Quick Overview

• Boil target: 90 °C (194 °F), not a rolling boil. Stir intermittently for even hydration and fewer burst kernels.
• Final moisture target: ~50–55% by weight (typical hydrated weight ≈ 2.1× dry oat weight).
• Surface dryness goal: Grains look matte (not shiny), don’t weep onto a paper towel, and don’t leave droplets when shaken in a colander.
• Sterilization: 15 PSI for 120 minutes for 3 lb spawn bags.

Supplies

• Whole oat groats (not rolled oats)
• Clean water (RO or tap is fine if low in sediment)
• Gypsum (calcium sulfate), optional at 1–2% of dry grain weight
• Filter-patch spawn bags (3 lb / ~1.36 kg final per bag), or wide-mouth jars with modified lids
• Large stockpot with reliable thermometer (to hit 90 °C / 194 °F)
• Stirring utensil (stainless or food-safe), measuring scale (grams), heat-proof gloves
• For straining/steam-drying:
  • Large stainless mesh colander or a pasta pot insert/steamer basket
  • Alternative: Food-grade bucket with a perforated insert, or a clean window-screen panel set over a sink/bucket
  • For steam-drying: Sheet pans + cooling racks (best); clean towels/paper towels underneath to catch drips
• Pressure cooker or autoclave (15 PSI / 121 °C)
• Clean workspace (SAB or flow hood) for inoculation

Batch Planner (Exact Weights)

This table shows precisely how much dry whole oats you need to produce 5 × 3 lb hydrated grain bags (common commercial size). Target factor assumes a reliable hydrated:dry ratio of ~2.1×.

Per-bag reference: Dry oats ≈ 648 g + gypsum 6–13 g (optional) yields ≈ 1,361 g hydrated oats after prep/steam-dry. The absorbed water per bag is ≈ 713 g. You’ll simmer in a much larger volume of water; only ~713 g ends up inside the grain.

Step-by-Step Process

1) Rinse

1. Weigh your dry oats: for 5 bags, weigh 3,240 g whole oats.
2. Rinse 2–3 times in cool water to remove dust/husk fragments. Pour off floating chaff.

2) Optional Soak (helps even hydration)

1. Cover oats with at least 2–3× their volume of water and soak 8–12 hours. (Optional but reduces simmer time.)
2. Drain fully before the simmer step.

3) Controlled Simmer to 90 °C (194 °F)

1. Fill a large stockpot with enough water so grains can move freely (at least 4–6 L for this batch; more is fine).
2. Heat the water and add the rinsed (and soaked, if used) oats.
3. Stabilize at 90 °C (194 °F)—avoid a vigorous boil. This reduces split/burst kernels, which can lead to stickiness and bacterial risk.
4. Stir intermittently every 2–3 minutes so all kernels see the same temperature.
5. Time window: typically 10–20 minutes at 90 °C. You’re aiming for kernels that are hydrated yet intact:
   • Cut a kernel: the center should be soft but not mushy; the hull remains intact.
   • If many kernels split or look “blown out,” reduce time/temperature next batch.

4) Strain Thoroughly

1. Pour grains into a mesh colander or lift a pasta pot insert/steamer basket to drain. Let water drain for 2–5 minutes.
2. Alternative: Dump grains onto a window-screen panel set over a sink/bucket; or into a food-grade bucket with a perforated false bottom. Gently shake to shed surface water.

5) Steam-Dry to “Surface-Dry + Internally Hydrated”

Move strained oats to cooling racks set over sheet pans (best airflow). Spread to a shallow layer (≤ 1 inch / 2.5 cm). Let steam escape for 20–40 minutes, flipping once halfway.

How to know the oats are dry enough

• Matte look: Kernels lose their wet sheen; surfaces look slightly dull.
• Paper-towel test: Press a handful onto a paper towel for 5 seconds. The towel should not show wet spots; a few faint dots are okay.
• Colander shake test: Put a scoop in a dry colander and shake for 10 seconds—no droplets should fling off; kernels shouldn’t clump.
• Finger roll: Roll a few kernels between dry fingers. They should separate easily and feel barely tacky, not slick.
• Tray check: After 5 minutes on a dry tray, you shouldn’t see puddles. A little condensation under the pile is fine; puddling means keep drying.

Tip: If you overshoot and the grain seems too dry (almost squeaky), lightly mist and mix, then rest 5 minutes before bagging.

6) Add Gypsum (Optional)

• Weigh gypsum at 1–2% of dry grain. For this batch (3,240 g dry), that’s 32–65 g total.
• Sprinkle evenly while turning the grain on the racks; gypsum improves flow, reduces clumping, and buffers pH slightly.

7) Bag (or Jar) and Weigh-Check

1. Label bags with date/species.
2. Weigh each empty bag (tare), then load ~1,361 g hydrated oats per bag for a 3 lb finished bag.
3. Expel excess air, then impulse-seal the top. Leave filter patch unobstructed.
4. Jar option: Fill wide-mouth quart jars 2/3–3/4 full; fit lids with self-healing injection ports and filters.

8) Sterilize

1. Add a rack or towel to the pressure cooker’s bottom; load bags upright with space around them.
2. Add water per manufacturer’s guidance. Vent the PC for 10 minutes (steady steam) before pressurizing.
3. Sterilize at 15 PSI for 120 minutes for 3 lb bags (90–120 minutes for jars depending on fill).
4. Allow to cool naturally to room temp—do not force-vent or move hot bags.

9) Inoculate & Incubate

1. Work in a SAB or under a flow hood. Sanitize gloves/tools.
2. Inoculate with liquid culture or a clean agar wedge. Reseal if applicable.
3. Incubate at 68–75 °F (20–24 °C). Shake or break up gently at 20–30% colonization for even spread.
4. Full colonization typically 10–21 days depending on species and temp.

Quality Checks & Troubleshooting

• Grain looks wet after sterilization: You didn’t steam-dry enough. Next time, extend the steam-dry by 10–15 minutes. (Slight surface moisture is okay; puddly is not.)
• Lots of burst kernels: Temperature too high or time too long. Hold at 90 °C and stir intermittently; shorten simmer by 3–5 minutes.
• Clumping blocks gas exchange: Add gypsum (1–2%) and ensure better surface dryness pre-bag.
• Bacterial smell or slime: Often from over-hydration or dirty workflow. Revisit rinse/steam-dry and sterile technique.
• Slow colonization: Check incubation temps (aim mid-low 70s °F), reduce bag compaction, verify culture vitality.

FAQ

How much water should be in the pot? Use plenty—enough for the grain to move freely (4–6 L+ for this batch). The grain will only absorb ~3.6 L total; the rest is just cooking medium.

Can I skip gypsum? Yes. It just improves flow and consistency. If you’re getting clumping, try 1–2%.

Why weigh instead of measuring cups? Oat groats vary in size and packing density. Weight gives repeatable hydration and yields.

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