Bitter gourd, cucumber, etc. are bitter to taste. Which chemical causes bitterness in them?
I found these when I searched online: this says that it is because of calcium, while this and this talk about physical effects.
Answer
Bitter taste is sensed by bitter sensitive gustducin receptors (T2R family). There are different types of bitter receptors and they can be triggered by different kinds of ligands. Different classses of phytochemicals that can trigger bitter taste, are reviewed by Drewnowski and Gomez-Carneros (2000).
The bitter tasting phytochemicals include phenols, terpenoids, alkaloids, flavonoids, glucosinolates and isothiocyanates.
Drewnowski and Gomez-Carneros' review (2000) and @anotherHomosapien's answer provide examples for these different classes of compounds. I guess they have missed alkaloids; so I give some examples: theophylline, theobromine (from chocolate) and caffeine (from coffee); these all are isomers of N,N'-dimethyl xanthine and taste bitter. Many alkaloids are toxic. Different bitter tasting alkaloids are reviewed in this chapter (Fattorusso and Taglialatela-Scafati, 2008. Chapter 3. Modern alkaloids. Weinheim: Wiley-VCH. ISBN:9783527621071).
High amount of salt may also taste bitter. The sensation of high salt concentration is transduced by amiloride-insensitive channels which are different from the amiloride-sensitive ENaC which sense low sodium concentration. Amiloride insensitive channels can also sense cations other than sodium. This taste is aversive in contrast to that due to low sodium concentration. Some of the bitter sensing taste cells have amiloride-insensitive channels (Lewandowski et al., 2016). However, I cannot comment if these channels elicit a bitter taste response.
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