Mycology 103: Substrate Science & Spawn Production
Master grain spawn preparation and bulk substrate formulation. Learn proper grain hydration, sterilization protocols, and substrate recipes for maximum yields.
Mycology 103: Substrate Science & Spawn Production
With clean cultures in hand from Mycology 102, it's time to scale up. This guide covers everything you need to know about grain spawn production and bulk substrate preparation—the critical middle stages that bridge culture work and fruiting.
Understanding Spawn
Spawn is colonized grain that serves as the inoculum for bulk substrates. Think of it as the mushroom equivalent of seeds—except instead of planting one seed, you're distributing millions of mycelial connection points throughout your final growing medium.
Why Grain?
Grain is the preferred spawn substrate for several reasons:
- High nutrition accelerates mycelium growth
- Individual kernels distribute evenly when mixed with bulk substrate
- Moisture retention keeps mycelium hydrated during colonization
- Availability and low cost make it practical at any scale
A higher spawn rate (more grain per pound of bulk substrate) means faster colonization and reduced contamination risk. Commercial growers often use 10-20% spawn rates, while hobbyists fighting contamination might go as high as 30-40%.
Choosing Your Grain
Not all grains are created equal. Each has characteristics that make it better suited for certain situations.
| Grain | Kernel Size | Colonization Speed | Notes |
|---|---|---|---|
| Rye berries | Medium | Fast | Gold standard, excellent moisture retention |
| Whole oats | Large | Medium | Cheap, widely available, forgiving to prepare |
| Wheat berries | Medium | Fast | Similar to rye, good alternative |
| Millet | Small | Very fast | More inoculation points, but harder to prepare |
| Wild bird seed | Mixed | Fast | Budget option, variable quality |
| Popcorn | Large | Medium | Easy to source, good for beginners |
Rye Berries: The Professional's Choice
Rye berries are the industry standard for good reason. They hydrate evenly, resist clumping, and colonize quickly. If you're serious about cultivation, invest in quality rye.
Where to buy: Feed stores, brewing supply shops, or online in bulk. Look for whole, unprocessed rye berries—not rye flour or flakes.
Whole Oats: The Forgiving Option
Oats are more forgiving of preparation mistakes. They're cheaper than rye and available at any feed store. The larger kernel size means fewer inoculation points, but this is offset by ease of use.
You need whole oat groats, not rolled oats or oatmeal. Rolled oats turn to mush and are unusable for spawn production.
Grain Preparation: The Critical Steps
Proper grain preparation is arguably the most important skill in spawn production. Get this wrong and you'll face either contamination (too wet) or stalled colonization (too dry).
Target Moisture Content
The goal is 45-50% moisture content. At this level:
- Grains are hydrated enough for mycelium to thrive
- Not so wet that bacteria flourish
- Kernels remain separate (no clumping)
The Soak Method
Rinse the Grain
Rinse grain thoroughly in a colander to remove dust and debris. This step removes surface contaminants and broken kernels.
Soak Overnight
Cover grain with water (at least 2 inches above grain level) and soak for 12-24 hours. Grain will absorb water and swell.
Simmer (Optional)
Bring grain to a gentle simmer for 10-15 minutes. This hydrates the interior and softens the hull without bursting kernels. Skip this if your grains are already fully hydrated.
Drain Thoroughly
Drain in a colander and spread on a clean towel or screen. Allow surface moisture to evaporate for 1-2 hours, stirring occasionally.
Test Moisture
The grain should feel dry on the outside but squish slightly when pressed between fingers. No water should drip when squeezed.
The No-Soak Method
Some cultivators prefer a faster approach:
- Rinse grain briefly
- Add grain to jars with water (roughly 1 part grain to 0.5 parts water)
- Sterilize with the water inside
- The grain hydrates during sterilization
This method works but offers less control over final moisture content. It's convenient for small batches but less consistent at scale.
