Protein concentration and bentonite requirement in Marlborough Sauvignon Blanc wines
Background and Aims
Protein instability has long been a technical issue in white wine production. The aims of this study were: to examine the variability of wine proteins from a single cultivar within a defined wine region over two vintages; and to investigate the correlation of the content of the total and individual proteins and protein haze with the bentonite dosage required for heat stability.
Methods and Results
Proteins in Sauvignon Blanc juices and wines from five selected Marlborough, New Zealand, vineyards with two pruning/training regimes (two- and four-cane vertical shoot positioned) were studied over two consecutive vintages. Proteins were quantified and characterised by Coomassie Brilliant Blue assay, sodium dodecyl sulfate polyacrylamide gel electrophoresis and sodium dodecyl sulfate capillary gel electrophoresis (SDS-CGE). The bentonite requirement for wine protein stability was determined by bentonite fining coupled with a hot/cold test. One vineyard consistently showed the lowest protein concentration and bentonite requirement regardless of pruning treatment and vintage, whereas others varied with pruning treatment and/or vineyard site and/or vintage. Two prevalent juice protein peaks at 22 and 28 kDa in SDS-CGE corresponded to two main wine protein peaks at 22 and 26 kDa, respectively. The 26 kDa fraction was reduced and became heterogeneous after fermentation, while the 22 kDa fraction remained unaffected. There was a good correlation between bentonite requirement and the 26 kDa fraction (R2 = 0.78).
The depletion of 26 kDa fraction in wine determined by SDS-CGE was a good indicator for protein stability by bentonite fining.
Significance of the Study
These results suggest that the method of protein quantification may have a strong influence on the utility of protein concentration measurement to predict the bentonite requirement of wine.