By Erin Holden
From grapefruit to coffee to arugula – lots of plants are bitter. And while some people will cross the street to avoid a bitter veggie, others can’t seem to get enough. Why is this? Join me on a two part journey as I delve into the science and use of one of nature’s most divisive flavors.
The What: How do you define a bitter? If you’re a chemist, they’re structurally unrelated compounds that, well, taste bitter. If you’re an herbalist, bitters are plants that are used for their therapeutic effect on the digestive system. And if you’re a bartender, bitters are aromatic flavoring agents made from various plant parts to enhance the taste of a drink. Based on these definitions it’s easy to see that bitters fulfill many roles in our lives. But why can we taste bitter to begin with?
The Why: The main reason plants create bitter compounds boils down to one simple thing: defense. Since plants can’t run and hide from predators, they devised their own chemical defense system – the first instance of chemical warfare, you could say. When an insect bites into, say, a broccoli plant, it gets a mouthful of glucosinolates, sulfur containing compounds that are toxic to insects and rodents, and gross to some people. Plants see us as essentially giant rodents that want to eat them, and create a wide variety of nasty tasting compounds to deter us from doing so. In turn, since some of these compounds not only taste bad but can also kill humans, we evolved taste receptors to detect them, therefore potentially skirting death.
The Where: So, where exactly do we have these taste receptors? I’m sure many of you learned the “flavor map” of the tongue in school, like I did, which outlines where we can detect sweet, sour, salty, and bitter (poor umami got left out on that one). Well I’m here to tell you, that map is bunk! It’s based on the misinterpretation of a graph from 1942, that plotted out areas of relative sensitivity of the tongue to the various flavors. Later on, low areas of sensitivity were mistakenly interpreted as having no sensitivity, leading to the incorrect map we’re all familiar with today. All flavors can be detected across the whole tongue, the back of the roof of the mouth, and the epiglottis. Other research has suggested that while areas of the tongue do show varying levels of sensitivity to the five known flavors, these differences are not significant (Wanjek, 2006).
Having taste receptors on the tongue is no surprise, but you may be surprised to learn bitter receptors have also been found in the brains, airways, gastrointestinal tract, testes, and pancreas of various mammals (including humans). This leads back to WHY? The prevailing hypotheses boil down to the same thing – protection. If our airways are full of bacteria that are producing bitter compounds, our cough response is triggered, so we cough up and expel the offending invader. If an ingested bitter toxin makes its way to our guts, it’ll essentially trip the “abandon ship” alarm, causing our body to purge itself of the potential poison in unpleasant ways.
I’ll stop there with that thought. Keep an eye out for the follow-up, where I’ll talk about the historical uses of bitters, their role in herbal medicine, and mention some specific bitter herbs.
Wanjek, C. 2006. The tongue map: Tasteless myth debunked. Live Science. From Livescience.com. Accessed January 31, 2019. Available from https://www.livescience.com/7113-tongue-map-tasteless-myth-debunked.html on 1/31/2019
Medicinal Disclaimer: It is the policy of The Herb Society of America, Inc. not to advise or recommend herbs for medicinal or health use. This information is intended for educational purposes only and should not be considered as a recommendation or an endorsement of any particular medical or health treatment. Please consult a health care provider before pursuing any herbal treatments.
Erin Holden is the gardener for the National Bonsai and Penjing Museum at the U.S. National Arboretum in Washington, D.C. She is a member of the American Herbalists Guild, United Plant Savers, and a member-at-large of the Herb Society of America.