Acetaldehyde - questions

[adambrau]

I'm new to brewing, having just bottled my second brew. Both brews when finished fermenting have had an overwhelming sour apple smell to them, the first brew I ended up chucking down the sink presuming it was infected. But after some research following that I determined it was most likely acetaldehyde.

My first brew was in the fermenter for 2 weeks, I checked gravity after around a week when fermentation had seemingly stopped and when having a taste of the sample it tasted ok. But after checking the gravity again a few days later it was starting to take on the apple flavour. By the time I opened it up for bottling a few days after that the smell was overwhelming, hence me presuming it was a goner.

Second brew, was sat in the fermenter for 3 weeks as I went away and forgot about it when I got back. It fermented down from 1.050 to 1.009 (prediction was 1.053 -> 1.010, so I was happy with that). But again pulling it out and taking the lid off the fermenter I was hit with the apple smell again. I pulled a sample and had a taste and it's very green apple. I've bottled this one to condition and see if it goes away. But was wondering if there's something I'm doing wrong.

One thing I've noticed is that once fermentation seems to be complete, my airlock seems to reverse and start pulling oxygen back into the fermenter. I'm assuming this is when some of the CO2 is absorbed into the beer or something and the air pressure changes, but it's happened on both brews. The kit I have at the moment only really allows me to do a 10L batch as the kettle can hold around 15L (i'm doing brew in a bag) but the fermenter is a 23L one I got in a starter kit. Is having so much headroom in the fermenter likely to be causing this issue with the airlock reversing?

I did read that the same process that converts acetaldehyde into ethanol can be reversed if oxygen is introduced post fermentation. So it would make sense to me that the oxygen getting in is causing excess acetaldehyde, unless anyone has had similar issues and has some other ideas, or what I read is incorrect?

 

[cheeseyfeet]

Acetaldehyde is usually caused by yeast stress I believe - are you pitching enough healthy yeast?

 

[PhilBrew]

Hi adambrau

One thing I've noticed is that once fermentation seems to be complete, my airlock seems to reverse and start pulling oxygen back into the fermenter.

... to me, combined with a fairly large headspace, that suggests your FV is too cool/cooling at times ... might it be somewhere where it's getting much cooler overnight, perhaps? Or might it have been somewhere where the heat generated during fermentation has warmed it to OK fermenting temps, but then as fermentation has slowed it's cooled? ... and it'll be the cooling/contraction of the gas in the headspace that is causing the "suckback"

As @cheeseyfeet says acetaldehyde is caused by "yeast stress" and giving them temps too low for them to get on with what they're doing is likely to cause them stress ... can you get your FV somewhere warmer or where the temp is more consistently warm?

Cheers, PhilB

 

[Sandimas]

I had this issue a couple of brews ago during the hot weather, FV got too hot and stressed the yeast I think. As mentioned above, temps too low or too high will do it, consistent temps really help a brew.

What temp did you ferment at, and did it vary a lot day/night?

 

[adambrau]

Acetaldehyde is usually caused by yeast stress I believe - are you pitching enough healthy yeast?

I think so, I'm using dry yeast, first time I pitched just over half a pack (7g ish) of US-05 into 10L, which was within the recommended pitching range.

Second time I pitched a full pack of BRY-97 into 10L, the pack said between 0.5-1g/l.

What temp did you ferment at, and did it vary a lot day/night?

This is the thing, I have it in a fermentation fridge outside in the shed, with an ink bird and a tube heater. I kept the temp set in the middle of the range recommended for the yeast, around 18C. Every time I checked, it was at that temp, cant check it in the middle of the night but cant see how it would vary by much.

