Grain Spawn Recipe.
Grain Spawn Recipe Calculator
Grain spawn is what colonizes your bulk substrate. Build batches in jars or bags. Hydrate, sterilize, inoculate, transfer.
Why grain spawn at all
Grain spawn is the bridge between agar/liquid culture (small-scale, sterile mycelium) and bulk substrate (large-scale, fruiting). Each grain becomes an inoculation point — colonized grain shaken into bulk substrate seeds it from hundreds of points simultaneously, dramatically reducing colonization time.
| Stage | Volume | Sterility required | Time to colonize |
|---|---|---|---|
| Agar plate / petri dish | ~50 mL | Lab-grade | 5-14 days |
| Liquid culture | ~500 mL | Lab-grade | 5-7 days |
| Grain spawn (jar) | ~1-2 lb | Pressure-sterilized | 14-21 days |
| Bulk substrate (bag/tub) | 5-50 lb | Pasteurized | 10-21 days |
| Fruiting | — | Clean environment | 5-21 days |
Grain selection
| Grain | Size | Hydration time | Best for |
|---|---|---|---|
| Rye berries | Medium | 12-24 h soak | Universal best; cheap; widely available |
| Millet | Tiny | 1-2 h boil | Maximum inoculation points; slow species (lion's mane, reishi) |
| Wheat berries | Medium | 12-24 h soak | Widely available; similar to rye |
| Oat (whole) | Medium | 24 h soak | OK; tends to clump if over-hydrated |
| Popcorn (corn) | Large | 24 h soak + 30 min simmer | Cheap; hard to over-hydrate; common in commercial production |
| Sorghum / milo | Small | 12-18 h soak | Cheap; small grain; good for slow species |
| Wild bird seed | Mixed | 12 h soak | Cheapest; mixed grains; backyard mycology |
Hydration is the critical step
The single most common failure mode in grain spawn is wrong hydration. Symptoms:
- Too dry: Mycelium colonizes slowly, looks sparse, "frosts" surface only. Grain stays separate. Lower yield.
- Too wet: Bacterial contamination almost guaranteed. Black/yellow/orange patches. Sour smell. Wet pooling at jar bottom.
- Just right: Mycelium colonizes through grain entirely; jar shakes loose; no free water at bottom; faint mushroom-y smell.
The water-to-grain ratio test: after hydration, drain grain in a colander. The grain should still be slightly tacky but no water drips when squeezed firmly. If water drips, dry the grain on towels. If powdery, hydrate longer.
Sterilization specifications
| Format | Temperature | Time | Notes |
|---|---|---|---|
| Quart jar (1 lb grain) | 15 PSI / 121°C | 90 minutes | Add 15 min for cool-down ramp |
| Half-gallon jar (2 lb) | 15 PSI / 121°C | 2 hours | Standard for production |
| 3 lb spawn bag | 15 PSI / 121°C | 2 hours | Bags must be vented properly |
| 5 lb spawn bag | 15 PSI / 121°C | 2.5 hours | Allow extra cool-down |
| Tyndallization (2-3 cycle pasteurization) | 200°F / 1 hour × 3 days | 3 days | Less reliable; works on small scale without pressure cooker |
Free under CC BY 4.0. Cite as "OAT Grain Spawn Recipe Calculator (openagriculturetechnology.com)".