Have any Questions?


Jun 08, 2022 View:

Possible Mash Infection Need Help

I have a customer who is having a problem.  I think that he has some kind of infection in his mash.  They are doing a grain in corn mash for Whiskey.  The problem is that their distillate has a very skunky smell and taste.  They have tried distilling very slowly and leaving the tails in the pot.  Also their mash has a dark color after distillation.  They say the mash does not have anything unusual about it when it leaves the fermenter but one thing that is very strange is the fact that if they try to ferment with the top open on the fermenter, the fermentation stops, no matter what point the fermantation is at.  They can only get their mash to ferment when they have a lid on the fermenter.  They say that they clean everything very well.  I think that they have some kind of infection.  Anyone have any ideas concerning what they can do to solve this issue?


I have an ethanol plant background so my statement may not apply, please take with a grain of salt: 

There were times when our mash would turn a darker color after distillation, and the resulting DDGS and syrup product would be much darker as well.  This was due to fermentation not finishing and sending a high sugar load through distillation/evaporation.  Is it possible these ferms are not fully finishing and the sugar is caramelizing?  

I do not remember there being any off odor when this occurred.

As far as top open/closed:  Any chance the yeast are entering the budding phase with the introduction of oxygen?  We would use an air sparger to introduce oxygen during yeast propegation, the fermenters were sealed and very quickly turned anaerobic.  We typically had little to no ethanol in the yeast prop, we didn't make ethanol until the fermenter.  That being said, I can not really believe just opening a top of a vessel compares at all to using an air sparger.  Open top fermentation produces ethanol all the I really do not have a good guess...just a wild one.........



Sounds like infection to me. I have had a few runs where the mash smelled bad and it resulted in 3 times the fores than usual and the distillate just had an off smell.  I want to know more bout what their process is especially the cleaning. Customer says they clean everything very well means absolute jack squat because to some people cleanin means nothing more than hosing it off.

Why do they want to open top ferment? Do they love fruit flies?

Reply:24 minutes ago, MGL said:
Reply:1 hour ago, AK2 said:

Any mold issues with the corn?


Aren't most large open top fermenters dosed with antibiotics, which is the reason the EPA wants to change the regulations on mixing distillers/Brewers grains into the commercial beef and pork industry ? So while many in fact use open fermenters, how many are dosing to prevent infection ?


There seem to be multiple problems here, not just one factor in play.  Sounds like possibly overdosing nutrient, coupled with a stalled fermentation.  Going to result in a lot of residual nitrogen and sugar in the wash, which together are going to create a ton of non-pleasant off-flavors (scorched, smoky, bitter, etc).  These are mostly Maillard reaction products, which require sugars (primarily monosaccharides/glucose, nitrogen/amino acids, and heat to generate, the dead giveaway being the dark color of the stillage.  @Ak2 is dead right here.  Running slower is just going to cause more time under temperature, thus more Maillard reaction products.  Whatever is causing the stalled fermentation, is likely causing significant yeast stress, and thus the sulfur aroma/flavors.  The yield in PG is probably abysmal, this would confirm it.

The open/closed top is a red herring, it's purely coincidental at best.  It stops fermenting when the top is open, because someone opened the top and noticed it stopped fermenting, or stopped fermenting shortly thereafter, or jiggled the tank a bit to release some co2 bubbles, saw it was bubbling, but then it stops.  Schrodinger's Cat Whiskey.

We need to know what the whole protocol is to help any more I think, otherwise just conjecture.

Reply:1 hour ago, Silk City Distillers said:
Reply:9 hours ago, Roger said:
Reply:2 hours ago, Silk City Distillers said:
Reply:26 minutes ago, indyspirits said:
Reply:10 hours ago, Huffy2k said:
Reply:37 minutes ago, Southernhighlander said:


Show me a snide remark. You should reread what I wrote.  I never made any snide remarks.  Now read what you yourself wrote.  I only disagreed with you.  I appreciate the help but not the abuse.  I will not retaliate in kind.


Thank you.




As far as my customers go I am absolutely loyal to them and I always take care of their needs.  I only disagreed with Roger.  I never disrespected him.  Please see the below to see  what my customers say about me.  I have never had a customer give me a negative review that I know of and I have a lot of customers in this industry and I gain more every day.


Reply:59 minutes ago, Southernhighlander said:

I am here to offer people the equipment that they need.  I have helped a huge number of people on here with their equipment needs and I will continue to do that.  Me disagreeing with you, did not mean that I had a negative attitude toward you, it just meant that I disagreed. You said that I made a snide remark, and I asked you to show it to me and you could not because I did not make any snide remark.  I only disagreed with you.   As far as distilling from corn my family has distilled from corn for well over 200 years My grandfathers distilled from malted corn for over 60 years.  How long have you been distilling from corn?  If I didn't know how to distill I wouldn't be in this business.  I help my customers with their distilling processes everyday.  But that doesn't mean that I know everything, there is always more to learn.  I have learned a tun of stuff on here, especially from Silk City and also from Roger and many others and I will keep doing so..  I have a great deal of respect for Roger.  He takes a lot of pride in what he does and he is all about craft and keeping craft pure.  Also he does everything from scratch when many others are rebubling.  I know that you use bulk spirits.   That's fine in my opinion as long as you are not putting craft on the bottle.  Do you put craft on the bottles of spirits that come from the bulk GNS that you buy?  If you are doing so then you are misleading your customers.  If you don't then I respect that.


