The first time a customer sent me a photo of what he called "endo," it was a quart jar of rye with a gray, glassy wet patch creeping up from the bottom and a smell he said reminded him of sour milk left in a hot car. That is endo contamination, and it is bacteria, usually heat-resistant Bacillus endospores, that survived a weak sterilization cycle and then woke up inside the grain once it cooled and got inoculated. It is not a rare new plague and it is not something floating in from your kitchen air. It starts inside the jar, which is exactly why it fools beginners, and the whole game in avoiding it is heat that actually reaches the core of the grain plus clean technique when you open the jar to inoculate.
Out-Grow Quick Answer
- Endo contamination is bacterial contamination from heat-resistant endospores, usually Bacillus, that survive an incomplete sterilization and germinate inside grain, liquid culture, or substrate.
- The tells are localized wet or slimy spots, translucent mushy kernels, a sour or rotten smell, and mycelium that stalls in those patches.
- The two root causes are under-sterilized grain and dirty inoculation technique. Fix both and endo largely disappears.
- Sterilize grain long enough for the container size, roughly 90 to 150 or more minutes at 15 pounds per square inch, and let it cool fully before you inoculate.
- Work in a still air box or under a laminar flow hood, wipe down with 70 percent isopropyl alcohol, and flame your tools every transfer.
What Is Endo Contamination in Mushroom Growing?
I get the word "endo" in support tickets more than almost any other contamination term, and nine times out of ten the grower using it is not totally sure what it means. That is fair, because it is a growers' nickname, not a textbook term. When people say endo they mean contamination that comes from endospores, the armored survival form that spore-forming bacteria like Bacillus switch into when conditions turn hostile. A vegetative bacterial cell dies easily in a pressure cooker. An endospore does not. It can ride out heat that would kill the parent cell, then rehydrate and germinate back into a growing colony once the grain cools, gets moisture, and gets nutrients, which is precisely what happens when you inoculate a freshly sterilized jar.
The reason it feels different from a green mold landing on your substrate is where it begins. A Trichoderma spore usually rides in on air or a dirty tool and shows up on a surface. Endospores were already in the grain, survived the cook, and bloom from the inside out. So growers gave internal, heat-surviving bacterial contamination its own shorthand. The mechanism is well documented in food and agricultural microbiology even though the nickname is not.
A dormant, heat-resistant form that certain bacteria such as Bacillus produce to survive stress. Growers shorten "endospore contamination" to "endo." The spore survives an incomplete sterilization, then germinates into active bacteria once the grain cools and is inoculated.
What Endo Contamination Looks and Smells Like in Grain Jars
A grower emailed me in the middle of a heat wave one July, panicking because two of his six rye jars had gone patchy while the other four ran clean white. He thought he had a mold outbreak. When he described the smell, sour and vinegary rather than musty, I knew before I saw the photos that he was looking at endo contamination, not mold. Bacteria and molds fail a grow in different ways, and once you learn the tells you rarely mix them up.
Healthy grain spawn is uniform white, cottony or ropey, anchored into and running through the kernels, and it smells like plain mushroom or wet earth. Endo contamination reads wet. You see glassy, translucent, mushy kernels stuck together in a slimy patch, often at the densest or wettest part of the jar, with the mycelium thin or absent right around it. The smell is the fastest confirmation. Bacterial wet spot is sour, vinegary, or flat-out rotten, described more than once to me as gym socks or garbage. Mold, by contrast, is usually a colored patch, bright green for Trichoderma, blue-green or gray for others, and a musty rather than sour odor.
| What you see or smell | Healthy mycelium | Endo (bacterial) contamination | Mold contamination |
|---|---|---|---|
| Color | Uniform white | Glassy, translucent, no color of its own | Green, blue-green, gray, or black patch |
| Texture | Cottony or ropey, anchored to grain | Wet, slimy, mushy kernels stuck together | Powdery or fuzzy on the surface |
| Smell | Mushroom, earthy, neutral | Sour, vinegary, rotten, gym-sock | Musty, moldy |
| Growth pattern | Even spread from every point | Stalled and thin around wet patches | Expanding colored colony |
One honest note on condensation, because it panics people for no reason. A little water beading on the glass from metabolic heat and temperature swings is normal and is not endo. Endo is a wet patch in the grain itself with a smell, not a few droplets on the wall of a jar that otherwise looks and smells fine.