Hi adambrau

... to me, combined with a fairly large headspace, that suggests your FV is too cool/cooling at times ... might it be somewhere where it's getting much cooler overnight, perhaps? Or might it have been somewhere where the heat generated during fermentation has warmed it to OK fermenting temps, but then as fermentation has slowed it's cooled? ... and it'll be the cooling/contraction of the gas in the headspace that is causing the "suckback"

Cheers, PhilB

I guess the exothermic heat generated by the yeast would go post fermentation this is good point. I can only currently tape the probe from the ink bird on the outside of the bucket where the liquid is, but whether it picks that heat up or not I dont know. This would explain the cooling inside the fermenter and the pressure change. But I'm not sure how I can stop that? Less headspace?

 

[Sandimas]

This is the thing, I have it in a fermentation fridge outside in the shed, with an ink bird and a tube heater. I kept the temp set in the middle of the range recommended for the yeast, around 18C. Every time Ichecked, it was at that temp, cant check it in the middle of the night but cant see how it would vary by much.

It might be worth sticking a max/min thermometer inside the fridge, I wonder if the tube heater was struggling with the temp variations in the night? Don't know where you live but we've already had a couple of frosts and the temp in my garage was only 8C at mid-day at the weekend. 18C is always the absolute lower limit I use for ale yeasts, I usually aim for 20C - appreciate different yeasts have different specs, but I've had problems in the past with ale yeasts when the temp dips to 17C or below.

 

[Brew_DD2]

I misdiagnosed oxidation time and time again as acetaldehyde, until I realised what was happening. Definitely look into reducing cold-side exposure to oxygen.

 

[An Ankoù]

Acetaldehyde is a precursor of acetic acid. Too much air getting to your brew at late fermentation stage. Stop the oxygen and you stop the problem. Once bottled, it won't get any worse and most vinegary smells will be carried off with the initial burst of "condition" when you pour the beer. You need to consider why your beer is infected with acetobacter, though.

 

[adambrau]

Thanks for all the replies guys, some good information to go on.

I misdiagnosed oxidation time and time again as acetaldehyde, until I realised what was happening. Definitely look into reducing cold-side exposure to oxygen.

Acetaldehyde is a precursor of acetic acid. Too much air getting to your brew at late fermentation stage. Stop the oxygen and you stop the problem. Once bottled, it won't get any worse and most vinegary smells will be carried off with the initial burst of "condition" when you pour the beer. You need to consider why your beer is infected with acetobacter, though.

Any tips for reducing the cold-side oxygren exposure? Different fermenter more suited to 10L batches, so there's less headspace? Increase temps slightly once fermentation is drawing to a close to try stop the gasses contracting in the headspace when the heat dips in the fermenter?

I think my sanitisation practices are ok, clean with sodium percarbonate, and sanitise with chemsan anything that will come into contact with the cooled wort, but acetobacter is everywhere from what I read, so surely it's impossible to be rid of completely? But as you say no oxygen, probably no problem.

I have ordered a fermenter king jr, as I like the look of fermenting under pressure and potentially using it as a uni-tank for serving once fermented as the bottling process is a PITA. Using a spunding valve I presume will stop any oxygren ingress. But I think at least getting one or 2 good brews just using a bucket would be nice

 

[Wynne]

Acetaldehyde is also a precursor of ethanol during normal fermentation. Its presence does not, necessarily, point to an acetobacter infection. Actetaldehyde in your beer could also result from a low yeast cell count (underpitching), low yeast vitality or an insufficiently vigorous fementation resulting in reduced CO2 stripping (e.g. from fermenting too cool) as well as oxidation. Increasing fermentation temperature a couple of degrees as @darrellm suggests or lengthening the fermentation time may help.

 

[adambrau]

Acetaldehyde is also a precursor of ethanol during normal fermentation. Its presence does not, necessarily, point to an acetobacter infection. Actetaldehyde in your beer could also result from a low yeast cell count (underpitching), low yeast vitality or an insufficiently vigorous fementation resulting in reduced CO2 stripping (e.g. from fermenting too cool) as well as oxidation. Increasing fermentation temperature a couple of degrees as @darrellm suggests or lengthening the fermentation time may help.

