Grain Spawn Calculator: Bags Needed and Sterilizing Time
Enter how much grain spawn you need and pick one published protocol. You get the number of bags that protocol's load size gives, plus the sterilizing treatment it states. Each protocol keeps its own time, and the calculator never borrows a time from another one.
Result
120 min at 15 psi
Utah State home grain bags, 2 to 3 bags for 5 lb of spawn
- Load
- Grain bag holding 2.25 to 3 lb
- Preparation
- Boil the grain carefully without bursting the kernels
- Treatment
- about 250 F and 15 psi for 120 minutes
- Colonization
- 2 to 4 weeks
- Bags
- 2 to 3 bags
Follow your pressure cooker's manual for water, venting, load and release. A time from one protocol does not carry over to a different vessel.
How the protocols compare
| Protocol | Load | Treatment |
|---|---|---|
| Utah State home grain bags | Grain bag holding 2.25 to 3 lb | about 250 F and 15 psi for 120 minutes |
| UF/IFAS seed spawn | Seed in spawn containers, no single load size given | 30 minutes at 15 psi, generally suggested |
| Penn State rye, 250 mL flask | 250 mL laboratory flask with its own grain and water formula | autoclave for 35 minutes at 121 C |
| Penn State rye, 1,000 mL flask | 1,000 mL laboratory flask with a different grain and water formula | autoclave for 45 minutes at 121 C |
How it works
Each Protocol Keeps Its Own Load and Time
Utah State's home method fills each grain bag with 2.25 to 3 pounds and pressure-cooks it at about 250 F and 15 psi for 120 minutes. The calculator divides your spawn weight by that bag size. For 5 pounds of spawn, you need 2 bags packed full or 3 lighter ones.
UF/IFAS soaks its seed for about 12 hours and generally suggests 30 minutes at 15 psi. It gives no single container size, so the calculator shows the UF/IFAS treatment without counting bags. That shorter time belongs to the UF/IFAS containers, and you cannot carry it over to a full Utah State bag.
Penn State's laboratory rye formulas tie each time to a flask size: 35 minutes at 121 C for a 250 mL flask and 45 minutes for a 1,000 mL flask. Those are autoclave formulas for a lab, and they are not home bag cycles. Even at that small scale, the bigger flask takes longer.
Heat has to reach the middle of the load, and a bigger, denser load takes longer to get there. Your gauge reads the steam around the bags and cannot show the temperature of the grain at the center. For the same reason, a pressure time set for one vessel does not transfer to another.
Your numbers, worked through
bags = spawn (lb) / grain per bag (2.25 to 3 lb), rounded up
- 5 lb / 3 lb per full bag = 1.67, so 2 bags at the fullest
- 5 lb / 2.25 lb per lighter bag = 2.22, so 3 bags at the lightest
- Each load gets the protocol's own treatment: about 250 F and 15 psi for 120 minutes.
Limits
What this tool cannot do
- It cannot tell whether your cooker reaches 15 psi. A button labelled high does not say so.
- It cannot see inside the load, so a gauge reading does not prove the middle got hot enough.
- It is not canning guidance and does not replace your cooker's manual.
Questions
How long do I pressure cook grain jars?
It depends on the protocol and the load. Utah State gives 120 minutes at 15 psi for a 2.25 to 3 pound grain bag. UF/IFAS generally suggests 30 minutes at 15 psi for seed in spawn containers without fixing a load size, so match the time to the source whose container you copy.
Can I just boil the grain instead?
UF/IFAS mentions boiling for at least 60 minutes as an option. It also warns that boiling may not reduce microbes on the seed enough. If one boiled batch comes out clean, that shows one batch grew, and it does not prove boiling can replace pressure.
How long does grain spawn take to colonize?
Utah State describes 2 to 4 weeks for its bags. The UF/IFAS oyster example gives around 10 to 20 days under its own conditions. Temperature and strain move both windows, and the grow planner puts dates on each stage from your start day.
Sources3
- Utah State University Extension, A Beginner's Guide to Growing Mushrooms at Home Grain bags of 2.25 to 3 lb, about 250 F and 15 psi for 120 minutes, colonization over 2 to 4 weeks. Checked 2026-10-20.
- UF/IFAS, Oyster Mother Cultures and Seed Spawn (SS663) Soak about 12 hours, 30 minutes at 15 psi generally suggested, boiling at least 60 minutes as an alternative that may not sufficiently reduce microbes. Checked 2026-10-20.
- Penn State Mushroom Spawn Preparation Rye spawn flask formulas: 35 minutes at 121 C for 250 mL and 45 minutes for 1,000 mL. As summarized in our grain spawn guide, checked 2026-10-20.