Losses during fermentation. Yeast cells adsorb hop bitterness, as a simple taste test proves. The degree of adsorption depends on a multitude of factors, not the least of which is the amount of cell growth.
To give some idea of how important these unknown and unaccountable factors can be, let me cite two beers we brew at The Saint Louis Brewery: our wheat ale and oatmeal stout. Both were analyzed by friends with access to a large and well-equipped brewing laboratory. Both beers are brewed using pelletized hops, and both are fermented using the same yeast strain, pitching rate, and method. For the oatmeal stout, we add all the hops 45 min before the end of the boil. Wort gravity is 1.056. Our hop utilization is 31.5%. For the wheat ale, we add 50% of the hops (by a-acid) 30 min before the end of the boil and the remainder at the finish. Wort gravity is 1.037. Our hop utilization is 26.8%.
You may ask, what in the world is going on here? Other things being equal, you would expect a little better utilization in the wheat ale because of the lower gravity, but in no way could that make up for the difference in boil times. Remember, by the article you are following, we should be getting only about 5% utilization from our finish hops.
The answer (in part) is that our kettle also is a whirlpool, and we whirlpool the wort for 20 min and then let it settle for another 10-15 min before knocking out. Obviously, 30 min of agitation at near-boiling is almost as effective as boiling for a similar period.
OK, this is a gross example, and you might argue that you could still use the formula if you simply include the 30-min whirlpool period in the total boil time. But that still will not get you anywhere near an accurate estimate. Figuring the finish hops as a 30-min boil and the first addition as 1 h, we should (again by the article you are following) get an average utilization of 30 + 15.3/2 = 22.7% - significantly below the actual figure.
Every published formula I have seen for estimating IBUs has more than an outside chance of giving you a grossly inaccurate estimate of the bitterness in your finished beer. The brewing process simply contains too many variables, and some of them are nearly impossible to quantify. Whenever a brewer tells me, "This beer has so many IBUs," I have to ask in turn, "What lab did the measurement?" There is a real need for a good, reasonably priced lab service that would measure bitterness for small-scale brewers.
In any case, I see no advantage in publishing recipes that require working backwards from an IBU figure through a complex formula to derive a hop rate. The simple old AAU (a-acid unit) or HBU (homebrew bitterness unit) system is just as likely to get you in the ballpark. Furthermore, brewing encompasses art as well as science, and setting bitterness falls on the art side of the process. You have to go by taste. All the calculations are only a means to let you use your knowledge and experience to get a beer that tastes right - the right amount of bitterness, in this case - in as few trials as possible, and then to repeat or fine-tune the flavor of that beer.
I hope it doesn't sound like old technical Dave is getting arty and muddle-headed. Flavor is the reason we brew and drink beer. Even the biggest and most technically proficient brewers in the world rely on taste panels as the ultimate analytical instrument. Numbers do not tell the whole story; they are only a means to an end. So if we cannot fix a recipe entirely by formulas, it's not the end of the world. We can still brew great beer.
Keg Carbonation
Q: I have a 5-gal Cornelius keg that I use for my homebrew. However, I have never been completely satisfied that I've achieved the correct amount of carbonation. Usually the brew is a little flat. What methods would you suggest for carbonating beer in a keg? I'm interested in both natural and artificial carbonation techniques, and please be as specific as possible.
DM: The surest way of achieving a precise degree of carbonation is to carbonate the keg artificially, using a carbonation temperature/pressure chart such as the one shown in Table I (2). This method also has the advantage of avoiding a keg fermentation, which means a clearer beer in the keg and less yeastiness on the palate. Of course, if you are making Hefeweizen or if you simply prefer natural carbonation, by all means prime the keg like a big bottle, using corn sugar (glucose) or gyle (wort).
If you wish to carbonate naturally, I believe that glucose will give more repeatable results because it is 100% fermentable. As I said on the last question, brewing is an art, so ultimately you have to go by taste and trial in getting the carbonation level where you want it. Start with 1/3 cup of corn sugar and, if that proves too little, increase gradually until you get it where you want it to be.
Artificial carbonation is simply a matter of following a few rules. First, chill the beer to near the freezing point before racking it into the keg. Second, take the temperature of the beer in the keg before sealing it up. Third, apply gas pressure and release it a few times to purge air from the headspace. Fourth, find the beer temperature on the chart and look along that row until you find your desired carbonation level. Then, look along the other axis to find the correct head pressure. Set the gas regulator to that pressure, and agitate the keg until you no longer hear gas flowing through the regulator during agitation. This can take quite a bit of shaking. At the end of the process, you should have achieved the desired volume of carbonation in the keg. Store the keg in a refrigerator, and, if the fridge is not set to the same temperature as the beer, adjust the head pressure if necessary to maintain carbonation.
