11.5 P.s.i. V's 15 P.s.i.???

[naja naja]

G'day all,

I just got given a pressure cooker that, on it's high setting, says it only goes up to 11.5 P.s.i. I know this will be much better than just steam sterilization, but does anyone know how it compares to 15 P.s.i. sterilization? Can I just increase the cooking time for the same results? Anyone who has tried these lower pressures b4 please let me know. I would like to know if it's worth the $200 for a 15 P.s.i. or weather 11.5 should be O.K. for most things?

Also, if I want to test my sterilization and sterile technique's, could I just run mock jars of diff substrates, re-enact the innoculation with sterile water, then wait a month to see signs of contamination? If no contam, then would suggest good sterility had been achieved? What is the most sussuptable substrate for me to run tests on? Birdseed, pop corn,cow poo/straw mix, etc

Any info would be great, especially about the 11 P.s.i.

Thanks naja

[genki]

it might be a good idea to do multiple sterilisations a couple of days apart, as you would if sterilising with a normal saucepan. thats just a guess from what ive read though, i dont have much experience with that type of sterilisation.

[occidentalis]

I just got given a pressure cooker that, on it's high setting, says it only goes up to 11.5 P.s.i. I know this will be much better than just steam sterilization, but does anyone know how it compares to 15 P.s.i. sterilization?

11.5 psi is obviously not as good as 15 but honestly I think a lot of people are being a bit fastidious about the 15 psi thing. I used a dodgy opshop PC that didn't come close to sealing and had reasonable results until I could get a better one. Now I have a less dodgy opshop PC which does seal fairly well but I have no idea what pressure it reaches. When I'm careful I don't get any contams. When I'm slack I might get 2 or 3 jars out of 10 contam'd. You may get a slightly higher contam rate but in my mind this is not a big problem. Just do more jars.

Also, if I want to test my sterilization and sterile technique's, could I just run mock jars of diff substrates, re-enact the innoculation with sterile water, then wait a month to see signs of contamination?

If you have nothing else to do with your life. Why not just add some spores to the sterile water? There's a good chance that at least some of your jars won't contam and then you'll be a month closer to fruition.

If no contam, then would suggest good sterility had been achieved? What is the most sussuptable substrate for me to run tests on? Birdseed, pop corn,cow poo/straw mix, etc

If you really want to do this, then use the substrate that you plan to use for your real spawn run. That will give you the best idea of whether or not your PC is suitable for the application you have planned for it.

[AndyAmine.]

All Hawkins pressure cookers will pull 15psi and are also under $200 when you buy them from http://www.pressurecooker.com.au/Pressure%20Cookers.html

If you can spare the extra cash I would advise you get a stainless steel one but if your only doing sterilization with you Aluminium is fine.,

[gerbil]

15 PSI+ results in temperatures of 121 degrees C which is the level at which full sterilisation should occur. At 121 C all living cells should be killed off, leaving a clean substrate for innoculation of desired species.

Anything lower than this and you more than likely won't achieve full sterilisation (even with extended cooks AFAIK). To compensate you can increase the cook time And/Or increase your innoculation rate. It will be situation specific, though more vigorous species and having a good isolate will increase your chance of success by narrowing the time frame for competitor fungal/bacterial spores etc. by having a shorter colonisation period.

It's alot easier and more forgiving when you have a healthy culture established. For example when you do techs. like paper pellet oven bags, the paper pellets are favoured by wood loving species narrowing the chance of competitor species (yet not eliminating), so you can 'super pasteurise' rather than fully sterilise the substrate which you then launch healthy spawn to (possibly at a higher spawn rate if you desire) which if you get at the right stage of growth, should colonise the substrate in the least amount of time, closing the window for contam and resulting in a clean culture of more spawn or a fruiting bag.

If you can afford to and are wanting to continue with this hobby for a long time, I'd highly suggest getting something that goes to 15 PSI in stainless steel. Further down the line you are going to want to be able to sterilise petri dishes, scalpels/handles, grain/sawdust spawn, agar, syringes, other lab stuff etc etc.

Also if you are wanting to get lots done in one day, extra cooking time is an absolute pain in the arse, it might not seem like a big deal but when you get into it you'll be wishing you didn't have to waste the extra time/gas/space etc.

[naja naja]

thanks for that gerbil.

Is there any mathematical meathod for me to work out what temperature my 11.5 p.s.i. cooker gets up to?

Thanks for the good info

[smogs]

there is but im too lazy to search for you...

my brother has a chart that has what temp water boils at under diff pressures, try search for steam pressure temprature chart.

i would recomend a pressure cook... wait 2 or 3 days then do it again

extended time wont really help.. the time is to allow for heat to penetrate into grain. It wont get any hotter by leaving for longer

[naja naja]

yeah I realized longer wouldn't equal hotter, just didn't want to point it out to people.

Also why bother sterilizing 2-3 days later again. It will only sterilize as good as the first time.

Unless some bacteria hatch then are killed by lower temps.

I've heard of acceptable results with BRF cakes just useing steam pasturization so imagine any pressure cooking will get better results than just pasturization. Grains I imagine would be a stretch. I think ill go a 15 p.s.i. one anyways, as soon as i got some spare $$$$$.

If for nothing more than peace of mind.

[Fenris]

Excuse my nerdyness.

Psi to Kpa conversion is this. 15 x 6.895 = 103.4 Kpa plus relative pressure of 101.3 equals 204.7 Kpa which equates to 2 bar

Most standard steam sterilisers (autoclaves) run at 2 bar. The ones I run operate at about 208 Kpa.

