Hop Polyphenols
An IBU is actually 50X the amount of light absorbed by a solvent-extracted beer sample containing all the “bitter stuff” that is chemically similar to isomerized alpha acid, but also includes oxidized beta acids, non-iso-alpha, hop polyphenols and flavanoids. This also means that, historically, a beer’s quoted BU number quantified a variety of bitter substances in beer, which collectively would have a different character than one of today’s beers built with today’s high-alpha hop varieties.
Today’s beers would typically have a more singular and sharper bitterness character compared to a beer brewed 40 years ago with the same measured BU level. Of course, no one was brewing 80 BU beers forty years ago, but they were brewing Pilsners and bitters with 20–40 BUs and the overall character would have been softer. By the way, the United States are apparently the only country that refers to this as an “International” unit, everyone else just calls it a BU.
First, let’s talk about hop polyphenols. Are hop polyphenols bitter or astringent or both? Bitter is an actual flavor that is detected by the tongue, but astringency is not. Astringency is a physical mouthfeel caused by the association and precipitation of large polyphenols with proline-rich proteins in the saliva. The precipitation of these compounds coats the mouth and tongue, resulting in a sensation of dryness and inhibition of taste. In the Journal of the American Society of Brewing Chemists (J.ASBC), 66(3): 174–183, 2008, McLaughlin et. al., published “Bitterness-Modifying Properties of Hop Polyphenols Extracted from Spent Hop Material,” in which they dosed an American light lager (3 BUs) with five levels of added iso-alpha and polyphenols, measured the resultant BUs and had the beers evaluated by two test panels as well. The experimental samples consisted of kegs dosed with either zero or 10 mg/L of iso-alpha acid extract, and zero, 100 or 200 mg/L of hop-derived polyphenols. The beers where then analyzed for BU (ASBC method), total polyphenol and flavanoid content (EBC method), alpha acids and iso-alpha acids (HPLC). A partial summary of their results are shown in Table 1. A negligible amount (< 1 mg/L) of un-iso-alpha was present in all the dosed beers, and this data was not included for brevity. In other words, the dosing extracts were essentially pure iso-alpha and/or hop polyphenols. A caveat is that the dosings were prepared solutions and were measured and added by volume, and that each condition was added to a different keg, and thus there is some variation in the constituent measurements between kegs.
First, look at the numbers for the base beer: nothing added and 3 BUs consisting of 1.6 mg/L of iso-alpha and 66 mg/L of polyphenols. In the second condition, 100 mg/L of polyphenols are added and the BUs jump up to 16. Doubling the amount of polyphenols in the third condition increased the measured polyphenols count from 66 to 276, and moves the BUs to 26. A similar behavior is seen in the second group of dosings where 10 mg/L of iso-alpha are added to the three levels of polyphenol addition. The iso-alpha levels increase in-step with each addition as do the polyphenol levels, although there is a little more variation in that measurement than with iso-alpha. The punch line to the whole experiment though is the BU measurement of the last beer (10–200) — it measures 40 BUs even though it contains only 12.5 mg/L of iso-alpha!
An eleven member tasting panel partially confirmed those results, ranking the beers from most bitter to least, averaged over three repetitions, as: 10–200, 1–100, 0–200, 10–0, 0–100, 0–0. This ranking is interesting in that there appears to be a synergistic affect between iso-alpha and polyphenol so that the combination is more bitter than iso-alpha alone. It also appears that 0–100 mg/L is less bitter to the palate than 10–0, even though the BUs indicated the reverse. (Note: the tasting panel had been trained to differentiate between bitterness and astringency before the tastings.)
Now, you might be thinking to yourself, “So what, I don’t add hop polyphenol extract to my beer . . .” Ah, but you probably do. The polyphenol additions in the study were based on typical levels of polyphenols that can be found in high-adjunct lagers (60–140 mg/L), and highly-hopped American Northwest craft-brewed ales (160–400 mg/L). High BU levels, particularly from large additions of low-alpha hops in the boil, can put a lot of polyphenol in the beer. Dry hopping can result in quite a bit of polyphenol being extracted from the hops into your beer as well if you leave the hops in the keg or fermenter for an extended length of time (>1 week). Polyphenols are haze formers and many West Coast brewers claim that a lack of haze in a West Coast pale ale or IPA means that you didn’t have enough hops in the recipe.
There have been many discussions among homebrewers in recent years about dry hopping and perceived bitterness. One group might assert that no isomerization occurs because the hops are not boiled, that there will not be any bitterness contribution, only aroma and flavor. Another group might say that some small amount of isomerization is occuring, that there is a perceivable difference in bitterness, and that this difference is reflected in the BU measurements of dry hopped beers. In fact they are both right and both wrong. Alpha acids will isomerize at fermentation temperatures, but only at a negligible rate. As noted above, the hop polyphenol levels in highly-hopped northwestern ales can be as high as 400 mg/L, and based on the results of the tasting panel, this will have a definite effect on the perceived bitterness. The BUs for dry-hopped beers do increase, and this rise is due to polyphenol and raw alpha acid carried into the beer. This raises another question for homebrewers using hopbacks or dry-hopping their brews, “Do un-isomerized alpha acids add bitterness to beer?”
According to the Fritsch and Shellhammer paper, “Alpha Acids Do Not Contribute Bitterness to Lager Beer,” in J.ASBC 65(1): 26–28, 2007, a triangle test of 11 taste panelists, wearing nose plugs, determined that there was no significant difference between a control beer, and beers dosed with 14 and 28 mg/L of 90.9% pure alpha acids. The nose plugs were necessary because the people could discern more hoppy aromas in the dosed beers, and could thus taste the added alpha. Interestingly, in a subsequent tasting the panelists were blindfolded and tasted the dosed beers without noseplugs, but they could not reach a consensus on which beer was the most bitter, which further validated this result. The conclusion of the authors was that un-isomerized alpha acid does not add bitterness to beer. The increase in flavors and aromas noted by the panel in the dosed beers may be due to the 9% of non-alpha acid material in the extract, so this study did not determine whether alpha acid has other flavors besides bitter.
In conclusion, we have determined that hop polyphenols do occur at high levels in craft brewed beer, that hop polyphenols are significantly bitter, and that raw alpha acids may come from dry hopping or hopbacks will increase the BU but not the actual perceived bitterness. We can use this information to help plan the kind of hop character we want in our beers. Using a lot of low-alpha hops, such as East Kent Goldings or Cascades, will tend to contribute more polyphenol to the beer than a high alpha/high oil variety such as Centennial or Simcoe. If you are going to dry hop with a classic aroma variety, you should use a greater quantity but limit the contact time with the beer to a few days to get the maximum aroma and flavor impact while minimizing the amount of polyphenol extracted.
