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34 p r a c t i c a l w i n e r y & v i n e ya r d J a n U a r y 2 0 1 5 W I N E M A K I N G process includes both hydrophobic inter- actions and hydrogen bonding, with the bottom line effect being that tannin likes to "stick" to protein. The precipitation of salivary proteins by tannins has been considered to create a loss of lubricity in the mouth, which is responsible for the sensation of astrin- gency. Studies have shown that tannins isolated from skins interact differently with proteins than tannins extracted from seeds. 20 Tannin structure variation has been shown to influence the extent of hydro- phobic interaction and it can be quan- titatively measured. It is hypothesized that the extent of hydrophobic reaction between tannin and salivary protein is related to the ability of hydrophobic sur- faces to interact (Figure 3). To conceptualize tannin stickiness, the following is proposed. A young tannin that would be typical of a pre-veraison grape would have a more rod-like structure that would have more surface available for interaction with salivary protein. However, more modified tannins in matured wines would contain more complex i n t r a m o l e c u l a r bonds and struc- ture thus devel- oping a more globular shape due to structure m o d i f i c a t i o n , resulting in less available sur- face for interac- tion. The goal of our research is to develop analyt- ics that can more routinely measure the ability of tannin to interact with a hydrophobic surface (hereafter referred to as stickiness). Recent developments in determin- ing the shape of tannin structures pro- vides supporting evidence for this shape transformation. 10 As stated above, tan- nin shape modification with red wine age is postulated to ultimately lead to a reduction in the surface available for interaction between tannin and protein, resulting in the tannin being less sticky (Figure 3). This transformation is con- sistent with sensory evaluation of older wines and the "softening" effect that occurs. With this interaction in mind, a high performance liquid chromatogra- phy (HPLC) method was developed to measure tannin stickiness. In an effort to measure stickiness, and based upon earlier findings, 20 an HPLC method that measures the extent to which tannin interacts with an HPLC column, was developed. 21 The HPLC method consists of a polystyrene divi- nylbenzene column that would only allow for hydrophobic interactions — the same chemical interaction during the initial stage of tannin-protein inter- action, and was modified from a previ- ously published method. 22 After initial success, this HPLC method was further modified and tested. 23 Tannin stickiness is concentration- independent and therefore adds to the analytical information available. To acquire stickiness information for the tannin in question, each sample is analyzed at four different column tem- peratures, and from this temperature response, the stickiness of the tannin towards the HPLC column can be cal- culated. 24 In addition to stickiness, this HPLC method can also provide concen- tration information. 23 Tannin stickiness is unique and different than concentration Operating under the hypothesis that stickiness is a property of the tannin rather than a function of its concentra- tion in the system; it was predicted that stickiness would not be affected by con- centration. To investigate this, wine or tannin solutions were serially diluted across a large concentration range (from 7408-225 mg/L) and the results showed ment in bottle is generally considered to influence perception by modifying tannin interaction through structure and shape modification. 10 Protein-tannin interaction, stickiness concept In order to better understand the role of tannin structure in red wine astringency, it is helpful to understand how tannin interacts with other molecules, including proteins. 11,12,13,14,15,16 From previous studies on the interac- tion of tannins with model proteins, 17,18,19 a three-stage process can be proposed where the initial stage of interaction is consistent with a hydrophobically-driven interaction (a chemical force that drives the molecules together). 20 Subsequent stages including aggregation of tannin- protein complexes (the second stage), and precipitation of aggregates (the third stage). Based upon current evidence, this Figure 2. The management of red wine mouthfeel generally encompasses the management of the attributes shown above. Figure 3. Model conceptualizing tannin hydrophobic interaction with salivary protein. In this model, a grape-derived tannin (left), a proline repeat that is particularly abundant in salivary proteins (middle) and an aged tannin (right). Purple ovals indicate regions available for surface interaction.