Effect of bentonite characteristics on wine proteins, polyphenols, and metals under different pH conditions

Studies have yet to evaluate how bentonite properties may affect the protein profile, the polyphenol content, the metal concentration, and the heat stability of a white wine under different pHs. Therefore, this paper assessed the proteins, polyphenols, metals and haze forming tendency when heating white wine samples before and after a fining treatment with four activated sodium bentonites in a typical wine pH-range (3.00-3.60). Soluble wine proteins were separated by SDS-PAGE. Gel images were compared using the QuantityOne software package. The wine haze forming tendency, metals, and polyphenols were measured using heat tests and O.I.V. methods. Low molecular mass proteins were efficiently removed by all of the bentonites, regardless of the pH. High and medium molecular weight proteins were generally less likely to be removed with a variable efficiency that depended on the pH. Reductions of vacuolar invertase (GIN1) and VVTL1 fractions of the thaumatin-like proteins (TLP) were induced by bentonites having a pH lower than 10. These bentonites were affected to a lesser extent by the negative effect of acidic pHs. The reduction in haze forming tendency of the unfined Erbaluce wine was particularly noticeable in bentonite fined samples heated in the 50-60°C, 60-80°C, and 70-80°C range at pH 3.17, pH 3.30 and pH 3.60, respectively. The lower removal of glycoproteins (YGP1 and Hmp1) at higher pHs contributed to an increased thermal stability. The exchange of cationic species, notably Na and K, between the bentonites and the wine was more related to the wine pH than to the clay type. Finally, the extent of polyphenol removal was strongly correlated to the amount of proteins removed. When protein removal did not occur, the reduction of polyphenols was driven by the specific surface area and the surface charge density of the bentonite.