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GETTING  THE  MOST  FROM YOUR  AUTOCLAVE
>From the April 1994 issue of:
     The Mushroom Growers' Newsletter
     P.O. Box 5065
     Klamath Falls, OR  97601
Copyright 1994 - All Rights Reserved
(Posted at gopher.teleport.com with permission)

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Our experience with the topic we are about to address includes a lot of
college level physics and engineering thermodynamics, several year's
experience operating a high pressure steam retort in the vegetable canning
industry, and recent experience helping Lloyd Jones, a local grower, resolve
some problems in his sterilization process.  In this article we'll cover the
key information you need to know to operate your autoclave or pressure cooker
successfully and economically.
 
Most growers of sawdust based shiitake or other exotic mushrooms use an
autoclave to sterilize their substrate. Generally these autoclaves were
originally designed for the vegetable canning industry.  In that industry
they are referred to as "cookers" or "retorts".    They are nothing more than
a very heavy steel pipe, typically five feet in diameter, and 10 or more feet
long, with a door on one or both ends, a small pipe for the entry of steam,
an exhaust port, and some instrumentation.  The instrumentation includes at
least a pressure gauge and, often, a thermometer.  

Other typical attributes include an emergency exhaust port, steam
distribution pipes within the retort, tracks on the floor of the retort, and
sometimes a water inlet pipe.  The water inlet pipe is used in the canning
industry to allow cooling of the finished cans while still under pressure.
This is necessary to prevent the sealed cans from breaking open when they are
removed from the retort. If your retort has an extra pipe, that's probably
what it was used for. It's not needed in substrate sterilization because the
bags are not sealed. Pressures can equalize without a problem.
 
The key difficulty experienced by mushroom growers is inadequate
sterilization of the substrate.  Some bags will typically show contamination
while others do not.  This problem is due to inadequate heat for sufficient
time to kill all contaminants. Sterilization requires temperatures of around
250 degrees F for about fifteen minutes.

Water boils at 212 degrees F at sea level without additional pressure.  To
get steam temperatures as high as 250 degrees F requires pressure.  That's
where the retort comes in. High pressure steam from the boiler is forced into
the retort, where the cool air is bled off, until the desired pressure and
temperature is achieved and can be maintained.  What is the desired pressure?
 It depends upon your elevation above sea level.  According to published
steam tables [1], an absolute pressure of 30 psi is required to get steam to
a temperature of 250 degrees F.  At sea level theatmospheric pressure is
about 14.7 psi so your retort pressure gauge must read 15.3 psi to assure
that an absolute (i.e. total) pressure of 30 psi is achieved.  Atmospheric
pressure varies and these gauges aren't terribly precise so you've probably
been working with advice that 15 psi is what's needed.  That's close enough
unless your location is above sea level.  The higher your elevation, the less
your atmospheric pressure and the more pressure is required in your retort.
 The following table shows how that works:

	    ELEVATION       ATMOSPHERIC             RETORT 
	    ABOVE SEA          PRESSURE               PRESSURE
		 (feet)                  (psi)                        (psi)
		       0                   14.7                          15
		1,000                   14.2                          16
		2,000                   13.7                          16
		3,000                   13.2                          17
		4,000                   12.7                          17
		5,000                   12.3                          18
		6,000                   11.8                          18
		7,000                   11.4                          19
Atmospheric pressure (p) is calculated as:
   p = 14.7 e^-(ELEV/27621)           where  e = 2.7183.
The  retort pressure is simply 30 psi, the desired absolute pressure, minus
the atmospheric pressure and rounded to the nearest pound.  If you're at 4000
feet elevation, like us, and have been relying on 15 psi steam to sterilize
your substrate, you'd better add a couple pounds. If your retort lacks a
thermometer, it would be a good idea to add one as a double check.  

Variations in weather can also affect the atmospheric pressure at your
elevation, sometimes by as much as 2 psi.  Pressure gauges are not perfect
either and pressure within the retort can easily vary as much as 2 or 3 psi
if not closely monitored. When all these factors are considered, your 15 psi
gauge pressure may be as much as 7 psi too low!  This would give you only 235
degree F steam - not enough to do the job.  You could invest in some nifty
gadgets to monitor and adjust your pressure or you can simply use higher
pressures to insure attainment of the desired temperatures. Lloyd tried
increasing his gauge pressure to 25 psi. That worked fine although something
a few pounds lower probably would work well too. Be sure your retort is
capable of handling these pressures before trying this. 

The cost of providing steam doesn't vary much whether you use 25 psi or 15
psi.  In fact, Lloyd reports that the natural gas to generate his steam costs
about $1.00 per hour.  That makes the price of steam an insignificant factor.
 At our elevation, a gauge pressure of 25 psi should produce an absolute
pressure of about 38 psi and a temperature of about 265 degrees F, if
everything is operating properly.  That's theoretically warmer than
necessary, but it shouldn't harm anything.

