Below you will find a couple of articles that George had written. I apologize for any errors, or questions that you may have - for I will not be able
to answer for him. I thought you might like to read them any way. George had written so many articles, that I am sure these appeared in
one of the many brewing magazines. I need to do some further research to give credit to them.
Laurie Fix
to answer for him. I thought you might like to read them any way. George had written so many articles, that I am sure these appeared in
one of the many brewing magazines. I need to do some further research to give credit to them.
Laurie Fix
Lagering - Is it worth the effort?
George J. Fix
To the average homebrewer, the term "lager" is a descriptor for beer styles; namely numbers 12 (Bock) through 17 (Vienna/Maerzen/Oktoberfest) on the AHA category description list. The one common factor in these styles is the type of yeast used, and not the way they are processed. On the other hand, historically the reverse is the case, and a "lagered beer" has generally been one which has been afforded an extended cold maturation, independent of the type of yeast used. Arnold[1] finds that extensive cold storage goes back to the very beginning of monastery brewing (approximately 50-100 A.D.), if not sooner. This possibly pre-dates the systematic use of what today is a genetically narrow band of microbes called lager yeast[2]. However, this begs the absolutely fascinating open question of exactly when lager yeast entered monastery brewing.
In Bavaria lager beers were also called summer beers because they were brewed from September to April, and cold lagered during the summer months[3]. When refrigeration was introduced near the end of the 19th century, brewing was possible year round. Yet the 3-6 month cold storage still found favor, and 6-9 month cycles were common for high gravity lagers[4].
An interesting twist occurred in the U.S. among ale brewers in the sense that early on extended cold storage was employed by them. Greves[5], an English brewer who visited the U.S. just before the turn of the century, commented on this point. He cited two reasons for this departure from traditional British ale brewing practice. First, he noted competitive pressure from lager brewers, who tended to promote the theme that beer clarity and beer purity were synonymous. Cold storage is one of the best ways to clarify beer, a point that is discussed below. Very likely these influences affected German ale brewers in the Rheinland as well. The second reason cited by Greves was the unfavorable climatic conditions in the U.S., and therefore the need for cold maturation to promote beer stability before distribution. Greves praised the overall quality of American ales, but he concluded that cold storage was not needed in the U.K.
One of the most obvious trends in brewing practice in the 20th century has been the gradual reduction of aging times. "Common beer" using short 2-3 week cycles have been present throughout this century, but it was not until the last part of the century that short cycles were used for premium products. Even Pilsner Urquel, argueably the flagship lager, has been affected by these trends. It was brewed on a six month cycle throughout much of the 20th century. This was cut to three months in the post World War II era, and it is now produced using cylindrical conical fermenters with a short brewing cycle. Critics of this trend cite competitive pressures, and a relatively flat beer market for putting a premium on plant efficiency. Lighter flavored beers were also increasing in popularity during this period. These beers tend to require less aging, the limiting case is water which requires none!
Defenders of short aging periods argue differently. First, given the increased understanding of beer fermentation that has occurred in the last few decades, it is now possible for brewers to reduce green beer characteristics in the main fermentation. In previous periods that was one of the main purposes of aging. Also, there has been vast improvements in yeast management as well as considerable improvements in the quality of brewing materials, most notably malt. These also reduce brewers dependence on aging.
As homebrewers we are free of many of the pressures facing commercial brewers. As a consequence, we can and do put beer quality above any other consideration. Thus, if extended cold storage will improve beer, it will be employed by most homebrewers. On the other hand, it makes little sense to cold lager beer beyond the point where improvements stop. This begs the central questions associated with this article. Namely, exactly what does cold maturation do for us, and how much is enough?
We shall define cold as below 2°C (36°F), although some of the mechanisms discussed below can also take place at slightly higher temperatures.
1. Beer Clarification
The most obvious benefit of cold maturation is the precipitation of haze active polyphenols and proteins. The cold conditions also encourages yeast flocculation. It is my experience that the beer should clarify within the first week of storage. This is possibly why two week cycles for ales, and three week cycles for lagers are so widely used in commercial brewing.
Failure to clarify during the first week of storage is usually due to technical errors. Poor quality malt and/or dysfunctional yeast are obvious culprits. Errors in mashing and sparging cannot be ruled out either. The solution in these cases is not to extend the aging period, but rather to correct original problem.
2. Chill Proofing
Extra measures are needed to chill proof beer. Additives like silica gels and polyclar can remove the relevant haze active constituents.[6] I have found that extended cold storage (say 8-12 weeks) at 0-2° C (32-36°F) will achieve the same effect. The recently developed ice brewing procedure provides an interesting alternative. In this process beer temperature is reduced to just below its freezing point so that very small ice crystals are formed. Extensive data[7] has shown that the beer obtained after separation from the ice crystals is fully chill proofed. In commercial practice, where this process is automated, the temperature is reduced to a couple of degrees centigrade below the beer's freezing point. The latter varies with alcohol content, but it is near -2.3°C, (27.9°F) for beers of normal strength. The contact time with the ice crystals is brief, typically less than one hour. In homebrewing higher temperatures and longer times are used. I have found that holding the beer at -3°C (26.6°F) for 48-72 hours is adequate.
3. Reduction of Diacetyl
The most widely studied green beer compound is undoubtedly diacetyl. There is good reason for this since it can be responsible for some highly unpleasant flavors, especially in packaged beer as it ages. There is ample evidence[8] that the long extended cold storage, in contact with yeast, was the primary tool used by turn of the century brewers to combat off flavors like with diacetyl. In modern practice, there is a decided preference for reducing diacetyl in the main fermentation. This is achieved through proper yeast management, and in particular using yeast which have very low bacterial and mutant levels. It can happen at the fermentation end point that diacetyl levels are slightly above acceptable levels. In this case, best results are usually obtained by kraeusening the beer with fresh wort and yeast, rather than relying on extended aging.
It should be noted that there is much more to flavor maturation than reducing diacetyl levels. For example, research on immobilized yeast reactors has shown that diacetyl can be reduced to normal levels with only a few hours of maturation. Nevertheless, the overall quality of beers produced with these systems has not been impressive.
4. Reduction of Sulfur Compounds
Fermentations conducted at ambient temperatures 18-20°C (65-68°F) should end with all relevant sulfur compounds well below their threshold. Exceptions are usually due to infection by sulfur producing gram negative microbes. These can be found in infected wort and/or in pitching yeast. With lagers fermented at 8-12°C (46-50°F) the situation is more complex[9]. First, the removal of volatile sulfur compounds in a cold fermentation is greatly reduced over what occurs at higher temperatures, and this can lead to a situation where several sulfur compounds are above their flavor threshold.
Lager brewers disagree about how much is too much. However, there is widespread agreement that lager beer will be insipid if all sulfur bearing compounds are reduced below their threshold. In addition, residual sulfur can act as an oxygen scavenger, and this may be responsible in part for the excellent flavor stability of traditional lagers. Nevertheless, most lager beer needs some maturation to reduce sulfur levels, and it has been my experience that objectionable sulfur levels can be reduced to acceptable levels within one week of storage at 0-2°C (32-36°F).
Failure to achieve this reduction can be due to several factors in addition to those cited above. A common culprit is high DMS levels in chilled wort, which may be due to the malt or wort production procedures used.
Yeast related issues tend to have more damaging effects. While there is a difference with respect to sulfur production among strains, pitching rate is even more important. Ideally, lager yeast should be pitched at a rate of 1-2 million cells per ml for each degree Plato; e.g., between 12 and 24 million cells per ml for 12°P (1.048) wort. Under pitching can lead to problems, but so can over pitching. For example, using very large yeast starters, and with this cell counts a factor 5 or more above the ideal, can lead to excessive sulfur levels[10]. It can also create a variety of off flavors due to yeast autolysis. Synthetic fuels are produced using elevated pitching rates, but flavor is not an issue with these products! As with the other defects mentioned above, the best approach is to correct the original problem and not to rely on ageing.
