Out-Grow Quick Answer
- Make a nutrient agar, usually malt extract agar, sterilize it in a pressure cooker, cool it to a safe pour temperature, and fill sterile plates in a clean, low-draft spot.
- Standard mix is 15 to 20 grams of agar plus about 20 grams of light malt extract per liter of water, which is roughly a 2 percent agar gel.
- Pour 20 to 25 milliliters into each 90 to 100 millimeter plate, which gives you a firm layer about 3 to 5 millimeters thick.
- Cool the molten agar to about 45 to 55 Celsius before pouring, hot enough to flow evenly, cool enough to limit condensation.
- Home-poured plates last roughly 2 to 4 weeks stored well. Store inverted, watch for cracking or any mold, and toss a plate that grows anything before you inoculate it.
To pour agar plates at home, prepare a malt extract agar of 15 to 20 grams of agar and about 20 grams of light malt extract per liter of water, sterilize it in a pressure cooker at around 15 pounds per square inch, cool the molten medium to about 45 to 55 Celsius, and pour 20 to 25 milliliters into each sterile plate in a clean, still-air area. Then let the plates set for 20 to 30 minutes, dry off any heavy condensation, and store them inverted until you are ready to inoculate.
That is the whole job, and none of the steps are hard. The reason people fail is almost never the pour itself. It is skipping proper sterilization, pouring at the wrong temperature, or storing plates so long they dry out or catch a stray spore. I have poured a lot of agar over the years at Out-Grow, and everything below is aimed at those three failure points.
What You Need to Pour Agar Plates at Home
The first time a customer asked me what gear he needed for agar, he assumed it was a whole lab. It is not. He set up on a wiped-down kitchen counter with a pressure cooker he already owned and a bag of empty plates, and he was pouring clean agar that same afternoon. The pour itself needs almost nothing. What matters is that everything that touches the molten agar is sterile and that the air around you is still.
- Agar powder and light malt extract, or a ready-made agar mix
- A pressure cooker that reaches 15 pounds per square inch
- Heat-safe bottles or jars with loose lids or foil for sterilizing the media
- Sterile Petri dishes, 90 to 100 millimeters is the usual size
- Seventy percent isopropyl alcohol for wiping surfaces and your hands
- A still, low-draft spot, or a still air box if you have one
A still air box, which is just a clear tote turned upside down with two arm holes cut in the front, cuts your contamination rate more than any single other thing at this stage. It is optional. Plenty of people pour fine on an open counter with the windows shut and the fans off. But if you find your plates keep catching mold, the box is the cheapest fix there is.
The Malt Extract Agar Recipe to Pour Plates at Home
I keep coming back to malt extract agar for home work because it is forgiving. A batch I mixed for a lion's mane clone a while back was nothing fancy, just malt extract, agar powder, and distilled water, and the mycelium ran across it clean and white. Most gourmet mushrooms, oyster, shiitake, lion's mane, grow happily on the same simple mix, so there is no reason to overcomplicate it when you are learning.
Per liter of water, use 15 to 20 grams of agar powder and about 20 grams of light malt extract. The agar is the gelling agent, roughly a 2 percent gel, and the malt extract is the food. For a half-liter batch, which is plenty for a first run, halve it to about 7.5 to 10 grams of agar and 10 grams of malt extract in 500 milliliters. Heat gently and stir until everything dissolves before it goes in the pressure cooker.
You do not need to fuss over pH for basic gourmet work. Malt extract agar naturally lands in the mildly acidic range that filamentous fungi like, around a pH of 5 to 6, without you adding anything. Distilled water keeps the batch consistent and skips whatever your tap water is carrying. That is the whole recipe.
What Temperature to Pour Agar Plates
The single mistake I see most with agar is pouring too hot. A grower emailed me once with plates that had a foggy film of condensation dripping onto the surface, ruining every isolation he tried to read. He had poured straight out of the pressure cooker while the agar was near boiling. Steam has to go somewhere, and in a closed plate it goes onto the lid and rains back down. Let the bottle cool first and that problem disappears.
Pour when the agar has cooled to about 45 to 55 Celsius. That is hot enough that it still flows into an even layer, and cool enough that it is not throwing off clouds of steam. Below roughly 45 Celsius the agar starts setting in the bottle and you get lumpy, uneven pours. Above about 60 Celsius the condensation gets heavy and the heat can damage any sensitive additives. The window is wider than beginners fear, and a good tell is simple: when you can hold the bottle against your inner forearm for a few seconds without wanting to let go, it is about right.
To see where that pour window sits against the rest of the process, it helps to look at the temperature the agar passes through from the stovetop to the fridge.
How to Sterilize Agar Before You Pour Plates
Early on I watched a new grower try to shortcut this by just boiling his agar on the stove and pouring it. Every plate fuzzed over with mold inside a week. Boiling does not kill the heat-resistant bacterial spores that live in malt extract and on glassware, and nutrient agar is a buffet for them. This is the step you do not get to skip. Full pressure sterilization, in a pressure cooker or an autoclave, is what makes agar work at all.