The Squeeze Test
The most reliable way to check moisture content:
- Take a handful of prepared grain
- Squeeze firmly in your fist
- Open your hand
Too wet: Water drips out, kernels clump together Too dry: Kernels don't compress, grain feels hard Just right: Kernels compress slightly, no water drips, grains separate when you open your hand
Wet grain is the primary cause of bacterial contamination. If in doubt, err on the side of slightly too dry—mycelium can tolerate this better than excess moisture.
Containers: Jars vs Bags
You have two main options for containing your grain spawn: jars and bags. Each has advantages depending on your scale and goals.
Quart Jars
Best for hobbyists and small-scale production.
Advantages:
- Reusable (cost savings over time)
- Easy to inspect for contamination
- Simple to shake for even colonization
- No special equipment needed
Setup:
- Drill or punch holes in lids
- Cover holes with micropore tape or synthetic filter discs
- Add prepared grain, leaving 1 inch headspace
Spawn Bags
Best for scaling up and commercial production.
Advantages:
- Higher volume per unit
- Filter patches built in
- Faster workflow
- Stackable for incubation
Considerations:
- Single use (ongoing cost)
- Harder to inspect
- Require impulse sealer
| Scale | Container | Typical Size |
|---|---|---|
| Hobby | Quart jars | 1 quart (2-3 lbs grain) |
| Intermediate | Half-gallon jars | 0.5 gallon (3-4 lbs grain) |
| Commercial | Spawn bags | 3-10 lbs grain |
Sterilization Protocols
Grain must be sterilized to kill all competing organisms. This is non-negotiable—pasteurization is not sufficient for grain.
Pressure Cooking
The standard method for hobbyists and small-scale operations.
| Container | PSI | Minimum Time |
|---|---|---|
| Quart jars | 15 | 90 minutes |
| Half-gallon jars | 15 | 120 minutes |
| 3 lb spawn bags | 15 | 150 minutes |
| 5+ lb spawn bags | 15 | 180 minutes |
Load the Pressure Cooker
Place a rack in the bottom (jars should not touch the bottom directly). Stack jars or bags, ensuring steam can circulate.
Add Water
Add 2-3 inches of water. Not enough water = dry cooking and scorched grain. Too much = jars sitting in water.
Bring to Pressure
Heat on high until the cooker reaches 15 PSI. Adjust heat to maintain steady pressure.
Time from Full Pressure
Start your timer only when full pressure is reached and maintained. Maintain consistent pressure throughout.
Natural Pressure Release
Turn off heat and let pressure drop naturally. Do not quick-release—this causes grain to burst and create a wet mess.
Cutting sterilization time is the second most common cause of contamination (after wet grain). Those extra 30 minutes in the pressure cooker save hours of dealing with failed jars.
Inoculation
Once grain is sterilized and cooled, you can inoculate from liquid culture or agar.
From Liquid Culture:
- Inject 1-2 ml per quart jar through a self-healing injection port
- Shake jar immediately to distribute LC throughout
- More inoculation points = faster colonization
From Agar:
- Work in your SAB or flow hood
- Cut agar into 8-10 small pieces
- Drop pieces into grain, distributing evenly
- Shake jar to spread pieces throughout
Incubation and Shaking
After inoculation, jars need warmth and patience.
Temperature
Most species colonize best at 75-78°F (24-26°C). Slightly warmer speeds growth but increases contamination risk. Slightly cooler is safer but slower.
The Shake Schedule
Shaking grain jars dramatically speeds colonization by redistributing colonized kernels throughout uncolonized grain.
| Colonization % | Action |
|---|---|
| 20-30% | First shake—break up mycelium and redistribute |
| 50-60% | Second shake—may not be necessary if growth is even |
| 100% | Ready to use or store |
Shaking too early (before visible colonization) can damage young mycelium. Wait until you see clear growth before the first shake.
Bulk Substrates
Once you have colonized grain spawn, you're ready to mix it with bulk substrate for fruiting. Different species prefer different substrates.
Hardwood Fuel Pellets (HWFP)
The modern standard for wood-loving species like shiitake and lion's mane.
