygen is critical for aerobic microbes.
Carbon dioxide and ammonia must be removed.
Phase 1 vs Phase 2:
In outdoor Phase 1 heaps:
Oxygen is quickly depleted.
Compost becomes anaerobic except near the surface.
In Phase 2:
Controlled ventilation ensures aerobic conditions.
Promotes growth of beneficial microbes (bacteria, actinomycetes, Humicola).
Conclusion
A successful Phase 2 depends on balancing:
Pasteurisation: Eliminate pests without harming thermophilic flora.
Conditioning: Support microbial transformation to enhance selectivity.
Ventilation and temperature control: Create optimal conditions for microbial succession.
Key takeaway:
"Bad compost can’t be corrected in Phase 2, but a good compost can be ruined by poor Phase 2 management."
Careful monitoring of temperature, time, oxygen, and microbial indicators is essential to produce high-quality, selective compost ready for mushroom spawning.
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