How to Make Agar Without a Pressure Cooker
Yes, you can make usable agar plates without a pressure cooker, but you have to be honest about what you are actually doing. You are not sterilizing the medium, you are heavily pasteurizing it with extended steaming or repeated boiling, then working clean enough to outrun the contamination that survives. I have shipped culture supplies since 2009, and the growers who succeed without pressure equipment are the ones who accept a higher loss rate, keep their batches small, and inoculate carefully. The ones who fail treat a single boil as a magic sterilization step. It is not.
Out-Grow Quick Answer
- Steaming or boiling at roughly 100 degrees Celsius kills most vegetative bacteria, yeasts, and molds, but not all bacterial endospores, so this is pasteurization grade, not true sterilization.
- Use small no pour containers, roughly 60 to 125 milliliters each, and steam them 90 to 120 minutes at a rolling boil, or run tyndallization, 20 to 30 minutes on three days in a row.
- Choose a light malt extract agar, about 5 grams of malt per liter, so contaminants grow slower on leaner food.
- Expect a higher contamination rate than a pressure cooker gives, and inoculate in a still air box, not open on the kitchen counter.
- Skip this method for grain spawn or bulk supplemented substrate. Those still need a real pressure cooker.
Here is roughly what that setup looks like on my own bench.
Can You Sterilize Agar Without a Pressure Cooker?
The first thing I tell people who ask this is that the word sterilize is where they go wrong. A pressure cooker gets media to 121 degrees Celsius at 15 pounds per square inch, and that combination is what reliably kills bacterial endospores inside a practical fifteen to twenty minute window for volumes up to about 500 milliliters. Drop the pressure and you cap out around 100 degrees Celsius at sea level. At that temperature you kill vegetative cells and weak spores, but some endospores shrug off boiling for hours. So the honest framing is that you can heavily reduce contamination without a pressure cooker, not sterilize.
That distinction is not academic. It sets your whole workflow. Because you cannot count on a sterile pour, you compensate everywhere else: smaller containers that heat evenly, leaner recipes, no pour jars that stay sealed, and clean inoculation in a still air box. Miss those and the surviving spores have everything they need to take over the plate.
| Factor | Pressure Cooker | No Pressure Cooker |
|---|---|---|
| Peak temperature | 121 degrees Celsius at 15 pounds per square inch | About 100 degrees Celsius at a rolling boil |
| Kills endospores | Yes, within 15 to 20 minutes for small volumes | Not reliably in one pass |
| Best container size | Up to 500 milliliters, or 1 liter with longer time | 60 to 125 milliliters only |
| Contamination risk | Low with clean technique | Higher, plan for losses |
| Good for grain spawn | Yes | No, sterilize grain properly |
How the No Pressure Cooker Agar Method Actually Works
Picture the two enemies you are fighting on an agar plate: fast growing molds and spore forming bacteria. Vegetative cells, the actively growing ones, die easily in a rolling boil. The problem is the dormant survival form some bacteria make, the endospore, which can sit through hours at 100 degrees Celsius and wake up later on your fresh nutrient plate. That single fact is why classical microbiology invented fractional sterilization more than a century ago, and it is why boiling agar once is not the finish line beginners think it is.
So the whole no pressure cooker approach is really a stack of small advantages that add up. Lean recipes slow contaminants down. Small containers heat through evenly so the center actually reaches temperature. No pour jars stay sealed from steaming to inoculation, so the only exposure is the moment you introduce your culture. And repeated heatings catch spores that germinate between cycles. None of those is sterilization on its own. Together they get you clean enough plates to work with.
A fractional sterilization method that brings media to about 100 degrees Celsius for 20 to 30 minutes on three days in a row. Between heatings, surviving bacterial spores germinate into vegetative cells that the next boil can kill. It is the closest a no pressure cooker method comes to real sterilization.
The second habit that pairs naturally with tyndallization is choosing the right container from the start.
