Brewing again – trying to solve the stuck fermentation problem

We haven’t brewed for a while. Since moving to all-grain brewing last year, we’ve had a couple of successes – a tasty mild and a blackberry wheatbeer, for example, as well as some drinkable if unremarkable bitters. However, we’ve suffered stuck fermentations in the last couple of brews, and it’s been rather depressing. All that effort, and you end up with sugary water.

(Note – a stuck fermentation is where it starts off fine, but stops fermenting before it reaches the expected final gravity. Final gravity should be around 25-30% of the original gravity. So our last brew had O.G of 1053, but only got down to 1025, i.e. 45%)

This has happened twice (and in fact, our third-from-last brew only got down to 35%), and we can’t understand why this has started happening – we’re not doing anything different with the yeast or aeration, which seem to be the usual culprits for stuck fermentation.

We consulted all our books, and searched the internet, and came up with the following theories (and counter-arguments).

1. Wrong temperature for fermentation, or sudden temperature shocks – probably not an issue, we have a max-min thermometer which confirmed that the room temperature was between 17-22 deg C, which should be fine for ale.
   
2. Percentage of alcohol too high for the yeast to cope with – we’d expect this to be a factor over 1070, but not for the brews we did, which were all around 1045-1055
3. Yeast not effective for various reasons – it’s happened with several different types of yeast. With one of our failed batches, we split the batch in two and tried two different types of yeast – Nottingham ale yeast (which we’d successfully brewed with in the past) and some yeast from a Young’s bottle we’d cultivated ourselves. Both gave exactly the same final gravity). We also tried to restart it with some new yeast – this did make it bubble again, but it only reduced the final gravity a further 1/2 percent.
4. Insufficiently aerating the wort, possible combined with underpitching the yeast (see article here) – possible, we only added around a bowl-full of re-activated yeast, gave it a stir and a bit of a shake; then again, that’s all we did for previous brews which worked.
5. Insufficient or incomplete nutrients required to allow the yeast to complete fermentation, i.e.we’re getting sugar from the mash, but not the right type. Possible, and worth further testing.

The only differences between our successful batches and the failures (as far as we can tell) are;

• filtering the mash liquor rather than boiling it to remove the chlorine;
• using a different source of malt (it’s still Maris Otter, but it came from a different supplier)

Both of these would point to a mash problem – is it possible we’re getting sugar, but it’s non-fermentable for some reason?

So today we’re going back to basics, using a couple of cans of extract, enhanced with some steeped malts, some malt extract and some fresh hops. If our problem stems from the mash, we expect this to work. If we still get a stuck fermentation, then we’ve at least narrowed down the problem.

We’ll update you on progress. (sorry, no beercam!)

If anyone else out there has some suggestions, do let us know!