November 1998 Volume 21, Issue 11

Experimental Determination of IBU

by Bob Jones

The Experiment

The Draught Board conducted an experiment at the September meeting on experimental IBU determination. The experiment would hopefully provide a means for homebrewers to experimentally determine the bitterness of their finished homebrewed beers. The basis on this experiment is rooted in Mark Garetz’s book on Hops. The technique revolves around taking a beer of unknown IBU level and a beer like Bud which has a know IBU level and then add bitterness (iso-aplha acid) to the Bud sample until the bitterness of the 2 samples taste the same. By knowing the amount of bitterness added to the Bud sample one can determine the bitterness of the unknown beer. For the details on this technique I would refer you to Mark’s book on Hops. By doing this experiment in a large group I was hopeful the experimental error would be smaller and therefore the results would be better.

Some Background

Several years ago the Draught Board did a similar experiment and actually had very good results. In this experiment we added bitterness to Bud until is tasted similar to Anchor Steam. The data we used for the bitterness of Bud and Anchor Steam were from published textbooks. The experiment was a blind one. The level of bitterness (IBUs) we determined was very close to that of Anchor Steam. There were several unknowns in this initial experiment. So we set out this time to determine just how accurate this technique really is, as well as trying to determine the technique’s validity.

The New Experiment

We sent off 3 beer samples to Siebles Institute for IBU analysis. One of these samples was Bud. Having Bud analyzed should further reduce the error, because it is the bases for determining the IBU of the tested beer. Buds reported IBU level is between 10-11 IBUs.

The 3 samples we sent off were a Brown Ale (BA) a Pilsner (pils) and Bud. Siebels analysis for these beers IBU levels were as follows:

Brown Ale (BA) = 37.7 ibu

Pilsner (pils) = 45 ibu

Bud = 10 ibu

As you can see Bud came in very close to the reported IBU level.

Each person at the meeting (9 judges) was given a sample of Bud and a sample of the experimental beer, which we were trying to determine the IBU level of. Each person then added drops to the Bud until its perceived bitterness was the same as the unknown beer. Each person was responsible for keeping track of the number of drops added. I collected the data and analyzed it after the meeting.

The Data

The column titled "drops" indicates the number of drops each judge required to get the 2 samples to taste similar in bitterness. This column is multiplied by the IBU per drop and then added to the IBU level of Bud to get the "est. ibu" column. Then the average (avg), median and standard deviation (sd) were calculated.

The Results

The results didn’t turn out as well as I hoped they would. In fact there is a huge error for both samples we tested. The judges IBU estimates were 49.1% higher for the Brown ale and 39.9% higher for the Pilsner than the Siebels measured IBU levels for the 2 samples! The standard deviation of the data doesn’t seem that bad actually. It did seem to get worse for the second test beer. Maybe fatigue played a role here.

I have double checked the calculations for the dilutions I used to make the bitterness drops that were added during the experiment, and I see no errors. I have called Mark at Hoptech and ask him if he might have made an error in his dilution of the iso-alpha acid solution we utilized. He said he personally did the dilution himself and had no idea where the error might be. He suggested I come in and get another sample and see if it produced a similar error. I will plan to do that in the future.

Summary

The experiment was fun and gave the Draught Board a chance to do some critical tasting with regard to just the bitterness of a beer. The large errors we encountered could be due to a dilution error by Mark, the company he gets the concentrated iso-alpha acid from or just plain experimental error in the technique. Overall I would have to say the experiment was a failure with respect to providing a low cost experimental method of determining the IBU of a homebrewed beer. So for now it seems the best way to really determine the true IBU of your homebrewed beer is to send it to Seibels Institute and have it analyzed. This analysis costs $39 per beer. I think we might consider trying this experiment again in the future, but I personally have less confidence in it now than I originally did. I will continue to look for the source of the error and will report it if I ever locate it.