covering starter question

[Bewbzout]

So if you want O2 to get in why cover your starter? so dirt and dust do not get in? If that is true then a S-type airlock without water in it would work perfect right? Thanks for your help.

 

[jsled]

Making a starter is just a mini-fermentation. You only want O₂ in before yeast activity begins; the yeast need the oxygen to build up their internal stores, and once it's consumed, they start converting sugars to alcohol; O₂ is required for good yeast health and activity. Once they've transitioned, though, you don't want to introduce O₂ … the yeasts will stop working, and you'll start oxidizing the beer.

So, you cover it for the same reasons you cover your beer: so dirt, dust, bugs, &c. don't get in. An empty S-type airlock covers the dirt and dust, but not bugs. A filled airlock gets all three. Many of us just use aluminum foil, which works as well.

 

[el_caro]

many of us just use aluminum foil, which works as well.

+1

 

[Bewbzout]

So what is a stir plate good for if you dont want to introduce O2?

 

[Revvy]

So what is a stir plate good for if you dont want to introduce O2?

You DO want to introduce oxygen in a starter, since you're not concerned with the beer above it, if it is oxydized or not it doesn't really matter, you're not going to drink the stuff. Since you are reproducing yeast, a stirplate is excellant because it agitates the liquid. It also keeps the yeast in suspension longer which keeps it in contact with all the sugar.

But since you are reproducing yeast, you want to give them a lot of oxygen.

Tinfoil or plastic wrap are excellent barriers for you starters, it can keep the bugs out, but still keep the o2 in the vessel longer.

An airlock, with or without water really isn't a barrier, it is meant to allow o2 and other gasses to escape when they exceed the volume capacity of whatever the are protecting.

 

[Ohio-Ed]

I use a 2 liter flask and a foam rubber stopper. I sanitize the stopper by saturating in starsan and wringing out the excess. It does let o2 through.

 

[el_caro]

Aeration is the name of the game.
Yeast can extract more energy from sugar when oxygen is present in their environment. In the absence of oxygen, yeast switch to a process called fermentation. With fermentation, yeast can still get energy from sugar, but less energy is derived from each sugar molecule.

In addition to deriving less energy with fermentation, the end products of sugar metabolism are also different. When oxygen is present, the sugar molecules are broken down into carbon dioxide and water (plus the energy that the yeast uses to grow and reproduce). In the absence of oxygen, the fermentation process produces alcohol, carbon dioxide and water (and less energy).

An airlock with water whilst presenting a barrier to unwanted nasties is a barrier to oxygen and hence is probably not a good idea. The bad guys cannot go uphill they say so a piece of sanitized foil is sufficient to keep them out, allow surplus CO2 to be expelled and oxygen to enter.