Agar that is portioned into its final small jars before heating, then inoculated directly in those jars instead of being poured into open plates. Sealing the medium before steaming removes the open air pour, which is the riskiest exposure in home agar work.
Here is that whole sequence laid out as the four stages I actually walk through.
A Light Malt Extract Agar Recipe for the No Pressure Cooker Method
When I am setting someone up for no pressure cooker work, I steer them straight to a light malt extract agar rather than a rich potato dextrose plate. The logic is simple. If you cannot guarantee a sterile medium, do not lay out a lavish buffet for whatever bacteria survive. A leaner plate still feeds mushroom mycelium fine while giving contaminants less to run on. Aim for roughly 5 grams of malt extract per liter for the lightest version, up to 10 grams for a standard plate, with 15 to 20 grams of agar per liter to set firm.
Here is the 500 milliliter batch I hand out. Use 7.5 to 10 grams of agar powder, 5 to 10 grams of light malt extract, and 500 milliliters of clean water, distilled or previously boiled and cooled. Hydrate the agar and malt in room temperature water for 10 to 15 minutes, stirring so it does not clump. Heat gently while stirring until it goes clear to slightly hazy, and do not hammer it at a hard boil, since scorching the sugars adds nothing and can leave a burnt smell. Pour into your small jars to a depth of about 10 to 20 milliliters each before you cap and steam.
- Agar powder and light malt extract
- Small jars, roughly 60 to 125 milliliters, with vented or loosely fitted lids
- A large stockpot with a lid and a trivet or rack
- Distilled or boiled and cooled water
- Seventy percent isopropyl alcohol for wiping surfaces, and a still air box for inoculation
- A thermometer to catch the pour window, roughly 45 to 55 degrees Celsius
Once that list is on the counter, here is roughly what it looks like laid out and ready to go.
While you are portioning ingredients, it helps to know why this particular recipe works so well for mushroom mycelium.
How Long to Boil or Steam Agar Without a Pressure Cooker
The most common question I get here is a plea for one magic number, and there is not one. Time depends on how big your containers are, because heat has to reach the center of the medium and hold there. That is exactly why the no pressure cooker route lives and dies on small jars. A 60 milliliter jar comes up to temperature fast and evenly. A 500 milliliter bottle does not, and steaming it longer does not fix slow heat penetration, it just cooks the outside while the middle stays underdone.
Read the bottom bar as a stop sign, not a longer time. Anything from 250 milliliters up is strongly discouraged without pressure, because the heat cannot penetrate reliably. If you want more agar, make more small jars, do not make bigger ones.
Is Tyndallization Good Enough for Agar Without a Pressure Cooker?
Tyndallization is the closest I have found to a real substitute when there is genuinely no pressure cooker on the bench. The idea is old and sound: heat, wait, heat again. You steam the jars at about 100 degrees Celsius for 20 to 30 minutes, let them sit at room temperature so any surviving spores germinate into vulnerable vegetative cells, then steam again the next day, and once more on the third day. Each cycle mops up what the last one woke. It takes three days and more fuss than a single boil, and for slow, delicate species it is worth it.
Is it good enough? For fast, aggressive species and short term culture checks, in my experience it is a workable stand in. For commercial spawn or contamination prone clones, no. Tyndallization has solid classical microbiology behind it, but it has not been rigorously demonstrated specifically for mushroom agar at home, so I treat it as a strong workaround rather than a guarantee.
Boiling agar once is pasteurization. Three cycles across three days is the closest a home kitchen gets to sterile, and it still is not a promise.
Pouring and Inoculating Your No Pressure Cooker Agar Cleanly
Here is where most no pressure cooker batches are actually lost, and it is not the heating step. It is the moment you open the jar. When your medium is only pasteurized, the inoculation event carries all the risk, so this is the one place I refuse to let a beginner cut corners. You do not strictly need a laminar flow hood, but you do need a still air box or a cleaned, draft free room, because pouring open on a normal kitchen counter with only partly sterilized agar is asking for green mold.
