Fermentation not starting after re-using yeast

[Blinky]

I brewed an IPA and bottled yesterday morning. I left the yeast in the fermenter and later in the afternoon brewed another smaller 10l batch of pale ale and pitched on top of the yeast around 7pm last night. I was expecting it to have taken off and be bubbling away but it aint! Do I need to go get fresh yeast or leave it longer? There is a slight displacement of water in the airlock which might indicate fermentation but I was expecting it to have taken off like a rocket!
Just conscious that if it does not take off it will be the 27th before I can purchase a fresh pack of yeast....

 

[DoctorMick]

I’m sure it’ll be fine. What temp was the yeast when you pitched on top of it? If it was cold it’ll need time to wake up.

 

[Blinky]

It had warmed up to 18 but just dawned on me since it’s a half batch there will be less sugar and less co2 getting produced? Suppose it should be fine till the 27 th anyway as it’s in a sealed fermenter. Just a teeny bit worried..

 

[jeg3]

At the end of fermentation yeast starts its dormant phase, and needs another propagation phase.

I have pitched straight onto yeast from a previous brew and was similarly surprised it didn't go off like a rocket. Then I read the Yeast book by Chris White...

As a comparison I've cropped from one batch that had been in primary for two days and pitched directly into a second batch; that yeast was sufficiently viable that the lag only lasted about two hours

 

[Blinky]

Wonder if I should stir it up, the old yeast cake is still sitting on the bottom?

 

[jeg3]

Give it a gentle stir and leave for a day or so. It'll start in its own time

 

[BeerCat]

I have been pitching quite cold lately and sometimes it can take 24 hours to get going. Beers are turning out fine so don't stress.

 

[Blinky]

Gave it a wee stir and it’s now bubbling away, reckon it was a combination of me putting a bit more water than usual in the airlock and maybe the lid was not perfectly tight, whew!

 

[Cwrw666]

I always get 2 brews from a packet of yeast - one sprinkling it on top of the brew (not stirring it in as all the oxygen is at the surface!) and one using a few spoonfulls of trub from the previous brew which is of necessity stirred in. The first brew always takes off quicker than the trub brew. It could be the oxygen thing, or it could be that as I leave a brew in the FV for 2 or 3 weeks, the yeast may have gone dormant.

 

[ExpatBrewer]

One of the issues I've read with pouring fresh wort over an existing trub is that this technically represents a gross over-pitch. In practice whether this has any detrimental effect on the beer is hard to say. What I prefer to do is simply collect around 250ml of the yeast slurry, jar it and put it in the fridge until I'm ready to pitch into my next brew. Even after a couple of weeks, the cold yeast once pitched get's going very quickly and produces a healthy krausen in a matter of hours.