NMR based studies on odorant melanoidin interactions in coffee beverages
Creators: name: Gigl, Michael
name: Frank, Oliver
name: Hofmann, Thomas
Publisher: name: Zenodo
Year: 2021
Subjects: NMR
offee aroma staling
sensory experiment
covalent interaction
π-π interaction

description: Freshly brewed coffee is appreciated by consumers all over the world because of its stimulating effect, its characteristic taste centring on sourness and pleasant bitterness and its alluring aroma with characteristic “roasty/sulphurous” odour notes, culminating in the unique flavour sensation of coffee. The molecules responsible for the olfactory sensation of roasted coffee beans and percolated coffee beverages, analysed by means of the molecular sensory science approach are well understood. With previous studies providing qualitative and quantitative data, using aroma extract dilution analysis (AEDA), gas chromatography-olfactometry (GC-O) and headspace GC-MS techniques resulting in comprehensive aroma recombinates consisting of not more than 30 odorants [1-3]. Although the aroma of coffee can be reconstituted rather well, the impact of the melanoidin containing high molecular weight fractions (HMW) on the sensory quality of coffee beverages, is still mostly unclear on a molecular basis. Whereas former studies clearly indicated that especially odour active thiols exhibit high binding affinity to high molecular weight melanoidin fractions of coffee, only covalent interactions have been considered so far. The impact of non-covalent π-π interactions on coffee flavour is still completely unclear [4, 5]. To get detailed insight into the molecular phenomenon of odorant polymer interactions and the sensory impact on coffee flavour perception, a quantitative 1H-NMR based screening approach was developed, which allowed the direct and non-invasive analysis of molecular interactions between key coffee odorants, like 2-furfurylthiol and high molecular weight melanoidin polymers (&gt;10 kDa). A clear distinction between covalent and non-covalent interactions was achieved by monitoring time dependency of odorant polymer interactions, with 2-furfurylthiol exhibiting π-π interactions as well as covalent bindings. In contrast, pyrazines and hydroxyphenols showed only non-covalent π-π stacking, whereas aldehydes incubated with HMW material showed only covalent interactions at prolonged incubation times. Furanones, as well as diketones showed no interactions with the HMW. Human sensory experiments with isolated HMW material &gt;10 kDa and a full aroma recombinate of coffee were well in the line with the findings from the NMR based approach. A drastic reduction of “roasty/sulphurous” aroma notes in combination with a decrease in overall coffee-like odour quality, as well as an increased “sweetish/caramel-like” flavour was perceivable upon incubation of coffee melanoidins with the aroma recombinate. The lack of binding affinity of the sweetish/caramel smelling 4-hydroxy-2,5-dimethyl-3(2H)-furanone in combination with the high binding affinity of coffee thiols provides explanation of the sensory evaluation and might be the reason for the fast disappearance of the “roasty/sulphurous” aroma impressions of a freshly prepared coffee brew.