55 Gallon Drum Sterilizer

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This page details construction of the 55 gallon barrel sterilizer, pictured at your left, that we are using at Mountain-Mushroom-Farm in Ferry County, Washington. Total cost for the entire system, including the propane burner and steel support frame was under $400, and it can sterilize 100 pounds of mushroom spawn or substrate at a time. I originally set up the drum sterilizer last December, when temperatures were below zero and the ground was frozen solid.

Once the weather warmed up, I dug out the ground to set concrete footings and a slab for the system to sit on. As we rebuilt the sterilizer, I took lots of pictures to document the process in hopes it would help other mom-and-pop mushroom farms find an inexpensive method of sterilizing grains and bulk substrates for growing mushrooms. Using this sterilizer and our underground colonization and fruiting area, Mountain Mushroom Farm is producing up to 70 pounds of Shiitake mushrooms daily.

The first thing that became obvious from using this system during the winter with no foundation other than concrete blocks for the steel frame pictured to your right, was that a concrete slab and footings would have to be built when the weather warmed up. The heat from the burner melted the ice, causing the thing to constantly settle and heave as the ground repeatedly thawed and then re-froze.

The first step in doing this correctly was to dig down 8 inches and then to insert steel mesh and rebar(in this case angle iron I had around that was left over from building the shelves in the mushroom farm fruiting room). When you live three hours from the nearest home improvement center, you use what you have!

I used 2 X 4 lumber as forms on three sides, giving the concrete slab a depth of up to 12 inches, once it was all level. That may be overkill, but at least I know it's not going to shift this winter under repeated heating and cooling cycles!

Once the concrete had cured 48 hours, I busted the forms off and then used two 8" X 16" concrete blocks on each side as the support structure for the bulk sterilizer frame. The concrete blocks were leveled and mortared into position, and then the voids were filled with concrete, giving a solid 8" X 32" concrete frame on each side. I cut steel plates to fit the top of the blocks, and then built and welded a mitered angle iron frame to each steel plate. This spreads out the load, preventing the concrete from being damaged by the frame and weight of the full 55 gallon drum boiler and sterilizer system.

On the left, you see the completed frame sitting on the steel-jacketed footings. Above it you can see the insulated copper pipes which feed steam to the underground growing areas for heat and humidity. Once the barrels were in position, the frame was adjusted on the blocks so the existing piping would attach back to their original fittings, which were unions soldered into place to facilitate later removal. The upper pipe takes steam from the exhaust on the sterilizer barrel, to utilize 'waste' steam to heat the slab in the growing room. The lower fitting can take steam directly from the boiler to provide heat for the growing rooms when the sterilizer is not in operation. Gate valves feeding a copper 'T' determine which direction the steam flows.

On the right is our propane burner used to heat the 55 gallon barrel sterilizer. It was purchased from Fungi Perfecti (scroll half-way down the page) and produces up to 160,000 BTU. The steel frame above holds the weight, so the burner is free to be slid on the concrete from one end of the slab to the other, allowing it to heat the boiler when steam is required, or to heat the second barrel, which is used for pasteurizing straw. The legs on the propane burner were too long to fit under the support system, so I cut 2" off each leg before welding them back on.

The Boiler

On the right is the 55 gallon barrel, modified to use as a low pressure steam boiler. As you can see, I welded two black iron pipe nipples to the barrel, to be used for filling and also for the sight glass. Attached to the one at the bottom is a 90 degree fitting, and a 'T' is attached to the top. The next time I take it apart, the fitting at the bottom is going to be changed out to a 'T' also, to facilitate easier draining. Plastic tubing is used as a sight glass so I can see the water level at all times. The hose fitting and valve at the top allow the system to be filled with water, even when steam is being produced. Since I only operate the system at 2 psi, my well easily delivers enough pressure to overcome the steam pressure and fill the boiler during operation, allowing it to run 24/7 during the coldest part of the winter months.

These pictures show the steam piping system from the boiler. The insulated pipe at left goes to the underground mushroom farm for heating. When the gate valve is closed, steam heat to the mushroom farm passes through the valve on the right, travels through the sterilizer barrel when it's in operation, and then the sterilizer exhaust is connected through another valve to supply heat. To your lower right is the piping to the sterilizer.

