When Fermentation Rears Its Dirty Head


Testing the Positions for and against Removing Kräusen during Fermentation

by Al Korzonas (Brewing Techniques - Vol. 4, No.3)

One inquisitive brewer designed an experiment to determine the effect of removing or not removing the dirty head during the early stages of fermentation. The results confirmed some expected effects — and revealed some surprises.

Few topics in home brewing perennially elicit as great a debate as that of whether to blow off or not blow off the dirty head from atop the kräusen that forms during fermentation. Arguments typically center on fusel alcohols and bitterness and the effect on the finished beer when the dirty head is removed. More recently, some brewers have raised the issue of head retention. Commercial brewers likewise debate whether or not to remove the dirty head during fermentation.

Supporters of removing the dirty head cite decreasing “coarse or rough bitterness” as the benefit (1). In the United States, the practice seems to be decreasing in popularity, supposedly because of “the use of clean wort and yeast” (1).

Opponents of the blowoff method often point to the significant loss of beer that is commonly associated with this practice. Depending on the yeast strain, temperature, and the original gravity of the wort, the amount of beer lost can vary from just a few ounces to as much as ¾ of a gallon in a 5-gal batch.

Another reason for choosing not to blow off the dirty head is loss of yeast; when using a strongly flocculating, top-fermenting yeast, a significant portion of the yeast can be blown off along with the dirty head, and the loss of yeast may slow fermentation. Furthermore, most yeast populations are made up of individual cells of various flocculating abilities. Removing the highly flocculent yeast through the blowoff method selects for less-flocculent yeast. If the yeast from this fermentation is not reused, however, then this selection process has no impact on future fermentations.

Home brewers loyal to the blowoff method argue that the finished beer tastes smoother and less harsh than if brewed without removing the dirty head. “Once you’ve tasted the actual blowoff,” some brewers have been known to say, “you will be sure to stick to the blowoff method.”

On 21 September 1993, I posted a hypothesis to the Home Brew Digest (an internet mailing list) that perhaps the blowoff method reduced the amount of small proteins, which cause a head to form and remain stable in a finished beer. This hypothesis was spawned by a batch of Lithuanian imperial stout (1.120 original gravity) that created so much thick blowoff, the blowoff jug overflowed with foam. Once the overflowed suds dried, they solidified into a brittle, brown foam. The incredibly long-lasting head in the blowoff jug suggested to me that perhaps foam-sensitive proteins were selectively blown off.

It became apparent that an experiment was necessary to test this hypothesis and to determine whether quantifiable differences existed between beers brewed with and without the blowoff method. Samples of beer brewed using each method could be compared by measuring the proteins, esters, alcohols, and bittering units of each and performing subjective taste tests.

Experimental Design

To experimentally compare the two methods, I brewed two batches of beer: an American pale ale using exclusively Cascade pellet hops, and an India pale ale using Liberty, Mount Hood, and East Kent Goldings pellet hops. I split each batch between two fermentors.

In both batches, I based the estimated IBUs on Jackie Rager’s formulas (adjusted 10% to compensate for utilization losses resulting from the use of nylon hop bags in the boil) (2). Each of the two sub-batches was brewed from half of a 1-L starter made from a single package of yeast.

For the American pale ale, I used a 5-gal carboy for the blowoff sub-batch and a 6-gal carboy for the non-blowoff sub-batch. In an effort to eliminate differences attributable to fermentor geometry, I chose fermentors that gave the wort the same aspect ratio (the ratio of height to width) for each sub-batch. Each carboy received 5.125 gal of pitched wort. The carboys were kept in a room with an ambient temperature of 66–68 °F (19–20 °C). I pressed a sanitized 1¼-inch o.d. plastic hose into the neck of the 5-gal carboy for diverting the blowoff. Active fermentation lasted about five days, but the beer was allowed to sit in the fermentors for a total of two weeks. No secondary fermentors were used.

