Mushroom Substrates: Pasteurisation vs. Sterilisation

The difference between pasteurisation and sterilisation confuses many new mushroom growers. Getting it right is key to a successful harvest, whether you’re growing with a >> DIY Oyster Mushroom grow kit in your NZ backyard or running a full-scale mushroom farm.

Correct substrate processing is critical, but with a few basic rules, it’s easy to do.

This blog’s tips will boost your success as a grower by guiding you to the best substrate prep method. You’ll learn how to choose based on your equipment, mushroom species, and production goals for your family or community.

Where the magic happens at Sporeshift: Sterilised grain spawn prepped for mushroom growing success!

Why Substrate Prep Matters

Choosing between pasteurisation and sterilisation is critical for a bumper mushroom crop.

Pasteurisation suits Oyster mushrooms, which are hardy and thrive with beneficial microbes -- Oysters are forgiving to work with, and tolerate an alkaline environment.

For Shiitake, Lion’s Mane, and other species, sterilisation is essential—leftover microbes can cause mould, ruining your harvest. I’ve seen new growers lose crops by skipping sterilisation! Proper substrate prep reduces contamination, boosts yields, and delivers nutrient-dense mushrooms for your table or community.

Pasteurisation Vs Sterilisation - The Basics

Pasteurisation and sterilisation both use heat or alkalinity to clear unwanted bacteria and microorganisms from mushroom substrates, but their methods, intensity, and results differ.

Pasteurisation reduces harmful microbes while keeping beneficial ones, perfect for Oyster mushrooms. Sterilisation wipes out all microbes, ideal for trickier species like Shiitake and Lion’s Mane.

Pasteurisation:

• Purpose - Pasteurisation reduces unwanted bacteria, yeast, and microorganisms that compete with your mushrooms for nutrients (usually sawdust or compost). This gives your mushroom spawn a strong edge to thrive.

• Method 1 - Steam or Hot Water Submersion - Heating at atmospheric pressure to bring the substrate to 70-99°C for 1-12 hours. Ideal for New Zealand beginners using basic equipment.

• Method 2 - Increasing Alkalinity - Using hydrated lime or wood ash to raise the substrate’s alkalinity, reducing harmful microbes.

• Method 3 - Anaerobic Fermentation - Submerging the substrate in cold water to ferment, eliminating unwanted microorganisms without heat.

• Outcome - When done right, pasteurisation kills most harmful microorganisms while keeping beneficial ones. This lets >>mushroom grain spawn colonise the substrate and grow edible mushrooms. Pasteurised substrates are typically packed into >>plastic bags with holes or into buckets.

• Species Match - All Oyster Mushrooms, King Stropharia outdoor garden beds.

• PROS:

  • Cheapest way to process substrate, especially for oyster mushrooms.

  • Can be done with no specialised equipment.

  • Pasteurised substrate can be inoculated in open air as the beneficial microorganisms keep unwanted species at bay for up to 2 weeks.

  • The best way for beginners to start growing bulk mushrooms cheaply using >>grain spawn.

• CONS:

  • Only works reliably for cultivating Oyster mushrooms,

  • Can be a bit water intensive.

  • Does not work for grain spawn production.

Sterilisation:

• Purpose - Sterilisation wipes out all unwanted bacteria, yeast, and microorganisms in the substrate, ensuring no competitors threaten your mushroom crop.

• Method - Autoclave or pressure cooker heat sterilisation at 15psi. The substrate is cooked for 1.5 hours, with the timer starting when the core reaches 121°C. For large cookers, an 8-hour cook at 15psi may be needed, while small home pressure cookers in New Zealand typically require a 3-hour cook to fully sterilise. Substrate is placed inside mushroom >>filter patch bags.

• Outcome - Sterilisation fully cleans the substrate, leaving no competing microorganisms. This creates a sterile environment for mushrooms, guaranteeing reliable yields.

• Species Match - All Oysters, Shiitake, NZ Lion's Mane, Tawaka, Enoki, Turkey Tail.

• PROS:

  • Ideal for grain spawn and culture work.

  • Required to grow more exotic species such as our Native Lion's Mane, Shiitake etc.

  • Gives a better yield than pasteurised substrate.

  • Offers a far more controlled and predictable incubation time and fruiting schedule.

• CONS:

  • Requires more equipment such as pressure cookers and >>laminar flow hoods.

  • Sterilised substrate is more prone to contamination after sterilisation so inoculations must be done in a >>still air box or in front of a laminar flow hood.

  • More energy intensive than pasteurisation

Super Pasteurisation

• Purpose - Super pasteurisation nearly wipes out all unwanted bacteria, yeast, and microorganisms. Sitting between pasteurisation and sterilisation, it’s ideal for growers aiming to cultivate all species. This is the method most mushroom farms use today.

• Method - Atmospheric steam cooking at 95-99°C for extended periods (12-24 hours). Substrate is placed in >>filter patch bags.

• Outcome - Super pasteurized substrates have minimal microorganisms, creating a near-sterile environment. This promotes strong mushroom growth with low contamination risk.

• Species Match - All Oysters, Shiitake, NZ Lions Mane, Tawaka, Enoki, Turkey Tail.

• PROS:

  • An easy way to process larger volumes of substrate.

  • Allows you to grow species that will not grow on pasteurised substrate.

  • More economical way to process substrate compared to pressure cooking.

  • Can give slightly better yields than pasteurised substrates for oyster growing.

• CONS:

  • Requires building a steamer/ boiler.

  • Need a laminar flow hood to perform inoculations after cooking.

