This was the most technical and time consuming operation of the whole growing process and to produce enough material to fill three by 11 litre buckets took us the greater part of a day.
Mixing and hydrating the substrate
For this process you need about 30 litres of substrate. The recommended substrate from the book is one third chopped straw (I believe 100mm is the usual straw length of chopped straw) to two thirds hardwood sawdust, although I have been told that the exact proportions are not critical and there is some flexibility here so don’t get too uptight about it.
For us the straw was relatively easy as we keep it for the chooks, but the hardwood sawdust was a bit more problematic. This is going to be used to grow stuff we were going to eat and so needed to be sources in the same way you would soil to grow veggies in, it needed to be hardwood, not softwood and no pressure treated (CCA) sawdust. Also, I am cheap and my first thought was to get some for free from wood machinists/saw mill somewhere. This proved to be more difficult than I thought and the only stuff I could source which seemed able to do the job came in one metre cubed bulkabags. OK, they were free but would be difficult to transport (it was pick-up only) and store.
I had just about shelved the mushroom plan for lack of substrate when a mate shouted me a 75 litre bag of hardwood sawdust from a butcher’s supply place. While it did not cost too much for the bag, the cost of delivery from Melbourne more than doubled the total cost of the material. So a shout out to John for his generosity!
When looking at substrates for your mushroom projects I think Nick’s words should be borne in mind – “There are many options for container substrate, and it’s important for the long term resilience of your mushrooms that you choose a source that is locally available, preferably a waste product” (Milkwood Page 81). Fortunately, with a ready supply of sawdust provided for me, I can start experimenting now while I research local sources.
Preparing the Substrate for Pasteurising
Note: builders lime is an irritant so when handling it you should wear the dust mask, gloves and eye protection.
To prepare the substrate it needs to be mixed, limed and hydrated. To do this I measured out a full 10 litre bucket of straw and a full 20 litre bucket of the hardwood sawdust into a 60 litre plastic tub. I then weighed out 250grams of hydrated or slaked lime, also called builders lime. I got the lime from the green hardware but because it was builders lime not garden lime (Calcium hydroxide vs calcium carbonate) the only bag I could get was 20kg. I have enough lime for mushroom growing for the next 100 years!
With the solid materials in the tub I gave them an initial mix and then started to add water. It is important to add the water slowly because you need to add enough to moisten the entire batch of substrate but not wind up with a soupy mess! The way I did this was to add water in small increments, then mix, and repeat. The whole mass of substrate needs to be moistened to allow the heat to penetrate during the pasteurisation process, otherwise there may be pockets of unpasteurised substrate which can contaminate the whole lot once the substrate is inoculated and left for the mycelium to run.
All in the bucket
After water and mixing
Pasteurising the Substrate
The substrate has been formulated to be ideal for growing fungi, but the idea is to prevent as much as possible other fungi from colonising it so we can grow the species of fungi we want, in this case oyster mushrooms. For the substrate to be pasteurised it needs to be kept at a temperature above 60⁰C for in excess of two hours by soaking it in hot water. To contain the substrate while this is happening it gets placed in cotton pillowcases, in this case three pillowcases with 10 litres of substrate per pillowcase.
Heating the water
To pasteurise 30 litres of substrate we needed about 40 litres of fresh water heated to 80⁰C, and this proved to be one of the most problematic parts of the whole process. One of the hints is to use an electric urn, which is what I intended to do because after holding some workshops at our place we had picked one up so we could provide tea and coffee easily when required, and then filed that issue as sorted. Unfortunately, as the day grew closer I realised that the volume of the urn was 10 litres so I had to find another 30 litres of hot water from somewhere.
I had intended to start off with water from the solar hot water so at least I was not starting with dead cold water, but true to form, the days leading up to ‘mushroom day’ were cold. Rainy and sunless, leaving only tepid water in the hot water system. Yes I could have turned on the electric boost but that kind of defeated the purpose of using the solar.
