Methods

Pasteurisation Techniques for Bulk Substrates

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Pasteurisation Techniques for Bulk Substrates

Look, everyone wants sterile substrate. But do you actually need it? Honestly, mostly no. Pasteurisation isn’t about nuking everything. It reduces microbial populations WITHOUT eliminating all organisms. Creates a selective environment where your fungus dominates. It’s the standard for bulk substrates.

Why Pasteurise Not Sterilise?

Sterilised substrate equals biological vacuum. First contaminant spore that lands colonises freely. Ever tried fighting trich in a sterile jar? Yeah, doesn’t go well. Pasteurised substrate retains heat-tolerant beneficial bacteria that compete with contaminants. Called “competitive exclusion” or “biological buffer.”

We see people sterilising straw all the time and wondering why they get contaminants. It’s because you killed the good guys too.

When to Pasteurise vs When to Sterilise

CriterionPasteurisationSterilisation
Substrate nutrient densityLow to moderate (straw, coir, manure)High (grain, supplemented sawdust)
SupplementationNone or lightModerate to heavy
Working environmentNon-sterile (open room)Sterile (flow hood or SAB)
Target speciesAggressive colonisers (Pleurotus, Stropharia)Slower colonisers (Hericium, Ganoderma)
Equipment neededLarge pot, drum, or vesselPressure cooker or autoclave

Rule of thumb: C:N ratio above 60:1 can pasteurise. Below 40:1 must sterilise. So if you’re working with coir, why are you buying a pressure cooker? Save the sterilisation for the high nutrient stuff where contaminants have a feast waiting.

Hot Water Immersion

Temp 65-82C (ideal 71-77C). Duration 60-120 min (best 75-90). Below 65C doesn’t kill enough. Above 82C kills beneficial bacteria too. It’s a narrow window but you’ll get it.

Temperature and Time Parameters

ParameterMinimumOptimumMaximumNotes
Water temperature65C71-77C82CMeasured at substrate core, not surface
Immersion duration60 min75-90 min120 minTiming begins when core temp is reached
Substrate : water ratio1:3 (v/v)1:4-1:5 (v/v)1:6 (v/v)Enough water ensures thermal mass

Protocol for Straw

  1. Chop to 50-100mm. Garden shredder or chaff cutter. Shorter lengths hydrate more uniformly.
  2. Load into mesh bag in vessel. Mesh bag makes drainage easier.
  3. Heat water to 77-82C. Slightly above target to account for heat loss when substrate goes in.
  4. Submerge fully. Weight down with lid, bricks, or metal plate. Air pockets create cold spots.
  5. Monitor core temp. Probe thermometer into the centre. Timing starts when core hits 65C+.
  6. Maintain 60-90 min above 65C. Wrap the drum in old sleeping bags or insulation to reduce heat loss.
  7. Drain. Cool to below 30C before spawning. Hot substrate kills mycelium.
  8. Field capacity test. Squeeze a handful firmly. Few drops of water = perfect. Dripping freely = too wet.

Common Errors

  • Insufficient water volume. Not enough thermal mass, temp drops below 65C. Use at least 3:1 water to substrate.
  • Cold spots. Air pockets in tightly packed straw. Agitate gently after loading.
  • Over-pasteurisation. Above 82C kills the beneficial bacteria. Then it behaves like a badly sterilised medium. Vulnerable without the microbial buffer.
  • Spawning too hot. Step 5 is where people mess up. You gotta monitor core temp. Surface temp means nothing if the inside is cold. And don’t spawn too hot. Ever tried spawning into hot substrate? Cooks the myc. Wait until it’s below 30C.

Cold Water Lime Bath

Hydrated lime Ca(OH)2 at 1-2g/L, pH 12-12.5 destroys cell membranes. No heat needed.

Protocol

  1. Prepare lime solution. Dissolve builders’ lime (NOT agricultural lime) at 1-2g per litre. For a 200L drum, that’s 200-400g of lime. Water goes milky.
  2. Submerge straw. Full submersion, weight down.
  3. Soak 16-24 hours. 12 hours minimum for finely chopped substrates.
  4. Drain 2-4 hours. Residual pH will be 8.5-9.5, which gradually neutralises as mycelium produces organic acids.
  5. Test pH (recommended). Should be 8.0-9.5. Above 10.0 means excess lime, which may inhibit growth. Rinse briefly if needed.

