Saccharomyces uvarum and its Role in Winemaking

Abstract

In wine making, fermentation is typically carried out by S. cerevisiae yeast, but S. uvarum can dominate in some 'cold climate' wines, even on occasion completing fermentation. Preliminary work in this laboratory found S. uvarum strains were often associated with an aroma in Marlborough Sauvignon blanc reminiscent of 'Weissbier'. Experiments were undertaken to establish whether S. uvarum was responsible for Weissbier-like aroma (WLA) in New Zealand Sauvignon blanc and to establish its chemical basis. Marlborough S. uvarum isolates were fermented in Sauvignon blanc juice and assessed for WLA in double-blind sensory tests. Correlation of WLA with S. uvarum wines was statistically significant, with the most experienced wine makers always associating the aroma with S. uvarum and never with S. cerevisiae. This result demonstrates that for these strains, S. uvarum was responsible for WLA. Weissbier's distinctive aroma is due to 4-vinylguaiacol (4-VG) and 4-vinylphenol (4-VP). No differences were found between S. uvarum and S. cerevisiae in levels of these volatile phenols for both Pinot noir and Sauvignon blanc wines. Fermentation of Sauvignon blanc with added ferulic acid identified the phenolic off flavour (POF) phenotype in all S. uvarum strains tested and several commercial wine strains of S. cerevisiae. An S. uvarum PAD1 deletion mutant was produced for which the levels of 4-VG and 4-VP were undetectable by GC-MS. Sensory testing of these mutants found good correlation between active PAD1 gene function and POF+ phenotype but no relationship to WLA. This experiment established that volatile phenols are not responsible for WLA in S. uvarum. Fermentations of S. uvarum in Sauvignon blanc and Pinot noir confirm it performs well in early fermentation in the cold and at temperatures under 30°C but residual fructose was found in some wines. Sensory testing of the wines from a commercial-scale trial of S. uvarum in Pinot noir showed lower scores for some fruit aromas and higher scores for 'green' and 'stemmy' for S. uvarum compared with S. cerevisiae. GC-MS and GC-O testing of wines found differences in aroma compounds between the species. In particular phenylethyl alcohol and 3-methylbutanoic acid were significantly higher in S. uvarum than S. cerevisiae wines and this combined with their sensory qualities make them possible contributors to WLA,