Belgian Ale Fermentation w/ sulphury smell

[John Coo's Brews]

Currently fermenting a 5 gal Belgian Pale Ale with Wyeast 3787, top-cropped from my last batch of a different recipe. Made up a starter to grow and revitalize the top-cropped yeast in order to pitch at an appropriate rate (~200 bn). Starter process went normally. OG of current batch was 1.056, and mash, boil, chill, and early stages of fermentation went normally. Yeast pitched at 66 F, and first two days were kept cool - estimated fermentation temps between 65 and 69 F. Based on prior experience with this yeast, it forms a very thick Krausen, so installed a blow-off arrangement once airlock activity was steady - and indeed, the blow-off was needed. Top-cropped from this batch as well, and the yeast sample smells normal (even now). The batch is now in its 4th day. Temp was allowed to rise, and then held at around 74F. Beginning Wednesday, the bubbling coming off the fermentor had a sulphury smell to it - reminiscent of past experiences with cider. Because the rate of fermentation has dropped (bubbling has decreased significantly), I'm worried that there's substantial sulphur compounds remaining in the beer. One difference in this recipe was that I added fresh orange zest at flameout, and some of the initial aromas coming off the fermentation had very pleasant orange hints. Could the orange also be the source of sulphur smells now that it's later stage fermentation? Any other thoughts on causes of sulphur odors?

 

[VikeMan]

If it's the classic "rotten egg" (H2S) I doubt that it's coming from orange zest. It's normally a yeast product, and chances are that O2 and/or trace levels of metals will clean it up over time. Raising the temp a little can accelerate that, as well help to blow off some of the volatile H2S (assuming CO2 is still being made).

 

[John Coo's Brews]

If it's the classic "rotten egg" (H2S) I doubt that it's coming from orange zest. It's normally a yeast product, and chances are that O2 and/or trace levels of metals will clean it up over time. Raising the temp a little can accelerate that, as well help to blow off some of the volatile H2S (assuming CO2 is still being made).

Thanks for reply. Fermentation has definitely slowed quite a bit now. I wouldn't call it rotten egg, but definitely sulphur of some form. Last fall I did a cider that had a very similar odor during fermentation. The early bottles had a hint of it, but over time I think the yeast in the bottles cleaned up whatever was remaining. I ended up enjoying the cider - when initially, I thought it was going to be a dumper. That said, I've never experienced it on beer, and I've used the 3787 strain a few times. I typically do smaller batches (3.5 gal), but did a 5 gal this time. I may not have gotten enough dissolved oxygen, which may have stressed the yeast early - but after the lag phase, it definitely performed normally until the sulphur off-gassing. Will be transferring to 2nd-ary this weekend, so we'll see how things go.