PROTEIN INSTABILITY OF WINES - INFLUENCE OF PROTEIN ISOELECTRIC POINT

Proteins in Gewurztraminer wine were separated by an improved method o f chromatofocusing using an FPLC system in conjunction with characteri zation by isoelectric focusing. Bentonite fining resulted in removal o f all the different protein fractions. There was no bentonite selectiv ity based on isoelectric point (pl). The amount of protein depletion c orrelated approximately linearly with the level of bentonite addition. Five groups of proteins of differing pl were assigned after character ization of 35 individual FPLC fractionated proteins by isoelectric foc using. The five groups of proteins were added back separately to prote in-free wine. Unfractionated protein was similarly added to fined prot ein but not phenolic-free wine and to tartrate buffer (pH 3.14). All a dditions 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 t o haze/sediment formation. Marked differences in the precipitation/coa gulation of the protein groups after a heat/cold test were observed. T he high pl (greater than or equal to 7.0) protein groups developed a c ompact sediment; the middle pl (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 pl groups, and the low pl (<4.65) group a suspended haze. The two samples containing all pl proteins also developed compact sedi ments. The interaction of proteins with other components in wine, prim arily phenolics, must be considered in order to determine factors resu lting in instability of wine.