In the picture above and to the right, you'll see a plug at the top of the steam manifold. This is where the 5 psi relief valve attaches, not yet installed at the time these pictures were taken during assembly. For safety sake, never run a boiler without a relief valve, even if you're standing by to babysit. It's just too dangerous.

The 55 Gallon Barrel Sterilizer

Our 55 gallon barrel sterilizer holds 100 pounds of mushroom substrate per cycle. We load 5 to 7 pounds of supplemented sawdust or rye berries into filter patch autoclavable bags for sterilization, as seen above left. The unit easily holds 20 bags without overcrowding. We place mason jar lid rings between each bag to keep them separate and allow steam to completely surround each substrate bag during sterilization.

To provide a good seal, wax paper is stuffed into the groove on the lid, as can be seen upper right. This provides an excellent gasket once the ring is tightened down on the barrel. About every ten cycles, the top layer of wax paper begins to wear out, tearing when the lid is removed. It only takes a few minutes to stuff a new layer of wax paper into the groove and it's ready for the next several cycles.

55 Gallon Drum Sterilizer Fittings

On the bottom-front of the sterilizer I welded a 'street L', which is a 90 degree iron fitting with male threads on one end and female on the other. This is the feed from the boiler. Steam enters here, passing through the barrel and substrates, and then exits through a fitting welded to the upper rear of the barrel as shown on the right.

When the spin valve on top is open, steam vents to the outside air. This is used at the start-up of each cycle to purge air from the unit to ensure it's full of pure steam. Once all the air is purged, this valve is usually closed and the gate valve is opened, allowing the 'waste steam' to flow to the mushroom farm slab, providing essentially free heat. If no heat is required in the underground grow areas, such as during summer, this small valve is left slightly open to provide a constant flow of steam over the substrates.

Controlling the steam

When we're sterilizing substrates and require heat in the underground mushroom farm growing room, the valves are positioned as shown on the left. Steam travels from the boiler to the sterilizer, passes through the sterilizer, and then down through the open gate valve and out to the mushroom farm. When we're not sterilizing substrates and need heat, the valves are positioned as shown in the picture on the right. The valve to the sterilizer(with the round handle) is closed, and the gate valve leading from the sterilizer exhaust is closed. The gate valve from the boiler to the 'T' fitting leading to the mushroom farm is open, thus that's the path steam takes.

Admittedly, the insulation on the sterilizer is getting pretty ratty looking, but I don't want to make it permanent with a steel jacket for the reason that after every cycle, I unwrap it to allow for cooling. If I left the insulation in place after sterilizing, the substrates would still be hot two days later. Since we sterilize a full load every day, I have to wrap and unwrap the insulation daily. These bats have been in use for over six months now, and while the paper backing is wearing out, they still perform like new. Obviously, they have to be kept well away from the flame source since the paper will ignite.

Shelving in the 55 Gallon Barrel Sterilizer

The shelves for the bulk mushroom substrate sterilizer sit in the 55 gallon barrel by gravity. The bottom shelf simply slides in the sterilizer barrel, and the top shelf sits on the frame welded to the bottom one.

The bottom shelf is always loaded first, and then a piece of hardware cloth followed by three lengths of angle iron are placed over the folded up bags to keep them closed during sterilization.

A tyvek sleeve is placed into each bag before folding as shown in the Sawdust-Woodchips video clip.
The tyvek sleeve allows the bag to expand and expel air during sterilization, but prevents the bag from drawing in contaminants from the air as it cools. Once the bottom shelf is loaded and weighted, the top shelf simply slides in on top of it and is loaded,

with another sheet of hardware cloth and three lengths of angle iron placed on it, before the lid is attached and steam is introduced.

Once all the air has been purged out of the sterilizer, the valves are closed or reduced in opening and pressure is allowed to climb to 2 psi, which is only slightly above ambient. I maintain the correct steam pressure by adjusting the regulating valve on the propane burner. This unit has provided our farm with 100% success, whether we use it for supplemented sawdust or grains. I always keep it at 2 psi for a full 8 hours before shutting off the steam supply and removing the insulation from the sterilizer for a slow cool down. Feel free to Contact-Us with any questions you may have.

Marc R Keith, July 2010