5. Flavor Maturation
This in many respects is the most interesting part of lagering in the sense that the goal is not damage control, but rather taking a sound beer and improving it. Two mechanisms are fundamental. One concerns polyphenols. Extensive data shows that anaerobic cold storage favors the precipitation of phenols in the higher oxidation states. That is, cold anaerobic storage will reduce the beers redox potential. This promotes rounded flavors and a smooth palate for conventional lagering (0-2°C, 32-36°F), discernable improvements will be seen through 6-8 weeks. In the ice brewing process, the times are shorter as noted above.
The second effect is a slow esterification of fusel alcohols. The reduction of higher alcohols promotes a greater elegance of taste, even when these alcohols are below threshold. The esters so formed tend to be of the desirable type, and add to the beer's complexity. This effect is enzymatic, and depends on having viable yeast present in maturation to keep the beer "alive." Ideally, this should be between 500,000 and 750,000 cells per ml. This effect also depends on time. When Pilsner Urquell was fresh and brewed on a 3-6 month cycle, it displayed these effects to perfection.
6. Dry Hopping
As Greves noted in his paper, the addition of hops during cold maturation was a uniquely American practice. The most important historical examples showing the benefits of this procedure was (arguably) the Ballantine Ales, back in the days when Ballantine was an independent family owned brewery. Their flagship product was Ballantine XXX, which was dry hopped during a cold three month maturation period[11]. This ale was noted for its full and attractive hop aroma, which was accompanied by a well defined but mellow hop bitter. Dry hopping will always achieve these effects, at least for a short period. The problem is that the aromatic compounds are unstable, and can quickly disappear. Ballantine XXX in its prime was a national beer, and subject to market abuse. Yet, typically around 5,000,000 bbls. were sold each year. The hop aroma was its signature, and hence the stability of it was a crucial point. The extended cold contact time played a fundamental role since it was during this period that the aroma compounds became bound up in the beer. Nugy8 suggests that one month of cold contact time with hops yields one month of stability for the aroma compounds. This is consistent with what has been reported about the Ballantine ales, and it is also consistent with my own brewing experiences.
7. Carbonation
The traditional carbonation of lagers takes place during cold storage. Beer is transferred from fermenters to maturation tanks with approximately 1% (by weight) of fermentable sugars present. The secondary fermentation which takes place creates sufficient CO2 to saturate beer. It has been my experience that 2.8-3.0 volumes of CO2 will be dissolved at equillibrium. Thus, it is desirable to take pressure and temperature readings so that adjustments can be made to desired levels. A 3 month cycle for this method is ideal if the beer is held under an appropriate counter pressure. I have found that 1 atmosphere (14.7 psi) is adequate.
I have personally not seen any difference in foam quality that can be attributed to the way beer is carbonated, either forced or natural. However, it has been my experience that people who are sensitive to carbonic acid, and dislike gassy beers, tend to prefer (by a wide margin) the flavor of the beers made with traditional carbonation when CO2 levels are in excess of 2.4 vols. The reasons for this are not altogether clear, but it could be that the traditional carbonation has CO2 more tightly bound up with other constituents.
Conclusion
To lager or not to lager is a question to which brewers will likely come to different answers. I recommend that brewers not blindly follow the rigid rules set up by others on this matter, but rather be guided by test brews. I have found the following to be useful.
(a) Do three batches of your everyday beer and lager for 3, 8, and 16 weeks, respectively.
(b) Do three batches of your favorite high gravity beer and lager for 3 weeks, 6 weeks, and 9 months respectively.
(c) Do two test batches, one being forced carbonated and the other carbonated by the traditional method.
It is of course desirable to control for extraneous effects. This is hard to do in a homebrewing content. However, the key items to control are yeast, brewing materials, and oxidation. In particular, the test brews should be staggered so that they can be bottled at the same time (or nearly so). If this is done, then it is my experience that what ever bias that may exist will not be significant enough to alter conclusions.
I realize that this program requires a lot of brewing, but then it also gives us an excuse to do more brewing, as if any of us needed such a reason!
[1] Arnold, J. P., Origin and History of Beer and Brewing, Wahl-Henius Institute, Chicago, 1911.
[2] Casey, G. P., Presentation at Rocky Mountain Microbrewing Symposium, Univ. of Colorado, Colorado Springs, Feb. 1999.
[3] One Hundred Years of Brewing, Arno Press, New York, 1974.
[4] A. Zimmermann, Brauereibe Briebslehre, Buffalo, New York, 1904.
[5] J. A. R. Greves, "On Some Recent Advances in Brewing in the United States," J. Institute of Brewing, Vol. III, 1897.
[6] G. J. Fix and L. A. Fix, "Analysis of Brewing Techniques,"
Br. Publ., 1997.
[7] G. J. Fix "Principles of Brewing Science," Br. Publ., to appear, October 1999.
[8] Nugy, Brewers Manual, Jersey Printing, 1948.
[9] G. J. Fix, Sulfur Flavors in Beer," Zymurgy, vol. 15, No. 3, 1992.
[10] Cahill, G., P. K. Walsh, D. Donnely, J. Am. Soc. Br. Chem., Vol. 57.2, 1999.
[11] George Lever, former Ballantine brewmaster, personal communication
George J. Fix
To the average homebrewer, the term "lager" is a descriptor for beer styles; namely numbers 12 (Bock) through 17 (Vienna/Maerzen/Oktoberfest) on the AHA category description list. The one common factor in these styles is the type of yeast used, and not the way they are processed. On the other hand, historically the reverse is the case, and a "lagered beer" has generally been one which has been afforded an extended cold maturation, independent of the type of yeast used. Arnold[1] finds that extensive cold storage goes back to the very beginning of monastery brewing (approximately 50-100 A.D.), if not sooner. This possibly pre-dates the systematic use of what today is a genetically narrow band of microbes called lager yeast[2]. However, this begs the absolutely fascinating open question of exactly when lager yeast entered monastery brewing.
In Bavaria lager beers were also called summer beers because they were brewed from September to April, and cold lagered during the summer months[3]. When refrigeration was introduced near the end of the 19th century, brewing was possible year round. Yet the 3-6 month cold storage still found favor, and 6-9 month cycles were common for high gravity lagers[4].
An interesting twist occurred in the U.S. among ale brewers in the sense that early on extended cold storage was employed by them. Greves[5], an English brewer who visited the U.S. just before the turn of the century, commented on this point. He cited two reasons for this departure from traditional British ale brewing practice. First, he noted competitive pressure from lager brewers, who tended to promote the theme that beer clarity and beer purity were synonymous. Cold storage is one of the best ways to clarify beer, a point that is discussed below. Very likely these influences affected German ale brewers in the Rheinland as well. The second reason cited by Greves was the unfavorable climatic conditions in the U.S., and therefore the need for cold maturation to promote beer stability before distribution. Greves praised the overall quality of American ales, but he concluded that cold storage was not needed in the U.K.
One of the most obvious trends in brewing practice in the 20th century has been the gradual reduction of aging times. "Common beer" using short 2-3 week cycles have been present throughout this century, but it was not until the last part of the century that short cycles were used for premium products. Even Pilsner Urquel, argueably the flagship lager, has been affected by these trends. It was brewed on a six month cycle throughout much of the 20th century. This was cut to three months in the post World War II era, and it is now produced using cylindrical conical fermenters with a short brewing cycle. Critics of this trend cite competitive pressures, and a relatively flat beer market for putting a premium on plant efficiency. Lighter flavored beers were also increasing in popularity during this period. These beers tend to require less aging, the limiting case is water which requires none!
Defenders of short aging periods argue differently. First, given the increased understanding of beer fermentation that has occurred in the last few decades, it is now possible for brewers to reduce green beer characteristics in the main fermentation. In previous periods that was one of the main purposes of aging. Also, there has been vast improvements in yeast management as well as considerable improvements in the quality of brewing materials, most notably malt. These also reduce brewers dependence on aging.
As homebrewers we are free of many of the pressures facing commercial brewers. As a consequence, we can and do put beer quality above any other consideration. Thus, if extended cold storage will improve beer, it will be employed by most homebrewers. On the other hand, it makes little sense to cold lager beer beyond the point where improvements stop. This begs the central questions associated with this article. Namely, exactly what does cold maturation do for us, and how much is enough?