One final thought - if you follow these procedures and still get consistently flat beers, a couple of other possibilities merit consideration. One is that your taste does not agree with the conventional wisdom on carbonation. You may prefer fizzier beer, and there is nothing wrong with that. If you have been going for 2.5 volumes, try carbonating to 3.0; that may suit your taste better. After all, it's your beer.
The other possibility is that you might have an equipment problem - for example, a slow leak in your gas system somewhere. This would cause loss of head pressure and carbonation over time. The best way to track down leaks is to spray a soap solution on all joints and connections and look for bubbling. Usually such leaks can be fixed merely by tightening a nut or screw, but sometimes you have to take the connection apart and replace a washer or other seal. In checking for leaks, don't neglect the keg itself: the lid and fittings (both gas and beer) on the Cornelius keg are frequently the culprits.
Yet another possibility, if your artificially carbonated beers are consistently flat, is that your regulator gauge is wrong. We have about 28 carbon dioxide pressure gauges at our brewery, and some of them are way off. You might be able to check this by substituting another gauge, but in any case trust your taste buds, not the readings. Carbonate your beers to the level you prefer, and if the numbers say that you are out of line, remember that instruments share with humans the capability of lying.
Diacetyl rest
Q: I generally perform the first two-thirds of my lager fermentation at 50 degrees F, rack the beer to a secondary fermentor, and lower the temperature 2 degrees F/day until I reach 40 degrees F. I hold it there until fermentation is complete.
I want to include a diacetyl rest, but I am not sure how this procedure is done properly. I understand that usually after racking to the secondary you allow the temperature to increase to around 60 degrees F; after some time you then take the temperature back down.
At what temperature should the diacetyl rest occur? Do you allow the temperature to rise freely, or do you step it up? How long is the diacetyl rest, and how do you know when it is complete? Once the rest is complete, do you step the temperature down, or drop it all at once?
DM: Your understanding of the diacetyl rest is basically correct. It should be done when fermentation is about two-thirds complete, as measured by beer gravity. For example, if you have a 1.054 beer and anticipate a final gravity of 1.012, that means a drop of 42 points. Two-thirds of 42 is 28, so when the gravity of the beer has dropped 28 points - to 1.026 - rack the beer to the secondary and let the temperature rise to 60 degrees F or so. Normally, a diacetyl rest lasts about 48 h, but it varies according to the requirements of the brewer. Once it is complete, the temperature is brought down gradually - 2-4 degrees F/day - until lagering temperature (usually 32 degrees F) is reached.
One thing to remember, if you incorporate a diacetyl rest, is to rack the beer - or in a unitank fermentation, draw off the yeast - before you allow the temperature to rise. This will help prevent the yeast dregs from autolyzing and giving a yeasty taint to the finished beer.
Most brewers just allow the beer to rise at its natural rate. As to the right temperature for the diacetyl rest, it is related to time and also to your question about how you know when the diacetyl rest is over.
The purpose of the diacetyl rest is to give the yeast a boost when it is in its corrective phase - that is, while it is reabsorbing and metabolizing the by-products that it excreted early in the fermentation. Anybody who has ever smelled a lager beer in the second or third day of active fermentation knows that it is a nasty smelling concoction, reeking of acetaldehyde and other things including, often, diacetyl - the butter or butterscotch note that is a part of the flavor profile of many ales. It is considered undesirable in most lagers, hence the inclusion of a diacetyl rest to reduce it.
Lager yeasts vary tremendously in their ability to remove diacetyl. Some of them, such as the famous Weihenstephan strain #308, need all the help they can get just to bring levels down to the taste threshold range. Others do quite well with no diacetyl rest at all. So to answer your question about times and temperatures, I would need to know what yeast you are using and how much diacetyl is in your finished beer.
Lacking that information, I would say that you have to experiment to get the results you want. Two days and 60 degrees F are fairly common, but they may be more or less than you need. Some breweries use higher or lower temperatures - anywhere from 55 to 65 degrees F - and longer or shorter times. You know when the rest is complete only by assaying the results. For most small-scale brewers, this means using our favorite lab apparatus, the human palate and nose.
The reason for dropping the temperature slowly after the diacetyl rest is that you don't want to chill the yeast too quickly. Fast chilling could put the yeast to sleep. More sugars remain to metabolize and more nasties to remove, and if you give the yeast a chance to adapt to the lower temperature it will happily continue to work slowly, even near the freezing point.
By the way, if you can manage it, I recommend a lager temperature of 32 degrees F. This temperature will give a clearer, cleaner flavored beer, especially if (as I recommend) you filter your lagers to remove yeast and chill haze.
REFERENCES
(1) J. Rager, Zymurgy 13 (4), 53-54 (1990).
(2) D. Miller, Brewing the World's Great Beers (Storey Communications, Pownal, Vermont, 1992), pp. 132-133.
