I'm amazed at how yeast go about doing their thing, and how hard it is to tell a lot based on appearances.
I had a vial of WLP090 (San Diego Super Yeast) that was several months past expiration. Three weeks ago I made a one liter starter with it. The starter never seemed to take off. After 36 hours on the stir plate and 12 cold crashing very little yeast had settled out. I made a 5 gallon batch of IPA and ended up using a pack of US-05 dry instead the starter.
I almost tossed the starter but left it in the refrigerator. I checked it a week later and slightly more cells had settled out. It was about half what I'd normally expect from a usual WLP001/002/007/etc. starter. I made a small batch of Pale Ale (sized to fill a 3 gallon corny) and decided to decant and use the starter.
I made the batch, pitched the yeast and then left town for about 10 days. When I returned the batch was quiet. There was dried krausen on the carboy but nothing on the beer. (There was still about a half inch of krausen on the IPA I'd made the week prior using US-05).
I did a gravity reading yesterday on the WLP090 batch. It was 1.012. (OG was 1.062). Better yet, the gravity sample tasted amazingly clean for a 2 week old beer. Normally I'll primary a beer for 4 to 6 weeks. But I racked the WLP090 beer to a keg for dry hopping. I did so because I was satisfied with the fermentation. More importantly I needed some yeast for a dark ale I made yesterday. (The decision to brew was last minute. I didn't have any dry yeast, or time to make a starter).
I pitched about 1/4 of the WLP090 cake. I oxygenated the wort, put it to rest, and hoped to see some activity in a day or two. To my surprise, this morning fermentation was already going strong. At 16 hours there was already a 3 inch krausen. The wort was swirling and airlock activity looked like WLP001 might look at its peak. The temp went from 65F at pitch to 68F.
I've never had a fermentation take off like this. My general rule when it comes to yeast is that it's better to over pitch than it is to under pitch. I hope I didn't go to far this time. My experience with WLP090 is limited. Yet it appears to be lively stuff.
I'm still perplexed about the starter, simply because I've made starters from other vials of yeast that were many months out of date and they were way more active than this one particular vial of WLP090. But even then it fermented the first batch out nicely.