Over attenuation

[Grealish]

I recently brewed using harvested Wyeast 1028. OG was 1.046. Expected FG was around 1.013, actual was 1.003. I fermented for just over two weeks at 19º. I was concerned it must be infected so I tried the hydrometer sample and it tastes fine. Maximum expected attenuation for this yeast is 77%, I achieved over 90? Can anyone throw any light on this? I use the extra large hydrometer supplied by the Malt Miller.

 

[Sadfield]

Overpitching or your yeast is contaminated with a diastatic strain of Saccharomyces. The latter is a big problem in the industry as it doesn't throw of noticeable off flavours and looks the same under a microscope, unlike Brettanomyces. Belle Saison, MJ French Saison, WY3711 are examples of this strain variation as are many wild Saccharomyces.

How did you reuse the yeasts? Did you build a new starter, pitch a measured amount of slurry, pitch a random amount of slurry or onto the whole yeast cake?

 

[Dutto]

Have you checked the hydrometer? It's very easy to dislodge the paper that sits inside.

 

[stz]

What was the recipe? How did you calculate your anticipated FG? Do you usually get normal attenuation with other yeasts? How is your mash? Thermometer calibrated?

 

[Grealish]

I checked the hydrometer, it was reading OK in plain water.

The story of the brew is slightly complicated. I used 2kg MO and 500g Vienna to make a ten litre brew, with a minimal sparge, so that I could make a small beer from the second runnings. Mash was at about 64C for an hour.

The yeast was a very old rinsed yeast which had been stored in the fridge. I built it up to a very healthy looking 2 litre starter. At every stage, I tried the wort, which tasted OK. I used about 2/3 of the starter for the main brew and a third for the small beer. The beer itself taste OK at bottling. I do think that could have been a bit of an overpitch.

I got the estimated FG from Beersmith.