The Phase H Chamber Steam Pasteurization

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A second method calls for the placement of straw in a highly-insulated room into which steam is injected. This room is known as the Phase II Chamber. Before the straw is loaded into the Phase II chamber, it must be moistened. This can be done simply by spread;ng the chopped straw over a large surface area, a cement slab or plastic tarpaulin to a depth no greater than 12 inches. Water is sprayed on the straw via sprinklers over a two to four day pe riod. The straw is turned every day to expose dry zones to the sprinkfng water. After several turns, the straw becomes homogeneous in its water content, approaching 75% moisture, and is reduced to about 1/2 of its original volume. Short stacking the straw is not intended to ac

Figure 147. After 1-2 hours of submerged pasteurization, the basket is lifted out. After draining excess water, the straw cools. Grain or sawdust spawn is broadcasted over the surface an/ n.ixed throughout the straw complish composting, but rather a way of tendering the straw fiber, especially the waxy, outer cuticle. In contrast to composting, the straw is not allowed to self-heat. Once evenly moistened, the straw is now ready for loading into the steam chamber.

An alternative method calls for the construction of a large vat into which straw is dunked. This tank is usually fitted with high pressure water jets and rotating mixing blades to assure full moisture penetration into the straw. If given sufficient agitation, finely chopped straw gains 75% moisture in the matter of minutes. Once moistened, the straw is loaded directly into the Phase II chamber.

One ton of wheat straw, chopped and soaked, occupies approximately 250 cubic feet

' cultivating gourmet mushrooms

Chamber Construction Mushroom Compost

of space, equivalent to 10 ft. x 10 ft. x 2.5 ft. Thi figure is helpful in sizing a pasteurization chamber. An additional 25% allowance should be made for variation in the chop s ^e of straw, air plenums, and handling needs. Five dry tons of wheat straw functionally fills a thousand square foot growing room. Most growers fill growing rooms to no more than l/4th of total a volume. I prefer to fill to only l/Mh of capacity This means that for every 8 air spaces, 1 space is occupied by substrate. (In other words, the ratio of air-to-substrate space is 7:1)

The classic Phase II room has a raised false floor, screened several inches above the true floor upon which steam pipes are situated (See Figures 148-150). The walls and ceiling are well insulated. The interior pan els are made of heat resistant, water-proof materials. Many convert shipping containers used to ferry cargo on ships into Phase II chambers. Others custom build their own steam rooms Another important feature is a floor drain fitted with a gate valve. This valve prevents contamination from being drawn in during and after pasteurization.

Boilers provide live steam, dispersed through the pipes, and into the straw, which can be filled to a depth as great as 8 ft. The greater the depth, the longer heat takes to penetrate to the center. Heat penetration can be enhanced with high-pressure blowers. Multiple thermometers are inserted in at least three, various locations: low (within 4-6 inches), midway, and high (within 12-24 inches of the top surface). 'hese temperature probes should be monitored periodically to gather data for the generation of a pasteurization profile specific to each run. Over time,, the temperature points of each successful batch are accumulated for establishing a baseline for future operations.

When steam is injected, the outer edges o the straw mass heat up first. An outer shell of high temperature forms, and over time increasingly enlarges towards the center. Early on in the Phase II process, three thermometers can read a range from room temperature to 160° F. (71° C.) simultaneously. This temperature differential must be monitored carefully. The

Steam Pasteurization
Figure 149. Newly constructed screened floor racks which allow the passage of steam underneath the mass of bulk material being pasteurized.

centers of densely packed Phase II steam chambers remain below 100° F. (38° C.) for several hours, lagging behind the hot outer shell, and suddenly race upwards. If steam output from the boilers is not reduced in time, the entire mass continues to heat at an uncontrollable rate. Without the cold core, which in effect is a heat sii.k, to deflect the spiralling increase in temperature, thermal momentum continues for an hour or two beyond the time steam injection is shut off. The minimum rec ommended time for steam pasteurization is two hours above 160° F. (71° C.).

A common oversight in steaming a mass of straw or compost ;s the failure to chart, every 30 minutes, the temperature profile of the mass. By measur ng and charting, trend analysis is possible. If the climb in temperature is not anticipated, and reduced at the right time the thermal momentum of the hot outer shell not only overwhelms the ever-shrinking cold core, but causes the entire mass to skyrocket to 200°+ F. (93° C.), a temperature above which disaster awaits. Above this temperature plateau. non-competitive, beneficial organisms are killed, and the substrate becomes an open habiiat for many competitors which would otherwise be held in abeyance.

