Proteins in White Wines: Their Interaction With Tannins And Aroma Compounds

The presence of protein in white wines represents a major problem for the wine industry mainly due to the fact that proteins generate haze in the bottled white wines. Protein instability, which results in wine haze formation, is due to some grape PR-Proteins that thanks to their intrinsic resistance survive the vinification process, pass into the wine where cause the appearance of undesirable haze and deposits, leading to rejection by consumers. Protein hazing of white wines is considered to be a three-step process, involving protein denaturation followed by aggregation into colloidal particles able to scatter the visible light and make the wine turbid. Because of the complexity and the variability of the wine matrix, the factors and mechanisms involved in this process are still largely unknown. Commonly, winemakers prevent haze formation by removing the proteins through the use of bentonite. However, this treatment causes loss of wine and, being unspecific, also the removal of some aroma compounds. It has been calculated that the total cost deriving from bentonite treatments corresponds to a worldwide total amount of 1 billion dollars per year. Therefore, basic and applied research is still needed to solve the problem of protein haze formation in white wines. Firstly, the present thesis faces the problem of the impairment of aroma due to bentonite fining. In particular, the study arises from a previous investigation which suggested the existence of an interaction between proteins and aroma compounds. In this context, the interaction of the main wine protein VVTL1 with some fatty acid ethyl esters (FAEE), which are important fermentative aroma compounds has been investigated. Due to the difficulty to determine this interaction at the molecular level, Synchrotron Radiation Circular Dichroism (SRCD) has been used to study the secondary structure of the wine protein as affected by the interactions with FAEE having different chain lengths. Subsequently, the research continued with the investigation of the role played by tannins in the phenomena leading to protein instability of white wines. To this purpose, the effects of several polyphenols (deriving from wine and not) on the stability of VVTL1 has been investigated using SRCD. In parallel, the capability of tannins to react with the proteins over time in bottled wine has been evaluated by Dynamic Light Scattering (DLS) studies in a model wine system. In addition, the thermal stability of two purified proteins, which are representative of the major classes of proteins in white wine (i.e. a class IV Chitinase and the VVTL1), has been investigated by Differential Scanning Calorimetry (DSC) in the presence of the tannins purified from wine at different times after bottling. Finally, the last part of the research focuses on the possibility to produce good quantities of grape proteins in pure form starting from the in vitro culture of berry pulp tissues. These proteins can be used for molecular and functional characterisation. In particular, with this technique it is possible to label the proteins by cultivating the cellular tissues in the presence of N15 which allows the study of their fine structure and interactions by spectroscopic methods.