Use of stir plate to build up yeast concentration

[bobukbrewer]

Given that a warmish solution of yeast, sugar, acid and nutrient is involved, is there not a significant risk or airborne bacteria contamination ?

 

[stigman]

Not if you don't let any in

 

[strange-steve]

Given that a warmish solution of yeast, sugar, acid and nutrient is involved, is there not a significant risk or airborne bacteria contamination ?

There is a risk which is why good cleaning and sanitising are essential when propagating yeast.

 

[stigman]

A good tip is to work next to an open flame, the hot air rising should stop floating nasties falling in.

 

[MyQul]

It's also a bit of a race to colonise the wort and use up all the nutrients and oxygen. Seeing as brewers yeast has 'evolved for wort' over thousands of generations it normally out out competes everything else so the nasties cant get a foothold before the yeast turns the wort into a hostile (low ph, no oxygen or nutrients left, etc) environment for the nasties

 

[IainM]

Normally the neck of the Erlenmeyer is covered with sanitised foil, so any nasties would have to get up from the bottom lip of the foil without sticking to it or the glass, then over the lip of the flask and into the wort, all the while going against the flow of air because the gas generated by the fermentation puts the flask in positive pressure. Not to mention that the yeast has long consumed all the oxygen, which immediately rules out infection from all obligate aerobes, including large swathes of Pseudomonas, Bacillus and Mycobacillus, as well as pretty much all algae and fungi. Not only that, but the yeast are producing alcohol, which is antibacterial, and are consuming most of the nutrients that bacteria can feed off. Given this, as long as the starting equipment and materials are clean and sanitised, then the chance of getting an infection are pretty low.

 

[bobukbrewer]

if fermentation is occurring, CO2 stops O2 getting to the starter AND the yeast is no longer growing in concentration...just my thoughts....

 

[Bigcol49]

Hi!
If there was a serious risk of contamination, homebrewers wouldn't employ this method of building a yeast starter. The fact that thousands of homebrewers do use this method suggests that there is an extremely low risk of contamination from airborne nasties.

 

[Fil]

if fermentation is occurring, CO2 stops O2 getting to the starter AND the yeast is no longer growing in concentration...just my thoughts....

thats where the regular swirls of the bottle come in, or if you have knocked up a stirplate, the vortex of liquid should create an air vortex too.

most airborne contaminates simply fall into the brew so a foil cap or loose ball of cotton wool, or special breathing bung stop this.. And any that do get sucked in with a gas exchange will be up against the significant population of yeast alredy pitched which should still out consume any invaders.

 

[Dutto]

Check out this video ...

[ame]https://www.youtube.com/watch?v=jMhFerNTwbQ[/ame]

Yes, there is a chance of contaminating the starter and the video shows you exactly how to avoid it. :thumb:

Don't for one second think that the gentleman in the video is in any way paranoid. He isn't!