Mr. Wizard’s Musings – Disseminating the latest hop research
I annually attend several brewing meetings and every so often I see or hear something that really gets me thinking. And I like to write about these topics to give fellow brewers something to noodle on. A few years ago, back in 2012, Dr. Tom Shellhammer from Oregon State University gave a presentation at the Craft Brewers Conference in San Diego, California about differences in the analytical methods used to measure hop bitterness that was very intriguing. Shellhammer explained how beer bitterness measured with the International Bitterness Unit method (iso-octane extraction, and absorbance at 275 nm) is typically higher, especially with late-hopped and dry-hopped beers, than iso-alpha-acids (IAA) reported by the high performance liquid chromatography (HPLC) method. There was a presentation given this year related to a similar topic that again perked my ears.
Andreas Gahr, from the St. Johann Research Brewery, gave a presentation based on work performed by the German Hop Growers Cooperative (HVG) titled “What Are Auxiliary Bitter Compounds?” Andreas presented this paper on behalf of a group that included himself and Drs. Adrian Forster and Florian Schüll from HVG in Wolznach. What follows is a summary of his presentation along with some take-home points about hopping.
Andreas established that there are compounds in beer that are measured by the IBU method that do not show up with the HPLC method used to measure IAA (this makes sense because the HPLC is specific for IAA, and the IBU method is not). This is the same topic that Tom Shellhammer covered at the 2012 CBC in San Diego. Andreas used the term “alternate bitter compounds”, or ABCs, as the difference between the HPLC IAA value and the IBU. For example, a beer with 20 IBUs and 17 IAA has 3 of these ABC units. A convenient way of comparing these differences is with the IBU:IAA ratio. When IBU=IAA, there are no ABCs and the IBU:IAA ratio is 1.0. When a beer has 20 IBUs and 17 IAA, the ratio is 1.18.
The point of this discussion is not the development of a new number, but an understanding of the discrepancy in these two values. These ABCs include deoxyhumulones, xanthohumol, iso-xanthohumol, humulinones (alpha acid derivatives), hulupones and hulupinic acid (beta acid derivatives), traces of beta acids, and alpha acids. All of these compounds are bitter, but are not typically considered significant sources of bitterness in beer. But brewers are changing the way beers are hopped, so the rules of the past are being changed by current brewing techniques.
Andreas cited the work of Hofmann, Haseleu and Dresel, and established that ABC bitterness is generally perceived as positive and mild. Although the compounds that contribute to ABC are usually below flavor threshold values, their effect on beer flavor may be additive and synergistic in nature. This is especially relevant when beer has higher levels of ABCs, as is the case with many new generation beers with very high hopping rates at the late-boil and/or post-boil stages.
Beers brewed using iso-alpha-acid extracts (common on the international scene) and/or high alpha hops for bittering, and no aroma hop additions contain very little to no ABCs, whereas beers brewed using multiple hop additions in the brewhouse and dry hopping can have an IBU:IAA ratio approaching 2, or about 50% ABC. Gahr presented a plot of IBU:IAA ratio versus the total of alpha acids, hulupones, and humulinones that showed a significant, positive correlation between the two measures. This correlation was used to validate the use of IBU:IAA ratio as an indicator of ABC’s concentration.
Aside from the numerical debate that can follow such differences, especially among brewers who like to brag about hop bitterness, the real question is how do these ABCs influence beer flavor and the quality of bitterness. This was the focus of Gahr’s presentation, and here are the major points.
1. Not all hop varieties have the same composition, and it seems that German landrace varieties (these are hops that developed by selection within a region and largely comprised of the so-called noble varieties) produce beers with higher IBU:IAA ratios compared to high alpha and newer generation aroma hops.
2. Bitterness quality scores were higher in beers with higher IBU:IAA ratios (2005, German Central Marketing Association for Agricultural Products study).
3. Gahr, Forster, and Schüll found significant correlation between IBU:IAA ratio and hop beta acid:alpha acid ratio (β/α). This correlation is of practical significance because it allows brewers to select hops that may have a positive influence on the IBU:IAA ratio. Gahr showed the following β/α ratios for German hops:
• β/α ratios of high alpha varieties = 0.3 to 0.5
• β/α ratios of Hüll aroma varieties = 0.7 to 1.0
• β/α ratios of German landrace varieties and
Saphire = 1.3 to 2.4
4. Sensory data from two sensory trials examining the hedonic response to low alcohol (<3.5% ABV) and alcohol-reduced beer (<1.2% ABV) also showed a clear positive correlation between sensory score and IBU:IAA ratio (in other words, ABCs improve sensory scores).
The take home points offered to brewers were pretty direct:
• Use other hop varieties if you currently exclusively use high alpha hops.
• Use plenty of aroma hops and landrace hops (β/α ratio > 0.7).
• Limit hop boil time by adding hops later in the boil; this increases the IBU:IAA ratio because ABCs dissolve quickly, whereas alpha takes time to isomerize.
• Use multiple aroma additions.
• Dry hop more.
This topic needs further exploration, using other hop varieties from other hop regions of the world, to determine if these metrics are robust. But the suggestions are aligned with what many brewers have empirically determined. One thing of note is most hop varieties have a fairly low and consistent beta acid content in the 3-5% range, so high alpha hops almost always have a low β/α ratio. This means most new American hops, do not fare well using this metric. Anecdotally, there are some old-school brewers who swear by using aroma hops for bittering purposes, and Andreas Gahr’s presentation implicitly gives credence to this thought.