We shall define cold as below 2°C (36°F), although some of the mechanisms discussed below can also take place at slightly higher temperatures.
1. Beer Clarification
The most obvious benefit of cold maturation is the precipitation of haze active polyphenols and proteins. The cold conditions also encourages yeast flocculation. It is my experience that the beer should clarify within the first week of storage. This is possibly why two week cycles for ales, and three week cycles for lagers are so widely used in commercial brewing.
Failure to clarify during the first week of storage is usually due to technical errors. Poor quality malt and/or dysfunctional yeast are obvious culprits. Errors in mashing and sparging cannot be ruled out either. The solution in these cases is not to extend the aging period, but rather to correct original problem.
2. Chill Proofing
Extra measures are needed to chill proof beer. Additives like silica gels and polyclar can remove the relevant haze active constituents.[6] I have found that extended cold storage (say 8-12 weeks) at 0-2° C (32-36°F) will achieve the same effect. The recently developed ice brewing procedure provides an interesting alternative. In this process beer temperature is reduced to just below its freezing point so that very small ice crystals are formed. Extensive data[7] has shown that the beer obtained after separation from the ice crystals is fully chill proofed. In commercial practice, where this process is automated, the temperature is reduced to a couple of degrees centigrade below the beer's freezing point. The latter varies with alcohol content, but it is near -2.3°C, (27.9°F) for beers of normal strength. The contact time with the ice crystals is brief, typically less than one hour. In homebrewing higher temperatures and longer times are used. I have found that holding the beer at -3°C (26.6°F) for 48-72 hours is adequate.
3. Reduction of Diacetyl
The most widely studied green beer compound is undoubtedly diacetyl. There is good reason for this since it can be responsible for some highly unpleasant flavors, especially in packaged beer as it ages. There is ample evidence[8] that the long extended cold storage, in contact with yeast, was the primary tool used by turn of the century brewers to combat off flavors like with diacetyl. In modern practice, there is a decided preference for reducing diacetyl in the main fermentation. This is achieved through proper yeast management, and in particular using yeast which have very low bacterial and mutant levels. It can happen at the fermentation end point that diacetyl levels are slightly above acceptable levels. In this case, best results are usually obtained by kraeusening the beer with fresh wort and yeast, rather than relying on extended aging.
It should be noted that there is much more to flavor maturation than reducing diacetyl levels. For example, research on immobilized yeast reactors has shown that diacetyl can be reduced to normal levels with only a few hours of maturation. Nevertheless, the overall quality of beers produced with these systems has not been impressive.
4. Reduction of Sulfur Compounds
Fermentations conducted at ambient temperatures 18-20°C (65-68°F) should end with all relevant sulfur compounds well below their threshold. Exceptions are usually due to infection by sulfur producing gram negative microbes. These can be found in infected wort and/or in pitching yeast. With lagers fermented at 8-12°C (46-50°F) the situation is more complex[9]. First, the removal of volatile sulfur compounds in a cold fermentation is greatly reduced over what occurs at higher temperatures, and this can lead to a situation where several sulfur compounds are above their flavor threshold.
Lager brewers disagree about how much is too much. However, there is widespread agreement that lager beer will be insipid if all sulfur bearing compounds are reduced below their threshold. In addition, residual sulfur can act as an oxygen scavenger, and this may be responsible in part for the excellent flavor stability of traditional lagers. Nevertheless, most lager beer needs some maturation to reduce sulfur levels, and it has been my experience that objectionable sulfur levels can be reduced to acceptable levels within one week of storage at 0-2°C (32-36°F).
Failure to achieve this reduction can be due to several factors in addition to those cited above. A common culprit is high DMS levels in chilled wort, which may be due to the malt or wort production procedures used.
Yeast related issues tend to have more damaging effects. While there is a difference with respect to sulfur production among strains, pitching rate is even more important. Ideally, lager yeast should be pitched at a rate of 1-2 million cells per ml for each degree Plato; e.g., between 12 and 24 million cells per ml for 12°P (1.048) wort. Under pitching can lead to problems, but so can over pitching. For example, using very large yeast starters, and with this cell counts a factor 5 or more above the ideal, can lead to excessive sulfur levels[10]. It can also create a variety of off flavors due to yeast autolysis. Synthetic fuels are produced using elevated pitching rates, but flavor is not an issue with these products! As with the other defects mentioned above, the best approach is to correct the original problem and not to rely on ageing.
5. Flavor Maturation
This in many respects is the most interesting part of lagering in the sense that the goal is not damage control, but rather taking a sound beer and improving it. Two mechanisms are fundamental. One concerns polyphenols. Extensive data shows that anaerobic cold storage favors the precipitation of phenols in the higher oxidation states. That is, cold anaerobic storage will reduce the beers redox potential. This promotes rounded flavors and a smooth palate for conventional lagering (0-2°C, 32-36°F), discernable improvements will be seen through 6-8 weeks. In the ice brewing process, the times are shorter as noted above.
The second effect is a slow esterification of fusel alcohols. The reduction of higher alcohols promotes a greater elegance of taste, even when these alcohols are below threshold. The esters so formed tend to be of the desirable type, and add to the beer's complexity. This effect is enzymatic, and depends on having viable yeast present in maturation to keep the beer "alive." Ideally, this should be between 500,000 and 750,000 cells per ml. This effect also depends on time. When Pilsner Urquell was fresh and brewed on a 3-6 month cycle, it displayed these effects to perfection.
6. Dry Hopping
As Greves noted in his paper, the addition of hops during cold maturation was a uniquely American practice. The most important historical examples showing the benefits of this procedure was (arguably) the Ballantine Ales, back in the days when Ballantine was an independent family owned brewery. Their flagship product was Ballantine XXX, which was dry hopped during a cold three month maturation period[11]. This ale was noted for its full and attractive hop aroma, which was accompanied by a well defined but mellow hop bitter. Dry hopping will always achieve these effects, at least for a short period. The problem is that the aromatic compounds are unstable, and can quickly disappear. Ballantine XXX in its prime was a national beer, and subject to market abuse. Yet, typically around 5,000,000 bbls. were sold each year. The hop aroma was its signature, and hence the stability of it was a crucial point. The extended cold contact time played a fundamental role since it was during this period that the aroma compounds became bound up in the beer. Nugy8 suggests that one month of cold contact time with hops yields one month of stability for the aroma compounds. This is consistent with what has been reported about the Ballantine ales, and it is also consistent with my own brewing experiences.
7. Carbonation
The traditional carbonation of lagers takes place during cold storage. Beer is transferred from fermenters to maturation tanks with approximately 1% (by weight) of fermentable sugars present. The secondary fermentation which takes place creates sufficient CO2 to saturate beer. It has been my experience that 2.8-3.0 volumes of CO2 will be dissolved at equillibrium. Thus, it is desirable to take pressure and temperature readings so that adjustments can be made to desired levels. A 3 month cycle for this method is ideal if the beer is held under an appropriate counter pressure. I have found that 1 atmosphere (14.7 psi) is adequate.
I have personally not seen any difference in foam quality that can be attributed to the way beer is carbonated, either forced or natural. However, it has been my experience that people who are sensitive to carbonic acid, and dislike gassy beers, tend to prefer (by a wide margin) the flavor of the beers made with traditional carbonation when CO2 levels are in excess of 2.4 vols. The reasons for this are not altogether clear, but it could be that the traditional carbonation has CO2 more tightly bound up with other constituents.
Conclusion
To lager or not to lager is a question to which brewers will likely come to different answers. I recommend that brewers not blindly follow the rigid rules set up by others on this matter, but rather be guided by test brews. I have found the following to be useful.
(a) Do three batches of your everyday beer and lager for 3, 8, and 16 weeks, respectively.
(b) Do three batches of your favorite high gravity beer and lager for 3 weeks, 6 weeks, and 9 months respectively.
(c) Do two test batches, one being forced carbonated and the other carbonated by the traditional method.
