Dave makes the point that batch size plays a key role in determining hop utilization and that formulas for hop utilization usually have a term for utilization percentage. Our experience as both home brewers and commercial brewers verifies Dave's statement that hop utilization percentages vary with wort volume and that the larger the wort volume the more efficient the utilization.

In defending his position that "every published formula I have seen for estimating IBUs has more than an outside chance of giving you a grossly inaccurate (emphasis ours) estimate of the bitterness in your finished beer," Dave cites two of his brews from The Saint Louis Brewery.

Lab results provided to The Saint Louis Brewery for their wheat ale show a hop utilization of 26.8% for two hop additions. Dave allowed some adjustment to the time factors due to postkettle boil procedures at the brewery and came up with a 60-min addition and a 30-min addition of hops. To answer the reader's question, Dave then cites utilization percentages that appeared in Zymurgy (1990 special issue, Table 2, p. 54). The percentages are as follows:

       Boiling Time	Utilization
	(min)		    (%)
	<5		     5.0
	6-10		     6.0
	11-15		     8.0
	16-20		    10.1
	21-25		    12.1
	26-30		    15.3
	31-35		    18.8
	36-40		    22.8
	41-45		    26.9
	46-50		    28.1
	51-60		    30.0

Research performed by our staff confirms our belief that the percent utilization figures listed in the table are for 5- and 10-gal batches, not a 7- or 14-bbl brewery. If wort volume is so important to hop utilization, why use percent figures for a 5-gal volume and apply them to a large brewery? If there is anything grossly inaccurate, it's Dave's argument and conclusions.

Based on our use of a simplified utilization formula with percent factors, as well as witnessing its use by our customers over an extended period of time, we have seen that these formulas are accurate and repeatable and produce top-quality beers. Brewers moving onto larger batches and commercial quantities must adjust their utilization factors for batch size. For those brewers who are put off by large algebraic expressions or calculators, products are now available that require only a look-up procedure for estimating hop utilization.

-The Beverage People Santa Rosa, California
Miller responds: I am gratified that The Beverage People have found something in my column worth objecting to. It is hard to overstate my respect and admiration for Nancy, Byron, and Paddy. They have done a lot for home brewing and their record speaks for itself. That does not mean I agree with them, or they with me, on every issue. This is clearly one such instance.

First, it is true that I took a formula designed for home brewers and applied it to microbrewing. But that is how the question was framed (the difference in hop utilization between a 10-gal and a 10-bbl batch), and the writer was using a homebrewing formula. I used my own experience because (a) I have hard numbers, which I never had when I was home brewing, and (b) to point out how different home brewing is from microbrewing in some fundamentals of procedure as well as results.

Second, many home brewers of my acquaintance have observed a significant change in hop utilization when moving from a 5- to a 10-gal batch size (and making no other deliberate changes in their procedures). Even though both of these volumes are minuscule compared with 7- or 14-bbl microbrewery batch sizes, one is double the size of (100% larger than) the other. I question whether a single formula can work for both batch sizes without a corrective factor.

Third, I don't doubt that The Beverage People have found that a simplified IBU formula gives repeatable results. So does the old alpha acid unit (AAU) system. As for producing top-quality beers, that is due to a lot of things, but most of all to the fact that they themselves are good brewers and good teachers of brewing. Getting the IBUs and the other numbers nailed is not the whole story by a long shot. You can brew a clean beer that meets all the specs for the style but still doesn't taste right.

Fourth, concerning accuracy I would like to see some lab results on beers made by a number of different home brewers, with different equipment and procedures, using the same formula to predict the IBUs. All beers would have to be analyzed in the same laboratory by the same method and personnel. If such an experiment confirms the accuracy of the formula, I will not only accept it, I will start using it. What degree of accuracy is required? A difference of 2 IBUs is detectable by most tasters. So all the tested beers would have to fall within the range of ±1 of the predicted number.

Fifth, and last, note that once again the human factor is the final test. Our taste buds are the ultimate analytical instrument, and the definition of accuracy must be keyed to its resolution. A difference in bitterness is significant if we can taste it. All brewers use some bitterness formula for their first attempt at a new beer style. But they then have to make a sensory analysis of their results and adjust their recipes accordingly. If your formula works for you and your customers, great! My own experience as a home brewer was that the IBU formulas that I tried didn't get me within shooting distance of the expected result, and I went back to AAUs because the system is simpler and works as well as anything else. But that's not the end of the world. The beer was good even though I had only a vague idea what its IBUs were.

-Dave Miller
St. Louis, Missouri

Priming Methods Compared