Lime Pasteurising Sawdust Pellets for Mushroom Growing

Cold water lime pasteurisation of sawdust pellets is an easy, affordable and efficient method of preparing bulk substrate for the growing of vigorous mushroom species like oyster mushrooms, using readily available materials. Sawdust pellets are in general a very clean substrate with low levels of mould spores or chemical residues.

This method utilises the high alkaline pH of hydrated lime (calcium hydroxide) to suppress the germination of mould spores in the substrate while allowing for growth of mushroom mycelium. The concept is very similar to lime pasteurised straw, however in this case, the sawdust is hydrated with a measured amount of lime water to reach the desired hydration, rather than being soaked and then drained as with straw. This also eliminates the need for disposal of excess lime water.

After the sawdust pellets have been hydrated with the lime water, the substrate is mixed with mushroom spawn, packed into bags or buckets, and then incubated until fully colonised before being placed in fruiting conditions.

Materials Needed

• Sawdust pellets (fire or BBQ pellets available from hardware stores etc.) In NZ the most readily available are pine fire pellets, which work well

• Hydrated lime (calcium hydroxide) 3g per litre of water used

• Mushroom spawn (available from MycoLogic website)

• Water (clean tap water is best)

• Scales or measuring jug / spoons for measuring out substrate, water and lime

• Clean storage tote or bin for hydrating and mixing substrate in

• Clean ‘chip scoop’ or trowel for loading substrate into bags

• Mushroom growing bags or clean buckets with holes drilled in to load the inoculated substrate into for incubation and fruiting.

• A clean workspace away from any likely sources of moulds and contaminants

Method

1) Calculate how many kg of sawdust pellets you will need, depending on how much spawn you have and how many bags or buckets you want to make. One kilogram of sawdust pellets turns into about 3kg, or 5 litres of finished substrate (depending on hydration and spawn rate). Check the substrate table for more info.

2) Weigh out or measure your sawdust pellets and put them into your mixing tub.

3) Measure out the required volume of water for the amount of sawdust pellets, and also measure out the proportionate amount of hydrated lime. Tip the lime powder into the water and stir it, wait a few minutes, then stir again, to ensure that the lime is dissolved in the water.

Safety Note: Hydrated lime, while an ‘organic input’, is very alkaline and can be an irritant to skin, eyes and airways. Avoid touching lime water or powder with bare skin, and avoid breathing dust when handling powder. Consider using gloves and a mask when handling.

Spawn rates suggested in the table are flexible. Higher spawn rates will result in faster colonisation and lower chances of contamination, the nutrient in the grain also serves as a supplement to boost ‘yield per kg of substrate’. Lower spawn rates are more risky for contam but can also result in higher overall mushroom yield with more substrate being used per kg of spawn.

4) Pour the lime water over the sawdust pellets. Allow to hydrate for about 30 minutes. After this time the substrate should be hydrated to a level where it feels moist but not gluggy. If the substrate feels overly dense with water or if water comes out when a handful of it is squeezed, add a few more dry sawdust pellets to soak up some excess. If the substrate feels too dry, add a bit more water.

5) a) If bulk mixing in a tub. With your desired amount of spawn on hand, break it up inside the bag so that the grains are separated. Sprinkle this over the substrate. Use your clean scoop or trowel to mix the grains evenly through the substrate. Then, scoop the inoculated substrate into your bags or buckets, and tamp the substrate down so that it is dense and uniform, not loose or airy. Seal the tops of the bags by twisting, then folding over and cable tying (or heat sealing).

5) b) If mixing in filter patch bags, close the end of the bag by twisting it over and cable tying tightly (or heat seal), then mix the grain spawn through the substrate by flipping and shaking the substrate within the bag, before tamping it back down into a block shape, avoiding air gaps around the sides of the bag.

6) Incubate your bags or buckets in a clean, dark space at room temperature (15 to 25°C) for about 7 to 14 days (until mycelium has spread throughout the substrate), then move into fruiting conditions. The top part of the bag can be folded over to remove the air space, and an X cut about 10cm wide can be made in the front face of the bag for mushrooms to fruit from.

Sawdust pellets and grain spawn

1kg of sawdust pellets in a filter patch bag, with 1.8L of hydrated lime water ready to add

Sawdust pellets after hydration, ready to have grain spawn added

Mixed substrate colonising with mycelium in filter patch bag

Colonised block with air space removed, ready for X cut for fruiting

Native phoenix oyster mushrooms fruiting from a sawdust block