Where Endo Contamination Actually Comes From
Years of reading support tickets taught me that endo contamination almost never has an exotic cause. It comes down to a short list, and the top two account for the overwhelming majority of the jars I get asked about. When I walk a grower back through their process, we usually find the heat never fully penetrated the grain, or something dirty went into the jar at inoculation. The chart below is how often I end up pointing at each cause, ranked from what I see most to what I see least.
The mechanism ties all of these together. Endospores are far more heat-resistant than the living bacteria they come from, so an under-time or under-temperature cook, or a pressure gauge that reads high while the real temperature sits low, leaves them intact. When the grain cools, hydrates fully, and you introduce your culture, those spores get exactly the trigger they were waiting for: water, nutrients, and a comfortable temperature. Laboratory work on Bacillus spores shows germination can begin within hours to a few days once favorable conditions come back after heat stress. That is the same window your mycelium needs to get established, so it becomes a race, and under-sterilized grain hands the bacteria a head start.
Laid out step by step, the biology behind that race looks like this.
It helps to see the grain's journey as a pipeline, because endo gets its two best openings at two specific stops.
Stop two, sterilize, is where surviving endospores get to live if the heat falls short. Stop four, inoculate, is where new bacteria walk in on an unflamed needle or a gust of room air over an open jar. Almost every prevention move below targets one of those two stops.
How to Avoid Endo Contamination: Sterilization, Grain Prep, and Sterile Technique
The cleanest run of grain I ever put up came from the least glamorous setup, a big stovetop pressure canner, well-hydrated but not soggy rye, and a still air box I wiped down twice. Nothing exotic. Avoiding endo contamination is really five habits working together, and if you hold all five you can run jar after jar without a wet spot. Skip one and the bacteria find the gap.
Grain prep is where I would start, because it is the cheapest fix. Get your kernels fully hydrated but drain them hard and let the surface moisture flash off so they are dry to the touch before they go in the jar. Standing water and soft, waterlogged kernels create the low-oxygen pockets that bacteria love. Broken kernels leak starch, which feeds them too, so quality grain with less debris runs cleaner.
Sterilization is the other half. For typical hobby jars and small bags in the half-liter to three-liter range, run 90 to 150 or more minutes at 15 pounds per square inch, and push the time up for larger or more densely packed loads because heat has farther to travel to the center. Let the load cool all the way to room temperature before you inoculate, both so condensation does not pool and so you are not opening a warm jar that is still steaming. Then keep the finished spawn at 68 to 77 degrees Fahrenheit while it colonizes, which is 20 to 25 degrees Celsius for most gourmet species, and expect a full colonization somewhere in the 10 to 21 day window at that temperature.
Here is the whole prevention sequence as one flow, start to finish.
Common Bad Advice About Endo Contamination and Sterilizing Grain
A new grower once told me, almost proudly, that his guide said 60 minutes at 15 pounds sterilizes any grain, so his gear could not be the problem. His gear was the problem. He was running dense quart jars for an hour and wondering why the bottoms kept going sour. The advice was not malicious, it was just a single number applied to every situation, and that is the trap.
A common rule you will see is that 60 minutes at 15 pounds per square inch fully sterilizes any grain jar and kills all endospores.
Time has to scale with the container size and how tightly the grain is packed. Small, loose jars may finish near that, but larger or densely packed loads need 90 to 150 or more minutes for heat to reach the core, where surviving spores cause endo.
Many beginner guides claim that if 90 minutes is good, running the cooker for hours longer is always safer.
