View Full Version : Fermenting - weird top layer appearance at WT

05-01-2009, 06:25
When I was at Wild Turkey last week I noticed that after several days of fermenting the top layer on the tanks was VERY thick and lumpy. I asked Jimmy Russell about this and mentioned that I had never seen the fermentaion so lumpy and thick at the top before at any other distillery. There is usually a layer at the top, but this was many times thicker and lumpier than I had ever seen. He said is was due to their mashbill. I asked if he meant the high rye content and he replied yes.

The next day I was at Four Roses and was expecting to perhaps see something similar due to their high rye usage, but their fermentations all looked pretty normal. (Unless all the tanks were fermenting the low rye mashbill that day.)

Has anyone else noticed this and have a further explanation? Chuck?


Bourbon Geek
05-01-2009, 08:46

The thickness of the grain cap on top of the fermenters is a function of a number of things:

1. What stage of fermentation is in progress. When the fermenter is first filled, the grain is evenly distributed, and the fermenter has no grain cap. There may even be a layer of frothy foam on top of the fermenter. As the fermentation proceeds, the carbon dioxide released by the yeast during fermentation begins to buble to the surface ... carrying the sticky grain residue with it. A grain cap begins to form, and gets progressively thicker as the fermentation progresses ... sometimes reaching a thickness of 4 to 6 inches. At first, as the grain cap forms, it will appear rather smooth and uniform, but as it continues to expand, it will crack and heave ... resulting in a jagged and uneven surface. As the process begins to wind down, the level of carbon dioxide bubbles begins to decrease ... and the weight of the grain cap will cause it to sink ... lessening, or almost completely diminishing the appearance of the grain cap. Right before pump out, the fermenter is stirred to disperse the spent grain more uniformly ...

So the appearance of the grain cap will depend on when you look at it.

2. The thickness of the grain cap is also dependant on the performance of the mill ... either through wearing out, or falling out of adjustment, or by choice of the distiller... The more coarse the grist, the thicker the cap will get ... at Maker's Mark, right before they replace or sharpen the mill rolls, the grain cap is very thick ... right after, it is much less thick.

3. As Jimmy Russell told you, the thickness of the cap will also depend upon the mash bill. this can be a fairly complex relationship ... However, the rye residue does tend to be "stickier" and clings better to the bubles as they rise ... so the cap gets thicker.

4. There are probably other factors that help determine the ultimate grain cap thickness and appearance ... like atmospheric conditions (high or low pressure ... humidity levels ... etc). It has been a long time since I did any Chemical Engineering math on the topic called "buble fractionation" ... but that is basically what's going on here.

Hope all this helps.

05-01-2009, 08:57
I've always wanted to time a distillery tour to coincide with the fermenting and distilling of the rye, but have so far been unsuccessful in this endeavor.

Bourbon Geek
05-01-2009, 09:07

I think it might also be instructive to take the same tour on 4 consecutive days ... making note of a specific fermenter or two and watching their progress throughout the fermentation process.

05-01-2009, 09:44
I've always thought it would be a good idea for the distilleries to put a sign on the fermenters telling the fill date of each fermenter, so the tourists could easily see that one was on day one, day two, day three, etc.

05-01-2009, 09:50

Thanks for the detailed response. That may be it, that I caught the fermentation right at the stage where the grain cap was at its most pronounced and before it sank back down. Some of the tours I've been on have shown us fermenters are various stages and none looked this dramatic to me, but perhaps the stage where the cap cracks and makes its thickness obvious is brief enough that a visitor would not catch it very often.

I'll try to post a few comparison pictures as I get to them.

05-01-2009, 10:36
I've always thought it would be a good idea for the distilleries to put a sign on the fermenters telling the fill date of each fermenter, so the tourists could easily see that one was on day one, day two, day three, etc.

Jimmy and I toured Maker's last week, and the guide pointed out the "approximate" age of what was in each. Didn't break it down by day, but simply said "Tank X is new, Tank Y is in the middle of the process, Tank Z is almost done." The tank I dipped my finger into was very globby, thick, and not producing very many bubbles. The guide said it was one near the end of the cycle.

Bourbon Geek
05-03-2009, 05:01
If it was globby and thick ... it couldn't have been too near done ... if you tasted it, you would have a better idea of completeness. Near the end, it is very sour and a hint of alcohol is present.