Bitterness is one of four individual tastes that are sensed by different areas of the human tongue. The others are sweet, sour, and salty, which require no explanation, but bitterness is often confused with the physical sensations of burning and astringency or dryness. Just as vintners must balance the acid:sugar ratio to produce a wine with drinkability, brewers must balance the sweetness derived from the malt with bitterness derived from hops and sometimes other sources. If the sweetness is not counterbalanced with bitterness, the beer will seem sweet and flabby tasting on the palate and the drinker will tire of the beer quickly. Whether very strong or barely perceptible, bitterness is critically important to beer flavor.
The alpha acids are the main source of bitterness from hops. Iso-alpha acids, produced from the alpha acids during the wort boil, are the main source of bitterness in beer.
Simple methods to determine iso-alpha acid content in beer are not used to measure bitterness because of the contribution of these other compounds. Industry dogma is that the standard IBU test is the best analytical method to measure perceived bitterness in beer.
Note that the term “alpha acids” is plural. This is because there are three main alpha acids and several minor ones. The major alpha acids are humulone, cohumulone, and adhumulone.
The beta acids are of secondary importance only when compared with the alpha acids. The beta acids are completely insoluble in cold water and beer and, unlike the alpha acids, do not isomerize to more water-soluble compounds when boiled. The beta acids themselves do essentially nothing in the brewing process, but their oxidation products formed as baled whole hops age are arguably more bitter than the iso-alpha acids, are quite water soluble, and end up in finished beer at high utilization rates.
This does not mean that the bittering profile from aroma hops does not change as the baled hops oxidize. The measured IBUs and the intensity of the bitterness of beer made with aged aroma hops will be roughly the same as that of beer made with the same amount of fresh hops, but the bitterness of the former will be harsher and more lingering. Just as there are qualitative differences in sweetness from different sources (aspartame, sugar, saccharine) and different kinds of spicy heat from various chilies, there are qualitative differences in bitterness from hops. The bitterness from the iso-alpha acids from fresh hops is very different from the bitterness of deteriorated hops or from so-called reduced iso-alpha products. These differences are not trivial.
As beer is exposed to light, it will develop a sunstruck or “skunky” aroma if the beer contains iso-alpha acids.
The relation between measured IBUs and perceived bitterness no longer holds in beers made with these products. Beers made with rho products tend to have lower perceived bitterness than their measured IBUs would indicate; beers made with tetra products have higher bitterness than expected.
Alterations in bitterness occur not only because of the aging of hops but also during staling of beer. The iso-alpha acids in beer are sensitive to oxygen and are removed as beer ages. The rate of deterioration is a function of time, temperature, initial oxygen content in the package, and how quickly oxygen enters the sealed package. Generally, beer will lose about 20% of the iso-alpha acids (and about 15% of the measured IBUs) after 8 months at room temperature. The resulting oxidation products are not detected by the IBU analysis and are not considered bitter. Hence, as beer ages, the bitterness decreases.
Both hops and malt are sources of polyphenols in beer. This is a wide class of compounds with a number of important characteristics. Some polyphenols can complex with proteins in beer and form haze. Others have antioxidative properties that may retard the staling of beer. Polyphenols also add astringency, or a quality of dryness, to beer. This latter property is often confused with true bitterness. Some polyphenols are believed to modify the bitter impression of the hop acids, mellowing the beer while adding complexity and body.
When hops are added to the brew kettle, most of the hop oil is distilled out quickly with the evolving steam. Enough may remain to contribute to the aroma of the beer, but not enough to contribute to bitterness. But, when hops are added during or after fermentation, hop oil is dissolved directly into the beer, resulting in a strong aroma of hops and often an oil “burn.”
The iso-alpha acids are weak acids. This means they exist in two forms in solution—the dissociated ionic form and the non-dissociated form. The ionic form, which is more abundant at higher pH, is more bitter than the non-ionic form. As a result, the bitter perception of beer is reduced at lower pH.
Many consider the taste of alcohol bitter. Higher concentrations of alcohol in wine are reported to increase perceived bitterness, but only at concentrations above 10%. Studies of this in beer are lacking, perhaps because the effect is small, below 10%. Alcohol would tend to suppress the dissociation of the weak iso-alpha acids mentioned in the previous paragraph, which would, in theory, reduce bitterness. This may counteract any increase from the alcohol itself.
The color compounds formed as malt is roasted will contribute to the bitter taste of beer, just as the roasting of coffee beans gives bitter flavor to espresso coffee. In beers made with very dark malts, this can be a significant portion of overall perceived bitterness. The color compounds from dark malts are more water soluble in general than the iso-alpha acids, so they do not contribute as much to the measured IBUs. It is rare that they would increase measured IBUs by more than 2–3 units. Their contribution to perceived bitterness can be much greater than this. Some stouts may derive as much of their bitter character from roasted malts as from hops.
Non-distilled water contains varying amounts of dissolved minerals. Many of these can have profound effects on the flavor of the water and also on the flavor of beer brewed with it. Brewers have long known that beers brewed from water containing high levels of calcium sulfate will tend to exhibit pleasantly crisp, snappy hop bitterness. Conversely, those brewed from alkaline water containing high levels of calcium carbonate will tend to show broader, coarser hop bitterness. This is among the reasons that many brewers of pale ales and India pale ales often add calcium sulfate to their brewing water or directly to the kettle; this is called “Burtonization,” after the famously sulfate-rich waters of Burton-on-Trent in the British midlands. Very soft water is said to give a delicate quality to bitterness, making such water suitable for brewing classic pilsner.
Lower temperature suppresses the perceived bitterness of all beverages—beer is no exception. For this reason, different styles of beer should be served at different temperatures for maximum enjoyment. The bitterness of a beer once served may increase, if it is allowed to warm substantially before it is finished.
Carbonation affects the flavor of beer in a number of ways. When carbon dioxide is dissolved in water, it forms carbonic acid. This increases the titratable acidity of the beer, increasing sourness. Also, as carbon dioxide bubbles are formed on the tongue, a characteristic “bite” is sensed, even in carbonated water. The higher the carbonation level, the more bite. The bite and sourness generated by high carbonation may increase perceived bitterness.