It is of course desirable to control for extraneous effects. This is hard to do in a homebrewing content. However, the key items to control are yeast, brewing materials, and oxidation. In particular, the test brews should be staggered so that they can be bottled at the same time (or nearly so). If this is done, then it is my experience that what ever bias that may exist will not be significant enough to alter conclusions.
I realize that this program requires a lot of brewing, but then it also gives us an excuse to do more brewing, as if any of us needed such a reason!
[1] Arnold, J. P., Origin and History of Beer and Brewing, Wahl-Henius Institute, Chicago, 1911.
[2] Casey, G. P., Presentation at Rocky Mountain Microbrewing Symposium, Univ. of Colorado, Colorado Springs, Feb. 1999.
[3] One Hundred Years of Brewing, Arno Press, New York, 1974.
[4] A. Zimmermann, Brauereibe Briebslehre, Buffalo, New York, 1904.
[5] J. A. R. Greves, "On Some Recent Advances in Brewing in the United States," J. Institute of Brewing, Vol. III, 1897.
[6] G. J. Fix and L. A. Fix, "Analysis of Brewing Techniques,"
Br. Publ., 1997.
[7] G. J. Fix "Principles of Brewing Science," Br. Publ., to appear, October 1999.
[8] Nugy, Brewers Manual, Jersey Printing, 1948.
[9] G. J. Fix, Sulfur Flavors in Beer," Zymurgy, vol. 15, No. 3, 1992.
[10] Cahill, G., P. K. Walsh, D. Donnely, J. Am. Soc. Br. Chem., Vol. 57.2, 1999.
[11] George Lever, former Ballantine brewmaster, personal communication
BEER OXIDATION – the how, where, and why of damage control
George J. Fix
INTRODUCTION
John Meier, senior brewer at Rogue’s Ales and former homebrewer of the year, was once asked what he feared the most as a brewer*. His answer was pointed and direct. "It is the A, B, Ds." The "A" on his list stands for air, and is the central topic of this paper. The interpretation of "B" and "D" is left as a homework exercise!
The key point is that successful brewers like John invariably display sensitivity to oxidative issues, and also have the skill to effectively deal with them. The goal of this article is to identify the flashpoints, i.e., the critical areas in the brewing process where care is needed, as well as some ideas on how one can operate a low oxygen system, be it on the homebrew or commercial level.
The major difference between brewing science and the biological and chemical sciences is that in the former, flavor is the final arbitrator of what is important and relevant. This is a particularly delicate point as far as oxidation is concerned, since there is a definite variation in how people perceive oxidized flavors. Thus, the first order of business is to establish criteria that characterize oxidized beer. This material is a sequel to Scott Bickham's excellent article in his column "Focus on Flavor" [1].
The next two sections deal with the parts of the brewing process where oxidation is likely to occur. A distinction is made between aeration in the "hot side," (HSA) and the "cold side" (CSA) of the brewing process. The former consists of mashin to wort-chilling. At that point, oxygen is added to wort, or more commonly these days to yeast, prior to pitching. Here oxygen serves as an invaluable yeast nutrient. Once the fermentation has started, oxygen returns to being a negative, and the cold side starts at this point and continues until the beer is consumed.
PERCEPTION OF OXIDIZED FLAVOR
Following Dalgliesch [2] three stages in beer flavor evolution can be defined:
FRESH 1 A 2
B
STALE 3 C 4
TIME
FIGURE 1
Stage A (points 1 to 2) is the period of stable "brewery fresh" flavor. Stage B (points 2 to 3) consists of a transition period where a multitude of new flavor sensation can be detected. Dalgliesch cites the following:
i. Decline in hop aroma
ii. Decline in hop bitter
iii. Increase in "ribes aroma" (or sometimes "catty" flavor)
iv. Increase in sweet/toffee-like/caramel tones
The term’s "ribes" or "catty" are widely used in the UK and Scandinavia. They generally recall over-ripe/spoiled fruit or vegetables. Some cite a "black currant" tone [3]. In truth, they are descriptors for a wide spectrum of flavors when beer is in Stage B. The toffee-caramel tones are a different matter. They are enhanced by residual diacetyl, and also by excess heat treatment of wort [4]. Boiling in pressure cookers will develop flavor tones like these along with other negative ones [12].
The Stage C products (points 3 to 4) are the classic flavor tones involved in beer staling. These are described below in conjunction with the part of the brewing process where they are created. Although, it is not treated in the references cited above, there is also a "Stage D," where Stage C flavors evolve into a kaleidoscope of flavors, which in very special formulations (Rodenbach’s Grand Cru comes to mind) recall the subtly and complexity of great wines. It is to be emphasized that this process takes years, not months of maturation. This effect deserves a paper all its own, and hence falls outside the scope of this article.
Axcell and Torline [3] in an interesting if provocative article argues that most beers are consumed during Stage B. This is a period where beer flavors are undergoing discernable change, and they suggest the changes are at the root of the consumer’s dissatisfaction. Among other things, they appeal to the "import paradox" as partial evidence. This paradox refers to the phenomena that a definite proportion of the beer consuming population actually prefer beers in Stage C. (Here "import" means any beer consumed at a significant distance from where it is brewed). These authors cited the stability of flavor in Stage C and "learned prejudices" (i.e., prestige of the beer, packaging, et al) as the key to this paradox. Statements like " … if Pilsner Ur-quell has HSA, then I want my beer to have HSA …", often seen in the internet discussion groups devoted to beer, illustrate this point [6]. Stage B flavors, on the other hand, appears to have very few advocates.
Meilgaard [7] is sharply critical of Stage C flavors because they are one dimensional. He states " … I think it ranks as an all-American scandal that fully half of all the interesting and unusual packaged beers that are on the market, get to us so oxidized that staleness and cardboard are the main flavor tones." I feel this is a very important point, and this paper is based on the premise that the best strategy for brewers (amateur or commercial), is to produce beers where Stage A flavors are as stable for as long as possible. If they, or their consumers prefer Stage B, C, (or D for that matter), then putting the beer aside for the required time is all that is needed. Staling, like entropy, can go in only one direction!
CSA
Oxidation on the cold side is the easiest to understand, and until the last decade has occupied all of the attention of those concerned about this issue. The major mechanisms are given in Table 1. Flavor thresholds in ppm (i.e., parts per million) were taken from Meilgaard [8].
TABLE I
CSA MECHANISMS
1. Ethanol + oxygen ® acetaldehyde
(14,000 ppm) (25 ppm)
2. Acetaldehyde + oxygen ® acetic acid
(25 ppm) (175 ppm)
3. Hop a -acids + oxygen ® valeric butric acids
(10 ppm) (1-2 ppm)
4. Hop oils + oxygen ® various oxygen bearing compounds (see [9]-[10]
(5) Unsaturated fatty acids from trub + oxygen ® oleic, linoleic acids
(less than 1 ppm)
The first redox reaction (1) in Table I is the start of the classic "vinegar process". It also marks the second appearance of acetaldehyde. The first is the main fermentation pathway where acetaldehyde emerges as a reduction product of pyruvic acid [9], [10]. At that stage it has a flavor which recalls freshly cut apples. As an oxidation product via (1) this changes to an "old apple" tone. It may also recall "rotten apples," although this effect is not nearly as strong as what occurs with Zymomonas infection.
Because of the high flavor threshold of acetic acid, the second mechanism ((2) in Table I) will rarely give a full vinegar taste. Generally, sherry like tones emerge. Massive amounts of oxygen and/or acetic acid bacteria (acetobacteria) infections can give vinegar flavors.
As noted above, the loss of hop flavors (taste and smell) is the easiest way to detect the start of staling. By the time stage C in Figure 1 is reached, the oxidation products of a -acids will start to appear. "Cheesy" is a widely used descriptor. The use of old hops at elevated levels will intensify this effect. The flavors from hop oils, on the other hand, will generally just disappear in Stage B. Sometimes residual tones recalling hay, sagebrush, or grass can be detected. Many do not see the latter as a defect, and highly valued hops like U.S. Columbus or U.K. Golding display these tones in attractive ways. Yet the variance among taste panels with respect to this issue can be great [11].