11.5 Psi equates to 180.6 Kpa or about 1.8 bar

There are lots of fancy definitions to work out the kill time for a 6 log reduction in spore count. The lethality @ 121.1 is measured in Fo values. A reduction in pressure will affect the duration to accomplish the 6 log reduction however you would probably only be looking at maybe 2 minutes extra over a sterilisation cycle to reach the required Fo value.

All this is worked out using a thermophilic bacterial spore such as Geobacilus stearothermophilus which is very hardy. Anything you find in a BRF cake will not provide such a microbial challenge.

So don't be concerned about your pressure cooker 'only' being 11.5 Psi it is still doing the job just fine.

All that is only relevant for steam sterilisers because there are things such as bacilli spores that will survive that temperature that havn't had adequate steam penetration. So make sure everything can breathe, tyvek is good for that it allows steam penetration and won't pass bacteria.

Hope that wasn't too boring.

[naja naja]

Wow!

Know ur equations huh?

Thanks heaps for that info. Just to check that I understood you properly.

11.5 is! sterilizing as good as 15p.s.i. aslong as I prolong the cooking time a few minutes every hour?The extra time is to allow for full penatration of the heat?Even if I was using say pop corn? or agar mixes?

I get, that there's not too much bacteria in a BRF, as to cause concerns between the diff pressures. Thats good news!

So is critical temp(>121) a factor or is it just getting the heat thats there(<121), to fully penatrate?

That was far from boring, great info

Cheers :D

[smogs]

the reason for the double cook is because most bacteria have a dormacy state where they dehydrate themselves and become VERY tough... if you hydrate (say pressure cook) then they come to life and start to grow and hence become weaker... so u hit them again and they die

[Fenris]

Gotta go to pub

but water boils at 100 Deg. Up the pressure and up the temperature it will boil at, so you can put more heat in at increased pressure. the pressure dictates critical temp.

You understood fine

Must get beer.

[naja naja]

b4 the vogons destroy the earth

Waiting for a lift from the dentracy cooks r we?

Don't forget your towel!

[reshroomED]

Ideal Gas Law

PV=nRT

ed

[Fenris]

D Values............ Don't talk to me about D Values.

http://www.micronews.info/articolo_detail.asp?AID=165&RID=22

Interesting read about sterilisation.

I can give you a full run down with formula regarding sterilising next week when I have my PDA journal of Steam sterilization in asceptic manufacturing Draft 13, and also the EU standards if anyone is really interested.??

Outside of industry its not that interesting.

All it will take is someone to ask for a sterilising sticky?? I dare you....

But please don't

But if you do I will try to make it painless and short.

[Rev]

Hey Fenris

We need a sterilising sticky

know anyone whos qualified to do it?

[Fenris]

I will see what I can put together.

[Fenris]

After a quick look through my sterilising bible I came to the realisation that a pressure cooker running at 11.5 Psi can only obtain a maximum sterilising temperature of 117.2 Deg C. This has approximately 1/3 the kill rate of a pressure cooker running at 121.1 Deg C. Having said that, the chances of encountering a thermophilic bacilli with a high D value is probably pretty slim. An extended sterilisation time would be recommended. The industry standards for Biological indicators for use in steam sterilisers often refer to G. stearothermophilus which is extremely hardy spore, and the usual population of a Biological Indicator is in the order of 10^6 which is a significant challenge.

Naja Naja I still have faith in your pressure cooker.

I will endeavour to put together a comprehendable sterilisation sticky but I might need a week or 2 to get it up to speed.

[naja naja]

Thanks heaps for all your info.

My mate bought an old, but 15p.s.i. pressure cooker for $1 at the op shop. Seals are good and he said I can borrow that till I buy a 15p.s.i. one myself.

Ill just cook some good stews in the 11.5

Cheers again

Naja

[mescalito]

One slight correction sorry gonz...up the pressure and LESS temperature is required to boil water.

It's inversely proprtional.

[Fenris]

I think you might be wrong about the pressure thing.

If you reduce the pressure, a liquid will boil at a lower temperature.

Water at 121.1 and 2 Bar is not boiling.

Industrial autoclaves incorporate a cooling cycle that gradually reduces the pressure with the temperature so the liquid product in a container does not boil. This is especially important in a sealed container.

The mountain climbers climbing everest were never able to get a good cup of tea because at the reduced pressure up the mountain water was boiling at something like 85 to 90 deg C.

[occidentalis]

I think you might be wrong about the pressure thing.

If you reduce the pressure, a liquid will boil at a lower temperature.

Yep, because the boiling point is when the vapour pressure of a liquid matches the atmospheric pressure

So if the atmospheric pressure is lower, the required vapour pressure will be lower

and therefore boiling temp will be lower

or something, haven't studied chem for 3 years

[mescalito]

Pressure and temperature are proportionately related but HEAT is a different thing and is measured in Kj not celcius.Latent heat is measured in Kelvin so it gets even more confusing.

Are we talking temperature needed or heat applied?

The reason the Everest guys had trouble was because of the lower pressure they required more heat and a higher temperature to boil the water which was nearly impossible to achieve considering the ambient temperature.

Most people spin out when you tell them that their refrigerator cools their food by boiling liquid gas!

The Kelvin factor gave Kelvinator their name as latent heat being measured in Kelvin is the immeasureable factor in any change of state ie.solid>liquid>vapour but it's calculated that it exists.

 

If you REDUCE the PRESSURE, a liquid WILL BOIL at a lower temperature.

.... that gradually REDUCES the PRESSURE with the temperature so the liquid product in a container DOES NOT BOIL.

You're contradicting yourself there bud ;)