I finally decided to use the urn, but to also heat the remaining 30 litres water I needed in a steel 50 litre drum I had, perched atop my rocket stove. That way I could use sticks and stuff I had gathered from the local park to heat the water. I had tried a similar stunt with a vacola bottling kit years ago, and it took forever and didn’t get much above 60⁰C due to (what I assumed to be) loss of heat through the sides. To reduce this I grabbed a spare glass fibre insulation batt, wrapped it around the sides of the drum and taped it on. It covered the majority of the sides so hopefully it would work!
The water started out at a balmy 15⁰C and while the rocket stove was starting out I filled and set the urn going. I set the 50 litre drum up and then slowly put 30 litres of water in it. It took a while but after half an hour the water was 25⁰C and the urn had boiled and been turned off. The my sweetie came up with the classic idea – take 10 litres out of the drum and put in the contents of the urn! Winner!
So I did that and refilled the urn and turned it on to reheat, the water in the 50 litre drum was now at 50⁰C and heating steadily, the insulation seemed to be doing its job. After about an hour the 50 litre drum was just over the 80⁰C mark and the urn had long since boiled and was keeping hot – we had hot water!
As a vessel to hold the substrate filled pillowcases I intended to use a cleaned out 55 litre plastic garbage bin I had been using as a storage container. When I put the pillowcases in I was a bit concerned at how much space they took up, would all the water fit? I moved the garbage can to where I had the rocket stove set up (I was NOT going to juggle 30 litres of hot water!) and placed it inside the 60 litre plastic tub. I did this for three reasons –
- It was a cold day and I wanted to see if the airspace between the garbage can and the side of the tub would act as an insulator to reduce energy loss,
- I was not totally sure about the structural integrity of the plastic garbage bin as it would have to have been at least 10 years old and if it sprung a leak the tub would contain it and hold it close to the substrate pillowcases, maybe allowing me to continue pasteurisation, and
- If all the water did not fit in the garbage can I would pour it into the tub so its heat could still contribute to the pasteurisation process.
I have a plastic dipper I use when topping up the batteries and I used that to transfer the water from the drum and the urn into the garbage bin as quickly as I could and in the end I was able to fit all the water in except for the last 2 or 3 litres, which dutifully went into the tub. I placed the lid onto the garbage can and then placed on top a couple of bean bag bean filled pillowcases I use as insulation in our stored heat cooker. This would hopefully keep the water in the pasteurising set up from falling below 60⁰C for two hours. Then, as Nick Ritar suggests, I had lunch!
After a nervous wait of two hours I removed the insulation cushions and the lid of the garbage tin and measured the water temperature, it was 68⁰C, we had pasteurisation!
Inoculating the Substrate
The next trick was to remove the soaking wet and still very hot substrate filled pillowcases, so I put the plastic washing up gloves on and washed my hands because cleanliness is important in preventing contamination. I pulled them out and squeezed them to remove as much excess water as I could and hung them up on hooks I had built in to the roof of the back deck to drain. I needed to move quickly to inoculate the substrate so I let them drain for 12 minutes only.
I set up a portable table in the back yard and wiped it over with vinegar as a disinfectant and left it to sit, and retrieved the pillowcases. I had removed the gloves by this stage and washed my hands thoroughly. I emptied the substrate out onto the table, it had cooled to the point where it only felt a bit warm to my hands. I spread it out so that it make a layer 75mm to 100mm thick.
I retrieved the spawn from the fridge and opened the pack and broke off chunks, crushing them up and distributing them over the surface of the substrate, then spent about 10 minutes mixing the spawn and substrate thoroughly together. When that was accomplished I packed the inoculated substrate into the fruiting containers and affixed the lids.
It’s winter here so I took the filled containers inside and placed them under our dining room table, so they would be somewhat warmer than storing them outside or in the garage or shed. It was then a 3 week wait to see how things had gone!