Your first point was what I had initially read, when trying to find out where it came from. My first brew was pitched at just over 0.5g/L, my second brew was pitched at slightly over 1g/L which was an overpitch as I was paranoid I'd maybe pitched on the edge of the low side on the first brew. I guess there are probably no guarantees on the vitality of the yeast. I can't tell how vigorous the fermentation seems as the bucket is opaque, though the krausen marks left on the side are sizable.

It does seem there are a lot of possible causes to consider, clearly I have a problem with oxygen ingress back into the fermenter currently when fermentation finishes that needs to be fixed. For the sake of a tenner I think a min/max thermometer to rule out large temperature swings in the fridge is wise too. It can't harm much to bump fermenting temps up to 20C either.

Again, thanks for everyone's input and suggestions on possible causes here.

 

[Brew_DD2]

I have ordered a fermenter king jr, as I like the look of fermenting under pressure and potentially using it as a uni-tank for serving once fermented as the bottling process is a PITA

This will make a big difference. Means you won't introduce oxygen when you cold crash. Will also allow for closed transfers to kegs.

 

[Tezz]

"My first brew was in the fermenter for 2 weeks, I checked gravity after around a week when fermentation had seemingly stopped and when having a taste of the sample it tasted ok. But after checking the gravity again a few days later it was starting to take on the apple flavour. By the time I opened it up for bottling a few days after that the smell was overwhelming, hence me presuming it was a goner."

I seem to have this issue now - tasted good when I tested it, now 3 days later its gone to tasting and smelling like apples. Didn't expect the change so quickly as there hasn't been any changes to the condition its kept at. I am still planning on bottling and hoping the conditioning phase will get rid of the taste and smell.

Maybe I should of bottled it sooner although it had only been a week.

 

[Sandimas]

Maybe I should of bottled it sooner although it had only been a week.

No, it'll be something to do with your process that is causing it, not the length of time in the FV. I regularly bottle my brews between 21 and 28 days after pitching and most are fine. You may not have detected it at first but it will have been there. Sadly my experience of this is that it only gets worse over time. Even us experienced brewers get it from time to time, you usually know as soon as you open the FV.

 

[adambrau]

I seem to have this issue now - tasted good when I tested it, now 3 days later its gone to tasting and smelling like apples. Didn't expect the change so quickly as there hasn't been any changes to the condition its kept at. I am still planning on bottling and hoping the conditioning phase will get rid of the taste and smell.

Maybe I should of bottled it sooner although it had only been a week.

Hopefully it works out for you in conditioning. It didn't for me, unfortunately. Opened a bottle recently and it was still as bad, so I dumped the lot. Nevermind though, this sort of stuff is all about practice, got to break a few eggs to make an omelette.

 

[Tezz]

No, it'll be something to do with your process that is causing it, not the length of time in the FV. I regularly bottle my brews between 21 and 28 days after pitching and most are fine. You may not have detected it at first but it will have been there. Sadly my experience of this is that it only gets worse over time. Even us experienced brewers get it from time to time, you usually know as soon as you open the FV.

Ahhhh well, only my second brew - so live and learn.

Wonder if its more the lager type brews that it happens to? or doesnt it make a difference.

 

[Sandimas]

One thing I do is replace the FV lid towards the end of fermentation with a solid one with no airlock. The main reason for this is that I can see when fermentation is topping when the top stops bulging, but it helps keen oxygen away from the brew.

Tell us more about your first brew. If it's lager then there are a number of other things to consider e.g. some lager require 12 weeks conditioning time, because they produce by-products that mellow out over time. So it actually might be OK. And was it a lager done with a lager yeast, or an ale yeast? And at what temp? etc.

 

[Tezz]

My first brew was a Diablo IPA - which seems fine and is conditioning .

This brew which am talking about is a Geordie Lager from Coopers:
Yeast - i presume it was Ale yeast as per comments I have read and the heat the instructions stated. 18-22
Temp - been kept at 20-21 degrees for 12 days
Air lock - bubbled for a week and has now died down to a murmur every now and then