When the steam output from the boiler is turned off, the Phase H box should be immed' tely positive-pressurized with con-:aminant-free air. By forcing air through a HEPA filter and ducting the air directly into the Phase II chamber, contaminants are prevented from being sucked in as the mass cools. For a steam box measuring 10 ft. x 10 ft. x 10 ft., a 1/8 HP blower pushing 200 CFM through a 12 in. x 12 in. x 6 in. HEPA filter (99.99 % @ 3jS) adequately positive-pressur-

Pasteurization Chamber For Mushroom
Figure 150. Inside a large Phase II chamber.

Steam Pasteurization Profile

Steam Pasteurization Profile

Pasteurization Chamber

00 1:00 ' 3:00 ' 5:00 Phase II Temperature Chart

» Maximum Temperature » Minimum Temperature

00 1:00 ' 3:00 ' 5:00 Phase II Temperature Chart

» Maximum Temperature » Minimum Temperature

Figure 151. Chart: Temperature Profile of a Phase II

izes the chamber. (Larger Phase II rooms will require correspondingly higher pressure tans able to push air over 2-4 inches of static pressure.) The substrate mass slowly cools in 12-24 hours to temperatures tolerable for inoculation, generally below 105° F. (38° C.).

Before the pasteurization chamber is opened, the inoculation area is intensively cleaned with a 10% bleach solution* To aid the cleaning process, venturi siphon mixers are ideal for drawing bleach directly into a hose line at the faucet connection. Conveyor belts, counter tops, funnels, ceilings, and walls are all cleansed with torrents of chlorinated water. Spraying down the room with such a solution is colloquially termed "bleach bombing" in the industry

* Most brand name bleaches have 5.25% sodium hypochlorite. One tablespoon of bleach in a gallon of water is roughly equivalent to 200 ppm chlorine. A cup of bleach per gallon of water is equivalent to 3200 ppm. Most mushroom mycelia are harmed above 200 ppm of chlorine.


Protective clothing is advised for the sanitation personnel.

Methods vary for the unloading of the Phase II chamber. Some remove the straw by hand with clean pitch-forks and throw the straw onto stainless-steel tables whereupon the inoculation occurs. Conveyors are favored for substrate handling by many growers. The largest Phase II chambers utilize a netted or "walking" floor which pulls the substrate mass into two outwardly rotating horizontally positioned, teethed cylinders. (See Figure 148). As the substrate mass is forced into the space between the two outwardly rotating, spiked cylinders, the straw is separated and ejected onto a depressed platform in whose center is a funnel or ramp that leads to conveyors. (Warning: the danger of personal injury here at this juncture is notorious. Special precautions must be implemented to prevent accidents.) After the straw is thrown into the conveyor belt, grain spawn is gravity fed or hand broadcasted onto the straw as it is being ferried away. Foot activated switches are helpful in controlling the off-loading of the substrate from the Phase II box with the conveyor.

When spawn is placed directly upon the surface of pasteurized straw, mixing is strongly advised. Cement and soil mixers, specially adapted funnels, ribbon blenders, and "Archimedes screws" suffice. If the spawn is lc1 id upon straw and not mixed through, growth layers form resulting in uneven colonization. The advantage of removing the straw and inoculating by hand is that the process can be interrupted and recurrent cleaning can occur. By intermittently disinfecting, cross-contamination can be prevented. With automated, continuous loop systems, the likelihood that contamination can travel throughout the facility unchecked is greater. Special attention to detailed disinfection is necessary with these systems to prevent disastrous results should pasteurization be incomplete. Once spawn has been sown throughout the straw, the inoculated substrate is placed directly into the "fruiting" containers, usually columns, trays, or bags. Each container must be vented so the mycelium can respire as it colonizes the substrate.

Several inexpensive, alternative methods can be used for treating straw (and other bulk materials) that do no :nvolve heat treatment. The first three are chemical; the last is biologi cal. Surely other alternative methods will be developed as imaginative entrepreneurs experiment. By sequencing a substrate through a combination of biological and chemical treatments, heat pasteurization can be entirely avoided. Small pilot-scale experimentation is strongly encouraged before cultivators attempt these techniques commercially. The future use of such methods is promising.

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