Load your bottles into the pressure cooker with the lids on loose, or foil over the top, so steam can move and the bottles do not build pressure and crack. Bring it up to about 15 pounds per square inch, which is roughly 121 Celsius, and hold it. Time depends on how much media is in each vessel, since a bigger volume takes longer for the heat to reach the center. When the cycle is done, let the pressure fall on its own rather than venting it fast, both for safety and to keep the agar from boiling over inside the bottle.
| Media volume | Pressure cooker time at 15 psi | Notes |
|---|---|---|
| About 100 mL | 15 minutes | Small teaching-lab volumes, heat-up time dominates |
| 500 to 1000 mL | About 30 minutes | Typical home batch, natural cool-down after |
| Over 1 liter | 25 to 30 minutes or more | Larger volumes need longer for heat to reach the center |
Those are practical home ranges. A certified lab autoclave running about 1 liter will often be done in 15 to 20 minutes, but a home pressure cooker heats and holds less evenly, so the longer end is the safer bet. The target is reaching and holding sterilization conditions for the volume in the vessel, not running the cooker for hours on end.
How to Pour Agar Plates Step by Step
The actual pour takes about a minute per stack once you have the rhythm. The batch I mentioned earlier, the lion's mane run, I poured a sleeve of twenty plates on a wiped counter with the fans off and lost none of them. The trick is to move steadily, keep each lid open only as long as the pour takes, and not let the plates sit open catching air. Here is the sequence I use.
Inverting matters more than beginners expect. Once the agar is set, you store plates upside down, meaning agar on top and lid on the bottom, so any condensation that forms collects on the lid instead of dripping onto the growth surface. It is a small habit that keeps your plates clean to read.
How Long Agar Plates Last and How to Store Them
A grower once wrote in worried that his freshly poured plates were fogged with condensation and asked if he had to throw them all out. He did not. Condensation right after pouring is normal, and as long as the plates are otherwise sterile and not dried out, they are fine to use. Knowing what actually ruins a plate, and what just looks alarming, saves a lot of wasted agar.
Home-poured plates have a shorter practical life than the commercial ones you buy. Plan to use your own within about 2 to 4 weeks when stored well, because they slowly dry out and the contamination odds climb the longer they sit. Commercially made, non-irradiated plates are documented to hold for roughly 8 weeks to 3 months, and irradiated ones up to about 6 months, but those are made and sealed under conditions a kitchen cannot match.
Where to store them is a real tradeoff, not a single rule, and this is where a lot of beginner guides oversimplify. The two honest options work for different reasons.
Refrigerated, 2 to 8 C
The common teaching-lab choice. Cold slows drying and microbial growth, so plates keep longer. The catch is condensation when you move a cold plate into warm air, so seal them and let them warm before opening.
Cool room, 10 to 25 C
Some commercial makers actually prefer this for their sealed plates, specifically to avoid condensation. It suits plates you will use quickly and keeps moisture from pooling on the surface.
As for spotting a plate that has gone bad, two signs matter. Cracking, shrinkage, and a widening gap between the agar and the dish wall mean the plate has dried out and should go. Any colony, discoloration, or surface film that shows up before you intentionally inoculate is contamination, and that plate goes in the trash. Heavy condensation by itself is not spoilage.
The growers who get clean plates are not the ones with the fanciest setups. They are the ones who respect the sterilizer and pour at the right temperature.
Common Bad Advice About Pouring Agar Plates at Home
Most of the bad agar advice floating around is not wrong so much as too absolute, stated as a single hard rule where the reality is a range or a judgment call. Here are the ones that trip up beginners most, and what is actually true.
Relevant Out-Grow Supplies
You can gather agar gear from a lot of places, but here is what fits this job honestly. Our culture media line covers the agar and media side if you would rather start from a ready mix than measure powders. If you want living cultures to streak onto your fresh plates, the liquid culture and broader mushroom culture collections are where I would point you. And if pouring your own agar feels like more than you want to take on right now, our all in one grow bags skip the lab work entirely and get you growing without any agar at all.
Related Resources
Pouring Your First Agar Plates
Pouring agar plates at home comes down to a clean recipe, real sterilization, the right pour temperature, and sensible storage. Mix 15 to 20 grams of agar and about 20 grams of malt extract per liter, pressure sterilize it, cool it to about 45 to 55 Celsius, and pour 20 to 25 milliliters into each plate in still, wiped-down air. Let them set, dry off the condensation, store them inverted, and use home-poured plates within a few weeks. After pouring more agar than I can count at Out-Grow, I can tell you the growers who get clean plates are not the ones with the fanciest setups. They are the ones who respect the sterilizer and pour at the right temperature.
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.