  • Does not work for grain spawn.

    
    

Pasteurising Mushroom Substrates

Pasteurisation is my top pick for growing Oyster mushrooms—it’s affordable, reliable, and yields great results with minimal gear. The three main methods are steam/hot water, hydrated lime, and anaerobic fermentation, perfect for beginners and experts alike.

I focus on hydrated lime, the cheapest and most reliable method in my experience. Sporeshift was growing 100kg of Oyster mushrooms weekly using this technique, with less than 1% contamination on cereal straws and >>firewood pellets, ideal for NZ growers chasing top results.

Straw Processing:

Hydrated Lime:

Check out my blog >>How to Grow Bulk Oyster Mushrooms on pasteurised straw for the full details on processing straw with hydrated lime.

Hydrated lime is my go-to for pasteurising straw and firewood pellet substrates for Oyster mushrooms—it’s cheap, reliable, and easy, needing no special gear. NZ growers can grab >>hydrated lime from Sporeshift’s shop.

Hot Water:

Straw can be pasteurised with hot water by heating water to 75-85°C and submerging the straw for 1-2 hours. Remove the straw, let it cool and drip dry for a few hours, and it’s ready to inoculate with grain spawn for packing into bags or buckets. This simple method is perfect for growers starting with Oyster mushrooms.

I personally started growing Oyster mushrooms this way, and it’s very reliable with great yields. However, hot water pasteurisation is energy-intensive—the water can only be used to soak straw twice before it becomes toxic from the straws tannins and must be discarded. You’ll use a lot of LPG when heating large volumes of water, so it’s only worth considering for smaller-scale growers.

Anaerobic Fermentation:

Anaerobic fermentation uses organisms to clean the substrate. The basic method is to submerge straw underwater for 4-5 days, during which anaerobic microorganisms rapidly grow and consume unwanted aerobic microorganisms. After 4-5 days, pull out and drain the straw; once exposed to air, the anaerobic organisms die off, leaving a pasteurised substrate ready for Oyster mushroom inoculation.

This method is easy and affordable, but it’s not my favourite. Anaerobic fermentation is slow, and the straw gets smelly, making it less pleasant to work with when mixing in grain spawn and bagging. It’s also limited to straw substrates for Oyster mushrooms only.

Wood Pellet Processing:

Wood pellets are simple to pasteurise—just add hydrated lime to the water used to hydrate them. Boiling water or anaerobic fermentation won’t work, as pellets absorb too much water and don’t drain properly for Oyster mushroom substrates.

Fire wood pellets available from Bunnings, Mitre10 and most supermarkets.

Growing mushrooms on wood fire pellets is easy and reliable. Our >>Oyster DIY kits teach you how to pasteurise wood pellets with hydrated lime for Oyster mushrooms.

Read my detailed blog on growing in buckets and bags with the hydrated lime and wood pellet method here: >>How To Grow Oyster Mushrooms in Reusable Buckets and Bags.

After seven years of mushroom cultivation, firewood pellets are my top choice. Any brand works, costing about $14 for 20kg. Skip pricey BBQ pellets for affordable Oyster mushroom substrates.

Sterilising Mushroom Substrates, Media and Utensils

Pressure sterilising substrates unlocks the full world of mushroom growing, with many species requiring fully sterilised substrates to grow at all. Sterilised substrates can also be supplemented with bran or soy hulls to increase yields, perfect for all mushroom we offer.

Agar and other mushroom culture work need pressure sterilisation, but the processing times vary depending on whether you’re sterilising utensils, agar, or liquid culture. Generally 30mins at 15psi is sufficient for this. Substrates will need 90mins at 15psi.

Sporeshift is New Zealand’s expert in sterilised substrates, with most of our farm dedicated to this specialty. We provide >>sterile grain, >>sawdust and >>compost to make growing any mushroom species easy, even if you don’t have expensive sterilisation equipment.

To reliably grow mushrooms beyond oysters, you’ll need 15psi sterilisation with a pressure cooker or autoclave, or super pasteurisation using a barrel steamer. These methods ensure success for any mushroom species.

When sterilising sawdust or compost-based substrates with pressure or atmospheric super pasteurisation, use filter patch bags to protect the substrate from outside air. After cooking and during incubation, sterile substrates can easily be contaminated by unwanted species like the common green mould, Trichoderma. Sealed filter patch bags let the substrate breathe while staying sterile.

Inoculate sterilised materials in a still air box or in front of a laminar flow hood to keep things sterile. Seal the filter patch bag after inoculation—don’t seal it before sterilising, as it will inflate like a balloon during cooking and burst.

Super Pasteurisation Mushroom Substrates

Like sterilisation, super pasteurisation involves packing the substrate into filter patch bags before cooking. The substrate is then steamed to 95–99°C and held at this temperature for 12–24 hours.

The simplest super pasteuriser to build is a barrel steamer—a steel barrel with a false floor at the base. Substrate bags are packed above the floor, while water below is boiled using an electric element to create steam that rises and heats the bags. A lid with a small hole prevents pressure build-up allowing the steam to flow out the top, keeping the process safe and effective.

If you made it this far you maybe interested in:

For anyone wanting to start a mushroom farm to supply their local markets we are happy to offer some advice. Growing oyster mushrooms on lime pasteurised substrates will be your bread and butter crop. Making bags weekly or fortnightly gives a constant supply of fresh mushrooms.

If you are interested in producing mushrooms for local supply fill out a >>contact us form and we will be happy to give some advice, we also offer commercial spawn at competitive rates.