Evaluation of colloidal stability in white and rosé wines investing Dynamic Light Scattering technology

Abstract

Proteins constitute one of the three main components of grape juice and white wine, phenolic compounds and polysaccharides being the others. A specific group of the total grape-derived proteins resists degradation or adsorption during the winemaking process and remains in finished wine if not removed by the commonplace commercial practice of bentonite fining. While bentonite is effective in removing the problematic proteins, it is claimed to adversely affect the quality of the treated wine under certain conditions, through the removal of colour, flavor and texture compounds. A number of studies have indicated that different protein fractions require distinct bentonite concentrations for protein removal and consequent heat stabilization. The lack of reliable tests assessing risk of protein clouding during bottle storage is a recurring problem of winemakers. The methods used to evaluate wine stability involve inducing haze formation (by heat, trichloroacetic acid, and ethanol) and then measuring of turbidity using spectrophotometers, turbidimeters or nephelometers. Currently used test assessing haze potential involves heating which often cause overdosing of fining agent. Moreover, it was shown that the composition of precipitate formed using above mentioned methods was not the same as naturally formed precipitate. A new and reliable method evaluating the haze potential, relevant to natural haze formation is needed. Different tests have been proposed to assess haze formation in wine. Most of these tests are based upon different types of procedures, leading to protein aggregation and precipitation. Heat stability trials, based on heat-induced precipitation, are the most common. These tests are empirical and do not necessarily reflect changes and destabilization phenomena liable to occur in real wine storage conditions. The fact that the same tests, associated with bentonite fining trials, are used to determine the bentonite doses needed to stabilize wines, leads to doses of bentonite much too high and consequently affecting wine quality. Given that the mechanisms underlying haze formation are still not fully understood, the aim of this work was to investigate the feasibility of using Dynamic light scattering (DLS) to understand the occurrence of haze formation, and the implication of wine compounds (protein, polyphenols, polysaccharides). DLS is a non-invasive, well-established technique for measuring the size and size distribution of molecules and particles typically in the submicron region, and with the latest technology lower than 1 nm. DLS directly measures fluctuations in scattering intensity due to Brownian motion, which are analyzed to determine the translational diffusion coefficient Dt and hence an effective measure of molecular size, the hydrodynamic radius Rh. DLS could also provide a rough measure of size distributions in order to assess populations of aggregates, and characterize the colloidal stability of wines.