This is EXACTLY what you need for outdoor grows. No propane, no heating elements. Just lime and water. The pH spike kills the bad stuff while you sleep. Basically free pasteurisation. Works great for Pleurotus species which tolerate the elevated pH. Not great for Agaricus.

Steam Pasteurisation

Atmospheric steam at 60-82C for 6-12 hours. Produces substrate at the correct moisture content without needing to drain.

Drum Method (20-50kg batches)

  1. Hydrate substrate to field capacity (65-75% moisture) before steaming
  2. Load drum. Perforated false bottom 100-150mm above bottom of 200L steel drum
  3. Add 20-30L water below the false bottom
  4. Seal and heat. Tight lid with small vent (not airtight). Propane burner or electric element.
  5. Monitor core temp. 65-82C for 90-120 minutes.
  6. Cool below 30C before spawning. Sealed drum protects from airborne contam.

Thermometer Selection

InstrumentAccuracySuitability
Dial probe±2CBasic, adequate for hot water baths
Digital probe±0.5CRecommended for all methods
Thermocouple with logger±0.3CIdeal for process validation
Infrared±2C (surface only)NOT suitable. Surface isn’t core temp.

See that infrared row? Don’t bother. Surface temp isn’t core temp.

Substrate-Specific Notes

  • Straw: Classic. C:N 75-100:1. Hot water 71-77C for 60-90 min. Chop to 50-100mm.
  • Coco coir: Bucket tek. Boiling water in insulated container, 4-8 hours. Dead simple and remarkably effective.
  • Hardwood pellets: Same bucket tek method. 1.5-2 parts boiling water to 1 part pellets. They expand into fine sawdust at field capacity.
  • Manure: Phase I composting at 60-80C for 7-14 days first, then Phase II at 57-62C for 6-8 hours. Don’t try to pasteurise raw manure without composting. Smells awful and invites contaminants.

Post-Pasteurisation

Spawn within 2-4 hours of reaching 30C. Why the rush? Because once it cools, contaminants start landing again. You want your myc established before the bad stuff wakes up.

  • Basic hygiene. Clean surfaces, tools, and hands. Not full sterile technique, but don’t be a slob.
  • 10-20% spawn rate. Rapid colonisation outpaces recovering contaminants. Up to 25% for sketchy conditions.
  • Seal and incubate at 22-27C until full colonisation.

So get your spawn ready before you start pasteurising. Right, get cracking then.

Tags pasteurization substrate preparation hot water lime bath

References

  1. Stamets, P. (2000). Growing Gourmet and Medicinal Mushrooms. 3rd ed. Ten Speed Press.
  2. Stamets, P. & Chilton, J.S. (1983). The Mushroom Cultivator: A Practical Guide to Growing Mushrooms at Home. Agarikon Press.
  3. Oei, P. (2003). Mushroom Cultivation: Appropriate Technology for Mushroom Growers. 3rd ed. Backhuys Publishers.
  4. Sánchez, C. (2010). Cultivation of Pleurotus ostreatus and other edible mushrooms. Applied Microbiology and Biotechnology, 85(5), pp. 1321–1337.
  5. Chang, S.T. & Miles, P.G. (2004). Mushrooms: Cultivation, Nutritional Value, Medicinal Effect, and Environmental Impact. 2nd ed. CRC Press.
  6. Zadražil, F. (1978). Cultivation of Pleurotus. In: The Biology and Cultivation of Edible Mushrooms. Academic Press, pp. 521–557.
  7. Royse, D.J., Rhodes, T.W., Ohga, S. & Sanchez, J.E. (2004). Yield, mushroom size and time to production of Pleurotus cornucopiae (oyster mushroom) grown on switch grass substrate spawned and supplemented at various rates. Bioresource Technology, 91(1), pp. 85–91.
  8. Naraian, R., Sahu, R.K., Kumar, S., Garg, S.K., Singh, C.S. & Kanaujia, R.S. (2009). Influence of different nitrogen rich supplements during cultivation of Pleurotus florida on corn cob substrate. The Environmentalist, 29(1), pp. 1–7.