Use of stir plates

[bobtheUKbrewer2]

Is there not a significant risk of airborne bacterial contamination ?

 

[myndflyte]

When you use a stir plate, you put an airlock in your flask. So with proper sanitation, nothing should get infected.

 

[Rhaop]

I sanitize a piece of foil and fold that over the top of the flask, no problems yet

 

[dstockwell]

Use a sanitized piece of foil, no need for an airlock.

 

[bobtheUKbrewer2]

but if air can get in, air contains bacteria, I thought the idea of a stir plate was to mix and oxygenate.....

 

[MapleGroveAleworks]

Air gets into wort too when you pitch your yeast, what's the difference?

 

[Rhaop]

The stir plate drives off co2, if you air lock your flask oxygen can’t get in, what i have read is that essentially the foil blocks bacteria from dropping in but allows oxygen to flow up and into the flask

 

[bobtheUKbrewer2]

Air gets into wort too when you pitch your yeast, what's the difference?

I use a starter jug to hydrate yeast before pitching, with the top "cling-filmed". Maximum time one hour. Stir plate would be at least 48 hours and open to the air (aluminium foil not a hermetic seal) so risk is hundreds of times greater imho.

If you airlock the flask, I cannot see how constant stirring of an oxygen free environment would help.

I am not saying don't use stir plates - I am trying to understand the risks and benefits.

 

[Horseflesh]

Of all the homebrew issues I read about here, infected starters aren't on the list. The common technique of sanitized foil or a foam stopper seems to be adequate in my experience.

 

[Wallachia]

I mean, bacteria are going to get in your beer -- what you want is an environment where the yeast will outbreed and outcompete inevitable additions of bacteria, and make their presence more or less meaningless (I don't care how sanitary your brewing system is, bacteria are getting in)

Typically in a starter, you're pitching billions of yeast cells into their preferred climate against a very, very small amount of bacteria that will flow through the air in a "foil lock"

If beer was that sensitive, it never would have been invented.

 

[D-roz]

I prefer foil with a straw over using a foam cork or foil alone. Your starter will benefit from oxygen. Part of the process is to avoid contaminating your yeast, foil and foam corks allow this to happen. IMHO the addition of a straw works even better. The bacteria and wild yeast we are trying to avoid merely float in the air around use, they do not fly.

 

[Jayjay1976]

I prefer foil with a straw over using a foam cork or foil alone. Your starter will benefit from oxygen. Part of the process is to avoid contaminating your yeast, foil and foam corks allow this to happen. IMHO the addition of a straw works even better. The bacteria and wild yeast we are trying to avoid merely float in the air around use, they do not fly.

Could you please elaborate a little on how you are positioning the straw? This sounds really interesting to me.

Also, anyone know where I can buy a foam stopper for a larger, 3L flask? The one I bought from my LHBS was too small and just falls in.

 

[mongoose33]

Is there not a significant risk of airborne bacterial contamination ?

Not if you cover the flask with loose-fitting aluminum foil.

You want there to be air exchange so oxygen can get into the starter wort. Yeast need oxygen to grow properly, and reproduce (it's used in the construction of cell walls).

You probably are "contaminating" your wort any time it is exposed to the air. When I boil my starters and then cool them in the flask, I keep them covered with aluminum foil, but as they cool, what happens to the air in the headspace of the flask? It cools and reduces in volume, meaning it's drawing in air from the outside--air that contains dust, and dust that has hitchhiking bacteria on it.

Much of contamination control is keeping it, well, under control. You have bacteria in your wort and your fermenting beer. But if you've taken care to carefully clean and sanitize, the amount of such bacteria will be very low--and they can't outcompete the billions and billions of yeast cells you're pitching into the wort.

But, if you haven't limited bacterial populations by cleaning and sanitizing properly, they can reproduce fast enough--starting from much higher populations--to cause an infection and a resulting loss of your beer.

So, do everything you reasonably can to reduce contamination, but don't obsess over it.

***************

PS: My son is finishing is doctorate in microbiology, which is where I get this from. He's also a homebrewer, and part of the reason I got into homebrewing myself. Sadly, his area of work is not with yeast.

 

[mongoose33]

Something more:

I do something very unusual (Day_tripper is the only other one I think who does this). I aerate, with oxygen, my starter before I pitch the yeast into it.

I believe it makes sense to give the yeast a head-start w/ O2, as boiling has driven off a lot of O2 from the starter wort.

But, I'm very cognizant of avoiding infection. When I'm boiling the wort on the stove, using a borosilicate flask, I'll spray the mouth of the flask w/ santizer just to be sure that nothing should be growing at the place where I'm pouring it out.

I'll also boil about a pint of water in a pyrex cup in the microwave, then immerse my aeration stone in it to kill off any bacteria that might be on it. I also do that before aerating in the 5-gallons of wort too. I have my aeration stone on a long metal tube through which the oxygen flows, and I'll hit it with sanitizer while it's in the boiled hot water.

When i'm aerating I'm adding gas inside the flask, so there's a positive pressure mostly keeping dust and microbiological uglies out of that flask. Then after 30 seconds, I'll remove the stone, cover loosely with foil, and put on the stir plate.

Too much? Everybody gets to make their own choices and decide how they want to do it. This is how I do it, and I don't find any difficulties with infections. <sound of wood being knocked on by knuckles>

 

[D-roz]

I use a bendable straw, shorten the shortest end, bend at 90 degrees then set on rim and rap with foil. Straw and foil get sprayed generously with StarSan prior to use. You will notice far less condensation versus a foam stopper.

