Heat-unstable proteins in grape juice and wine. II. Characterization and removal by ultrafiltration

White Riesling and Gewurztraminer wine and juice were ultrafiltered with Romicon and Millipore systems operated with membranes of nominal molecular weight cut-offs (MWCO) of 10 000 to 100 000 daltons. A progressive increase in membrane retention of soluble protein was observed with decreasing MWCO, up to 99% of the protein in wine being retained with membranes of 10 000-dalton MWCO. In the order of 3 to 20 mg/L protein frequently remained in ultrafiltration (UF) wine permeates; this correlated with the periodic detection of heat instability and of a low bentonite requirement for protein stability as determined by sensitive heat testing. Protein stability could be obtained with MWCO of 10 000 and 30 000; however when not obtained, reductions in the order of 80% to 95% in the bentonite requirement were achieved. The nature of unstable proteins and their removal by UF was further characterized by determining protein molecular weights (MW), isoelectric points (pl), and glycoproteins by lithium dodecyl sulfate polyacrylamide gel electrophoresis (LDS PAGE), two-dimensional isoelectric focusing-lithium dodecyl sulfate (IF-LDS) gel electrophoresis, silver staining, and protein blotting. Protein stabilization of wines by UF is similar to that by bentonite fining in that it is not until the lower MW (12 600 - 30 000) and lower pl (4.1 - 5.8) fractions and glycoproteins are removed that wines become protein stabilized to heat testing. Protein fractions in juices and wines behaved differently with UF. UF membranes were more effective in retaining wine proteins than juice proteins. Membranes were capable of retaining all glycoproteins in wines but not in juices. UF juices were more susceptible to heat-induced haze formation than UF wines.