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From what I've read, when you grow oyster mushrooms you can either sterilize or pasteurize the substrate, and pasteurization is supposed to be better because it preserves micro organisms that help reduce contaminants.

Recommendations for pasteurization seem to require heating for a long time, why is this - don't things die quickly in heat ?

I didn't have a thermometer to get the proper temperature so I decided to sterilize instead.

I experimented with microwaving my substrate in a jar to sterilize it, which people I've read on forums say isn't a good way to sterilize.

If I microwave substrate until it's sizzling, which I did, what is going to survive ? Why isn't this a sure fire way to sterilize anything ?

thanks.

Jimmy Widdle
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The reason that pasteurization is used when growing oyster mushrooms is that the media (usually straw) doesn't support contamination as easily as grain media since it's not as rich in nutrients. This means it's also much forgiving in terms of sterile technique when innoculating the media. Grain media readily contaminates in the home so needs to be sterilized.

The reason you have to keep the media heated for such a long time to pasteurize is that the media is usually quite bulky and you need to be sure that the heat is penetetrating the entire bag of media. You can see that pasteurizing milk takes 30 mins with shorter times depending on technique.

A microwave can't reach temperatures much higher than 100 deg C (because it's agitating water molecules) and therefore can not sterilize. If those water molecules are not contained, they turn into steam and leave the substance. If contained, such as in an egg, they tend to explode!

The substance to be heated also needs water content and fungal spores have a low water content making them hard to heat with microwaves.

A pressure cooker is thus pressurized to 15 psi to reach a sterilizing temperature of 121 deg C, and the 90 mins duration for jars is to make sure that that temperature reaches evenly throughout the jars. Bags of substrate such as wood chips have to be heated for more than 2 hours.

Note that straw can be pasteurized without heat using hydrated lime or other alkali techniques, and it's possible that a microwave could be used if you did fractional microwaving where you heated, cooled and allowed the spores to germinate once they had become hydrated, and then heated again in a number of cycles. But this sounds very painful.

Graham Chiu
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  • I see. I was using coffee grinds and cardboard and just microwaving it to sizzling point for a while. I figured if it was good enough for chicken it's good enough for this. That's a good point about spores being dry, though I thought they would still be getting heated, after all even the jar I used got really hot. So, digging further, there are organisms which can survive 100C ? – Jimmy Widdle Jan 21 '18 at 20:06
  • Coffee grounds need to be sterilized as it's a high nutrient substrate but usually when you get them freshly made they're sterile as the beans were roasted. The trichoderma spores survive 100 deg C as do other viruses etc. That's why medical instruments need to be autoclaved which is just a fancy pressure cooker. – Graham Chiu Jan 21 '18 at 20:13
  • You don't need to sterilize cardboard. It's a poor substrate for trichoderma so just soaking overnight in boiling water that's allowed to cool is sufficient. – Graham Chiu Jan 21 '18 at 20:16
  • Experience seems to be bearing all this out. All my coffee grind stuff is going green, but I tried some with just cardboard and that seems to be doing much better. Thanks for the info. Would it be safer to grow on cardboard first, then throw some coffee in when it's established ? – Jimmy Widdle Jan 21 '18 at 20:43
  • Buuut, what about microwave pressure cookers ? I have a microwave rice cooker that I think works up a pressure. Maybe it's worth experimenting with that. – Jimmy Widdle Jan 21 '18 at 21:14
  • Never heard of a pressure microwave cooker. Yes, start with cardboard and alternate layers of coffee with cardboard – Graham Chiu Jan 21 '18 at 21:55
  • To microwave, one needs water. Spores are not so watery. Additional, there is a misconception that microwave is uniform and heat the content. Microwave just heat the edges, doesn't go very in depth. Liquids will flow, so you have nearly uniform heat. For solid it is more complex, but one needs water inside, and some trick not to have the edge to dry. Then with some time, the heat will enter. A substrate is much more tricky [peat was used as good house insulation]: there is much air (microwave doesn't heat air), but not a nice flow. – Giacomo Catenazzi Jan 22 '18 at 07:54
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    For thin objects (e.g. herbaria), it was noticed that microwaving damage DNA, so it should be effective to sterilize, but a very thin substrate at a time. – Giacomo Catenazzi Jan 22 '18 at 07:56
  • Well, I did cardboard and coffee in an oven bag, sealed the top and microwaved. Coffee grinds are damp and It inflated with steam, which I hoped would steam sterilize everything. Yes a rice cooker is sealed and has steam pressure but I don't know what PSI etc. probably not great as they're plastic not metal. They're very cheap, you can google them. – Jimmy Widdle Jan 22 '18 at 11:59
  • So, apart from cellulose type food, do oysters need anything else ? There's not much mineral content in cardboard I would thnk. – Jimmy Widdle Jan 22 '18 at 12:04
  • The hydrated lime method is definitely way to pasteurize your straw for small-scale oyster mushroom cultivation for the following reasons. The lime raises the pH rapidly, killing most pathogens, and then the pH tapers down as it "ages" in the straw. The mushroom mycelium handle it just find, but molds and bacteria do not. The lime also conditions the straw to absorb water. Finally the lime adds calcium, which boosts fruiting production. This is an open air method that requires no heat and can pasteurize a half a bale or more of shredded straw in a 35 gallon trash can without any difficulty. – That Idiot Jan 22 '18 at 19:43
  • Why would calcium boost production ? I assume there is no calcium in cardboard or straw, and oysters can live off that. So what do oysters actually need to eat, other than hydrocarbons ? Maybe this is a separate question. – Jimmy Widdle Jan 26 '18 at 13:51
  • Oysters do pretty well in the wild, do they really need coddling ? – Jimmy Widdle Jan 26 '18 at 14:00
  • Update. I microwaved a mixture of coffee and cardboard in plastic oven bags a week ago and no sign of green yet. I sealed it with rubber bands and it hasn't been opened since, and the oyster mycelium is growing. – Jimmy Widdle Jan 26 '18 at 17:58
  • But you haven't sterilised with the microwave, just got something less than that. – Graham Chiu Jan 26 '18 at 22:30
  • Another update, the substrate I microwaved in a bag shows no sign of green but has a lot of oyster mycelium growing though it. It's looking good. – Jimmy Widdle Feb 11 '18 at 07:22
  • Update: I got some calcium hydroxide and used that to pasteurise a load of straw. It seemed to work OK, I wouldn't say it turned out better than heating, but with a big tub you can do a lot of straw at once. In Paul Stamets' book he gave a range that worked out at 6-10 grams per litre. I went for 8gms/litre. I also found that some of the seeds in the straw germinated. – Jimmy Widdle Apr 18 '19 at 15:21
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Pasteurizing coffee grounds in a microwave works.

I used 7.5 minutes on three litre buckets of coffee grounds, with lid fastened when it cools to just warm. When left unopened no mould grew, then after opening I would sometimes get Trichoderma (green mould).

Of course this process is not "sterilizing", but for this purpose killing off the mould spores is all that is required - the survival of various bacteria is not a problem.

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Polypipe Wrangler
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A microwave can definitely sterilize and will go above 100C. The fact that water boils at 100c does not stop the microwave from inducing continued energy on the water molecules though they will be in gas format as steam. 100c is also sufficient to kill all life forms if given enough time. The encapsulated spires are somewhat protected but once the proteins needed for life get denatured the mechanisms of life are broken.

Mats Hagstrom
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