How to Do a Grain-to-Grain Transfer (G2G): A Step-by-Step Guide

About 1 hour active work, plus colonization time

A grain-to-grain transfer (G2G) is the fastest way to multiply mushroom spawn. You take a fully colonized jar of grain and use a small portion of it to inoculate one or more fresh sterilized jars. Because every grain you add is already a fully active inoculation point, the new jars colonize roughly twice as fast as a liquid culture inoculation: 7 to 10 days for cubensis-style genetics versus the 14 to 21 days you would expect from LC. One quart of clean spawn can comfortably seed 8 to 10 fresh quarts in a single session.

That speed comes with a tradeoff. G2G compounds whatever is in the source jar across every downstream batch. If your source has a single hidden contaminant, you copy it ten times. This tutorial covers how to pick a source jar that is actually clean, how to run the transfer in a still air box without dumping spores into your new jars, and how to read the new jars through colonization. By the end you should be able to take one verified jar and turn it into a queue of spawn that can keep multiple grows going at once.

What you'll need

Materials

1 fully colonized quart jar of grain spawn (the source jar; 7 to 14 days post-full-colonization is ideal)
Fresh sterilized grain jars (the destination jars; 1 to 10 quarts per source jar, depending on ratio)
70% or higher isopropyl alcohol, plus a spray bottle and lint-free wipes
Aluminum foil, optional, for covering jar mouths between handoffs
Permanent marker, for labeling
Nitrile or latex gloves

Equipment

Still air box (SAB) or laminar flow hood
Small clean bowl or funnel, optional, for pouring
Large mason jar ring or a cleaned bench surface, for resting lids
Permanent marker and masking tape, for relabeling

Step 1: Pick a clean source jar (the most important step)

G2G is a multiplier on whatever is in the source jar. Spotless source, spotless downstream batch. Source with a single corner of cobweb hiding behind the label, ten contaminated downstream jars. Spend more time on this step than on any other. It is where beginners blow out a whole queue at once.

A jar is ready to be a G2G donor when it meets all five of these checks. If any one fails, do not use it. Save it for a bulk tub instead, where some hidden contamination is at least diluted by the substrate.

100 percent colonized. Every visible grain is wrapped in white mycelium. No uncolonized kernels in the corners, against the bottom, or under the lid. A 95 percent jar still has uncolonized grain that gives bacteria a head start in the destination jar.
7 to 14 days past full colonization. Wait at least a week after full coverage so the mycelium has time to consolidate. A jar that just hit 100 percent yesterday is still fragile and breaks into a paste when you try to redistribute it. After 14 days, the mycelium starts forming primordia (small pin-like bumps) and the jar enters its declining window. The 7 to 14 day band is the sweet spot.
No off smells. Crack the lid in still air and inhale gently from a few inches away. Healthy spawn smells faintly mushroomy, like a damp forest floor. Sour, vinegary, or wet-sock notes mean wet spot bacillus. Do not transfer it.
No off colors. Rotate the jar 360 degrees under a bright light. Any green dots are Trichoderma; black sporulating heads are Aspergillus; wispy gray fuzz that fills empty space is cobweb mold. The full visual ID set lives on the contamination guide. Light yellow droplets between grains are mycelial metabolites and are fine. Bright slimy yellow patches are bacteria and are not.
The grain still moves. Pick up the jar and shake it gently. The colonized grain should break apart into loose clumps. If the entire jar is locked into one solid brick, you waited too long; the mycelium is over-consolidated and will not redistribute cleanly. Spawn it out to bulk instead.

Generation drift safety note

Every G2G generation increases the risk of phenotype drift and senescence. T1 (your original jar from LC or agar) and T2 (one G2G off T1) are reliable. By T5 or T6, most genetics start showing slower colonization, lower yields, and occasionally sectoring or reversion. Track the generation number on every jar so you know when to go back to agar for a fresh start. The convention is T1, T2, T3... and so on.

The fresh destination jars also have to be ready. They should be cooled to room temperature (under 90°F / 32°C) after their full 90 minute pressure cycle, with the foil still on the lid ports. A hot jar will kill the mycelium on contact. If you have not sterilized your destination jars yet, see how to prepare grain spawn for the full grain prep, lid mod, and pressure cooker recipe. G2G assumes you already have fresh sterilized destination jars on hand.

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