I have been brewing for years and I did not realize until just recently that liquid yeast packs (or vials) become increasingly ineffective as they grow older.
I was reading some articles the other day and one of them mentioned that if you buy a yeast pack that was produced on that very day you can only expect about 97% of the yeast cells in the pack to be viable. If the yeast is a month old it is only about 75% viable. If you are brewing a strong beer with old yeast (or even a somewhat strong beer) you are probably going to need either a second packet of yeast or you will need to make a yeast starter to build up a sufficient count of yeast cells to tackle the strong sugars in a high-gravity wort.
When I stopped at the home brewing supply store last night to pick up yeast for my next batch I paid attention to the production date on the yeast. I was a little bit annoyed that the newest packet in the style that i needed was a month and a half old. There were some packets that were almost 3 months old. It was a strain that was somewhat less common that other brewing yeasts, but still…
I have experienced under-attenuated batches of beer in the past, and this might be why.
In any case, there is a pretty handy calculator at MrMalty.com that helps you determine how many packets of yeast you need under different conditions.
Have you ever experienced any problems with brewing yeast?
If you are interested in starting up a home beer brewing hobby then you will need to invest in some basic equipment. This equipment can be found at almost any home brewing supply store or online.