The final CSA mechanism is often underestimated because unsaturated fatty acids (UFA) from trub in wort play a highly beneficial role in the fermentation. Their value on the cold side is the exact opposite. They are highly reactive with oxygen, and oxidize to a broad range of products, none of which are attractive. These effects are usually not seen in beers produced from reasonably clarified worts. The relatively low flavor thresholds of these oxidation products are a special concern.
Prevention of CSA is clearly a prevention of oxygen ingress in the post fermentation processing of beer. As far as beer transfer is concerned, an important step is to first remove as much air as possible from the receiving tank. Purging the tank with CO2 will do this in part. If, in addition, the transfer is achieved by the application of CO2 pressure to the sending tank, then by allowing a slight gas bleed in the receiving tank one can eliminate oxygen pickup. In fact, I have found [12] that the use of CO2 pressure throughout the post fermentation process can result in null dissolved oxygen readings at least with standard meters. The next best option is transfer via gravity flow. Even here, it is crucial to keep a CO2 cover over the beer to protect against oxygen ingress. Maintaining a smooth laminar flow is also important.
In commercial brewing, the above two procedures are not always an option. CO2 is expensive, and a horizontal layout precludes gravity flows. Thus, the use of pumps are a necessity. To say that there are good ones and bad ones is a mild understatement. Operator error is also an issue. In this context, the availability of a DO meter is crucial to evaluate how well the system is working in each transfer. Pumping beer through filters is a particular concern, and dissolved oxygen should be measured on a regular basis.
The greatest concern about CSA has correctly pointed at the packaging of beer. The deleterious effect of headspace air has been evident ever since the human race started putting beer into small bottles. Fortunately, modern technology has made it possible to dramatically lower oxygen pick-up during filling. For example, double pre-evaculation bottle fillers routinely keep bottle air levels below .2 milliliters air per 1/3 liter beer. This is in striking contrast to the situation that existed a few decades ago, where criteria ran as high as 1.0 ml/in 1/3 L [13]. In fact, most of the reasonable homebrew hand fillers on the market today, if properly operated, can get below .5 ml/1/3 L.
Nevertheless, headspace air remains a concern, and the indited
co-conspirator is thermal abuse. Extensive data illustrating this is given in [12]. This data was carefully controlled both in respect to other forms of CSA as well as HSA discussed below, and so displays the direct effects of thermal abuse and bottle air levels. To cite one example, a beer which has .1 ml/1/3L of air at fill will start to display staling (first part of Stage B in Figure 1) in 140 days if it is stored at 6° C. This drops to 80 days if the storage temperature is raised to 30° C. On the other hand, a beer which has 1.5 ml/1/3L of air at fill will start staling after 20 days at a storage temperature of 30° C and 80 days at 6° C. In other words, beers with very low headspace air - - something that requires a million dollar plus filling machine - - that has been subject to thermal abuse are no more stable than beers that have been afforded proper storage yet have the sort of air one expects from cheap homebrew hand-fillers.
HSA
The first hint that oxygen uptake during wort production could be a problem was found in the fundamental work of N. Hashimoto, one of the leading experts on beer staling (see e.g., [14]) His prime concern was volatile aldehydes like 2-nonenal*. This compound has a characteristic papery/cardboard flavor that can evolve into leathery/woody tones in Stage C of Figure 1. It is a primary concern because of its very low flavor threshold, which for most people falls in the range of .1-.4 parts per billion. The defining step of Hashimoto’s work came from the observation in his now classic 1975 paper [15] that as far as the formation of nonenal is concerned "… molecular oxygen does not take a part… in bottled beer." In later work [16] the oxidative processes during wort production was identified as a serious concern.
This was a new finding at the time. The only previous study involving oxygen uptake during wort production was by deClerck [17]. He found that the total elimination of oxygen during wort production increased the finished beer’s tendency to form chill haze. In a follow up study [18], it was demonstrated that massive oxygen uptake prior to sparging greatly enhanced colloidal stability. This study did not take into account finished beer flavor, and in fact illustrates the point that the promotion of physical stability is not always in harmony with the promotion of flavor stability.
After Hashimoto’s work appeared, manufacturers of brewing equipment (most notably in Germany) funded additional research into this topic [19]. The result was a long series of articles, most of which appeared in Brauwelt. All concurred with Hashimoto’s basic findings (see [12]). I am not aware of any articles that have appeared in professional peer reviewed brewing journals, which has found the contrary.
The first mechanisms studied by Hashimoto involved malt derived compounds like polyphenols and melanoidins. Oxygen uptake on the hot side immediately sets in place a number of redox reactions. Temperature is crucial, since the speed of these reactions increases exponentially with temperature. For example, introducing say 1 ml per liter into wort at 70° C will start reactions that take place in seconds. Doing the same for wort at say 20° C will have virtually no effect. The oxygen fraction will typically remain an inert gas until consumed by yeast. Hashimoto showed that the oxidation of melanoidins and polyphenols was an important consequence of HSA. These oxidized compounds are normally held in check during fermentation by the strong reducing power of yeast metabolism. During maturation and Stage A (Figure 1), these compounds undergo a complex series of electron exchanges. The net effect is the oxidation of beer alcohols, and the creation of volatile aldehydes. This is sometimes called oxidation without molecular oxygen since these reactions can take place without the latter being present. These aldehydes do not display their presence until a lag period is over. It is felt that the reasons for this are bonds between the aldehydes and natural sulfur compounds from yeast metabolism [20]. These bonds, alas, are temporary. When broken a wide range of flavors appear. All have a grainy-astringency associated with them, and metallic undertones are often present. Sherry-like notes have sometimes been identified as well, this being one flavor note common to both CSA and HSA [10].
Unfortunately, these mechanisms do not explain how 2-nonenal is created, since it has been demonstrated that there is no alcohol relevant to beer that is a precursor to 2-nonenal [10]. Current research (see for example [21]) points to fatty acids in wort, along with malt based enzymes such as Lipase and Lipoxygenase. Heat and oxygen stimulate the enzymatically induced creation of hydro-peroxides which are the precursors of 2-nonenal. In any case, oxygen uptake in the brew house remains a culprit along with a still to be determined effect of malting. An extensive review of this research is given in [10].
Some of HSA is easy to avoid. An example is the minimization of sheering forces from excessive raking or stirring, a remarkably important effect [12]. A more perplexing problem is the HSA created by splashing during hot wort transfer. Following a presentation I gave at the Atlanta microbrewing conference in April of 1998, I was approached by an instructor of a domestic brewing school, who has been very skeptical about the importance of HSA. He correctly noted that there are a lot of commercial systems in operation that have very high oxygen uptake due to splashing. He then asked if HSA is sufficiently important to require the purchase of an entirely new brewhouse. This is also a perplexing problem for the manufacturers of brewing equipment. I was once associated with such a firm, which because of its basic designs, had extensive HSA. At the same time these designs permitted this firm to manufacture their equipment more cheaply than most of its major competitors. Again, the issue arises if HSA is really that important to take possibly expensive corrective action.
I feel, for at least the commercial brewers the answer is definitely yes. The comparative advantage of craft brewers in the market place is the perception of a dedication to excellence in all aspects of brewing. It is very hard for even ardent supporters of craft beers to maintain this viewpoint for beer’s whose primary flavor tone is cardboard (to quote Milgaard). Moreover, corrections are not always expensive. A particularly elegant "fix up" can be seen in the pilot brewery at Coors in Golden, Colorado. The once 100% copper brew kettle now has a stainless steel adjunct in the form of a tube leading from the original kettle inlet at the top of this vessel to its bottom. What was once a splashing flow, is now replaced with a gentle laminar fill. I do not know how much the modification cost, but it clearly is a minute fraction of the price of the kettle.
