As a globally traded commodity with significant economic and social influence, coffee stands out due to its unique flavor profile, marked by roasted aromas and bitter taste. At temperatures above 220 °C, green coffee beans are roasted to the final product of roasted coffee beans. However, during the roasting process, new compound classes form from naturally abundant precursors in green coffee beans, potentially contributing to coffee's bitter taste.
The bitter-tasting diterpene glycoside mozambioside is an Arabica-specific compound that degrades partly during roasting into new products. The newly formed roasting products were previously unknown, and their potential to contribute to coffees bitter taste was studied. Structural investigations were conducted to predict bioactivity of similar analytes from this compound class.
Mozambioside (1) was isolated from green Arabica coffee beans by methanolic extraction and preparative liquid chromatography. The pure isolated 1 was roasted without any matrix to generate roasting products, leaving only about 20% of the original mozambioside. By semi-preparative LC, five new degradation products were isolated in pure form and structurally elucidated using NMR spectroscopy, MS, UHPLC-PDA, and –ELSD detection: 17-O-β-D-glucosyl-11-hydroxycafestol-2-on (2), 11-O-β-D-glucosyl-16-desoxycafestol-2-on (3), 11-O-β-D-glucosyl-(S)-16-desoxy-17-oxocafestol-2-on (4), 11-O-β-D-glucosyl-15,16-dehydrocafestol-2-on (5), and 11-O-β-D-glucosyl-(R)-16-desoxy-17-oxocafestol-2-on (6). In an in vitro bitter taste receptor activation experiment roasting products 4, 5, and 6 revealed the most potent activation for TAS2R43 and -R46, showing even greater potency compared to the educt mozambioside. In silico molecular modeling explained how the small changes in the molecules and its functional groups greatly impact the interaction between analytes and the binding sites of bitter taste receptors. Using a 3-AFC experiment, a human panel of 10 participants studied the in vivo bitter threshold of potent activators 1, 4, 5, 6, and their aglycone 11-hydroxy-cafestol-2-on (8). Quantitative analysis was conducted using a UHPLC-QTrap-MS/MS device to detect the content of the target compounds in commercial coffee grounds, coffee brews, and coffee roasting series.

Dr. Roman Lang

Leibniz Institute for Food Systems Biology

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