Toward the optical tongue: flow-through sensing of tannin-protein interactions based on FTIR spectroscopy.

The interaction of polyphenols (tannins) with proline-rich proteins (gelatin) has been studied using an automated flow injection system with Fourier transform infrared spectroscopic detection to gain insight into chemical aspects related to astringency. In the perception of astringency, a major taste property in red wines and other beverages such as beer, tea, or fruit juices, an interaction between proline-rich salivary proteins and tannins present in the sample takes place. To study this interaction, agarose beads carrying gelatin (a proline-rich protein) were placed in the IR flow cell in such a way that the beads were probed by the IR beam. Using an automated flow system, we injected samples in a carrier stream and flushed over the proteins in a highly reproducible manner. Simultaneously, any retardation due to tannin-protein interactions taking place inside the flow cell was monitored by infrared spectroscopy. Tannins of different sources (grapes, wooden barrels, formulations used in wine making) were investigated, and their flow-through behavior was characterized. Significant differences in their affinity toward gelatin could be observed. Furthermore, because of small but characteristic differences in the IR spectrum, it is possible to distinguish condensed from hydrolyzable tannins. Nonastringent substances such as alcohols, sugars, and acids did not show retention on gelatin. The selectivity of the flow-through sensor was also demonstrated on the example of red and white wine. In contrast to white wine, where no interaction could be observed, in red wine a major interaction of the red wine tannins was found.