Stuck Imperial Stout

[Reuben]

Hi,

I'm relatively new to all grain brewing, ~8 brews in.

I currently have my first big dark beer fermenting and I'm trying to decide if my fermentation is stuck or if it has just finished.

Recipe: "Imperial" Stout
Batch size in fermentor: 15L
Mash Temp: 68C

Grains:
Maris Otter: 6Kg
Crystal Malt: 0.3Kg
Roasted Barly: 0.3Kg
Chocolate Malt: 0.3Kg
Honey Malt: 0.3Kg
Flaked Wheat: 0.3Kg
Lactose: 0.2Kg @ 20 min

Hops:
30g Magnum @ 60 min
20g Magnum @ 20 min

Yeast:
NBS Classic English Ale Yeast 12g
2L starter with 200g DME for 72 hours

Additions:
Yeast Nutrient
Whirlfloc

Original gravity read at 1.097. It has been fermenting for a week and a half at 20C, the first few days showed a lot of activity but I took a gravity reading 5 days ago and 2 days ago both at 1.042. I have raised the temperature to 24C and given the fermentor a swirl but no movement.

I used the Brewers Friend online calculator which predicted an OG of around 1.100 and an FG of 1.025.

What could be the reason be that the fermentation has stopped? Too high a level of unfermentable sugars (grain bill/mash temp) / yeast health/something else?

Currently, it’s not tasting too bad, a bit sweet but it’s being balanced out by the bitterness from the grains to an extent. My plan for this beer was to be an imperial cherry stout, I was going to add some “Mangrove Jack's Natural Flavour Boost – Cherry” along with cocoa nibs to the secondary before bottling. I’m now thinking of adding some sour cherry concentrate to take off some of the sweet edge. Any advice?

Thanks in advance

 

[BeerCat]

Try some high alcohol yeast from crossmyloof with some beer nutrient and give it a stir. Some have had success using amylase (gluco300) probably best used in conjunction with new yeast.

 

[strange-steve]

Lactose is an unusual addition for an imperial stout, low attenuation is probably one of the biggest problems when brewing this style. I'd probably have mashed a couple of degrees lower too. That being said, the attenuation is still lower than I'd expect at only 55%.

 

[An Ankoù]

You're approaching 8% abv. Does this yeast have a higher alcohol tolerance? If not, I'd make up a starter of champagne yeast and stick it in there. Using champagne yeast to dry out your beer is not the same as using it from the start. You won't lose any of the esters produced by your original yeast.

 

[stz]

I brew my imperial stouts with a fairly basic grain bill in comparison to my lower abv offerings because otherwise they will finish very high. Get enough crystal etc in a brew, mash warm, perform an extended boil and even a 4.5% stout will stop at 15-16. My imperial stuff is usually a 90m mash at 64C, up to 4% sugar, similar amounts of speciality malts to my lower abv brews, but proportionally less and a medium high attenuating yeast because I want to get them down below 18!