Awkward... Are they taking gravity readings during the fermentation? That will give them a more definitive answer if the fermentation is working while closed, and if there are any sugars left, instead of just observing bubbles from an airlock, which it sounds like may be the case.

Thanks for all the replies, if find I learn a ton from these trouble shooting posts, even if the answer isn't the solution of the specific problem.


Hi Adam,


Sorry for the BS.  They will be sending me a complete discription of their process by the end of the day and I will post it.  I know that he said that they are starting with a brix of 19 and ending with a brix of 4.



Has anyone heard of Kahm yeast?  I'm wondering if it might be the culprit.


Okay everybody.  Please see the below and thanks again for the help.  Also, see the pic below.  My Customer thinks that the problem is a wild yeast called Kahm yeast.


Here is our current process for making bourbon whiskey.  Just FYI, I’ve experimented with just about every aspect of this process trying to eliminate the cause of our skunky flavor.  Also another FYI, I brought a sample of this “skunk” out to Rusty, and his opinion seemed to coincide with what you mentioned earlier about heads bleeding through the run.  Rusty thought that our skunky sample had some characteristics of heads, although the sample was taken about 1 hour after I cut the heads off in the run. I’m generally getting about 10% heads to the expected overall yield per run.  Once it seems the heads are about done, I do notice a definite (expected) change in flavor and temperature in the still, but the change goes directly into this super skunky flavor.  I don’t monitor the proof of the heads so when I make the cut I’m generally running a little cold in the dephlegmator so I’m a little high in proof at the start of the hearts cut.  It seems like this skunk is more prominent at higher proofs, until I make adjustments to the dephlegmator temperature to knock the proof down to about 140 where the flavor seems to be the most tolerable.  From what I could tell at Rusty’s, it seems like our mash is getting scorched in the still, which I think would probably have a major effect on flavor.  Also, seems like too high of a temperature could have all the cuts bleeding together as well.  I haven’t had a chance yet to experiment with alternate heating times and methods.  I’ll be distilling early next week.


So basically we have 3 major issues:

    Kahm yeast in the fermenter if left to open air for any length of time. – Resorted to covering fermenter tanks with clear plastic shrink wrap.  No wild yeast at all when covering fermenters.  Problem solved? IdkStuck fermentation- At first I thought the wild yeast and the stuck fermentations were related.  I’ve eliminated the wild yeast but still have fermentations stop at 4-5 Brix.  Ferment still looks, smells and tastes great though.  “Skunky” Flavor in distillate- Super sour, astringent, skunky, medicinal, and also metallic flavor in distillate especially at higher proofs.  Sometimes the skunk works it’s way out once we settle into a lower proof, sometimes it stays through the whole run



I’m always paranoid about mold/bacteria/infections so I make every effort possible to clean all equipment the best I can.  I’ve use cleaners such as sodium percarbonate, PBW, citric acid, vinegar, etc.. I also sanitize after rinsing with Star San.  I’ve also experimented with NOT using these chemicals just to see if that made a difference in bacterial/mold growth or flavor.  The result is always the same- if I cover my fermenters no abnormal growth happens.  If I leave my fermenters open for any length of time I get what I’ve come to find out is Kahm Yeast.  Pic included.

I’ve experimented with taking the entire column apart and all the plates apart and cleaning all parts with brushes and Q-tips. Even had them soaking overnight in vinegar.   I’ve also experimented with just rinsing the column with hot water after a run just to see if the cleaners were causing any problems.  Same results, clean or dirty.



Using RO water.  City water here is terrible. 

Pot temp to about 120 F

Add 2 of 3 enzymes, then corn and rye then let rest for 30 mins

Ramp up to 150 F, add number 3 enzyme, add barley, let rest for 90 mins

Ramp up to 175 F, let rest for 90 mins

Cool to 80-85 F, add Fermaid K (1 g/gal- had trouble with stuck fermentation earlier then tried this Fermaid K and seemed to work for a few fermentations)

Add Yeast (Red Star Distillers active dry, also experimented with 2 other brands of active dry yeast with same results)

Brix reading with Brix hydrometer: 19 (I’ve experimented with all different ways of cooking as well with the same result of 19 brix)

Mash PH is around 5.5- 5.8



Mash goes in at 80-85 F (Fermenting on-grain)

Cover fermenter tank with clear “pallet wrap” plastic

Everything seems very active, smells and looks great, a cake develops within a few hours

Ferment temp gets to around 90 F

Cake remains for about 3-4 days

Check Brix on 5th day

Distill on 5th or 6th day (when we’ve had successful fermentations they usually took 5-6 days)