Not sure where to find a larger foam stopper. The one I purchased is 1 3/4" in diameter if that helps.

 

[Rhaop]

I use a bendable straw, shorten the shortest end, bend at 90 degrees then set on rim and rap with foil. Straw and foil get sprayed generously with StarSan prior to use. You will notice far less condensation versus a foam stopper.

Not sure where to find a larger foam stopper. The one I purchased is 1 3/4" in diameter if that helps.

Very nice ideal, will have to try this on my next starter, i only have a 1L flask right now so I generally make 500-600 ml viability starters not sure that i get tremendous growth at that size but seems to help my batches start really fast

 

[LarMoeCur]

Do not put an airlock on your starter when using a stir plate! You are trying to put Oxygen in to the starter. The stir plate job is to increase the surface area of a liquid allowing more O2 to be absorbed based on contact area. The air lock is trapping CO2 in the starter container and CO2 is toxic to yeast. You are not helping to grow yeast by trapping the CO2. CO2 will displace O2 so once the yeast start producing it you have cut off their oxygen supply.

Tinfoil over the top is all you need. Air and CO2 can move freely under the foil. No need for the straw snorker either. Bacterial, wild yeast, and any critters you care about CANNOT fall up. These things fall with gravity...they do not craw or move on there own they are transported on dust and airbone particals. Leave the container uncovered and something will fall in. Cover the container with foil and nothing can fall in.

A stir plate will grow 100% more yeast then just putting DME in a jug. They are worth the effort if used correctly.

 

[Bombo80]

I love my Maelstrom stirplate !!!! I finally got a couple flasks to use for yeast propagation. When I bought the flasks, the also had the foam stoppers. I really like those. They are a very soft foam material. I just sanitize everything, including the foam stopper. I just squeeze out any remaining startsan in the stopper before using. I figure enough O2 is getting in when I add more wort to the mix. I just aerate that before adding to the flask.

 

[Gavin C]

but if air can get in, air contains bacteria, I thought the idea of a stir plate was to mix and oxygenate.....

A sanitized sponge works well. Air in CO2 out. Nasties won't get in that way.

 

[bobtheUKbrewer2]

If fermentation is occurring in the flask, CO2 will keep O2 from the starter, besides not allowing yeast growth. Surely the idea is to add malt solution frequently enough to keep the yeast in the growth phase ?

 

[Dog House Brew]

I use a coffee filter over my starter flask. I had a fruit fly ruin my starter once. It was a Yeast I couldn’t replace. Never again!
I also use O2 when I pitch into my starter. I’ve been moving toward the shaken not stirred when I have fresh yeast.

 

[ericbw]

Lots of good explanations here. I do worry about it occasionally, just because there is so much bacteria and yeast in the house (that we deliberately cultivate with kombucha, sourdough, pickles, and other fermentation). I've had various fermentations cross contaminate, but never a problem with a yeast starter.

And there is definitely a difference between a 1-2 day starter and just putting it in a jug with DME for an hour. I would call that "waking it up" or "reviving" it, but it won't have time to reproduce much.

 

[parrothead64]

Sanitized foil will protect a starter. I boil mine in a the flask and put the foil on for the last 5 min. When adding yeast. I shut off any fans and use a burner to create an upwards draft. [/ATTACH]

 

[DurtyChemist]

Not if you put foil over the top of the flask. Bacteria will only get in if it is left uncovered. See the experiment above for the reasoning. Bacteria can’t come from underneath to get in.

 

[mongoose33]

If fermentation is occurring in the flask, CO2 will keep O2 from the starter, besides not allowing yeast growth. Surely the idea is to add malt solution frequently enough to keep the yeast in the growth phase ?

I've never read that--unless you're doing a stepped starter, the point is to get the yeast active and reproducing so as to increase the population of yeast cells.

I do a variation that few, I think, do--I begin my starter about 16-18 hours before I plan to pitch. I use 100 grams DME, add RO water to a total of 1000 ml in the flask. I add about 1/8 teaspoon of yeast nutrient to the flask, a drop of Fermcap, and boil it for 10 minutes on the stove. I cool to room temp, and meanwhile the yeast has been sitting out coming up to room temp.

Then--here's what few do, though I know of two others here on HBT who do this--I use an oxygen stone to oxygenate the starter wort prior to adding the yeast. Yeah, a bit of a pain, but I believe it helps the yeast get going, as yeast needs O2 to build cell walls, and boiling drives O2 out of the wort.

I pitch the yeast, then put on a stir plate at about 65-67 degrees for 16-18 hours.

Then--I just pitch the flask right into the fermenter after cooling the wort in the fermenter to about the same as the yeast. No cold crashing, no decanting of the "beer" produced--just right in.

Yeah, it adds a liter of volume to the mix, but that just drops the OG by about 1 point.

That yeast usually takes off like a rocket. Did this Friday, and I had krausen forming in the fermenter within 6 hours.

**********
I also have been using the accelerated fermentation schedule for lagers. Used this Friday as well. Pitch into the fermenter about 65 degrees, then I held that for 6 hours before beginning to drop the temp to 50 using my ferm chamber. I only make a 1-liter starter for lagers, but I get essentially another doubling in the active yeast going into the fermenter.

Based on what I think I know about yeast, I'm not allowing it to finish and drop out of solution in the stir plate, so it's still very active in the flask when I pitch it. The yeast drop into the fermenter, say "Yippee! More food!" and they keep on keeping on.

Seems to work. The beer tastes great.