Dry heat sterilization

[Izzie]

I have read in a few articles that baking media for more than a certain amount of time can cause toxins to develop/build up.

Has anyone heard this, or found it to be true?

In the mean time, I'm sticking with boiling it for 30 min and soaking in physan.

[BobInBonita]

From a microbiological point, dry heat is less efficient than moist heat for sterilization. It takes a higher temp for a longer time to achieve either pasteurization or sterilization.

The material being sterilized usually defines the best method. I believe something like LECA can take almost any amount of wet or dry heat. If you try to give bark mix the same dry time and temp that LECA will take, you will first char it (creating all kinds of partially oxidized hydrocarbons) and may eventually set it on fire. Materials like plant hormones that have "biological" properties usually require very gently treatment, and solutions can be filter sterilized if needed.

Very short answer to a really complex subject - sorry

[Izzie]

Thank you Bob, that is very similar to what I had suspected and heard.
I'd assume that boiling counts as wet sterilization? Is the 30min enough to kill the nasties?

[Royal]

Boiling doesn't sterilize, but it does sanitize. That should be good enough for a bark medium, and is probably more than most of us do. I just soak mine with Physan.

I've never heard of dry heat forming toxins, but Bob's explanation makes sense. I can see how really high heat could change the structure of some things (a la fried egg).

[Izzie]

Sound good Royal. I boil mine to get the new bark to hold water better from the start.

[BobInBonita]

Izzie is 100% correct about boiling just being a "sanitization", although it is a wet heat process that is still way better than dry heat (at the same temperature) at reducing the bacterial load.

True wet heat sterilization would take autoclave temperature AND pressure for at least 15 minutes. At this point there are theoretically no surviving organisms that will grow under normal conditions. A pressure cooker is the home equivalent of a lab or industrial autoclave.

With flask cultures, any yeast, mold or bacteria surviving can overgrow and cause problems, so the medium needs to be as close to sterile as possible, hence the pressure cooker.

I think the mega-question that should be probably be debated by casual growers is this: With natural growing conditions there is a normal flora. When we heat treat, we shift the flora towards spore forming bacteria and destroy first the yeasts and molds, then the gram negative, and gram positive bacteria, and finally spore formers (some of which are VERY resistant). The normal flora in many cases is in balance or equilibrium (provides certain pH limits, nutrient availability, etc). When this is shifted, how much do we affect the ongoing culture? Do we create a need for physan, peroxide, listerine, etc. that we wouldn't need in nature? Are we sometimes too clean?

I don't know the answer to my own questions. Since I'm just growing for pleasure and don't do flasking or stem propagation (yet) I generally just soak media to wet it, and let nature take its course. Once it's out in the Florida environment, birds, squirrels, geckos, and everything else is going to instantly ruin any attempt at controlling the media.

[Izzie]

Originally Posted by BobInBonita

Izzie is 100% correct about boiling just being a "sanitization", although it is a wet heat process that is still way better than dry heat (at the same temperature) at reducing the bacterial load.


I don't know the answer to my own questions. Since I'm just growing for pleasure and don't do flasking or stem propagation (yet) I generally just soak media to wet it, and let nature take its course. Once it's out in the Florida environment, birds, squirrels, geckos, and everything else is going to instantly ruin any attempt at controlling the media.

I agree with your thoughts about being too clean- I've wondered the same thing myself.

Do you think the boiling sanitizes enough that I would not have to worry about mold? That has been my biggest problem- it is frustrating that it is so resistant to my efforts.


How long have you been doing the soak and plant method? You've really not had any bad results? I'm still a bit leary, but it certainly would be more convenient...I grow mostly indoors though.

[BobInBonita]

Izzie,
Mold vegetative cells (and most mold spores) are killed at well below boiling temperatures. All mold spores that I can recall are easily killed at boiling temperatures.

I have been doing the "soak and plant" for about three years now. I have had problems, but I do not think even sterilization would have prevented them, because of how I grow - outdoors in SW Florida. We are surrounded by mold, and (I think) any attempt to eradicate it leads down a path of increasing chemical use and weaker plants. When I have had a slightly more serious problem than a small spot on a leaf, I do use Daconil spray. It doesn't cure the problem, but it lets me feel I've done something until the leaf ages and falls off.

The problems I have had are directly related to choosing a medium that will hold too little or too much water in my part of the world. Indoors where you are in total control of watering you can eliminate many uncertainties.

Again, just my opinion, but I think orchids are beautiful and rugged plants, designed to live in a dirty moldy world. If you let them be, they will adapt and survive. If you change their conditions too often, they never adapt and they get weaker. Example - today I repotted two phals that have lived outside since 2002 (guessing - it was before my recordkeeping started). They were inexpensive Costco Noids. I was so inexperienced when I got them, I didn't know about pot hangers, and I wired them up with copper electrical wire. They were still in their original clay pots with a plastic pot inside. Over these rainy and dry seasons they hung in a large guava bush and received marginal care. Even today they both had active nodes on a couple of spikes. The original media had almost rotted away and been replaced by roots, and the roots had filled the space between the pots and had circumnavigated the pots many times over. There was evidence that on one or two occasions they had been stressed (root "centers" without the surrounding tissue, which had rotted with the media). They hadn't been repotted for so long because they always seemed to either be in bloom or in spike. If I had had the knowledge to respond to every stress and attack, I question whether they would have given nearly the joy they did for these past years. I've attached pics (I hope) of before the repot, after extricating the roots, looking up into the root ball, and after the repot.

Obviously, everyones situation is different. If I were a grower or competitor, those who judge would have frowned on my plants. As someone who wants to look out my kitchen window and smile, with as few headaches and as little work as possible - it works for me.

http://www.orchidboard.com/community...68x1024_4_.jpg

http://www.orchidboard.com/community..._768x1024_.jpg

http://www.orchidboard.com/community..._768x1024_.jpg

http://www.orchidboard.com/community..._768x1024_.jpg