FINAL COMMENTS
One final issue that sometimes arises is the relative importance of CSA compared to HSA. This topic has been flooded with sweeping generalizations ranging from "the flavor stability of beer is determined in the brew house," to "no other oxidative effect is as important as bottle head space air." I have found in my personal brewing, that whenever I buy into such broad assertions, I seem to quickly run into a 500 pound version of its exception at the next dark alley! The best advice in problem solving associated with oxidation, is not to bring preconceptions into the process. Objective tests will always lead to the best solutions. In this regard I have found the following to be useful.
1. Anyone using pumps to transfer fermented beer should have their procedures checked with a D.O. meter. Respectable versions can be obtained for $400-500. This may be steep for the average amateur, but not for a medium to large brew-club. D.O. meters are not useful for HSA detection, since dissolved oxygen at high temperatures will react before it can be measured.
2. An air tester such as the classical Zahm and Nagle unit is also of great value. In addition to measuring the head space air at fill, this unit can also be used to track headspace air as the beer ages. The speed of its attenuation gives a good measure of stability with respect to CSA. Again, these instruments may be too expensive for the average homebrewer, but again, not necessarily for brew-clubs. For commercial brewers producing bottled beer, air testers are crucial.
3. The classical indicator time test ITT ([22], [23]) is a good way to monitor HSA. Special note should be made of the improved media recently recommended for this procedure [24].
4. The best test of all is to stress beer and allow tasting to determine the relative importance of CSA or HSA. I use a water bath at 30° C, and taste samples twice a month for 3 months. Higher temperatures give results in less time, but sometimes the flavors are more difficult to evaluate.
REFERENCES
[1] Bickham, S., "Staving off the Staling Compounds," this Journal,
Vol. 6, No. 2, pp. 20-27, 1998.
[2] Dalgliesch, C. D., "Flavor Stability," Proc. 16th European Brewing
Conference, Amsterdam, pp. 623-659, 1977.
[3] Hardwick, W. A., "Beer Flavor Stability," Brewers Digest,
pp. 42-44, 1978.
[4] Narziss, L. H., Miedaner, and F. Schneider, "Wort Boiling,"
Brauwelt, 1992, Vol. 4.
[5] Axcell, B. and P. Torline, "Some Alternate Views on Beer
Flavor," MBAA Tech. Qr., Vol. 35, No. 2, pp. 91-94, 1998.
[6] Homebrewers Digest - various issues
[7] Meilgaard, M. C., "The Flavor of Beer," MBAA Tech. Qr., Vol.
28, pp. 132-141, 1991.
[8] Meilgaard, M. C., "Flavor Chemistry," MBAA Tech. Ar., Vol. 12,
No. 3, 1997.
[9] Fix, G. J., "Principles of Brewing Science," Brewers Publications,
1989.
[10] Fix, G. J., "The New Principles of Brewing Science," to appear,
Brewers Publications, 1999.
[11] Stucky, G. J. and M. R. McDaniel, "Raw Hop Aroma Qualities
by Trained Panel - Free-choice Profiling," Journal Am. Soc.
Brew. Chem., Vol. 2, pp. 65-78, 1997.
[12] Fix, G. J. and L. A. Fix, "Analysis of Brewing Techniques,"
Br. Publications, 1997.
[13] Broderick, H. M. ed, "The Practical Brewer," MBAA, 1977.
[14] Hashimoto, N., "Flavor Stability of Packaged Beer," Ch. 6 in
Brewing Science, Vol. 2, JRA Pollock, ed., Academic Press,
1981.
[15] Hashimoto, N. and Y. Kuroiwa," Pathways for the Formation
of Volatile Aldehydes During Storage of Bottle Beer," Rept.
Res. Lab. Kirm Br. Co., No. 18, pp. 1-11, 1975.
[16] Hashimoto, N. etal, "Flavor Stability of Packaged Beer in
Relation to the Oxidation of Wort," Br. Digest, pp. 18-23, June
1986.
[17] de Clerck, J. "Textbook of Brewing," Vol. 1, Chapman and Hall,
1957.
[18] Morton, B. J. and M. R. Stat, "Production of Colloidally Stable
Beers through Wort Conditioning," MBAA Tech., Qr., Vol. 23,
1986.
[19] Seidl, C. "Steeped in Tradition," American Brewer, pp. 60-65,
1998.
[20] Barker, R. L. et al, "Liberation of Staling Aldehydes during
Storage of Beer," J. Inst. Brew., Nov-Dec., Vol. 89,
pp. 411-415, 1983.
[21] Cantrell, I. C. and D. L. Griggs, "Malt: Its Role in Oxidation,"
MBAA Tech. Ar., Vol. 33, No. 2, pp. 82-86, 1996.
[22] de Clerck, J., "Textbook of Brewing," Vol. 2, Chapman and
Hall, 1957.
[23] Chapon, L., "Oxygen and Beer," Ch. 7 in Brewing Science,
J. R. A. Pollock, ed., Academic Press, 1981.
[24] Kaneda, H., et al, "Chemical Evaluation of Beer Flavor
Stability," Vol. 32, No. 2, 1995.
George J. Fix
INTRODUCTION
John Meier, senior brewer at Rogue’s Ales and former homebrewer of the year, was once asked what he feared the most as a brewer*. His answer was pointed and direct. "It is the A, B, Ds." The "A" on his list stands for air, and is the central topic of this paper. The interpretation of "B" and "D" is left as a homework exercise!
The key point is that successful brewers like John invariably display sensitivity to oxidative issues, and also have the skill to effectively deal with them. The goal of this article is to identify the flashpoints, i.e., the critical areas in the brewing process where care is needed, as well as some ideas on how one can operate a low oxygen system, be it on the homebrew or commercial level.
The major difference between brewing science and the biological and chemical sciences is that in the former, flavor is the final arbitrator of what is important and relevant. This is a particularly delicate point as far as oxidation is concerned, since there is a definite variation in how people perceive oxidized flavors. Thus, the first order of business is to establish criteria that characterize oxidized beer. This material is a sequel to Scott Bickham's excellent article in his column "Focus on Flavor" [1].
The next two sections deal with the parts of the brewing process where oxidation is likely to occur. A distinction is made between aeration in the "hot side," (HSA) and the "cold side" (CSA) of the brewing process. The former consists of mashin to wort-chilling. At that point, oxygen is added to wort, or more commonly these days to yeast, prior to pitching. Here oxygen serves as an invaluable yeast nutrient. Once the fermentation has started, oxygen returns to being a negative, and the cold side starts at this point and continues until the beer is consumed.
PERCEPTION OF OXIDIZED FLAVOR
Following Dalgliesch [2] three stages in beer flavor evolution can be defined:
FRESH 1 A 2
B
STALE 3 C 4
TIME
FIGURE 1
Stage A (points 1 to 2) is the period of stable "brewery fresh" flavor. Stage B (points 2 to 3) consists of a transition period where a multitude of new flavor sensation can be detected. Dalgliesch cites the following:
i. Decline in hop aroma
ii. Decline in hop bitter
iii. Increase in "ribes aroma" (or sometimes "catty" flavor)
iv. Increase in sweet/toffee-like/caramel tones
The term’s "ribes" or "catty" are widely used in the UK and Scandinavia. They generally recall over-ripe/spoiled fruit or vegetables. Some cite a "black currant" tone [3]. In truth, they are descriptors for a wide spectrum of flavors when beer is in Stage B. The toffee-caramel tones are a different matter. They are enhanced by residual diacetyl, and also by excess heat treatment of wort [4]. Boiling in pressure cookers will develop flavor tones like these along with other negative ones [12].
The Stage C products (points 3 to 4) are the classic flavor tones involved in beer staling. These are described below in conjunction with the part of the brewing process where they are created. Although, it is not treated in the references cited above, there is also a "Stage D," where Stage C flavors evolve into a kaleidoscope of flavors, which in very special formulations (Rodenbach’s Grand Cru comes to mind) recall the subtly and complexity of great wines. It is to be emphasized that this process takes years, not months of maturation. This effect deserves a paper all its own, and hence falls outside the scope of this article.
