Pilsner with slight banana flavour

Joe1980

Hi, so I have tried to produce a Czech style pilsner but it currently has a bit of banana ester flavour to it.
I cooled wort to 13C and pitched a single packet of Mangrove Jacks M76 Bavarian Lager Yeast straight on top of the wort. (Clearly underpitched, oops!).
It took several days to get going in my fermentation fridge at 13C and was still bubbling along three weeks later. I ended up giving it three and a half weeks in primary before raising temp to 18c for a diacetyl rest.
I then cold crashed and kegged.
First taste is now two weeks in the keg and the banana ester and taste is noticeable but not completely overpowering.
Apart from the under pitch, what else would have caused the banana ester? I realise this is Isoamyl acetate so my question is really what causes this to form? was temperature the cause or is it more likely the under pitch of yeast causing stress?
Will the flavour improve with time? It is still drinkable but not the best, oh well you live and learn!
Cheers

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Dland

My guess it the the under-pitch could cause this. Have not used that yeast though, so so not know how clean it is either. Three and a half weeks is a pretty long primary even at that temp with lager yeast, so that kind of points to underpitch or lack of oxygen to get yeast going.
Off taste should decline with age, waiting for all the yeast to crash out and settle will help too.

Jack-Arandir

From White & Zainasheff's Yeast book, dry yeast should be rehydrated in water before pitching, or you can lose up to 50% of the cells. That, combined with the underpitch you mentioned creates an environment of high yeast stress. The yeast had to rebuild a large colony from a small start, then chew through your sugars, then clean up any off flavors created. Stressed yeast produce extra off flavors, especially esters and phenols.
That said, an underpitch creates an environment where other yeast cultures can compete. It's possible some other strains made it into your fermenter, such as a phenol off-flavor positive strain or a hefeweizen-like strain. That could add the banana flavor you're tasting.
There's a chance the flavor will improve with time. You could repitch with a neutral lager strain like 34/70 (WLP830), and hold that at 3-6C and hope it cleans up the esters. That may be your best bet for cleaning it up. Your existing yeast are probably too exhausted to do much cleanup.
And when all else fails, relax, don't worry, and have a homebrew!

VikeMan

Why do you think that adding a lager yeast (or any strain) and holding at 3-6C would clean up esters? The only reason I can think of to try adding yeast would be hoping the new yeast, containing no/few esters, might absorb and hydrolyze some of the beer's esters, until a synthesis  hydrolysis equlibrium is reached. But I doubt that the new yeast would have the incentive/capacity to do this, given that they would be quite inactive in the now food and nutrient poor finished beer. And the low temperature would also suppress activity.
Have you done this, and how did it work out for you?
(Also, 34/70 and WLP830 are not the same strain, but that's probably not relevant.)

Jack-Arandir

I am suggesting using krausening to attempt to remove off flavors. Add healthy yeast and some fermentable sugars and lager at low temperature. It is a commonly used technique in German lager brewing and has demonstrated effectiveness in removing off flavors.
Since isoamyl acetate has a low flavor threshold (one of the lowest among esters), it may not take much cleanup to push its concentration below the threshold.
There's no guarantee it will work, and the isoamyl acetate may drop enough over time without intervention. But it's a suggestion that could improve the beer.

Joe1980

The instructions on the packet suggest that dry pitching into up to 23L of wort is sufficient. I will double down next time though.
Now the first couple of pints have gone the beer is clearing up and the banana is fading a bit.

dmtaylor

Strange, getting banana in a lager. You might have a little contamination. Fortunately, banana (isoamyl acetate) is a weak ester and will disappear with age. How much age, depends. Two to six months is probably in the right ballpark.

VikeMan

Thanks for clarifying. I would not have guessed you meant krausening from "repitch." You are correct in that krausening can clean up some off flavors. These include diacetyl and acetaldehyde. But krausening will actually add esters (and/or fusel alcohols destined to become esters). I suppose if you krausen with a high enough volume of low fusel/ester wort, you might be able to dilute the beer's higher ester concentration, maybe to something below threshold, if it's close. But it wouldn't be the krausening per se that did the trick, it would be the dilution, sort of like blending beers.
I've never seen any textbook or paper suggest krausening as a way to fix excess esters. Can you point to any?
Also, krausening is not normally done at 3-6C, and certainly not at the lower end of that. i.e. krausening should be done at fermentation temps.