How to grow yeast?

I made a yeast starter directly in a bottle of beer after I poured it into a glass. I let the bottle with dregs warm to room temperature, then added dry malt extract with water. 10% DME in boiled water cooled to room temperature. Shake it up to oxygenate, then put airlock on. 12 hours later, I pitched into beer and made a lovely hefeweizen. Yeast was from an imported german hefeweizen. 2 weeks later, racked beer into bottles and poured slurry into mason jar and put in fridge. Will brew again in a couple weeks, and use slurry warmed to room temp. should save 3.99 per 5 gallon batch from here on out. Brewdog333

I haven't actually harvested or grown my own yeast yet, though I really want to. It's about $7.95 per batch for me, and I brew about 3-4 times a month. Adds up for sure.

BYO Magazine had a great issue with a bunch of yeast techniques. If you can, find the September 2009 issue. They have a bunch of other articles online.

Hopefully TimWeber will chime in here. I know he got a 5000mL flask recently.

The first thing you must learn how to do is make a yeast starter. This is simpler than making beer but your sanitation must be very good, don't be intimidated.

Yeast Starter

1. Mix malt extract and water to make two quarts of 1.030 to 1.040 gravity starter wort. Consider optionally adding yeast nutrient.
2. Sterilize this by boiling for 10 minutes.
3. Cool to pitching temperature - in the low 70's F
4. Sanitize a little fermenter - a half gallon glass jug or similar
5. Mix the starter wort and your yeast. Shake 4-5 minutes to oxygenate
6. Cover with sanitized tin foil, or an airlock
7. Let sit in a cool, dark place for 8-12 hours.

Pour a little off the top and taste it before you pitch the starter into your beer. If it tastes off, do not use. Re-evaluate your sanitary technique.

Having a 2-3 liter Erlenmeyer flask is great for this because you can heat it directly and transfer right into a ice bath to cool, eliminating step 4 above. I cracked a flask on an electric burner, so be careful.

See also: Mr Malty's yeast starter FAQ

Quick Techniques

Now that you know how to grow yeast outside the beer fermenter, there are a few easy things you can do to stretch your yeast. The easiest way is if you buy White Labs pitchable tubes only use half or a third of a tube to make a starter. Spray the cap and threads with sanitizer and re-cap. Try not to get too much sanitizer into the tube.

If you buy the WYeast smack packs you can do something similar with a sterilized mason jar. Boil a quart mason jar & lid for 10 minutes, cool and pour the contents of the smack pack into it.

Yeast Washing

This is another easy way to get more out of your yeast. Instead of splitting and growing yeast before you make beer, washing cleans trub off yeast you have already used. There is an illustrated post on HomebrewTalk which I will summarize.

1. Boil four pint mason jars and a two quart jar about 20 minutes
2. Remove jars & cap, keeping them filled with the boiled water
3. Cool
4. After transferring beer off the trub, pour the (cooled) sterilized water from the big jar into the fermenter & swirl
5. Let this mixture settle for 20 minutes. The mixture should begin to separate
6. Without disturbing the bottom layer fill the larger jar with the mixture
7. Let the large jar settle for another 20 minutes
8. Pour out the water from the pint mason jars
9. Without disturbing the bottom layer in the large jar, fill the mason jars from the larger jar.
10. Cap the pint jars, label them, and put them in the refrigerator
11. Clean up

When you are ready to reuse the washed yeast, make a starter from one of the pint jars.

Culturing Yeast From Bottles

As topfermented explained here you can grow yeast out of bottles of beer. You need very careful sanitary technique because you are working with small quantities of good yeast.

HA!

This is the best discription I've found yet on how to culture your own yeast. http://www.homebrewtalk.com/f163/slanting-yeast-133103/

I also have this book which teaches you handling techniques http://www.northernbrewer.com/brewing/first-steps-in-yeast-culture.html

You can also use yeast from batch to batch, though I'd recommend washing the yeast to prevent over pitching and remove trub. http://www.homebrewtalk.com/f163/yeast-washing-illustrated-41768/

I've grown my own yeast out of bottles before. Basically, all you're doing is making a very small yeast starter and pitching the dregs of a bottle into it and growing it up from there. I usually start with 25ml - 50ml out of a bottle and then step it up a couple of times until I'm making a 1000ml starter for a batch of brew.

I've had luck doing this with English-style ales and Belgian-style ales, both.

You'll want to make sure that you can sterilize the mouth of the bottle before you pour out of it, since that's the place that's most likely had some sort of other bacteria get to it. You can swab the mouth of the bottle with a small amount of isopropyl alcohol and burn it off for quick sterilization.

A few important things:

1) You must do this from bottle-conditioned beer. If the yeast has been filtered out prior to packaging, there will not be enough yeast in the solution for you to grow.

2) Not all bottle-conditioned beer is bottled with the same yeast that it was originally fermented in. Many breweries use lager yeast for bottle-conditioning, since they can then put the bottles into cold storage or cold shipping and it will still condition, ferment, and carbonate, just more slowly.

3) It has to be fairly fresh beer, as the yeast will suffer quite a bit of stress under packaging conditions (high pressure, not optimal temperature). You may not be able to culture yeast out of something that's been fairly aged, or what you do culture out may be a mutated strain.

4) Over time, yeast strains will mutate. Most commercial breweries will re-pitch yeast batch to batch for a few generations (6 - 10 is normal, I think), but at some point, if you're looking for consistency from your yeast strain, you'll want to wash your yeast and grow a pure culture up from scratch again, and this will be a pain in the ass without some sort of lab. Mind you, in a homebrewing environment this may not be much of an issue, so long as the mutations you get are pleasant.

I'm on my 4th generation of yeasts that I've grown out of bottles, now, and everything still tastes great. I have a yeast library of four yeasts that I've grown up from scratch - two belgian yeasts, one english-style (which I think is Whitbread), and one English-style combination (two bottles of different ales together).

That said, I still buy yeast for probably half of my batches, just for variety in flavor profile.