How to determine if over attenuation is due to infection?

[mmonacel]

My last three batches finished 4 points lower than the expected FG using the max attenuation figures for each yeast. What's odd is that it happened both for an AG and an extract. While I'm very good with sanitation (never had an infection and I follow good procedures), I don't rule out the possibility it could be an infection. I haven't experienced any off flavors or tell-tale signs of infection however. Is there a way to tell for sure?

Here are the detail on those batches:
AG batch - using chico strain with starter fermented at 67 (temp controlled). 4 points low (expected 12, got 8). Found that thermometer was a few degrees off so I chalked this up to mashing lower than expected. Took off the yeast once I found it was low.

Extract batches - same wort split into two using two different yeasts with starters - Wyeast ESB and British Ale. Fermented both at 68 (temp controlled). Both are four points low after a week and there still appears to be some airlock activity. Given this is extract, I can't think of a reason for the low FG other than potential infection. No signs of infection yet however.

All batches got 60 seconds of pure O2 at pitch time (same as most of my other batches) and all started within a few hours.

 

[Molybedenum]

I would validate my hydrometer at that point. Does it still read the same value as before with plain water?

 

[theredben]

2 things:

1 - Make sure you are reading the hydrometer with beer that is at the appropriate temperature (it says on the side of the hydrometer what temperature it is calibrated at), and that you have checked the hydrometer in plain water at the right temperature. Sometimes the little paper slip inside moves around a little.

2 - All beers are contaminated to some extent (the literature I have read says that anything below 1000 contaminants/mL is not detectable by taste), but for most people if you cannot taste anything, then it is not considered "infected". The only way to know for sure, is expensive lab testing.

 

[jonmohno]

Could be the extract is more attenuable than expected,i wouldnt worry the lowest i got was 1.004.As above do make shure your hydrometer is correct and temp difference.A few points is pretty common.Yeast are not perfect they have ranges.

 

[ThreeDogsNE]

They smell okay...how do they taste? Pathogenic bacteria don't survive in beer, so a taste test should be safe. If they taste okay, the yeast won. Wort is not sterile, and your equipment is presumably sanitized but not sterilized. You don't have a pure culture, but rather such an overwhelming predominance of Saccharomyces cerevisiae, then a 5% alcohol solution, so whatever else is there is struggling. There are some alcohol tolerant bacteria that will grow, given time, so enjoy your beer in it's prime, and don't keep a pale ale for a year (please don't ask me how I know).

 

[jonmohno]

I had a very fine ipa/pale over a year,not as aromatic shure but fine? Yes. Held up? Yes. Within my first few brews and extract batches also .Also a mistake of making a low abv ipa or pale which turned out great surprisingly.Ive had many light or esb low abv beers hold up very nicely over a year with extract also.
Thats just my testimony. Honestly ive had 0 of any beer turn out dirt terrible holding on to it over a year-anything.Iv'e dumped a few commercial brews but never of mine.Yes not all my beer is great either.Just like every brewery though you may not like all of them or the style anyways.

 

[mmonacel]

I usually use my refractometer for readings (even fermented wort) but in this case I also used my hydro. Didn't think to revalidate my hydro, however I just did and it's spot on given temp correction. (never thought I'd be unhappy to have an accurate hydrometer!) The refract readings I was getting were one point off from my hydro readings so that might be due to my aging eyes or the software calculations. I reported my hydro readings in the post FWIW.

No off flavors that I can taste or smell at this point. My primary concern is making sure I don't have some bugs / sanitation issues entering my process. My secondary concern assuming that those are good, I can repeatedly hit my target FG (within a point or two). Four points for me is a bit much as it has a noticeable impact on the beer. Sure it still tastes good, but it stinks to work so hard and then not get what you were going for. What's worse though is unpredictability.

FWIW I'm using the Mr. Malty pitching calculator. I'm not sure if pitching rates contribute to over attenuation. I'm sure it does for the other end of the spectrum!

 

[mmonacel]

The only way to know for sure, is expensive lab testing.

Good to know - I was hoping there's some post-fermentation "George Fix sanitation test" type of thing that could be done potentially. Well if I start seeing / tasting things then I'll know for sure of course. I won't be paying money to find out - just drinking fast. Ha!

I guess as a recourse I could add some maltodextrine to up the FG back to where I want it.