Final PH on Ferment is around 3.8



Distilling on-grain

Heat still to a vapor temp below column (4 plate copper) to about 200 F

Let column reflux for about 30 mins with full dephlegmator flow

Cut flow to dephlegmator to allow a nice steady stream (column head usually goes straight for 174 F, don’t know if that’s a coincidence or not.  Can’t get anything to flow below that temp)

After 10% of expected total yield, the heads start to change over

At this point I make the cut and observe my parrot hydrometer

Adjust the dephlegmator flow to achieve at least below 160

Find “sweet spot” for flavor at particular proof, usually around 140

Let run to 110 or until taste is too “tailsy” (which has happened before 110 before)



If I missed something, or if you need more details on something please let me know.  Thanks Paul.



Ok, lots of work to do here.

That is not "Kahm" yeast, it's a fairly typical pellicle.  It would be impossible to identify the specific microbes from the pellicle alone, but given the mash protocol, there are a good number of potential candidates.  Achieving that kind of pellicle in 5 days, or 1-2 days after the cap drops, is fairly impressive, that's a substantial bacterial load.  Congratulations.

Here goes.

1. You can not mash with RO water, unless you add necessary minerals back to the water.  This is the source of the yeast stress I mentioned.  Yeast will not be able to effectively reproduce in a wash made with only RO water, as it's missing a number of crucial minerals.  I suspect this fermentation is taking a long time, in addition to stalling, due to the yeast ultimately dying out and failing to reproduce effectively.  In addition, the lack of calcium ions are going to significantly reduce the efficacy of the enzymes.  Need to fix the source water.  As a test, you can try using 50% RO and 50% tap water to reduce whatever the offending issue is, until a better water treatment solution can be found (carbon treatment, nano/ultrafiltration) or adding back minerals (building water from scratch can be very costly in the long-run).

2. Related to the RO issue.  If he is hydrating the active dry yeast with RO water, he is likely killing a good amount of yeast cells in the process due to the osmotic shock before even pitching.  Alone this wouldn't be a massive problem, but in conjunction with #1 it's making it worse.

3. The mash protocol sucks (sorry, it does), I'm not sure who trained him on that approach, or if he developed it himself, but it's suboptimal.  We don't know the total grist used to hit 19 brix.  If it's around 500lbs (per 265g batch), it might be ok.  I'm not going to dwell here except to say that the way the mash is being done, it will create lots of unfermentable dextrins, which are loved by non-yeast bacteria as food (like brett), and will likely result in what looks like a stuck fermentation.

4. Everything above tells me that bacteria are out-competing the yeast here.  The yeast and the bacteria are standing on the starting line of a race, and before the race starts, someone goes up to the yeast and beats the hell out of him with a baseball bat.  The starter then proceeds to throw tacks all over the lane the yeast is running in, wishes the bacteria good luck, and then starts the race.

5. Proof of this is the fact that adding 1g per gallon of Fermaid K improves the situation.  While I wouldn't call this overdose, in an all grain mash this amount should be entirely unnecessary.  So adding the Fermaid K is fixing some of the issues caused by the RO water.  It's adding back some of the missing minerals, it's also giving a slight boost to the yeast.  We're talking about 2 pounds per 1000 gallon mash here, that's quite a bit.  Given the fermentation stalling, and limited yeast reproduction, I'm going to hold my ground here and say that at distillation time, we have an abundance of nitrogen left in the wash that is causing a problem.  Once the RO issue is corrected, he can likely dial back the Fermaid K additions substantially.  I would suggest continuing to use it, at about 1/4 the current dose rate, as "insurance" until the mash and fermentation protocol is dialed in well.

6. Don't ferment at 90 degrees.  The poor yeast.  This is causing major yeast stress.  This is going to result in a massive amount of heads, and combined with the already high stress environment, create a very sulfury wash.  You don't make good whiskey at 90f.  Until he can resolve the other issues, I would recommend keeping the fermentation in the low 80s.

7. Drop the pH at the time of pitching to closer to 5.2 using acid.  The increased acidity will help the yeast outcompete the bacteria.  5.8 is a very happy place for bacteria.

Once these things are taken care of, I would suspect the "bleeding heads through hearts" issue to go away - this is a direct result of acid production by the bacteria.  Also the skunkiness will be eliminated as well, this is the direct result of yeast stress.  Yield should likely improve substantially, and reduction of the nitrogen will result in reduction of any of the remaining off flavors.

Get the mash protocol straightened out too, it's an inefficient use of grain and enzyme.


A quick recipe for building mashing water from scratch (RO), if the water is so bad that it can't be used untreated.

Per 100 gallons:

20 teaspoons Gypsum

20 teaspoons Calcium Chloride

20 teaspoons Epsom Salt

20 teaspoons Baking Soda

In addition to the mineral additions, he should try using 5-10% liquid backset/stillage to help boost mineral and nutrient content.


Um, I don't recommend he adds backset from those past fermentations...