Sensory and Instrumental Astringency Determination in Wine.

This thesis is concerned with factors affecting the perception of astringency in wine and in model wine solutions. Since wine composition is complex, model solutions were also studied, so that the observations would contribute to the elucidation of the astringency mechanism. The introduction reports current understanding about astringency and astringent compounds published in literature. Many of the natural astringent compounds of wine are not commercially available. Accordingly, chapter 2 explores an extraction method of phenols from grape seeds and demonstrates the effect of different solvents. Chapters 3 to 5 report the contribution of small molecules and the effect of structure and pH on sensory astringency. In chapter 6, the saliva composition before and after assessing astringent compounds has been correlated with time-intensity parameters of astringency. Chapter 7 quotes the non-sensory phenol-protein interactions. Finally, in chapter 8 the factors which influence the perception of astringency and phenol-protein precipitation are discussed. The sensory studies suggest that astringency is a complex sensation elicited by large phenols, monomers, and even non-phenols such as acids. The mechanism of astringency is probably common among the phenolic compounds whereas in astringency of acids, a second mechanism might be involved. Stereochemistry of molecules and pH affect the perception of astringency. Phenol precipitation in vitro depends on whether phenols are in a pure solution or in a mixture where they must compete for proteins. However, phenol precipitation is not a good indication of astringency. Astringency is probably related to a phenol-mediated salivary protein precipitation, but it is not correlated with total protein precipitation. The relative concentration of individual saliva proteins might be more important for astringency than the total protein concentration. pH and protein to phenol ratio can modify phenol-protein precipitation. At high pH and excess protein, soluble complexes can be formed which could mask astringency