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Under-sterilized grain is the most common reason spawn contaminates. Pick what you’re sterilizing and how many jars or bags are in the load to get a time at 15 psi, with a pressure adjustment if you live at altitude.
Sterilization time chart (15 psi / 121 °C)
| Item | Time at pressure |
|---|---|
| Quart (1 L) grain jars | 90 min |
| Half-gallon (2 L) grain jars | 120 min |
| 3 lb (1.4 kg) grain bags | 150 min |
| 5 lb (2.3 kg) Master’s Mix / sawdust bags | 150–180 min |
| Agar, 500 ml | 30–45 min |
| Liquid culture, 500 ml | 20–30 min |
Why altitude matters
A dial gauge measures pressure relative to the air around it. At altitude the outside pressure is lower, so the same gauge reading gives a lower temperature. Home canning guidance raises dial-gauge pressure above 1,000 ft; weighted gauges are typically run on the 15 psi weight at altitude.
FAQ
Can I sterilize grain in a normal pot?
Boiling tops out at 100 °C and won’t kill bacterial endospores. It can work for pasteurizing straw, but grain and supplemented sawdust need a pressure cooker.
How long do sterilized jars keep?
Sealed with filter lids and kept clean, most growers inoculate within a few days to a couple of weeks. Always inoculate once jars are fully cooled.
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