Grain spawn in bags

What it is

Filter-patch bags are heat-tolerant plastic bags with a breathable patch. Growers use 5 lb and 10 lb filter-patch bags for spawn and substrate alike¹. The grain inside is prepared and sterilised just as it would be in jars².

The advantage is shaking. Cornell’s guide describes "through-spawning", shaking the bag after inoculation so spawn is spread through it, which shortens the spawn run¹. In one accidental trial at Fungi Ally, through-spawned bags were fully colonised after 14 days while top-spawned bags took until day 24¹.

What it suits

Every species, at a scale where jars get tedious.

Equipment

• Pressure cooker or larger steriliser
• Filter-patch spawn bags
• An impulse heat sealer, or a reliable fold-and-clip method
• Whole grain
• Clean agar or liquid culture, still air box or flow hood

Time to first harvest

10–20 days to colonise. Grain spawn colonises in about 10 to 20 days²; shaking after inoculation shortens the run¹.

Contamination and other risks

• Grain contaminates easily and needs lab-like conditions¹.
• Torn or melted filter patches from contact with the cooker walls.
• Leaving spawn on top instead of mixing it through gives contaminants more time to take hold¹.

Steps, in overview

1. Prepare grain. Soak and drain as for jars², then fill bags with the patch above the grain.
2. Sterilise. Pressure-sterilise, keeping bags off the cooker walls.
3. Inoculate and seal. In the still air box or flow hood, add culture and seal the bag.
4. Shake. Mix the culture through the grain¹.
5. Incubate. Keep warm and dark; shake again once growth is visible.

Common mistakes

• Overfilling so the bag cannot be shaken.
• Letting bags touch the side of the cooker, which can melt the patch.
• Skipping the shake¹.