For the India pale ale, I brewed 6 gal of wort and split it between a 3-gal carboy and a 5-gal Cornelius keg (lid removed, top covered with HDPE sheeting). I pressed a sanitized 1¼-inch o.d. plastic hose into the neck of the 3-gal carboy for diverting the blowoff. Because of the differences in fermentor material (glass versus stainless steel) used to brew the India pale ale, I attached Fermometers (Tkach Enterprises, Castle Rock, Colorado) to the two fermentors and monitored them carefully. The temperatures displayed on the two Fermometers were within 1 °F (0.5 °C) of each other throughout fermentation and were never more than 72 °F (22 °C), even at high kräusen. Active fermentation lasted about five days, but the beer was allowed to sit in the fermentors for a total of two weeks. No secondary fermentors were used.

In both batches, the blowoff sub-batches expelled a significant amount of kräusen. In the American pale ale blowoff sub-batch, approximately 48 oz of beer was lost to blowoff from an initial volume of 5.125 gal (a 7.32% loss). In the India pale ale blowoff sub-batch, approximately 24 oz of beer was lost to blowoff from an initial volume of 3 gal (a 6.25% loss).

Kräusen and the Dirty Head

What is the dirty head and the blowoff method?

The dirty head is the brown, sticky matter that rides atop the kräusen — the large mass of foam that develops on top of the wort during the early stages of fermentation. If the dirty head is not removed, some of it will adhere to the walls of the fermentor, some will redissolve in the beer, and some will settle to the bottom.

Brewers have used various methods for removing the dirty head, including closed fermentors designed such that the foam will adhere to their “ceilings.” The two most common methods of removing the dirty head are skimming and the blowoff method. Skimming involves the use of some sort of sanitized utensil or tool and is the only option for open fermentors, whether large commercial fermentors or plastic homebrew buckets. The blowoff method is used to remove the dirty head without the sanitation risks associated with skimming. With this method, a closed fermentor (a glass carboy or commercial-scale fermentation vessel) is filled with wort to within an inch or two of the top, and a hose is fitted to the opening at the top of the vessel (the larger the inside diameter, the less chance of clogging). The other end of the hose (or blowoff tube) is submerged in a few ounces of water in a jug or bucket. When fermentation begins, the kräusen pushes the dirty head out of the fermentor and into the receiving vessel.

The great debate over the effect of this matter redissolving into the beer and the value of removing it was the impetus for the study outlined in this article.


Table I: Fermentation Comparisons, Blowoff vs. Non-blowoff Method


Original Gravity

Estimated Bitterness (IBUs)

Temp (°F)


Final Gravity

Batch 1










Wyeast American Ale






Wyeast American Ale


Batch 2










Wyeast British Ale






Wyeast British Ale


After fermentation, I bottled several samples of each sub-batch without priming and sent them to the Siebel Institute of Technology (Chicago) for analysis. I also bottled several samples of each sub-batch with priming. The primed samples were taste-tested by BJCP (Beer Judge Certification Program) national and certified judges and one non-BJCP judge. None of the beers tested or judged were dry-hopped.


Fusel alcohols: The levels of fusel alcohols are different for the two fermentation methods; however, the levels in all samples were far below the average taste thresholds for fusel alcohols (see box for a discussion of fusel alcohols). Oddly, in batch 1, the level of isoamyl alcohol in the blowoff sub-batch is higher than in the non-blowoff sub-batch, whereas in batch 2 the reverse is true. It is conceivable that with less well-behaved yeasts — yeast that tended to produce significantly higher levels of fusel alcohols — the blowoff method might have a more pronounced effect on the flavor of the finished beer.

Esters: Of the two esters measured, one (ethyl acetate) was found to be higher for the non-blowoff method and the other (isoamyl acetate) higher for the blowoff method. Only ethyl acetate in the non-blowoff half of the second batch was above the average human threshold.

