So here’s the story: I have a 15 gallon barrel, and decided to brew a RIS to age in it. I knew I would need a large amount of yeast, so to start I brewed a 1.5L starter at 1.035 and pitched a pack of Wyeast 1056. I brewed a stout with an OG of 1.057, and pitched the starter. The beer fermented fine, and settled out in about a week. Sure enough, life intervened and I didn’t get around to brewing the RIS until this past weekend. I ended up with about 16 gal of 1.110 wort. The starter beer was racked over into a secondary, and the yeast cake was split between three fermenters, with about 1.5 cups of slurry added to each. I also oxygenated with pure oxygen for 2.5 minutes. I woke up this morning expecting to see some activity, but there was none. No airlock activity, and no sign of krausen. Once I got home from work I checked, and now 24 hours after pitching there is slow airlock activity and some krausen starting to form. Should I take action to increase the yeast cell count, or are they just a bit sluggish due to being dormant for a while?
There's a lot of missing information... like how old is the yeast cake??? You will likely see activity soon but the fermentation will not be the best. I'd throw a pack or 2 of Fermentis US-05 into each carboy; it can't hurt and might save your beer.
A yeast cake sitting warm for a month won't be at peak health, but it might do the trick. Typically you get 300% - 500% growth from a fermentation. So, assuming your 1.057 stout was also a 15 gallon batch and your original pitch for it was adequate (roughly 600 billion viable cells), I'd say you definitely have enough cell for a 15 gallon batch of 1.110 RIS which would require about 1100 billion viable cells.
The lag time is said to be able to last up to 12 hours. Suppose they could have simply had a slow start perhaps from being dormant for a bit, as you suggested.
Just as a follow up: I ended up adding a couple packs of US-05 into each fermenter. The yeast cake was from just a 5 gallon batch, and after sitting for so long I didn't want to risk a stuck fermentation. The activity had really picked up the day I decided to add more yeast; the bottom of the chest freezer I use as a temp controlled chamber had a couple pints of sanitizer/blowoff sitting in the bottom from the intense bubbling. Today I measured the FG and it's at 1.028. 75% attenuation and it tastes good!
