Major Bitter-Tasting\nCompounds from the Dichloromethane\nFraction of Bitter Gourd (Fruit of Momordica charantia L.) Extract and Their Precursors

The unripe fruit of the plant Momordica\ncharantia L., commonly known as bitter gourd or bitter\nmelon, is a popular\nvegetable and medical herb in many parts of the world and is characterized\nby its strong bitter taste. In our endeavor to decode its bitter taste,\nthe dichloromethane fraction of 75% methanol extract of bitter gourd\nwas found to be intensely bitter. Combining sensory analysis-guided\nfractionation and newly developed comparative high-pressure liquid\nchromatography (HPLC) analysis-guided purification led to the isolation\nof five known compounds including momordicoside L (<i><b>1</b></i>), (23<i>E</i>)-3β-<i>O</i>-malonyl-7β,25-dihydroxycucurbita-5,23-dien-19-al\n(<i><b>2</b></i>), 3-<i>O</i>-β-d-allopyranosyl-7β,25-dihydroxycucurbita-5,23(<i>E</i>)-dien-19-al (<i><b>3</b></i>), momordicine\nIV (<i><b>5</b></i>), and charantoside B (<i><b>6</b></i>) and three new compounds 3-<i>O</i>-β-d-3-ketoglucopyranosly-7β,25-dihydroxycucurbita-5,23(<i>E</i>)-dien-19-al (<i><b>4</b></i>), 6′-<i>O</i>-malonylmomordicoside L (<i><b>7</b></i>), and 6′-<i>O</i>-malonylmomordicine IV (<i><b>8</b></i>) from bitter gourd. Sensory analysis revealed\ncompounds <i><b>3</b></i>–<i><b>8</b></i> had strong bitter taste with their bitter taste recognition\nthresholds in the range between 3.6 (<i><b>4</b></i>) and 13.6 ppm (<i><b>6</b></i>) in 3% ethanol. UPLC-MS/MS\nquantification showed that their concentration in bitter gourd ranged\nfrom 16.5 ± 1.3 mg/kg (<i><b>4</b></i>) to 214.8\n± 14.0 mg/kg (<i><b>6</b></i>) on a dry weight\nbasis. Calculation of the dose-over-threshold (DoT) factor showed\nthat momordicine IV (<i><b>5</b></i>) and charantoside\nB (<i><b>6</b></i>) should be considered as bitter\nprinciples of bitter gourd. In addition, the study also demonstrated\nthe ubiquity of the isomerization reaction in the side chain of cucurbitane-type\ntriterpenoids. Many isolated compounds were the isomerized products\nof their natural precursors, and these precursors should be the primary\nbitterness contributors of fresh fruits. In addition, comparative\nHPLC analysis-guided purification could be a practical approach for\nthe fast isolation of acid-labile precursors.