In reviewing the data, Joe Power from the Siebel Institute of Technology warned: “The data on fusel alcohols and esters are a little confusing and hard to interpret. … Both batches showed a decrease in total esters with blowoff, but there is no clear pattern with the fusel alcohols. It can also be confusing to place too much emphasis on individual taste threshold values for esters and fusel alcohols. Their effects on flavor can be cumulative, and the ratio of one group to the other can have subtle effects on the overall flavor which have nothing to do with being able to pick out the individual compound as it increases above the threshold level.”

Fusel Alcohols and Their Effect in Beer

Fusel alcohols (also called higher alcohols) are larger molecules than ethyl alcohol (ethanol); ethanol’s chemical formula is C2H5OH, whereas propanol’s, for example, is C3H7OH and butanol’s is C4H9OH. Several types of fusel alcohols can be found in beer; some, such as tyrosol, are phenolic in structure. Amyl alcohols such as 2-methyl-1-butanol (4) are also common.

Fusel alcohols are commonly thought to be a source of hangovers, though no scientific study has proven that connection. Despite their association with hangovers, fusel alcohols play an important role in the flavors of many beer styles, particularly in such specialty beers as Bière de Garde and Belgian ales such as saîsons, some Trappist ales, and Belgian strong ales.

Fusel alcohols make significant aroma and flavor contributions to beer. Despite the fact that the typical level of ethanol in beer is many times that of the fusel alcohols, the aroma and flavor thresholds for the more intense fusel alcohols are considerably lower, which means they can be detected when present in smaller quantities.

Propanol, butanol, and similar alcohols have the same aroma and flavor that we associate with ethyl alcohol, but just a small amount of any of them can make an otherwise low-ethanol beer smell and taste “alcoholic.” Amyl alcohols vary in flavor from “alcoholic” to “banana” to “solvent-like.” Phenol alcohols vary in flavor from “roses” to “medicinal.” Also, just as beers high in ethanol have a warming character in the mouth and throat, fusel alcohols also produce this sensation, though even more intensely.


Table II: The Analytical Results


Human Threshold*

Batch 1

Batch 2





Fusel Alcohols:†


















Isoamyl alcohol






Amyl alcohol












Ethyl acetate






Isoamyl acetate






Bitterness Units












*Threshold data for fusel alcohols and esters taken from reference 4.

†AII values shown are in mL/L except protein and bitterness units.



Bitterness: The difference in bitterness between the blowoff and non-blowoff sub-batches turned out to be significant. With the systems used for this experiment, the blowoff method resulted in a 13–17% loss in bitterness. As an interesting aside, the data show that the estimated bitterness using Rager’s formulas for pellet hops (after factoring in an additional 10% to compensate for the large nylon hop bags used) was quite close to the measured bitterness for the non-blowoff method.

Protein: Protein levels were measured to determine the amount of protein lost due to blowoff. Although it is impossible to tell from this one experiment what percentage of the lost proteins consisted of small, head-retaining proteins and what percentage consisted of larger, haze-forming proteins, the losses were found to be negligible.

Final gravity: Note that in Table I, the final gravity of both of the blowoff sub-batches was 0.001 lower than in the non-blowoff sub-batches. The opposite might have been expected. Had the final gravity been lower in the non-blowoff sub-batches, a likely explanation would be that perhaps a significant amount of yeast was blown off and therefore the blowoff sub-batches did not fully ferment. This was not the case, however.

Two explanations are possible for the differences in attenuation, though neither are definitive. First, the difference in protein levels, although small, may be part of the reason for the difference in final gravity; the higher final gravities in the non-blowoff batches could be attributable to proteins retained in solution. Second, since blowoff would have a tendency to expel more-flocculent yeast, perhaps the higher proportion of less-flocculent, more-attenuative yeast would result in a lower final gravity. If the latter was a major factor, eventually the final gravity would have equalized.