Advantages:
- Pre-sterilized by the pellet manufacturing process
- Consistent particle size
- Easy to hydrate and prepare
- Widely available (sold as stove fuel)
Preparation:
- Weigh dry pellets
- Add equal weight of boiling water
- Cover and let absorb (pellets will expand and break apart)
- Confirm proper hydration (squeeze test)
Masters Mix
A 50/50 blend of hardwood fuel pellets and soy hull pellets. Higher nutrition than straight HWFP means bigger yields.
| Component | Purpose |
|---|---|
| Hardwood pellets | Structure and lignin |
| Soy hull pellets | Nitrogen and fast nutrition |
Masters mix must be sterilized, not pasteurized. The high nitrogen content from soy hulls makes it a perfect breeding ground for contamination if not properly treated.
Straw
Traditional substrate for oyster mushrooms. Cheap and effective.
Preparation options:
- Hot water pasteurization (160-180°F for 1-2 hours)
- Cold water lime bath (24 hours)
- Cold fermentation (7-10 days submerged)
Straw only requires pasteurization, making it accessible for beginners who don't have pressure cooking capacity for bulk substrates.
CVG (Coir, Vermiculite, Gypsum)
Common substrate for species that fruit on composted materials.
| Component | Amount | Purpose |
|---|---|---|
| Coco coir | 650g brick | Base substrate |
| Vermiculite | 2 quarts | Moisture retention |
| Gypsum | 1 cup | Calcium and pH buffer |
| Boiling water | ~4 quarts | Pasteurization |
Mix components in a bucket, add boiling water, cover, and let cool overnight. Pasteurization happens via the heat.
Spawn to Substrate: The Mix
The actual mixing process is straightforward but timing matters.
Prepare Your Workspace
Clean your mixing container and work surface. While you're not working in sterile conditions, cleanliness still matters.
Check Substrate Temperature
Bulk substrate should be at room temperature. Hot substrate will kill your spawn.
Break Up Spawn
Shake or manually break colonized grain into individual kernels. Better distribution = faster colonization.
Mix Thoroughly
Combine spawn and bulk substrate. Aim for even distribution of grain kernels throughout. Mix by hand (with gloves) or with clean tools.
Load Containers
Transfer mix to fruiting containers (bags, trays, or buckets depending on species and method).
Spawn Rates
| Situation | Spawn Rate | Trade-offs |
|---|---|---|
| Contamination-prone environment | 30-40% | Faster colonization, higher spawn cost |
| Standard hobby | 15-25% | Balance of speed and economy |
| Commercial production | 5-15% | Economical, requires clean environment |
Log the spawn rate for each batch alongside colonization time and yield data. Over time you'll dial in the optimal rate for your environment and species.
Colonization of Bulk Substrate
After mixing, substrate needs time to fully colonize before fruiting.
Environmental Conditions
- Temperature: 72-78°F (22-26°C)
- Humidity: Not critical during colonization (substrate contains moisture)
- Light: Not required
- Air exchange: Minimal (CO2 buildup is acceptable during colonization)
Timeline
Full colonization typically takes 10-21 days depending on spawn rate, species, and temperature. The substrate should be completely white with mycelium before introducing fruiting conditions.
Once your substrate is fully colonized, you're ready for Mycology 104: Environmental Control & Fruiting Mastery, where we'll cover the fruiting triggers and conditions that turn colonized substrate into mushrooms.
Quick Reference: Substrate Preparation
Grain Spawn Checklist
- Quality grain (rye, oats, wheat, or millet)
- Proper hydration (45-50% moisture)
- No visible water when squeezed
- Adequate sterilization time (90+ minutes at 15 PSI)
- Natural pressure release
- Sterile inoculation technique
- Labeled with date and strain
Bulk Substrate Checklist
- Appropriate substrate for species
- Proper hydration (field capacity)
- Correct treatment (sterilization or pasteurization)
- Cooled to room temperature before mixing
- Even spawn distribution throughout
- Documented spawn rate and recipe