Incomplete fermentations; incorrect mash temp?

[orford]

Greetings all,

Hope everyone had a great Christmas. I have had about five batches in a row that have failed to reach the target gravity. These have covered the range of styles, starting gravities and yeast strains/forms. Everyone of them stuck at 1.02 to 1.018, and all had a noticeable sweetness/heaviness. I have tried increasing the fermentation temp, agitating the fermenter a bit after a few days, adding more yeast, etc... I have never had an issue like this before so I do not believe it is an issue of underpitching or aeration.

The one thing I can think of is that the thermometer I use for measuring my mash temp is off and thus the fermentable sugars availability was off. I checked the thermometer (digital) against a candy thermometer we had in the kitchen. If the latter is to be trusted then my normal mash thermometer is actually reading too high thus my mash has been running too low. This is surprising to me as I typically mash in at 152 and in a couple of recent cases (for a Scottish ale and belgian blonde


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[orford]

Sorry, dog bumped the iPad and sent that early. What I was getting at was that the thermometer was off by 5-8 degrees C, thus a mash target of 154 would have been at 146-149 in reality. Wouldn't this have resulted in a highly fermentable wort rather than the interment able one I am seeing?

In the end I am probably going to just buy a new thermometer today. I am still not entirely sure this is the problem though.


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[Poobah58]

At 146-149 conversion takes a bit longer. It's possible you were not done.

 

[BlueHouseBrewhaus]

A couple of possibilities. First is definitely your thermometer. Boil some water and check the temp (100C) and then put some crushed ice in a cup with a little water. Stick it in the freezer until it is almost frozen and check that temp (0C).

Another possibility is oxygenation. Are you thoroughly aerating your wort before fermentation? Also, are you pitching enough yeast? Check your pitching rate with one of the yeast calculators. I use YeastCalc.

You don't mention if you are doing AG or partial. If you do partial mash, it is fairly common for brews with extract (LME or DME) to finish a little high. One more thing would be to check your fermentation temps. Too cool and the yeast can go to sleep. Racking to secondary too soon may also interrupt theyeast before they are done. I usually start at the low end of the yeast's temp range and ramp up a few degrees when fermentation slows down.

 

[orford]

Thanks for the feedback guys and sorry for the lack of info. I am an AG brewer.

New thermometer further confirms that the old one was reading about 5 degrees too hot (read 155, actually 150). Again, this does not make any sense, I was anticipating that it was reading too cold... Anyway, ran a mash yesterday and another today with the new thermometer.

I don't think it is aeration. I typically brew about 30 batches per year and have never had this problem until the last five or so batches. Also don't think it is underpitching as two batches used slurry from a previously successful fermentation, two were made from starters wherein I calculated the pitch rate as you suggested, the other was with dry yeast. For pretty much all five batches I tried warming them up, gave them a bit of a stir and let them all go for at least two weeks. One of them I even tried pitching another couple packets of US05 to no avail...


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