Axcell and Torline [3] in an interesting if provocative article argues that most beers are consumed during Stage B. This is a period where beer flavors are undergoing discernable change, and they suggest the changes are at the root of the consumer’s dissatisfaction. Among other things, they appeal to the "import paradox" as partial evidence. This paradox refers to the phenomena that a definite proportion of the beer consuming population actually prefer beers in Stage C. (Here "import" means any beer consumed at a significant distance from where it is brewed). These authors cited the stability of flavor in Stage C and "learned prejudices" (i.e., prestige of the beer, packaging, et al) as the key to this paradox. Statements like " … if Pilsner Ur-quell has HSA, then I want my beer to have HSA …", often seen in the internet discussion groups devoted to beer, illustrate this point [6]. Stage B flavors, on the other hand, appears to have very few advocates.
Meilgaard [7] is sharply critical of Stage C flavors because they are one dimensional. He states " … I think it ranks as an all-American scandal that fully half of all the interesting and unusual packaged beers that are on the market, get to us so oxidized that staleness and cardboard are the main flavor tones." I feel this is a very important point, and this paper is based on the premise that the best strategy for brewers (amateur or commercial), is to produce beers where Stage A flavors are as stable for as long as possible. If they, or their consumers prefer Stage B, C, (or D for that matter), then putting the beer aside for the required time is all that is needed. Staling, like entropy, can go in only one direction!
CSA
Oxidation on the cold side is the easiest to understand, and until the last decade has occupied all of the attention of those concerned about this issue. The major mechanisms are given in Table 1. Flavor thresholds in ppm (i.e., parts per million) were taken from Meilgaard [8].
TABLE I
CSA MECHANISMS
1. Ethanol + oxygen ® acetaldehyde
(14,000 ppm) (25 ppm)
2. Acetaldehyde + oxygen ® acetic acid
(25 ppm) (175 ppm)
3. Hop a -acids + oxygen ® valeric butric acids
(10 ppm) (1-2 ppm)
4. Hop oils + oxygen ® various oxygen bearing compounds (see [9]-[10]
(5) Unsaturated fatty acids from trub + oxygen ® oleic, linoleic acids
(less than 1 ppm)
The first redox reaction (1) in Table I is the start of the classic "vinegar process". It also marks the second appearance of acetaldehyde. The first is the main fermentation pathway where acetaldehyde emerges as a reduction product of pyruvic acid [9], [10]. At that stage it has a flavor which recalls freshly cut apples. As an oxidation product via (1) this changes to an "old apple" tone. It may also recall "rotten apples," although this effect is not nearly as strong as what occurs with Zymomonas infection.
Because of the high flavor threshold of acetic acid, the second mechanism ((2) in Table I) will rarely give a full vinegar taste. Generally, sherry like tones emerge. Massive amounts of oxygen and/or acetic acid bacteria (acetobacteria) infections can give vinegar flavors.
As noted above, the loss of hop flavors (taste and smell) is the easiest way to detect the start of staling. By the time stage C in Figure 1 is reached, the oxidation products of a -acids will start to appear. "Cheesy" is a widely used descriptor. The use of old hops at elevated levels will intensify this effect. The flavors from hop oils, on the other hand, will generally just disappear in Stage B. Sometimes residual tones recalling hay, sagebrush, or grass can be detected. Many do not see the latter as a defect, and highly valued hops like U.S. Columbus or U.K. Golding display these tones in attractive ways. Yet the variance among taste panels with respect to this issue can be great [11].
The final CSA mechanism is often underestimated because unsaturated fatty acids (UFA) from trub in wort play a highly beneficial role in the fermentation. Their value on the cold side is the exact opposite. They are highly reactive with oxygen, and oxidize to a broad range of products, none of which are attractive. These effects are usually not seen in beers produced from reasonably clarified worts. The relatively low flavor thresholds of these oxidation products are a special concern.
Prevention of CSA is clearly a prevention of oxygen ingress in the post fermentation processing of beer. As far as beer transfer is concerned, an important step is to first remove as much air as possible from the receiving tank. Purging the tank with CO2 will do this in part. If, in addition, the transfer is achieved by the application of CO2 pressure to the sending tank, then by allowing a slight gas bleed in the receiving tank one can eliminate oxygen pickup. In fact, I have found [12] that the use of CO2 pressure throughout the post fermentation process can result in null dissolved oxygen readings at least with standard meters. The next best option is transfer via gravity flow. Even here, it is crucial to keep a CO2 cover over the beer to protect against oxygen ingress. Maintaining a smooth laminar flow is also important.
In commercial brewing, the above two procedures are not always an option. CO2 is expensive, and a horizontal layout precludes gravity flows. Thus, the use of pumps are a necessity. To say that there are good ones and bad ones is a mild understatement. Operator error is also an issue. In this context, the availability of a DO meter is crucial to evaluate how well the system is working in each transfer. Pumping beer through filters is a particular concern, and dissolved oxygen should be measured on a regular basis.
The greatest concern about CSA has correctly pointed at the packaging of beer. The deleterious effect of headspace air has been evident ever since the human race started putting beer into small bottles. Fortunately, modern technology has made it possible to dramatically lower oxygen pick-up during filling. For example, double pre-evaculation bottle fillers routinely keep bottle air levels below .2 milliliters air per 1/3 liter beer. This is in striking contrast to the situation that existed a few decades ago, where criteria ran as high as 1.0 ml/in 1/3 L [13]. In fact, most of the reasonable homebrew hand fillers on the market today, if properly operated, can get below .5 ml/1/3 L.
Nevertheless, headspace air remains a concern, and the indited
co-conspirator is thermal abuse. Extensive data illustrating this is given in [12]. This data was carefully controlled both in respect to other forms of CSA as well as HSA discussed below, and so displays the direct effects of thermal abuse and bottle air levels. To cite one example, a beer which has .1 ml/1/3L of air at fill will start to display staling (first part of Stage B in Figure 1) in 140 days if it is stored at 6° C. This drops to 80 days if the storage temperature is raised to 30° C. On the other hand, a beer which has 1.5 ml/1/3L of air at fill will start staling after 20 days at a storage temperature of 30° C and 80 days at 6° C. In other words, beers with very low headspace air - - something that requires a million dollar plus filling machine - - that has been subject to thermal abuse are no more stable than beers that have been afforded proper storage yet have the sort of air one expects from cheap homebrew hand-fillers.
HSA
The first hint that oxygen uptake during wort production could be a problem was found in the fundamental work of N. Hashimoto, one of the leading experts on beer staling (see e.g., [14]) His prime concern was volatile aldehydes like 2-nonenal*. This compound has a characteristic papery/cardboard flavor that can evolve into leathery/woody tones in Stage C of Figure 1. It is a primary concern because of its very low flavor threshold, which for most people falls in the range of .1-.4 parts per billion. The defining step of Hashimoto’s work came from the observation in his now classic 1975 paper [15] that as far as the formation of nonenal is concerned "… molecular oxygen does not take a part… in bottled beer." In later work [16] the oxidative processes during wort production was identified as a serious concern.
This was a new finding at the time. The only previous study involving oxygen uptake during wort production was by deClerck [17]. He found that the total elimination of oxygen during wort production increased the finished beer’s tendency to form chill haze. In a follow up study [18], it was demonstrated that massive oxygen uptake prior to sparging greatly enhanced colloidal stability. This study did not take into account finished beer flavor, and in fact illustrates the point that the promotion of physical stability is not always in harmony with the promotion of flavor stability.
After Hashimoto’s work appeared, manufacturers of brewing equipment (most notably in Germany) funded additional research into this topic [19]. The result was a long series of articles, most of which appeared in Brauwelt. All concurred with Hashimoto’s basic findings (see [12]). I am not aware of any articles that have appeared in professional peer reviewed brewing journals, which has found the contrary.
