Discovery of major bitter-tasting compounds from bitter gourd (fruit of Momordica charantia L.) by application of the sensomics approach

Bitter gourd (fruit of Momordica charantia L.) is a popular vegetable and medical herb known for its intense bitter taste. However, its major bitter-tasting compounds have not been fully characterized. This study aimed to decode its bitterness by application of the sensomics approach. Combining solvent extraction, sensory analysis-guided fractionation and purification with medium-pressure liquid chromatography and high-pressure liquid chromatography, nine bitter-tasting cucurbitane-type triterpenoids were isolated from bitter gourd. The structures of isolated compounds were determined by high-resolution mass spectrometry and nuclear magnetic resonance spectroscopy. The bitter taste recognition thresholds of isolated compounds were in the range from 1.4 mg/L ((23E)-3(1-O-malonyl-7(1,25-dihydroxycucurbita-5,23-dien-19-al, 9) to 19.1 mg/L (yeojooside C, 5) in 3% ethanol as determined by trained human assessors. Quantification with UPLC-MS/MS demonstrated their natural concentrations in bitter gourd fruits were in the range between 14.2 +/- 0.9 mg/kg (yeojooside C, 5) and 245.6 +/- 22.1 mg/kg (3-O-malonylmomordicine I, 10) on a dry weight basis. Calculation of the dose-over-threshold (DoT) factors showed three compounds, namely 3-O-malonylmomordicine I (10), momordicine I (3), and (23E)-3(1-Omalonyl-7(1,25-dihydroxycucurbita-5,23-dien-19-al (9) were the bitter principles in bitter gourd fruits. On the other hand, momordicoside K (2), which had been known as a bitter principle of bitter gourd for more than forty years, had been proven to be an extraction and isolation artifact rather than a natural component of the plant. Besides, an acid-catalyzed isomerization reaction in the side chain of cucurbitane was discovered, which might explain the formation mechanism of many cucurbitane-type triterpenoids from bitter gourd.