Best Practices For Bulk Pasteurizing With Boiling Water Pour-Over?

Our current process for Kiez Pilz involves pasteurizing bulk-substrate with the following process:

  • pickup coffee grounds from a local coffee shop, produced day of
  • large sacks of sawdust from a local woodshop that have been sitting in the basement for months
  • mix 10% coffee, and 80% sawdust together into 28L coolers that look like this

    [image of blue cooler]
  • pour boiling water into the cooler, nix it all up quickly with a spoon, and then let it sit over night. We pour water at a volume of 40% of the total substrate volume, which we found leaves the substrate close to field capacity
  • then inside of a (somewhat goofy) giant still air box, we mix spawn into the cooled substrate, frequently spraying the air and our hands with 70% alcohol

    [image of giant still aribox, made of a rectangular wood frame and plastic sheeting with arm holes]

Using this process, with some small variations, we’ve had 3 successful bulk innoculations, and 2 bulk innoculations where everything was contaminated with green mold.

I’m sure there is some small tweaking we could still experiment with,
… how much coffee, how frequently we spray alcohol, how well sealed is the still air box, do we let the substrate sit in the cooler over night or just for a couple hours etc…

but I’m wondering for any advice on our process, and how to make it more reliable and reduce contamination.

I particularly wonder about the pasteurization.

  • is it important that all the substrate is wet and hot? or just that there is steam and its hot in there?
  • should we be looking into some way to submerge the substrate completely in boiling water and then strain it afterwards? (I’m not sure how we would do this easily in our space)
  • are there other pasteurization techniques that could be practical in the city?

If we only have 3/5 success that is not a great rate. Maybe we just need a little fine-tuning, but posting this here with the hope to sanity check our approach.

We may also get access to a large clean bench soon (which a lab is getting rid of for free) which would give us a nice chance to test if we have better results doing the innoculation with the clean bench.


Do you have access to a meat thermometer? That will allow you to check the temperature of the substrate after one hour and ensure that the core is reaching a sufficient temperature for pasteurization. You might find that there are pockets of sawdust which are not receiving sufficient heat due to lack of hot water / steam penetration.

When I’ve worked with sawdust in the past I’ve always pre-hydrated it to field capacity and then packaged it into heat-resistant plastic bags for hot water pasteurization (for example, in a big pot). Another approach is to hydrate the sawdust and add it to a large pillowcase to be placed in the pot (then you can hang it or leave it in a sink to drain overnight). You may want to consider these methods (hot water in pot) if you’re unable to improve your results using the pour-over approach.

Another question: what spawn ratio are you using? You might be able to outrun any contams by increasing the amount of spawn you’re using.

Personally, I wouldn’t bother including the coffee for (re)pasteurization if you’re using it on the same day you’re getting it from the source. As long as you’re keeping it in a clean and sealed container it should be good to go.


@glyph <3 thanks for the tips…will see if I can dig up a meat thermometer somewhere

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