Work in a still air box that you have wiped down with 70 percent isopropyl alcohol. Wipe the jar lids, your gloves, and your tools. Keep movements slow so you are not stirring up settled spores. If you are using no pour jars, you never open to a plate at all, you simply introduce your culture into the sealed jar and reseal, which is why the no pour approach pairs so well with this whole method.
Here is what that clean inoculation step actually looks like in practice.
How to Store Agar Plates You Made Without a Pressure Cooker
Once a batch sets clean, storage is the same as for any agar, with one extra habit: inspect harder, because you started from pasteurized medium. Keep sealed plates or jars in the refrigerator at 2 to 8 degrees Celsius. Unused medium holds well for about 2 to 4 weeks before drying and contamination risk climb, so I make agar in the small batches I will actually use rather than stockpiling. At room temperature, uninoculated plates dry out within days to a few weeks depending on how well they are sealed.
A colonized, well sealed culture is a different story and lasts far longer. Sealed and refrigerated, a working culture can hold for several months, and many gourmet cultures stay viable longer on well sealed slants. Sealing with Parafilm slows drying and keeps stray spores out. Watch for the warning signs: pronounced drying and cracking or spontaneous colonies in unused plates, and off colors, slimy or powdery textures, or halo like satellite colonies in cultures, which point to bacterial contamination.
That scorecard is the honest picture. It is cheap and accessible, which is the whole appeal, but reliability is where it pays for that. Some growers report contamination rates in the rough range of 5 to 20 percent per batch with no pressure methods, against single digits with good pressure sterilization, though those numbers are self reported and not rigorously studied, so hold them loosely. If a pressure cooker is within reach, it remains the better tool.
Common Bad Advice About Making Agar Without a Pressure Cooker
Two claims cause more failed batches than anything else. The first is that a single 20 to 30 minute boil fully sterilizes agar. It does not. Boiling kills vegetative cells but leaves resistant endospores alive, and those are exactly the contaminants that ruin plates. That is the entire reason tyndallization exists. The second is the flip side, the belief that you cannot do agar work at all without a pressure cooker. Not true either. Careful, repeated heating plus clean technique grows perfectly useful cultures, just with more effort and more losses than pressure gives.
You need a pressure cooker for every mushroom project, agar included, or it will fail.
The method depends on the substrate and species, not one rule. Coir based bulk substrates can be pasteurized, and agar can be made with extended steaming or tyndallization at higher risk. Grain spawn and supplemented sawdust still need full sterilization, so keep a pressure cooker in your future if you scale up.
One more that trips people up: growers panic when light malt extract agar browns a little during heating and toss the batch. Mild caramelization from extended heat can shift the nutrition slightly, but it does not automatically ruin the medium. As long as it still sets firm and there is no burnt smell, it is usable. And do not fall for the opposite extreme either, that longer sterilizing is always safer. Past the point of reaching temperature for your container, extra time just wastes fuel and can degrade the medium.
Relevant Out-Grow Supplies
You can source what this method needs from the same shelves I stock for my own lab. Everything here genuinely fits no pressure cooker agar work, and I have flagged where a product is a better match than a workaround.
For agar powder, malt extract, and prepared culture media, start with our culture media collection. If you would rather skip homemade plates entirely and reduce your contamination risk, a ready to use liquid culture gives you a clean starting point without any heating step on your end, which is honestly the safer path for a beginner without a pressure cooker. For everything else on the bench, from alcohol to still air box supplies, browse the broader mushroom supplies range.
The Bottom Line on Agar Without a Pressure Cooker
You can absolutely make agar without a pressure cooker, as long as you call it what it is. Steam small jars 90 to 120 minutes, or run three day tyndallization for a cleaner result, use a light recipe, and inoculate in a still air box. Accept a higher loss rate and keep batches small. It is a real, workable path when a pressure cooker is out of reach, and a fine way to learn. But the day you want reliable, low contamination plates or you move on to grain spawn, a pressure cooker earns its place fast.
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.