The first mechanisms studied by Hashimoto involved malt derived compounds like polyphenols and melanoidins. Oxygen uptake on the hot side immediately sets in place a number of redox reactions. Temperature is crucial, since the speed of these reactions increases exponentially with temperature. For example, introducing say 1 ml per liter into wort at 70° C will start reactions that take place in seconds. Doing the same for wort at say 20° C will have virtually no effect. The oxygen fraction will typically remain an inert gas until consumed by yeast. Hashimoto showed that the oxidation of melanoidins and polyphenols was an important consequence of HSA. These oxidized compounds are normally held in check during fermentation by the strong reducing power of yeast metabolism. During maturation and Stage A (Figure 1), these compounds undergo a complex series of electron exchanges. The net effect is the oxidation of beer alcohols, and the creation of volatile aldehydes. This is sometimes called oxidation without molecular oxygen since these reactions can take place without the latter being present. These aldehydes do not display their presence until a lag period is over. It is felt that the reasons for this are bonds between the aldehydes and natural sulfur compounds from yeast metabolism [20]. These bonds, alas, are temporary. When broken a wide range of flavors appear. All have a grainy-astringency associated with them, and metallic undertones are often present. Sherry-like notes have sometimes been identified as well, this being one flavor note common to both CSA and HSA [10].
Unfortunately, these mechanisms do not explain how 2-nonenal is created, since it has been demonstrated that there is no alcohol relevant to beer that is a precursor to 2-nonenal [10]. Current research (see for example [21]) points to fatty acids in wort, along with malt based enzymes such as Lipase and Lipoxygenase. Heat and oxygen stimulate the enzymatically induced creation of hydro-peroxides which are the precursors of 2-nonenal. In any case, oxygen uptake in the brew house remains a culprit along with a still to be determined effect of malting. An extensive review of this research is given in [10].
Some of HSA is easy to avoid. An example is the minimization of sheering forces from excessive raking or stirring, a remarkably important effect [12]. A more perplexing problem is the HSA created by splashing during hot wort transfer. Following a presentation I gave at the Atlanta microbrewing conference in April of 1998, I was approached by an instructor of a domestic brewing school, who has been very skeptical about the importance of HSA. He correctly noted that there are a lot of commercial systems in operation that have very high oxygen uptake due to splashing. He then asked if HSA is sufficiently important to require the purchase of an entirely new brewhouse. This is also a perplexing problem for the manufacturers of brewing equipment. I was once associated with such a firm, which because of its basic designs, had extensive HSA. At the same time these designs permitted this firm to manufacture their equipment more cheaply than most of its major competitors. Again, the issue arises if HSA is really that important to take possibly expensive corrective action.
I feel, for at least the commercial brewers the answer is definitely yes. The comparative advantage of craft brewers in the market place is the perception of a dedication to excellence in all aspects of brewing. It is very hard for even ardent supporters of craft beers to maintain this viewpoint for beer’s whose primary flavor tone is cardboard (to quote Milgaard). Moreover, corrections are not always expensive. A particularly elegant "fix up" can be seen in the pilot brewery at Coors in Golden, Colorado. The once 100% copper brew kettle now has a stainless steel adjunct in the form of a tube leading from the original kettle inlet at the top of this vessel to its bottom. What was once a splashing flow, is now replaced with a gentle laminar fill. I do not know how much the modification cost, but it clearly is a minute fraction of the price of the kettle.
FINAL COMMENTS
One final issue that sometimes arises is the relative importance of CSA compared to HSA. This topic has been flooded with sweeping generalizations ranging from "the flavor stability of beer is determined in the brew house," to "no other oxidative effect is as important as bottle head space air." I have found in my personal brewing, that whenever I buy into such broad assertions, I seem to quickly run into a 500 pound version of its exception at the next dark alley! The best advice in problem solving associated with oxidation, is not to bring preconceptions into the process. Objective tests will always lead to the best solutions. In this regard I have found the following to be useful.
1. Anyone using pumps to transfer fermented beer should have their procedures checked with a D.O. meter. Respectable versions can be obtained for $400-500. This may be steep for the average amateur, but not for a medium to large brew-club. D.O. meters are not useful for HSA detection, since dissolved oxygen at high temperatures will react before it can be measured.
2. An air tester such as the classical Zahm and Nagle unit is also of great value. In addition to measuring the head space air at fill, this unit can also be used to track headspace air as the beer ages. The speed of its attenuation gives a good measure of stability with respect to CSA. Again, these instruments may be too expensive for the average homebrewer, but again, not necessarily for brew-clubs. For commercial brewers producing bottled beer, air testers are crucial.
3. The classical indicator time test ITT ([22], [23]) is a good way to monitor HSA. Special note should be made of the improved media recently recommended for this procedure [24].
4. The best test of all is to stress beer and allow tasting to determine the relative importance of CSA or HSA. I use a water bath at 30° C, and taste samples twice a month for 3 months. Higher temperatures give results in less time, but sometimes the flavors are more difficult to evaluate.
REFERENCES
[1] Bickham, S., "Staving off the Staling Compounds," this Journal,
Vol. 6, No. 2, pp. 20-27, 1998.
[2] Dalgliesch, C. D., "Flavor Stability," Proc. 16th European Brewing
Conference, Amsterdam, pp. 623-659, 1977.
[3] Hardwick, W. A., "Beer Flavor Stability," Brewers Digest,
pp. 42-44, 1978.
[4] Narziss, L. H., Miedaner, and F. Schneider, "Wort Boiling,"
Brauwelt, 1992, Vol. 4.
[5] Axcell, B. and P. Torline, "Some Alternate Views on Beer
Flavor," MBAA Tech. Qr., Vol. 35, No. 2, pp. 91-94, 1998.
[6] Homebrewers Digest - various issues
[7] Meilgaard, M. C., "The Flavor of Beer," MBAA Tech. Qr., Vol.
28, pp. 132-141, 1991.
[8] Meilgaard, M. C., "Flavor Chemistry," MBAA Tech. Ar., Vol. 12,
No. 3, 1997.
[9] Fix, G. J., "Principles of Brewing Science," Brewers Publications,
1989.
[10] Fix, G. J., "The New Principles of Brewing Science," to appear,
Brewers Publications, 1999.
[11] Stucky, G. J. and M. R. McDaniel, "Raw Hop Aroma Qualities
by Trained Panel - Free-choice Profiling," Journal Am. Soc.
Brew. Chem., Vol. 2, pp. 65-78, 1997.
[12] Fix, G. J. and L. A. Fix, "Analysis of Brewing Techniques,"
Br. Publications, 1997.
[13] Broderick, H. M. ed, "The Practical Brewer," MBAA, 1977.
[14] Hashimoto, N., "Flavor Stability of Packaged Beer," Ch. 6 in
Brewing Science, Vol. 2, JRA Pollock, ed., Academic Press,
1981.
[15] Hashimoto, N. and Y. Kuroiwa," Pathways for the Formation
of Volatile Aldehydes During Storage of Bottle Beer," Rept.
Res. Lab. Kirm Br. Co., No. 18, pp. 1-11, 1975.
[16] Hashimoto, N. etal, "Flavor Stability of Packaged Beer in
Relation to the Oxidation of Wort," Br. Digest, pp. 18-23, June
1986.
[17] de Clerck, J. "Textbook of Brewing," Vol. 1, Chapman and Hall,
1957.
[18] Morton, B. J. and M. R. Stat, "Production of Colloidally Stable
Beers through Wort Conditioning," MBAA Tech., Qr., Vol. 23,
1986.
[19] Seidl, C. "Steeped in Tradition," American Brewer, pp. 60-65,
1998.
[20] Barker, R. L. et al, "Liberation of Staling Aldehydes during
Storage of Beer," J. Inst. Brew., Nov-Dec., Vol. 89,
pp. 411-415, 1983.
[21] Cantrell, I. C. and D. L. Griggs, "Malt: Its Role in Oxidation,"
MBAA Tech. Ar., Vol. 33, No. 2, pp. 82-86, 1996.
[22] de Clerck, J., "Textbook of Brewing," Vol. 2, Chapman and
Hall, 1957.
[23] Chapon, L., "Oxygen and Beer," Ch. 7 in Brewing Science,
J. R. A. Pollock, ed., Academic Press, 1981.
[24] Kaneda, H., et al, "Chemical Evaluation of Beer Flavor
Stability," Vol. 32, No. 2, 1995.