Protein Instability of Wines: Influence of Protein Isoelectric Point

Abstract

Proteins in Gewürztraminer wine were separated by an improved method of chromatofocusing using an FPLC system in conjunction with characterization by isoelectric focusing. Bentonite fining resulted in removal of all the different protein fractions. There was no bentonite selectivity based on isoelectric point (pI). The amount of protein depletion correlated approximately linearly with the level of bentonite addition. Five groups of proteins of differing pI were assigned after characterization of 35 individual FPLC fractionated proteins by isoelectric focusing. The five groups of proteins were added back separately to protein-free wine. Unfractionated protein was similarly added to fined protein but not phenolic-free wine and to tartrate buffer (pH 3.14). All additions of protein were at a concentration of approximately 75 mg/L, a level at which considerable haze was visible after heating bentonite fined wine. All protein groups were thermally unstable contributing to haze/sediment formation. Marked differences in the precipitation/coagulation of the protein groups after a heat/cold test were observed. The high pI (≥7.0) protein groups developed a compact sediment; the middle pI (predominately 5.94 to 5.36 and 5.36 to 4.65) groups a flocculated precipitate 4 to 5 times larger than that of the high pI groups, and the low pI (<4.65) group a suspended haze. The two samples containing all pI proteins also developed compact sediments. The interaction of proteins with other components in wine, primarily phenolics, must be considered in order to determine factors resulting in instability of wine.