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DIY Grains: The Ultimate Guide!

Updated: Jul 3, 2023

After purchasing spores, you'll need to inject them into sterilized grain jars.

Grain jars are the backbone of the MiniTub system, which requires 3 quart sized jars. Each purchase of a MiniTub or MiniTub refill comes with 3 quart-sized sterilized grain jars.

However, many customers simply want to create their own grain jars. We don't blame them and we gladly give away our insight.

This 2-step process creates sterilized grains each and every time.

Recipe for 12 perfectly sterilized grain jars

We've tried dozens of recipes and "TEK"s. And this is the easiest and best way to make grain spawn, ever.

We use, and recommend, whole oats because they have always been the easiest, most cost-effective, and reliable source of grains. Some people use rye, popcorn kernels, bird-seed, or millet. At thirty-cents a pound and good track record for proper sterilization, it's hard to argue against it.

This recipe is for 12, so if you want to make fewer, then adjust the recipe accordingly.


  • Aluminum foil

  • 5-gallon bucket

  • Funnel

  • Water

  • 12 quart sized jars (narrow mouth) with modified airport lids. If you’ve previously purchased spawn jars from us, you can reuse them!

  • 41-quart Autoclave - I got mine from Amazon

  • Oats – 1.8 kilograms or about 4 lbs

Step 1: Soak your grains

  • Soak grains for 12 hrs at room temperature

Step 2: Sterilize your grains

  • Drain your grains and load each jar with 1.5 cups of oats.

  • Tighten the lids and wrap the tops of the jars with aluminum foil.

  • Load them into your autoclave or pressure cooker. Make sure to read the user manual for important safety instructions

  • Sterilize at 15 PSI for 1 hour.

Sterilized oats in a quart-sized jars
Happy Little Sterilized Grain Jars

Airport Lids

Airport lids are modified lids. They include an injection port and a filter. The filter allows for gas exchange. The injection port allows you to safely inject spores directly into the grain jars.

You can purchase a set of 12 injection ports and filters to make your own lids.

This is a pint-sized jar with a modified airport lid. The airport lid is a modified narrow-mouth jar lid with an injection port and a small filter patch.
Pint-sized jar with modified Airport Lid


Most people use a small pressure cooker to sterilize their grain jars. I use an All-American electric autoclave.

The reason Happy Little Fungi uses the All-American electric autoclave is to ensure 100% sterilization and it cuts down on production time because it can hold more jars. If you want to use a smaller pressure cooker for fewer jars, just half or quarter the recipe and follow the steps accordingly.

I highly recommend using a pressure cooker with a pressure gauge. Something like this Presto seems to be popular with small growers. If you want to level-up and get an All-American electric autoclave, this is the one I use.

What's the next step?

Inoculate your grain jars with mushroom spores, then wait 2 weeks. Your grain jars will become covered with mycelium. This is the most interesting step in the whole process. You get to watch a little bit of nature unfold before your eyes. It's truly a wonderful experience and has personally taught me a lot about patience.


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