Better wine through novel and better informed application of microbiology

Abstract

This project sought to provide a better understanding of the potential influence of yeast and lactic acid bacteria (LAB) in fermentation and the resulting wine.

Summary

This project sought to increase the knowledge base specifically in relation to the contribution of microorganisms to fermentation kinetics and wine composition. In addition, it sought to define the basis for these contributions and to develop strategies and strains or treatments which could be used to tailor winemaking in a predictable manner. The key findings are listed below.

• Extensive expertise in yeast and bacterial functional genomics has been developed through this project along with an impressive inventory of strains and specialist research infrastructure. These form the foundation of further work to benefit the industry including, but not limited to, efforts recently initiated through project UA 11/01.
• Nutrient efficient yeast were isolated and/or characterised. The extent to which these proof-of concept strains are also metabolically distinct (with regard to their output of sensorily significant metabolites) is still being evaluated.
• A transposon mutagenesis strategy has successfully been applied to identify yeast genes whose manipulation alters fermentation performance. Deletion of one such gene, ECM33, specifically increased nitrogen efficiency (in low nitrogen media) and fermentation rate (in nitrogen sufficient media). These outcomes have been confirmed for laboratory and wine yeast strains in a chemically defined grape juice medium and grape juices. Efforts to attain analogous (nonrecombinant) strains are warranted as they will offer winemakers greater reliability during fermentation, especially in low nitrogen juices.
• Further characterisation of other High Nitrogen Efficiency (HNE) strains, namely deletants Δngr1 and Δgid7, confirmed their nitrogen efficiency as single deletants. However, unlike the laboratory strain background, deletion of both genes in wine strain DR4 was not synergistic in providing further  eductions in fermentation duration compared to the respective single deletion mutants.
• The ngr1 and gid7 mutants were further charactersised through a number of approaches including phenotypic arrays and metabolomic analysis to provide a greater understanding of the metabolic perturbation in these strains. The precise link between these genes and nitrogen efficiency remains to be determined.
• We demonstrated for the first time the existence of haploproficiency in a wine fermentation context. Heterozygous diploid deletants (i.e. halving of the gene copy number) were found to be haploproficient for wine fermentation, completing fermentation under FAN-limited conditions in reduced time compared to the parental strain. Genes highlighted were IRC9, RMD6, HSP150 and RPL2B. Further investigation exploiting these genes could yield enhanced strains according to this entirely novel concept.
• Optimised strains previously isolated via adaptive evolution strategies were characterised to a high degree using fermentation screens, phenotypic arrays, genome sequencing and metabolomics analysis. The fermentation efficient phenotype of evolved strains FM16, FM5 and L2056-C7 is not due to a single gene modification but rather a number of gene modifications. However single deletion mutants of key candidate genes did display partial fermentation efficient phenotype. These genes can therefore form the basis of optimised strains with increased fermentation robustness.