One can hardly speak about the chemical complexity in cocktails without mentioning bitters, the spices of cocktails according to some. Extracting aroma- and taste-carrying chemicals with high-proof alcohol from, mostly, plant material results in an array of concoctions which can give a simple drink an entire layer of flavor depth with a mere dash or two. After a hiatus of several decades, bitters have resurfaced in recent years and diversified exponentially from the few initial commercial bitters. While most of the candidates produced by this Cambrian explosion-like phenomenon will presumably peter out, some innovative and crafty cocktail bitters are here to stay. As the panoply of available bitters can be confusing at first, zooming in on the constituents as well as functions in a given cocktail may result in an improved usage when preparing a drink.

If bitters are the spices of cocktails, then Angostura bitters surely would be the equivalent of salt. It is both the oldest bitters which was widely commercially available (since the early 19th century) as well as the most frequently used bitters in cocktails. In fact, some people have developed such a taste for it that they developed cocktail recipes with Angostura bitters as their base spirit. These include the Angostura Sour from Kirk Estopinal as well as the Trinidad Sour from Giuseppe Gonzalez. As bitterness to a certain extent quenches acidity and therefore allows all the other aromatics to shine, the usage of Angostura bitters in sour cocktails seems like a natural choice to derive synergy.

Classification of cocktail bitters and their molecular characteristics.

Traditionally, bitters mostly use gentian or cinchona bark as a source of bitter taste (additionally contributing aroma molecules) and then incorporate additional spices, fruits, herbs etc. for their final flavor. The aromatic focus of the respective bitters allows for a classification into several broad bitters categories (at least for the majority of available bitters). Aromatic bitters (such as Angostura) showcase spice aromas such as cinnamon or cardamom, New Orleans-style bitters (such as Peychaud’s) have a heavy anise note, citrus bitters (such as Regan’s Orange Bitters) are characteristic of the respectively used citrus fruit, mole bitters (such as Xocolatl Mole Bitters) focus on cinnamon, chili and chocolate aromas, tiki bitters (such as ‘Elemakule Tiki Bitters) evoke falernum and allspice dram aromatics and celery bitters make use of the characteristic aromatics from celery seeds.

Scientists at UC Davis recently analyzed the volatile components of 16 commercially available bitters from the abovementioned categories both by sensory analysis through panelists as well as quantitatively by mass spectrometry. On average, each bitters sample contained 46 different aroma molecules (ranging from 19 up to a staggering 78) which make up their complex aroma and taste. Interestingly enough, from the 148 compounds they monitored only 23 were unique to one sample of their bitters array. Some molecules, such as decanal (described as having a ‘waxy orange peel’ aroma) were even present in all 16 bitters. However, as you might expect, a true orange bitters such as Scrappy’s orange bitters contained about 450 times more decanal than Peychaud’s bitters for instance. This implies that the majority of aroma molecules is at least partially shared between bitters, but of course they can drastically differ in their respective concentration. Thus, the proportions between aromatic ingredients seem to matter more than their absolute presence or absence.

Of course, some aromatics also stem from the source of bitterness used for the bitters. Decanal, limonene (citrus) and anethole (anise/fennel), which are present in nearly all tested bitters, can also be detected in the volatile profiling of gentian and cinchona bark, explaining their prevalence in bitters. In fact, the number of different aroma molecules has been shown to correlate with the perceived odor complexity as well as the number of aroma descriptors used by the panelists. If you have ever asked yourself what the ‘average’ bitters tastes like, aromatic bitters such as Angostura and Boker’s pretty much fit the bill. On a diagram with the different aroma descriptors, they are located squarely in the middle, with no aroma descriptor dominating the mix. Based on these diagrams plotting the aroma dimensions of the bitters, the authors of this study also suggest new potential avenues in the world of bitters. Combining ‘aromatic opposites’ in their diagram would for instance yield bitters with pronounced notes of chocolate and grapefruit or with vegetal ‘green’ aromatics and notes of ginger.

The aromatic interplay of whiskey & bitters in an Old Fashioned.

However, it is rarely the case that bitters are consumed on their own. Thus, their interactions with other substances in a cocktail have to be considered. This is why in 2017, researchers at UC Davis and Penn State conducted a study how bitters interact with other ingredients in a basic cocktail such as the Old Fashioned. For this they used four bitters (Angostura, Peychaud’s, Xocolatl Mole and Regan’s Orange Bitters) and four whiskeys (rye and bourbon, for each a pricey and a budget version) and prepared as well as analyzed the 16 Old Fashioned combinations. One immediate conclusion that they drew is that the chosen bitters mattered quite a bit: Peychaud’s and Xocolatl Mole Bitters completely dominated the Old Fashioned, preventing the panelists to taste differences between the four whiskeys that were used. In contrast, Angostura and Regan’s Orange Bitters accentuated the differences between the four whiskeys and resulted in eight quite different Old Fashioneds.

Evidence for the interaction of the bitters with the whiskeys can be for instance seen in the interaction of the orange bitters with the expensive rye, where it suppressed oak aromas and synergistically enhanced cola-like aroma. On another note, preparing the Old Fashioneds with some bitters made the expensive whiskeys more differentiable and complex than the cheaper whiskeys (more so than their comparison in the pure form), thereby belying the notion that one should not mix top-notch spirits. Considering their major impact on the drinks we make, bitters are surely something that needs to be explored further in the future.