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, polyphenol content, metal concentration, and heat stability of a white wine at different pH values. Therefore, this work assessed the proteins, polyphenols, metals, and haze forming tendency when heating white wine samples before and after a fin- ing treatment with four activated sodium bentonites in a typical wine pH range (3.00 to 3.60). Soluble wine proteins were separated by sodium dodecyl sulfate - polyacrylamide gel electrophoresis, and gel images were compared using the Quantity One software package (Bio-Rad Laboratories, Inc., Hercules, CA). The wine haze forming tendency, metals, and polyphenols were measured using heat tests and International Organisation of Vine and Wine (OIV) methods. Low molecular mass proteins were efficiently removed by all of the bentonites, regardless of the pH. High and medium molecular mass proteins were less likely to be removed and the efficiency, which depended on the pH, was variable. Reductions of vacuolar invertase (GIN1) and VVTL1 fractions of the thaumatin-like proteins were induced by bentonites with pH values less than 10. These bentonites were affected to a lesser extent by the negative effect of acidic pH. The reduction in haze forming tendency of the unfined Erbaluce wine was particularly notice- able in bentonite fined samples heated at 50 to 60°C, 60 to 80°C, and 70 to 80°C at pH 3.17, pH 3.30, and pH 3.60, respectively. The poor removal of glycoproteins (YGP1 and Hmp1) at higher pH values contributed to an increased thermal stability. The exchange of cationic species, notably sodium and potassium, between the bentonites and the wine was related more to the wine pH than to the clay type. Finally, the extent of polyphenol removal correlated with the amount of protein 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.