Once the whole load has truly reached and held sterilization temperature, extra time only wastes energy and can degrade some substrates. The target is full heat penetration for the load, not endless time at temperature.
Side by side, the difference between the two processes is exactly why grain spawn cannot skip straight to pasteurization.
Pasteurization
Holds substrate at 149 to 167 degrees Fahrenheit, which is 65 to 75 degrees Celsius. Knocks back living organisms and works for tolerant species like oyster grown on straw.
Full sterilization
Runs grain at 15 pounds per square inch for 90 to 150 or more minutes. Reaches the temperature needed to kill heat-resistant endospores too, not just living cells.
Can You Save a Grow With Endo Contamination?
I want to be straight with you here, because hope costs beginners a lot of wasted weeks. When a grower asks me whether to rescue a sour, wet-spotted jar, my honest answer is usually no. A small, early wet spot in a jar with an otherwise vigorous run can sometimes get out-competed by the mycelium, and I have seen it happen. But a jar that is broadly slimy, strongly sour, and stalled is not worth your incubation space or your risk to nearby clean grows. Here is how I weigh it.
For beginners especially, erring toward tossing a heavily contaminated jar is the safer call. When you do discard, close the jar before you move it, so you are not shaking bacteria into the air near your clean projects. You can pressure-cook a contaminated jar once more to kill everything before you open and wash the container, or seal it and put it in the household trash, or bury the spent grain outside well away from your grow area the way you would toss moldy food. Do not count on a splash of bleach or a kettle of boiling water to disinfect a dense jar core, because those work on surfaces, not deep into packed grain. If you are handling a lot of contaminated material, wear a mask and gloves and wash up after, since heavily contaminated substrate can throw a lot of bacteria and mold spores into the air.
Relevant Out-Grow Supplies
The gear that prevents endo contamination is not fancy, it is just the difference between heat reaching the core and clean hands at inoculation. A few things I point growers to:
- If your sterilization is the weak link, starting from professionally sterilized grain removes the biggest variable. Our sterilized rye berry quart jars and Mega Mix sterilized grain bags come cooked and ready to inoculate, so endospores never get a foothold before you do. Browse the full range in the grain bags collection and the mushroom spawn collection.
- When you want to clean up a strain or start from a known-clean culture rather than a suspect one, agar is the tool. Our mushroom culture plates and culture media let you isolate healthy mycelium away from bacteria before it ever reaches grain.
Related Out-Grow Guides
Endo contamination sits inside a bigger cluster of grain and cleanliness topics. These guides go deeper on the pieces:
- How to grow mushrooms at home for the full beginner workflow that keeps contamination low from the start.
- How to grow gourmet mushrooms for species-by-species substrate and sterilization needs.
- Growing mushrooms for the wider introduction to cultures, spawn, and substrate.
Endo contamination looks scary the first time you meet it, but it is one of the most preventable failures in cultivation because it comes from two things almost entirely within your control. Sterilize your grain long enough for the container size so no endospore survives, and open that cooled jar with clean technique so no new bacteria walk in. Do those two things and the sour, wet-spotted jar mostly stops showing up. When it does show up badly, read the smell, trust it, and toss the jar rather than fight a battle the mycelium is already losing.
About the author
Mike Wiberg is the founder of Out-Grow, a mushroom cultivation supply company he has run since 2009. He developed the original all in one mushroom grow bag in 2014, a format now used across the hobby, and over more than 20 years of hands-on mycology he has grown hundreds of species and researched substrate formulations from the lab bench. He maintains one of the largest commercial culture libraries online, including a working collection of more than 300 species under a USDA permit. His cultivation work has been cited in peer reviewed research published in Frontiers in Cellular and Infection Microbiology, and he is one of only a handful of growers with a documented successful clone of Matsutake, one of the most difficult species in cultivation. He has helped growers troubleshoot cultivation problems for nearly two decades, and he writes from the lab and the grow room, not from a keyword list.