Heat-Unstable Proteins in Wine. I. Characterization and Removal by Bentonite Fining and Heat Treatment

Abstract

The effect of bentonite fining on the total proteins and the heat-unstable (80°C, 6 h; 4°C, 12 h) proteins in Gewürztraminer, White Riesling, Sauvignon blanc, and Sylvaner wines was investigated. Protein molecular weights (MW), isoelectric points (pl), and glycoproteins were determined by using lithium dodecyl sulfate polyacrylamide gel electrophoresis (LDS PAGE) and two-dimensional isoelectric focusing-lithium dodecyl sulfate (IF-LDS) electrophoretic techniques with silver staining as well as protein blotting for glycoprotein detection. Relative concentrations of proteins in stained gels were determined by laser scanning densitometry. Bentonite fining tends to remove the higher pl (5.8 - 8.0) and intermediate MW (32 000 - 45 000) protein fractions first. However, these represent only a small proportion of the soluble proteins. In general, it is necessary to remove the lower pl (4.1 - 5.8), lower MW (12 600 and 20 000 - 30 000) fractions, which contain glycoproteins and represent the major component of the proteins, to protein stabilize wines to heat testing. Protein fractions with MW of 60 000 to 65 000 and having a wide range of pl (4.1 - 8.0) were highly resistant to removal by bentonite fining and remained in protein-stabilized wine. In addition, trace amounts of fractions with MW of 28 000 may remain in Gewürztraminer and White Riesling wines and 25 000 in Sauvignon blanc wines. Unstable proteins precipitated by heat tests were recovered and analyzed. Protein fractions with MW of greater than 14 000 were more heat sensitive than lower MW fractions. The heat-precipitated proteins found in sediments were mainly of low MW (< 30 000) and primarily glycoproteins. It is concluded that the protein fractions of lower MW (12 600 and 20 000 - 30 000) and lower pl (4.1 - 5.8) and glycoproteins are the major and most important fractions contributing to protein instability in wines.