HELP! Any way to stop a beer from further souring?
 

Hey guys,

So I brewed my first maple syrup beer last month. A buddy working on a syrup farm in the northeast shipped some grade C stuff (darker than anything in stores) and I added it after primary (because I wanted to minimize the amount of maple syrup flavor lost in fermentation bubbling), but I assumed I didn't need to boil the syrup first.

Unfortunately, somewhere along the way - probably during the syrup addition - I infected the beer. I first became suspicious when the second fermentation lasted almost three weeks with a steady 2.5" krausen cap, but when the krausen fell and left splotches of milky white on top, I was sure something was wrong. Checking the gravity confirmed my suspicion: the gravity went from 1.047 to 1.012 in the first round of fermentation (4.5% ABV - normal), and then from 1.044 all the way to 1.007 in the second fermentation. I'm no zymurgist, but I'm fairly certain there are no ale yeasts that can achieve 90% attenuation (since the adjusted OG would be ~1.079).

The beer is actually wonderful right now - woody, maple, roasted nose, and a toasted flavor full laced with hot esters and fusels, but I'm worried it will continue to sour and end up tasting too vinegar-y or phenolic or some other dominant off-flavor.

My questions:
1. Any guesses as to what bug got into the beer? Maybe Strepto?
2. Can I stop wild bug activity using practical means? I don't have a filter and I wouldn't dare boil the beer, but I was hoping a temperature drop or a "magical" additive could do the job. Thoughts?

Thanks!

Your beer is fine

The wort has more of an effect on attenuation than yeast.
Maple syrup is about 100% fermentable so thats why your beer is going so low.

If there was a bug in the beer you would surely taste it.

Or if you are still worried you could dose it with sulfite.

Combining the posts of the two above: Your beer is most likely fine, but if it will ease your mind you can add sulfites.

Doing a fermentation in "stages" like you did can often have the effect of drying out a beer much more than if you started with all the sugars in the beginning.

Sounds like your beer is probably fine. "Splotches of white" sounds like normal yeast behavior to me. What strain did you use?

I agree that I would expect noticeable off-flavors, but I still have a hard time believing that a second fermentation pushed the FG a full 5 gravity points lower than the first fermentation...

And I thought sulfites were only used for stabilizing wine by stopping oxidation. In fact, in this post it's mentioned that sulfur doesn't kill yeast. In any case, I'm not sure I want to risk tainting the batch with a sulfurous flavor, so I guess I'll go ahead and bottle this batch and cross my fingers and hope y'all are right.

Has anyone actually seen the gravity drop really low like this in a second fermentation?

Thanks again.

The yeast is a London Ale Yeast. I've used it quite a few times and never seen anything like this on the beer. If you want, I can snap a photo.

I was also thinking it might be some non-sugar compoind from the maple tree that got orphaned during fermentation.

Take a picture, but the fact you added sugar can easily drop it to 1.007. When I mash with minute rice, I've gotten even lower than that! A lot of people add adjuncts and sugar to purposefully dry a beer out.

40% of your fermentables were simple sugars, it's not surprising that it dropped so low.

Brewer's and winemaker's yeast are for the most part resistant to sulfites, bacteria definitely are not and I believe wild yeast are also not resistant. Although sulfites also help with oxidation, a lot of winemakers use sulfites almost exclusively as their equipment sanitizer as well.

I use secondary additions of simple sugars whenever I have a higher than normal gravity beer and want it to dry out. If I were you, I wouldn't be concerned. But if you are, then sulfite it.