Yeast Cluturing

[Scott H]

Thinking about giving this a crack but from what I have read on the internet I have to sterilise my slant tubes and Petri dishes in a pressure cooker. Is there an alternative way to do this (other than buying pre sterilised?) I'm assuming that starsan doesn't fit the bill in this instance?

Cheers

[Ralph]

There probably is an oven bake method that will sterilise as well? Problem is you may overcook any seals on lids etc.

Pressure cooker is your best option.

[Scott H]

To trademe it is then

[DMAC]

I used to use test tubes which I put upright in a pot with lid and boiled for 5 minutes with water part way up the sides. The test tubes had malt plus agar growth medium in them. During the boil the bungs were in but only loosely. After cooling I pushed the bungs in tight. I hope the guys working in infectious disease labs do a better job but this was sufficient for yeast propagation. Anyone know a beer spoiler that can survive higher than 100c (serious question - they may exist although they would live through the boil)? I gather this is why they suggest a pressure cooker (to get over 100).

[Sam DT]

The ability to survive high temps depends on the form of the bacteria, if they're hanging out like usual then a good boil or even 70+ is fine but in tough conditions they tend to form spores and bio-films which basically create protective layers which mean they can withstand higher temps.

Humidity and temperature combine to give a good kill, 170 deg dry heat for one hour or 160 deg dry heat for two hours will give you a similar kill to 134 deg for 5 minutes or 121 for 15 with steam. There's also a method where you heat to 80+ degrees three times in a row about 24 hours apart - basically you the bacteria are like sweet - threat is gone, I'll jump out of my spore form and then you hit them again - looking at using this to try and sterilise a plastic fermenter I've had an infection in.

I've never done slanting but I would assume you would want to be as sterile as possible as your yeast won't be burning through all the available nutrients and pumping out toxic alcohol like you would when brewing. If the spores wake up in a bunch of nutrient and nothing but dormant yeast hanging around they could well have a field day. All that said if you're keeping things relatively sanitary I'd guess you'd probably have a good chance of not having issues even with just a good boil - seems to have worked for you?

[DMAC]

Yeah it did work out alright. I had 10 or so yeasts that I kept for about a year and did around three brews off each with no issues.

Thinking about it, there is probably more risk in the inoculation and harvesting stages where they are exposed to air (some home brewer in the states will have a clean hood to work under!). Even after autoclaving the slants are still exposed to some level of airborne bacteria. I recall flaming the rod for inoculation and wiping the tubes down with alcohol prior harvesting.

Also, I guess the effort you go to is dictated by how much you are risking. If you run a 25000l batch then the stakes are pretty high.

[Ryan de Raat]

So a bit of back ground, my girlfriend that has be helping me with this did her studentship in wine sciences, I'm still tying to get my hands on her sav blanc yeast strain to try in a cider. Anyway what we are doing is using a pressure cooker to sterilise a schott bottle of growth media (agar + DME), then we poured into plastic disposable Petri dishes. It is quite important to pour the plate near a heat source ie. A Bunsen burner. This heat source creates a up draft of air that helps stop infections. Then once those are poured and cooling hit em with a germicidal UV light. Have you had a read of "yeast"

Ps You can pick up cheap pressure cookers from little Indian marts