To Repitch or Not to Repitch

When I brew a new batch of beer, I usually just buy a fresh pack of yeast. To save money, however, I've looked at reusing the yeast. I've done this a couple of times. Half of the times I tried it, I got strange off flavors. I'm guessing this was because I saved the yeast too long in the fridge before reusing, and that I didn't refresh it with a starter before pitching.

I'd like to understand what exactly the right way to reuse yeast is. It seems like the following guidelines are a start, but a definitive guide would be nice. Can anyone recommend a good book to learn more about yeast chemistry, etc., with a focus on brewing beer?

Possible Guidelines for Reusing Yeast

• Repitching fresh wort onto leftover yeast slurry at the bottom of your primary fermenter (after racking or bottling the last batch) is an easy way to reuse yeast. You should probably throw out half of the yeast slurry to avoid over-pitching, and only reuse the same strain 3 times or so.


• Saving yeast into a sanitized beer or soda bottle, as explained in the Joy of Home Brewing, is ok, but don't keep it more than a couple months in the fridge. If you keep it more than a week, make a small wort starter of the same or lower gravity as the beer you are making to refresh the culture.

Off flavors.
In hopes of learning more about repitching yeast, I started a discussion of how to properly reuse yeast in home beer brewing at the homebrewexchange.net forums.