Once you get the desired temperature inside your retort,it takes awhile to
transfer the heat entirely through each block of substrate.  How long depends
upon a number of factors.  Dry sawdust is a pretty good insulator and it
takes a long time to move the heat all the way into the center of a bag. The
addition of water greatly improves heat transfer and makes this type of
sterilization practical.  Still, the time required to move the heat into the
center of the substrate bag varies considerably depending upon the species of
the sawdust, the size of the sawdust granules, and the type and quantity of
amendments you have added to the substrate.  Paul Stamets reports [2] that a
pure hardwood sawdust substrate may take 2 to 3 hours to sterilize while
sawdust supplemented with rice bran may take 4 to 5 hours.  Lloyd found that,
at 23 psi, four hours was sufficient for his substrate. He did let the bags
cool for another hour inside the retort.  That should also improve his
results. There are also other variables to consider.

When bags in one part of the retort are adequately sterilized while bags in
other parts remain contaminated, one factor to consider is the distribution
of the steam within the retort.  Larger retorts usually include steam
distribution pipes inside the retort.  These perforated pipes allow the steam
to be sprayed into the retort along its whole length as well as on both top
and bottom.  Carefully run a little steam into your retort while the doors
are open to see if these pipes are functioning properly.  You may need to
unplug some of the distribution holes.  The way you pack your bags into the
retort also affects the distribution of steam and the time it takes to move
the heat through all the bags.  If you pack as many bags as you can into your
retort, you have eliminated the space between bags that would allow steam to
freely circulate. This requires the heat to be transferred through the
substrate a much longer distance and that takes time.  

The way you prepare your substrate bags also plays a role.  Air is a great
insulator so you want to remove as much of the air within your substrate as
possible.  Simply pack the substrate tightly into the bags.  This will
eliminate air pockets and improve your sterilization results.

You can try these few simple adjustments and check the results by using
sterilization indicator strips.  Lloyd used some manufactured by Propper
Manufacturing Co. Inc. in Long Island City, New  York.  The strips are made
of paper and are about 1/2 inch wide and 8 inches long.  They have a small
area of heat sensitive material which turns black when sterilization
temperatures are held long enough.  The figure (in the paper version of he
Newsletter) shows a strip before and after sterilization.  A strip can be
packed inside your substrate bags with one end near the center of the
substrate and the other outside the substrate, but still inside the bag.
 That allows a quick check of the visible part of the strip to determine if
the air inside the bag got hot enough.  If it didn't, the substrate didn't
either. Lloyd found that the strips tend to tear when pulled from the
substrate after sterilization so he wrapped the strip in clear waterproof
tape to give it some strength.  To check out your retort's effectiveness,
pack the retort as usual, but add sterilization indicator strips to bags
scattered evenly throughout the retort.  When sterilization is complete, a
check of these strips will reveal problem areas within your retort.  These
difficulties can probably be resolved by adjusting the way you pack the
retort so that these cool locations are more accessible to the circulating
steam.  

If your testing shows that all the bags are being sterilized, but you still
have contamination, you will need to look at your post-sterilization
procedures.  Are contaminants getting into your substrate between the time
you turn off the steam to the retort and the time the bags are inoculated and
sealed?  Procedures very so much from farm to farm that you will have to
think this through for your specific situation.  

Is contaminated air pouring into the retort when you open the door?
 Pressures can be equalized by allowing filtered air to enter the retort
through a special valve or you can open the retort while the air temperature
inside is still warmer than outside.  This will minimize the problem.

Are your bags staying in a semi-sterile environment as you cool them and move
them from the retort to your inoculation area?  Some growers blow filtered
air onto their substrate bags to cool them faster while keeping contaminants
away. Are you following good lab procedures under your laminar flow hood as
you inoculate and seal the bags?  Remember that contaminants on the outside
of the bag can enter the bag when it is opened to insert the spawn whether or
not it is under a laminar flow hood.  Simply wiping the top edges of the bag
with alcohol before opening it under the hood may be enough to prevent stray
contaminants from entering the bag.  Care in all of these areas will reduce
contamination problems and allow you to move more clean bags from your
incubation room into your growing cycle.  That will improve your profit.

REFERENCES

1. Obert, Edward F.; Young, Robert L. 1962. Elements of thermodynamics and
heat transfer. 2d ed. New York: McGraw-Hill Book Company; 538 p.

2. Stamets, Paul. 1993, Growing gourmet and medicinal mushrooms. Berkeley:
Ten Speed Press;552 p.

