Krausen dropped then rose again after dry hopping?

[MattyIce]

Cliff notes: Brewed a big IPA, three weeks of primary, beer was just about clear, dumped in hop pellets for dry hop. Three days later, I check on it before cold crashing and I have another krausen. Have you ever seen this: Details below:

IIPA AG BIAB Recipe:

Mashed at 150 for 75 min. Pitched yeast on to entire yeast cake from a session IPA. Pure o2 at pitching

Ferm at 66, three 12 ounce sugar additions each time krausen dropped. Another dose of o2 at first sugar addition. Three total weeks in primary at 66, beer started to clear. Two ounces of pellet dry hops for three days simply dropped into beer. I checked on it after three days of dry hop to see what looks like another krausen rose. Pic:

I was going to cold crash today, but I decided to wait this krausen out and take a gravity reading when it drops.

 

[BigFloyd]

That's very likely dissolved CO2 collecting around the hops to form bubbles.

Why would you add O2 after fermentation has already begun???

 

[MattyIce]

CO2 forming around hop bits makes sense.

On the second O2 dose...it is a recommendation per Yeast by Chris White and Hamilton Zainasheff on high-gravity brews.

 

[BigFloyd]

CO2 forming around hop bits makes sense.

On the second O2 dose...it is a recommendation per Yeast by Chris White and Hamilton Zainasheff on high-gravity brews.

O2 assists yeast cell aerobic respiration during the reproductive phase of fermentation, but may cause oxidation issues if introduced too late in the process so you have to be careful about it. It can also, if introduced at the wrong time and in sufficient concentration, cause yeast that is fully engaged in anaerobic fermentation to flip-flop back to aerobic.

Since you pitched on a whole yeast cake, you had a very healthy (probably with some to spare) cell count to begin with and really didn't need to oxygenate beyond the pre-pitch O2 bubbling.

 

[ArkotRamathorn]

Maybe the hop pellets droping in and creating nucleation sites, and releasing the dissolved CO2, caused enough of an up draft to cause some yeast to kick back up into suspension. Maybe they found some sugar they liked and re-krausened?

 

[MattyIce]

Disregard. I believe this was just hop matter as mentioned above. Clear beer underneath. Racked to keg this afternoon