Taste testing: In typical competition form, all the beers in the qualitative judgings were presented to the judges without any indication as to the differences between them. The judges were told only that the beers were distinguished by a single difference in the ingredients or methods used in production and that the flavor/aroma differences may or may not be significant. In the case of batch 1, in one of five judging sessions four judges tasted from each of four bottles (two from each sub-batch). The judges’ comments proved to be quite consistent between the two bottles.

Every judge consistently detected higher bitterness in both of the non-blowoff sub-batches. Some noted slightly higher malt flavor in the blowoff sub-batches, but this may be a result of less competition from hop flavor and bitterness. Clarity was deemed equivalent between the blowoff and non-blowoff sub-batches of the first batch.

In both batches, the judges decided that head retention was indistinguishable between sub-batches. A long-lasting head formed upon pouring all four sub-batches, and more than 50% of the head remained after 20 minutes. Lacing was strong, as the foam adhered well to the sides of the glasses.

Despite the fact that bitterness was both analytically and subjectively determined to be different and that hops are known to improve head retention, the difference in head formation and retention between blowoff and non-blowoff batches was effectively nil. A possible explanation may be that the hop components that were blown off are not responsible for increasing head retention. Or, perhaps when sufficient proteinaceous material is available for foam formation, the effect of hops is less of a factor; if insufficient protein had been present for foam formation, then perhaps the hops would have had a greater impact on head formation.

Three judges noted a slight harshness in the finish in the blowoff sub-batch and, even then, in only one of two identical beers. No judge indicated that one beer was “better,” “smoother,” or “less harsh” (overall) than the other, but rather that the primary differences were in the balance between bitterness and maltiness. In all cases, the differences (except bitterness) between the blowoff and non-blowoff sub-batches were extremely minor.

In the other four judging sessions, other judges were given only one sample from each sub-batch, but their comments were consistent with those of the judges who were given two.

In the second batch (India pale ale), all the judges indicated that the non-blowoff sub-batch had a slightly fruitier nose. This observation is consistent with the analytical data. Also, the non-blowoff sub-batch had a slight haze, whereas the blowoff sub-batch did not. The relative clarity of the latter may be a result of larger, haze-forming proteins being blown off.


The analytical and taste test results indicate that the differences between beers brewed with the dirty head blown off and not blown off are much smaller than previously believed by some brewers. The differences may be more significant if less well-behaved yeast, or perhaps harsher hops, are used.

Differences in protein levels were far less than initially expected. Correspondingly, head retention appears to have been unaffected by the difference in method.

If a highly flocculent top-fermenting yeast is used, it may be advisable to forego the blowoff method for fear that too much yeast will be blown out of the fermentor.

Note that racking the fermenting beer into a secondary fermentor during high kräusen, as some home and commercial brewers do (called the “dropping method” in England [3]), or skimming off the dirty head, would have a similar physical effect (primarily the reduction in bitterness) as the blowoff method.

Perhaps the most important result of this work is that we now have an indication of the extent to which bitterness is lost when using the blowoff method. An added benefit of this work was analytical data supporting the validity of Jackie Rager’s formulas, at least for one brewing system. The data clearly show that fermentation is a significant factor in the final bitterness level of beer and must be accounted for in bitterness estimation calculations.


I would like to thank Bill Siebel and Ilse Shelton of the Siebel Institute of Technology for their help with the analyses and judges Ray Daniels, Dennis Davison, Greg Hall, Steve Hamburg, Karen Korzonas, Randy Mosher, and Tim Norris. I would also like to thank Dr. George Fix for help with the analysis of the initial data as well as the planning of the tests. Finally, thanks to Joe Power of the Siebel Institute of Technology for his comments on the data and their analysis.

This research was funded by the Association of Brewers / zymurgy magazine.

All contents copyright 2024 by MoreFlavor Inc. All rights reserved. No part of this document or the related files may be reproduced or transmitted in any form, by any means (electronic, photocopying, recording, or otherwise) without the prior written permission of the publisher.