Ultrafiltration (UF) of White Riesling Juice: Effect of Oxidation and Pre-UF Juice Treatment on Flux, Composition, and Stability

Abstract

Investigations over three vintages indicated that White Riesling juice processed without SO₂ and clarified by ultrafiltration (UF) tended to develop sediments upon storage. This study investigates the possible effect of oxidation (processing with or without sulfur dioxide) and pre-UF treatment of juices with enzymes and fining agents on juice flux, color (browning), composition, and stability. White Riesling juice was ultrafiltered with a Romicon Lab-5 pilot-scale hollow-fiber unit, operated in a batch mode, with membrane of nominal molecular weight cut-off (MWCO) of 10 000 daltons. Grapes were processed with and without SO₂, and the effects of treatment of settled press juice with Rohapect VR Super (VRS, mainly pectinase and protease) and of fining with bentonite, gelatin, and silica sol before UF were investigated. Juice parameters evaluated included total protein, pectin, phenol, color (A_{420 nm}), and stability to heat/cold testing. Pre-UF treatment with enzymes and fining increased flux. Sediments were found to contain large amounts of proteins and phenolics and trace amounts of pectin and neutral polysaccharides. Sediment formation and instability to heat testing of UF permeates processed without SO₂ were prevented with pre-fining. Up to 99% of protein, 90% of pectin, 84% of color, and low variable phenolics were retained by the 10 000-dalton-MWCO membrane. However, pre-UF enzyme treatment can increase protein and pectin in permeates. During UF, there is a significant increase in the soluble protein and water-soluble pectin passing through the membrane with increasing volume concentration ratio (VCR, process time). It is concluded that it is not only the amount but the nature/state of compounds such as proteins, phenolics, and pectins and their interaction that results in instability.